High-Strain-Rate Superplasticity Materials And

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ISIJ International. Vol. 36 (1 996), No, 12, pp. 1423-1438 Revie w High-Strain-Rate Superplasticity in Metallic Materials and the Potential for Ceramic Materials

Kenji HIGASHl.MamoruMABUCH11)and Terence G. LANGDON2)

Department of Mechanical Systems Engineering, College of Engineering. Osaka Prefecture University. Gakuen-cho. Sakai. Osaka-fu, 593 Japan. 1) National Industrial Research Institute of Nagoya. Hirate-cho. Kita-ku, Nagoya. Aichi-ken, 462 Japan. 2) Departments of Materials Science and Mechanical Engineering. University of Southern California, Los Angeles, CA90089-1453, USA, (Received on June 10. 1996, accepted in final form on September9. 1996)

High-strain-rate superplasticity (i.e,, superplastic behavior at strain rates over IO' s~ ') has been observed in manymetallic materials such as aluminum alloys and their matrix composites and it is associated with an ultra-fine grained structure of less than about 3/sm. Its deformation mechanismappears to be different from that in conventional superplastic materials. Experimental investigations showedthat a maximum elongation was attained at a temperature close to the partial melting temperature in manysuperplastic materials exhibiting high-strain-rate superp]asticity. Recent]y, a newmodel, which wasconsidered from the viewpoint of the accommodationmechanismby an accommodationhelper such as a liquid or glassy phase, wasproposed in which superplasticity wascriticaliy contro[led by the accommodationhelper both to relax the stress concentration resulting from the sliding at grain boundaries and/or interfaces and to limit the build up of internal cavitation and subsequent failure. Thecritical conditions of the quantity and distribution of a iiquid phase for optimizing superplastic deformation wasdiscussed and then applied to consider the possibility of attaining high-strain-rate superplasticity in ceramic materials. KEYWORDS:grain boundary sliding; accommodation helper, fine grain size; cavitation; Iiquid and amorphousphases.

typical forming rate used for the conventional super- 1. Introduction plastic materials, but rather close to the commercial hot Polycrystalline materials, when pulled in tension, working rates of lO1 to 102 s~1. The high-strain-rate generally break after rather modest amounts of plastic superplasticity phenomenonwas originally observed in deformation, about less than 500/0 in elongation. Under 1984 in a metal-matrix composlte,6) then found in 1985 superplastic conditions of tensile testing, however, very in a mechanically-alloyed material2) as well as metallic high elongations of morethan 500 o/o or in a specific case alloys produced by moreconventional methods.29) Since over 5OOOo/o can be obtained. Superpiasticity is defined 1990, the high-strain-rate superplasticity phenomenon as the ability of a polycrystalline materials to exhibit, in has been studied extensively in somedetail, in metal- composites,7 ~27) jn a generally isotropic manner, very high tensile elonga- matrix mechanically alloyed materi- tions prior to failure. 1) Usually superplasticity is attained als,5) and in moreconventional alloys.30.31) in the low strain rate range from l0~5 to lO3 s~ 1. The Technologically, high-strain-rate superplasticity is of superplastic strain rate range is rather low for commer- great interest because it is expected to result in eco- clal forming of structural materials and therefore the nomically-viable, near-net-shape forming techniques for commercial viability of superplastic materials is limited. the automobile, aerospace, and even semi-conductor In recent years, however, research into superplasticity industries. Therefore, in order to focus on the factors has developed substantially, to the extent that there are contributing to the attainment of superplasticity at high now several important directions for future investiga- strain rates and to discuss the deformation mechanisms tions. Onenewarea is superplastic behavior at relatively in high-strain-rate superplastic materials, workshopson high strain rates over l0~2 s~ I in metallic alloys2 ~21) in- high-strain-rate superplastic forming were held twice at cluding metal matrix composites,6~21) which are pro- LawrenceLivermore National Laboratory on March15, 1994,32) duced by a combination of powder metallurgy and/or and at OsakaPrefecture University on May22, 1995,33) some advanced processing methods.22~27) This high- as shown in Figs. I and 2. Clearly from the strain-rate superplasticity is defined from JIS H7007 by drawings indicating the relationship betweenelongation strain for JapaneseStandards Association as superplasticity which and rate several aluminumalloys with various appears whenthe rate of strain is l0~2s~1 or over.28) grain sizes produced by different processing routes (Fig. 1) size These strain rates are considerably higher than the and a graln dependenceof the superplastic strain

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1ooOO cavitation, It will be proposed that the dominant de- Al AIloys formation mechanismfor al] the high-strain-rate [~~~ ~~~] super- 'e 2oeo, 500'c plastic materials is grain sliding, special ~~l supral' 450'c// boundary and a AI 7475, 504'c ~~l ~~.' 1000 Ai 21 24-0,e~1~zr' 47s'c accommodationprocess by an accommodationhelper ,: llquid is for o such as a phase suggested to be responsible ,p in fine a, the observed hlgh-strain-rate superplasticity very t: o grained materials. This is requlred only to attain UJ [~] not an IOO AI 21 24-20e/~Si d~ 4 IN 902i &9021 ultra-fine grain size, but also to relax the stress 475'C concen- ~~] trations at triple junctions and around the hard particles 'U 21 24~O~~~ilJ'T]1$SiCw 52s'c ' such as relnforcements in the meta] matrix composites 10 in d3 .2 J1 O1 02 order to attain superplasticity at very high strain ld 5 104 1 10 1 1Oo I 1 103 rates. Strain Rate (~~1 ) Further consideratlon for a special process by an ac- Fig. l. Thedrawing indicating high-strain-rate superplasticity commodationhelper such as a liquid phase is discussed, In aluminum alloys with various grain sizes, taken and will include a quantitative estimation for the opti- from the front of the final report of the first page volume and distribution of a liquid phase to relax workshopon high strain rate superplastic forming. 32) mum the stress concentrations during high-strain-rate superplastic fiow. Finally, this review will give guidelines for 104 Consolldated Alloys from the future research and development of materials with A Amorphousor Nanocrystalline capabilities for high-strain-rate superplastic forming 103 Powders Al¥Ni¥Mm¥Zr D Physlcal VaporDeposltlon operations such as metallic alloys and composites, as Mechanlcally Alloyed well as intermetallics or ceramics. 102 @AlumlnumAlloys

IN9021 IN905XL U, 2. Microstructural Features in High-strain-rate Super- 101 ¥CJO ¥ IN9052 plastic Metallic Materials SeN4p'AI¥Zn¥Mg-CuO CSICp/lN9021 O SeN4p/Al¥Mg¥SIO IVQAl¥Cr-Fe ,11 100 p!Al-Zn¥Mg of the superplastic properties of S~N4 O/ Al-N i¥Mm¥Zr Asummary a number S~N4p!Al¥Mg Al¥Nl-Mm = Os~N4p/Al-Cu-Mg of advancedstructural materials is reviewed in Table 1. CD i3N4w/AI-Zn-Mog 1o'1 S (C)~MAI-Zn-Mg-Cu¥Zr The materials include mechanically alloyed materials, U, ~rv~*,PM Al-Cu¥Zr O PMAl-Mg-Zr consolidated alloys from amorphouspowders O SI3N4w/Al-Cu¥M SieN4p/AI-Cu-Mg or nano- u) ¥2 crystalline alloy, ,:; 1o powders, a vapor quenched metal ,:L MAl-Mg-Mn matrix composites, as well as powder (PM) and ingot ,D 1o'3 (IM) metallurgically processed alloys. Top represents the a= Cl) PowderMetallurglcally superplastic Ti the incipient IQ' Processed Alloys optimum temperature, ¥4 point, T~ the solidus the 1o Ingot Metallurgically meitlng temperature and d grain l~ processed Alloys is size. It noted that all the materials have very fine 1o¥5 grained structures from O.5 to 3pmin size, except IM o.ol 0.1 1 10 loo 7475 which has a coarse grain size of about 20,am. The '1/um'l Inverse graln size, d deformation behavior of a material at elevated tem- Fig. 2. The drawing Indicating a relationship between the peratures can be represented by a constitutive equation strain size in superplastic superplastic r'ate and grain which incorporates an activation energy term as given aluminumalloys, taken from the front pageof the final by34'35) pamphlet of the second workshop on high-strain-rate superplastic formlng.33' ._AkGTb(~)P( (T G(T n -Q o Doexp (1) it is RT rate in manyaluminumalloys (Fig. 2), nowwidely is is is is acceptable that high-strain-rate superplasticity asso- where i the strain rate, h the Burgers vector, (T ciated with a very small grain size. It appears, however, the flow stress, (ro is the threshold stress, Gis the shear that the relationshlps between the microstructure and modulus, Do is the pre-exponential factor for diffusion, the mechanical properties of the materials exhibiting Qis the activation energy for superplastic fiow (presum- high-strain-rate superplastlcity are not fully understood. ably Qs~pe'plasti.=Qdirr~sion)' R is the gas constant, T is Currently, the origin of hlgh-strain-rate superplasticity the temperature in degrees Kelvin, n is the stress exponent is under considerable discussion. at typically 2 for superplastic fiow, p is the grain size The first purpose of the present review is to explore exponent at 2-3 depending on the dominant diffusion the possible deformation mechanismsof high-strain-rate mechanism, k is Boltzmann's constant, and A is a superplastic materials. The tensile properties of high- constant which is principally a function of the deforma- strain-rate superplastic materials with fine grained tion mechanism.Fromthe grain size dependenceof the structures of typically 0.05 to 5,tm are characterized over superplastic strain rate from Eq. (1), also as shownin a wide range of strain rates and temperatures. Onthe Figs. I and 2, it is clearly concluded that high-strain-rate basis of the experimental results reported from the superplasticity requires a very small grain size. temperature dependenceof the flow stress, elongation, The microstructural features in high-strain-rate strain rate sensitivity exponent, activation energy, and superplastic materials are as follows; a typical micro-

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Table l. Superplastic properties of advancedand convcntional materials 23.25)

Material Top Tj Ts Strain rate Stress* m Elong. d (s-1) (MPa) va]ue (~{c) atm) IN9052 863 837 866 10 15 O6 330 O.5 IN905XL 848 818 851 20 12 0.6 190 0.4 IN9021 823 754 852 50 18 0.5 1250 0.5 SiCp/IN9021 823 751 866 5 5 0.5 600 O5 Al-Ni-Mm 885 897 1 15 0.5 6SO I.O AJ-Ni-Mm-Zr 873 898 1 15 0.5 650 0.8 VQAl-Cr-Fe 898 896 1 20 0.5 505 0.5 Si3 N4p(lpm)/Al-Cu-Mg 773 784 853 O. 1 5 0.3 640 ¥_.O Si3 N4p(1,lm)/Al-Mg 818 819 866 1 6 o.3 700 1.0 Si3N4p(O2,Im)/Al-Mg-Si 833 830 855 2 5 03 620 l.3 Si3N4p(0.5pm)/Al-Mg-Si 833 829 858 1 6 0.3 350 1.9 Si3N4p(1,4m)Al-Mg-Si 818 822 853 O. 1 5 0.3 450 3.0 Si3N4wA:1-Mg-Si 833 843 858 O. 1 11 0.3 480 3.3 PMAl-Cu-Zr 773 10-2 8 0.3 600 16 I]M Al-Mg-Cr 848 10-2 3 0.5 510 3.4 I]M Al-Mg-Zr 773 l0-l 21 0.3 570 1.1 PMAl-Zn-Mg-Cu-Zr 788 7x IO-'- 8 03 lO60 l.2 PMAl-Mg-Mn 848 845 3x I0-3 07 0.5 660 3.5 IM 7475 806 808 4xl0-5 0.3 O5 2400 19.8

*; stress at e=0.1. structure of the mechanically alloyed materlals, i,e., IN9052 alloy (Al-4.0wto/oMg-1,lwto/oC-0.8wtoloO), IN905XL alloy (Al4.0wto/oMg-1.5wto/oLi-1.2wtoloC- 0.4wto/oO), IN9021 alloy (AI~LOwto/oCu-1.5wto/oMg- l.1wto/oC-0.8wtoloO) and 15volo/o SiCp/IN9021 composite, consisted of very fine grains with a 350-500nm mean size. Carbon and oxygen were present as fine ( carbide (Al4C3) and oxide (AI203, MgO,or Li02) particles. The estlmated volume fraction of these particles in each material wasalmost about 5volo/o. The structures of the mechanically alloyed materials were very stable at high temperatures for long times.5,22) The bulk materlals of A1-14wto/oNil4wto/oMm and Al-14wto/oNi-7wto/oMm1wto/oZr consolidated only by extrusion from amorphousor nanocrystalline powders consisted of nano or near-nano scale structures from 50 to 100nmin grain size. However, these fine structures unstable above about 773 K, Ieading to the were com- fffff :f fffflIIIIIIlifl'lllfjllfllfffl frrrylnl I[Iiprfff I~jnf[ ~ln7jjf~IflJIJI~f Ifffll petition between grain growth and superplasticity that I I exists at high temperatures. It noted that grain was 10 '* *' *' " 15 " " " growth of these materials depends both the annealing on Flg' 3' The high-strain-rate superplasticany formed souvenir time, also temperature and the holding and heating rate of the retirement celebration for Pror. T. Masumoto, up to the annealing temperature. For example, a typical TohokuUniversity. 39] microstructure of the Al-Ni-Mm-Zr alloy, heated up to an annealing temperature of 873 Kwith a very rapid rate uniform dispersion of iron-rich precipitates 3-5nm in and then annealed for a holding time of 30s, consisted diameter. The Al-Cr solid solution was unstab]e at of grains in a large size of about I~mand particulates high temperatures of more than 577 K, resulting in the 0.5 umin size and 30-40 o/o in volume fraction, but nano formation of (Cr,Fe)A17 partic]es of sizes more than or near-nano scale grained structures and particulates lOOnm.An operation near high temperatures of 898 K less than 200nmin size rernained even after annealing for the optimum superplastic flow produced coarser for 30 s at 773 K with a rapid heating rate.22,36~38) The dispersion of (Cr,Fe)A17 With a size of 500nm(30~POo/o high-strain-rate superplastically formed sample used the volumefraction) and a solute-depleted matrix. Thegraln Al-Ni-Mrn alloy, for which the forming time was less size of the matrix was determined to be approximately than 0.5sec, as shown in Fig. 3, was produced as a 500nm.22) souvenir of the retirement celebration by YKKCorp.39) It wasfound frorn typica] microstructures of the high- A typical microstructure of an as-received vapor strain-rate superplastic composites after annealing at quenched(VQ Al-7wto/oCrlwto/oFe) alloy from RAE each optimum superplastic temperature for 1.8ks that (Farnborough, England) consisted of a very fine grained all the grains of the composites were equiaxed and the structure A1-Cr matrix about 100nmin size and a sizes were about I to 3_ ,Im. Scanning electron micro- 1425 Q 1996 ISIJ ISIJ International, Vol. 36 (1 996). No. 12 graphs revealed that the whiskers in the composite were one or the other accommodationprocess in conjunc- aligned parallel to the extrusion direction and the par- tion with grain boundary sllding. The accommodation ticulates in the composites were distributed homoge- mechanismsconsidered can be divided into three groups, neously. Also no voids were observed at the matrix- diffusional accommodation,accommodationby disloca- reinforcement interfaces In all composites.25 ~27) tion motion, and combined models having elements A group of the PMprocessed Al-Cu. A1-Mgand of both dlslocation and diffusional accommodation. It Al-Zn-Mg system alloys with a large amountof Cr, Mn has been demonstrated that sixteen different structural or Zr wasmadeto a fine grained superpiastic structure alloys and composltes exhibit an excelient agreement of less than 5~mby optimum powder metallurgical between the optimum superplastic temperature for the processings.29,3 1) It is noted that the size of the dispersed maximumelongation and the incipient melting point of particles in PMA]-Mg-Zr alloy is one order of magni- the material.40) This broad observation confirms the tude finer than that in other Al-Mg alloys with Cr or suggestion that the presence of a small amountof liquid Mn. The numbersof the observed particles in each PM phase at grain boundaries and interfaces plays an es- alloy increase with increasing content of Zr, Cr or Mn. sential role in the superplastic flow at high strain rates, The values in diameter of the maximumnumbersof the and not only enhances the strain rate but also assists particles are nearly less than 30 nmfor PMA1MgZr, strain accommodation and thus delays the fracture A1Cu-Zrand Al-Zn-Mg-Cu-Zr alloys, about 0.2,tm process. for PMAI-Mg-Cr alloy, and about 0.4~m for PM Very recently, a newmodel48) wasproposed from the Al-Mg-Mnalloy. viewpoint of the mechanismsin the accommodation It is interesting to note in Table I that larger elonga- process by a liquid phase wlth a direct observation of a tions in manymaterials, Including a relatively coarse melting phase along grain boundaries and interfaces grained IM 7475 alloy, are found near the measured using an in situ TEMtechnique. This suggests that incipient melting point. Therefore, it is suggested that a superplastic flow is controlled by a grain boundary slid- liquid phase mayplay an important role in superplastic ing mechanismaccommodatedthrough relaxing the deformation for somealuminumalloys, independent of stress concentration by an isolated liquid phase at grain size.40) interfaces between matrix and reinforcements of the composites exhibiting high-strain-rate superplasticity.48) 3. High-strain-rate Superplastic Deformation Mecha- Superplastlc elongations strongly dependon the level of nisms internal cavitation occurring during superplastic flow. For metal matrix composites, stress concentrations 3.1. Microstructural Observation are caused at interfaces during deformation, so that cavita- Although the detailed mechanistic origins of high- tion is excessively developed at the interfaces, resulting strain-rate superplasticity not yet fully understood, are in premature fracture. This model suggests role interesting experimental observations have been new a some for the liquid phase in high-strain-rate superplasticity noted recently. It initially pointed out by Nieh very was such that the liquid phase serves both to relax the stress et al.4 l) that fine grain size is but insufficient a a necessary concentrations and to limit the build of internal condition for the high-strain-rate superplasticity phe- up cavitation and subsequent failure. This model is sup- and that the observed high-strain-rate super- nomenon, ported experimentally by direct evidence of the presence plasticity be related to the of liquid phases may presence of the liquid boundaries by in-situ transmission electron at grain boundaries interfaces. Especially, in metal or microscopy and differential scanning calorimetry49) and matrix composites this be result of, at least may as a or also investigations on cavitation.50 - 57) accompanied by, the segregation of solutes to such Also very recently, Koike et al.58) have provided regions. Nieh et a/.41) originally proposed rheological a strong support for this hypothesis with direct observa- model for the deformation mechanismsof high-strain- tions of solute segregation at interfaces or grain bound- superplastic composites the semi-solid rate where ma- aries and the preferential melting of these enriched grain terials behavelike non-Newtonianfiuid. It pointed a was boundaries and interfaces. They pointed out, as shown by et al.19) that this is interesting out Mabuchi model in Table 2, that segregation of Si at the grain boundaries but not in agreement with experimental results, some and Mgat the interfaces was identified by electron and it concluded that the deformation mechanisms was energy loss spectroscopy and this segregation was of high-strain-rate superplasticity not be satisfac- can proposed as a key reason for partial melting. It is also torily explained by the rheological view. important to note that the tendency of melting was The deformation mechanisms for high-strain-rate found to dependon the nature of the grain boundaries, superplastic materials to be different from those appear probably because of the observed dependenceof seg- for conventional superplastic materials. In conventional is superplastic materials, grain boundary sliding a Table 2. Concentrationofsegregants in at'/, at the interrace dominant deformation process, and the grain compati- and grain boundary. bility during grain boundary sliding is maintained by M Si N O concurrent accommodationprocesses which mayInvolve interface 9.3 3.2 grain .O 3*9 grain boundary migration, grain rotation, diffusion or boundary ** 2.5 1 matrix 2.7 O. dislocation motion. Most of the models proposed in the 15 * Strong peaks from aSi3N4particle are detected. superplasticity42~47) literature for generally consider ** Thepeak is not detected in ail seven grain boundaries.

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(a) (b) (c) Si3N4

Fig. 4. (a) TEMbright-field, (b) dark-field images and (c) selected area diffraction pattern of AlMgcomposite taken at 821 K.58)

1ol xl02 20 I 833 K Si3N4p(d=0.5Hrn, /Al-Mg-Si CI 783 K Si3N4p(d=1.0Hm)/AI-Cu-Mg 8MPa 1.0 :~:(G' 10 e- Oo c~, u' i (D E > E CTS ,,r O G) 15 ~:5 '; O (D ':! 200 E o ~5 0~1 o > 1 G, 8MPa Q 8 = 0.2 E 100 Si3N4p/Al-Mg-Si Composite ~ O s~1, (~ =8MPa,t=2 T=833K) e 7475 Alloy 0~4 ~,1 T=788K) ((;=5MPa, e=5xl , 1o~ o o.o 0,5 1,0 1.5 2,0 0.1 0.2 0.3 0.4 05 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 Strain Diameter ot cavity, um Fig. 5. The variation in the cavity volume fraction as a Flg. 6. Histograms showing the distribution of cavity dia- function of true strain for the Si3N4./AlMg-Si meters in two composites at strains of 0.2 and 1.0.54) composite and the 7475 alloy.50) composite was comparedwith that of a conventional regation on the grain boundary structure, and the results superplastic 7475 A1 alloy and it was concluded, as cavity obtained were explained by a decrease of the solidus shownin Fig. 5, that the rate of increase in the temperature due to segregation whoseextent dependson volume for the composite wasmuchlower than for the the type of the grain boundary structure. As shownin alloy, where the testing temperature for the composite Fig. 4, the tendency of melting appears to be dependent was slightly above the onset temperature for partial on the type of grain boundary structure characterized melting and the testing temperature for the alloy was by the misorientation betweenneighboring grains. In the below the solidus temperature.50) These results dem- bright-field image, two types of grain boundaries are onstrate that a liquid phase iimits development of it is liquid observed and marked as gl and g2' In the dark field cavitation. Therefore, likely that the role of a image, only the gl grain boundary has a bright contrast, phase in high-strain-rate superplasticity for metal matrix indicating melting. Quantitatively, the misorientation composites is to accommodatethe siding process and angle, determined from Kikuchi diffraction patterns, is relax the stress concentrations so that cavity develop- is 6, qualitative 18 degrees for the gl boundary and 3degrees for the g2 ment minimized. As shown in Fig. boundary. Theresults suggest therefore that the tendency analysis of cavitation for two high-strain-rate super- of melting along the grain boundaries is dependent on plastic composites revealed that most of the cavities were the misorientation angle between adjacent grains. very small (~I ktm), and nucleation might occur readily On the cavitation behavior, furthermore, it was at interfaces at low strains but the cavities grew relatively reported by lwasaki and his colleagues53) that the rate slowly with strain.54) The very small sizes of cavities of increase in the cavity volumeat a temperature slightly with strain are associated with a limitation of cavity is above the onset temperature for partial melting was development due to the presence of a liquid phase. It applications significantly lower than at a temperature below the onset interesting to note for superplastic forming static temperature for partial melting for a high-strain-rate that such small cavities can be easily reduced by superplastic composite. The cavitation behavior of the annealing after deformation.s5)

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flow whenthe stress concentrations are not sufficiently 3.2. Sliding, Stress Concentration and Critical Strain- relaxed by a conventional accommodationprocess. They rate proposed the concept of critical strain rate, ~*, i.e., the It is accepted from manyexperimental results that accommodationmechanismis diffusional flow and/or grain sliding plays role boundary an important to obtain diffusion-controlled dislocation movementin a strain large elongations in is superplastic materials and a rate range below the critical strain rate, on the other dominate deformation superplastic mechanismduring hand, a special accommodationprocess by an accom- flow.59) of the dorninant microstructural features in is One modation helper such as a liquid phase required in a is superplasticity the role p]ayed by grain boundary strain rate range above the critical strain rate because sllding. The grain compatibility during grain boundary stress concentrations are caused around reinforcements. sliding is maintained by concurrent accommodation An estimate of the accommodationmechanismsusing processes which mayinvolve grain boundary migration, the critical strain rate is not a rigid estimate since the grain rotation, diffusion or dislocation motion. If the surface energy, the morphology of the reinforcements sliding large displacements are too to be accommodated and the presence of threshold stresses are not taken into elastically, the sliding dif- must be accommodatedby consideration. However, some trends of the accom- fusional or plastlc flow. Therefore, the shorter distance modation mechanismscan be approxlmately examined with the refinement of grain size can enhance the ac- for superplastic flow in the metal matrix composites commodationby diffusional or plastic flow. The origin especially. of high-strain-rate superplasticity Is associated with The critical strain rate, i., Is defined in the following ultra-fine sizes grain of the materials. However,advanced forms for grain boundary or lattice diffusion-controlled ultra-fine grained materials exhibiting high-strain-rate superplastic flow respectively superplasticity contain a large volume fraction (5-30

volo/o) finer Vl.,,,.,,___,~f of second phase particles pinning graln )2( 8ald )2 DaG~~~'2B ~c 1'18 8 l +5 )( growth, in Sec. 2. In these materials, =: x 10~ as mentioned a high dp concentration in local stress by grain sliding, boundary '(2-a) if it is not fully accommodatedby diffusional or plastic (for grain boundarydiffusion-controlled superplastic flow) flow, causes excessive cavity nucleation at the interfaces between particles and the alloy matrix or at triple junc- 5.92xl0~10(QE) (1+5~alclp )(2 1 2 tions. In fact, microstructural observations from D many kT dp Vf L superplastically deformed materials indicate that cavities .(2-b) are initiated at the po]es of the particles and triple junc- (for lattice diffusion-controlled tions of grain boundaries and subsequent growth and superpiastic flow) coalescence invariably leads to premature failure.s2,60) where DL is the lattice self-diffusion coefficient, DGI; is In order obtain large superplastic to elongations to the grain boundary diffusion coefficient, a is the ratio of failure for ultra-fine grained superplastic materials with the diffusion coefficient (=DGB/DL), E is Young's manypinning particles, including for the metal matrix modulus, dp is the particle diameter, Qis the atomic composites, special mechanismsare required for the volume, b~ is the grain boundary width and Vf is the accommodationprocess to relax stress concentrations volumefraction of the reinforcements. Thecritical strain near interfaces as well as grain boundary triple junctions. rate, ~*, dependson the particle size, dp and the volume Especially in metal matrix composites having a high fraction, Vf' of the reinforcements. The variation in i* volume fraction of hard reinforcements, high-strain-rate as a function of the value of T/T~ in the aluminumalloy superplasticity is not necessarily attained only by ultra- matrix composites having a meangrain size of I and fine sizes is um grain becauseexcessive cavitation consider.ed a volumefraction of the reinforcements of 20 o/o is shown to be causeddue to the high stress concentrations around in Fig. 7for reinforcement particle sizes of 0.2 and I ~m the reinforcements intersected by grain boundaries. In in the case of lattlce diffusion-controlled and grain particular, it be difficult appears to to accommodate boundary diffusion-controlled superplastic flow respec- grain boundary sliding at high strain rates by diffusion tively. The T~ value is taken to be the absolute melting processes, also including diffusion-controlled dis]ocation point of pure aluminum. Thecritical strain rate strongly movement,becausethe times are too short,s) High-strain- dependson the particle size of the reinforcements, and rate superplasticity seemsto be critically controlled by increases with increasing temperature and with decreas- the accommodationprocess to relax the stress concentra- ing reinforcement particle size, dp. Furthermore, the tion resulting from sliding at grain boundaries and/or critical strain rate is dependenton the dominant diffusion interfaces, invo]ving an accommodationhelper such as process, and especially in the higher temperature range a liquid phase.23) Therefore, it is important to makeclear the critical strain rate in the grain boundary diffusion- the accommodationprocesses for superplastic flow at controlled superplastic flow is higher than in the lattice high strain rates, especially in the metal matrix com- diffusion-controlled superplastic flow. It is therefore posites. suggested that it is important to use reinforcements It was reported very recently by Mabuchi and having small size in order to limit cavity formation Higashi61) if that the above view is correct, the ac- caused by the stress concentrations around reinforce- commodationhelper such as a liquid phase is required ments for the given constant matrix grain size and as an additional accommodationprocess for superplastic constant volume fraction of the reinforcements. r.(*:, 1996 ISIJ 1428 ISIJ International, Vol. 36 (1996), No. 12

It is worthwhile to compare the predicted crltical is a fact that high-strain-rate superplasticity in the strain rate obtained in the present analysis with the composites reinforced with the smallest slze of O.2~m experimental resuits for some high-strain-rate super- was attained at a high stain rate of lO1sl In a solid plastic aluminum matrix composites. In this study, state including no liquid phase. Asshownin Table 3, the Si3N4p/A125) and SiCp/AI compositesl8,22,60) are em- higher critical strain-rates of more than l0~1 s~ I were ployed for comparison between theoretlcal analysis and obtained in the composites reinforced with the smallest experimental results because the mechanical properties slze of O.2 ,lm. In fact the predicted critical strain rate of and melting behavior of these composites are known. the 0.2/tm Si3N4/A1-Cu-Mgcomposite is 4x lO~ I s~ l The predicted critical strain rate and the experimental which is very close to the experimentally measuredstrain data of these composites: the optimumsuperplastic strain rate of 3x lO~ I s~ 1. This trend is probably due to the rate for maximumelongation, the grain size, the particle fine grain size of I ,am in the matrix and the lower stress size, the volume fraction of particles, the flow stress at concentration around the reinforcements with the 8= 0,1, are listed in Table 3. The optimumsuperplastic smallest size of O.2 ,lm which can be predicted from Eq. temperature for a maximumelongation and the onset (2), indicating that the stress concentration around temperature for partial melting are also listed in Table reinforcements decreases with decreasing the relnforce- 3. It is noted that partial melting occurred for all of these ment size and therefore Is not caused because of rapid aluminummatrix composites. relaxatlon by the diffuslon accommodationprocesses in High-strain-rate superplasticity in the composites, a solid state. The result that the local stress around the except the 0.2~m Si3N4p/Al-CLu-Mg composite, is reinforcements by the stress concentration is muchlower basically attained at a temperature very close to the onset than the experimental flow stress seemsto indicate that temperature for partial melting. The high-strain-rate the stress concentrations are sufficiently relaxed in a superplasticity in the 0.2 ~mSi3N4p/A1Cu-Mgcompos- solid state for the 0.2,am Si3N4p/Al-Cu-Mgcomposite. ite is obtained at a temperature of 773K, which is more In that case, however, the elongation obtained is not so than 40 KIower than the partial melting temperature. It large at less than 300 o/o, comparedto morethan 3000/0 in a solid and liquid state for other aluminum matrix 19) 10 3 composites. Aluminumalloy matrix composites On the other hand, for the other composites the predicted critical strain rates fundamentally 10 2 Vf of reinforcement, 20 vol% are much M~trix grain size, I um lower than the experimental superplastic strain rates. It tO ,. is special 10 1 therefore suggested that a process by an ac- C' IL / ,~, 1 commodationhelper such as a liquid phase relaxes the dp=0.2 l um is 10 a high stress concentrations, which required to continue '~: / superplastic flow. is essential only Cl) / This information not ~~,,, 10 ~1 / in clarlfying the cause of partial melting but also in ,5 -aL / / considering the deformation mechanismduring high- G' -2 / It is GJ 10 I~ / strain-rate superplastic flow. important to examine :F CO / ~l ~ the nature of the interfaces of current superplastic ~ 10 -3 / materials in order to understand the origin of high- f strain-rate superplasticity. O / 10 -4 / lattice di frusion-controlled 3.3. Accommodation Helpers and Their Optimum / superplasticity Amountand Distribution 10 '5 grain bouT]dary diffbsion- liquid controlled superplaslicity The optimum amount of phase maydepend the precise material composition and the precise 10 *6 upon 0.5 O.6 O.7 O.8 0.9 1.O nature of a grain boundary or interface, such as local TrTm chemistry which determines the chemical interactions

f;. in the liquid in the Fig. 7 The variation in as a function of the value of TIT* between atoms phase and atoms in aluminumalloy matrix composites having a mean neighboring grains, and also the magnitude and the type grain size rein- of I,Im and a volmne fraction of the of the misorientation. Only a small amount of liquid forcements of 20 "/, for the reinforcement particle sizes phase be present at temperatures close to the of 0.2 and I ,tm in the case of lattice diffusion-con- may incipient melting point, and it would be expected to trolled and grain boundary diffusion-controlled superplastic flow, respectively. 61) segregate to grain boundarles and particularly at grain

Table 3. Comparison between the experimental data and the predicted critical strain rate for high-strain-rate superplastic aluminumalloy matrix composites.61) Material Critical Optimum Grain Particle Volume Flow Optimum Partial melting strain superplastic size, size, of stress, superplastic temperature, rate, s~1 strain rate, s~ m m article,% MPa tem erature, K K PM-Si3N4p/Al-Cu-Mg 4xl0-l 3xl 0'1 1.1 0.2 20 8 773 816 PM-Si3N4p/Al-Cu-Mg 7xl0'4 4xl0-2 2.3 1 20 2 788 774 MA-SiCp/Al-Cu-Mg 6xlO'z 5 0.5 2 15 15 823 75l PM-Si3N4p/Al-Mg-Si 4xl0-l 2 1.3 0.2 20 5 833 830 PM-Si3N4 /Al-Mg-Si 8xl0-4 10-1 3.0 20 5 818 822

1429 @1996 ISIJ ISIJ International, Vol. 36 (1 996), No. 12

triple junctions. However, Iarger volumes of a liquid, achieved. Very recently, Jeong et al.,63) jnvestigated the or a continuous liquid iayer, can not support normal nature of the matrix/reinforcement interfaces of the tractions, and therefore can not contribute to large high-strain-rate superplastic Si3N4~/Al-Mg-Si com- elongations. Thus, intergranular decohesion at a liquid posite by high-resolution electron microscopy. HREM grain boundary leads to intergranular fracture and very observations revealed that newphaseswere found at the limited elongation. This increase in liquid explains the interfaces, and these phases grow epitaxially with an drop in elongation observed at the highest temperatures FCCstructure. ATEMimage of a Si3N4~crystal in the above the melting point in manyhlgh-strain-rate super- samp]e pulled at 818K is shown in Fig. 8. The EDX plastic materials.5,19,20.21,38) There apparently exists a results and HREMobservations suggest the following critical amountof liquid phase for the optimization of reaction process during tensile testing. Segregation of grain/interface boundary sliding during superplastic Mgand Cu from the Al-matrix or somecompounds, deformation. e.g., Mg2Si, during the extrusion process serves as start- Fromthe quantity of heat released in DSCinvestiga- ing points of a reaction betweenthe Si3N4~crystals and tions,20,49,57,62) the volume fraction of a liquid phase Al-matrix, and the reaction produces regions enriched can be approximately estimated to be 4-18 o/o, also the with Mg, Si and Cuat the interfaces. The concentration thickness of the liquid phase along the interfaces can be of Mgand Si lowers the melting point of regions near estimated to be about morethan I ,lm from the observa- the interfaces, and finally appear to result in partial tion by in-situ transmission electron microscopy for the liquid regions at the interfaces at high temperatures. In high-strain-rate superplastic aluminum matrix com- the cooling process after the extrusion, Al primary solid posites in the temperature range for partial melting.49) solution crystals including Mgand Si are solidified from However, the estimated values in both the volume the liquid regions at the surfaces of Si3N4~crystals and fraction and the thickness of the liquid phases seemto epitaxially grow into the matrix. It is easily understood be too large for superplastic deformation by tensile from the above speculation that the local liquid regions straining because decohesion at a liquid phase is easily also appear at the Si3N4~/Al-matrix interfaces testing at

0=1H-m

Fig. 8. (a); TEMimage of a Si3N4~embeddedin the A1-matrix in the elongated sample. (b) and (c); HREMimages of the places indicated by the B and C rectangles, respectively.63)

C 1996 ISIJ 1430 ISIJ International. Vol. 36 (1 996), No. 12 an optimum superplastic temperature and cause the Tgb, from eachother. 38) Thegrain boundarycan therefore high-strain-rate superplasticity. tensile force. Theobserved newphases sustain an applied Also, shear stresses 1: can were concluded to result from partial melting. Hencethe be transferred across the boundary. However, it was areas for partial melting at the optimum superplastic pointed out for many high-strain-rate superplastic temperature can be estimated by investigating the new materials that, at higher temperatures close to the solidus phases. temperature, macroscopic melting begins to occur, the TheHREMinvestigations revealed that the thickness liquid phase is thick, and atoms across two neighboring of the liquid phases at the optimum superplastic tem- grains can no longer experience traction from each other, perature is about less than 30nm, and the distribution and therefore larger volumes of a liquid or a continuous is discontinuous. A Iiquid phase at the optimumsuper- liquid layer can not contribute to large elongations. plastic temperature is rather thin by comparison with a Thus, intergranular decohesion at a liquid grain bound- llquid phase in the temperature range measuredby DSC ary leads to intergranular fracture and very limited for partial melting. The estimated volume fraction of a elongation. As already noted, this increase in liquid liquid phaseat the optimumsuperplastic condition based explains the drop in elongation observed at the highest on the HREMobservations is probably about I */* or temperature above the melting point.5,20,21,38) It is less. Thus, the volume fraction of a liquid phase is important to note that a critical amountof liquid phase required to be very smali in order to attain superplas- exists for the optimization of grain/interface boundary ticity. sliding during high-strain-rate superplastic deformation. Theoptimumsuperplastic strain rates strongly depend The superplastic elongation strongly depends on the on the refinement of grain structure. Also the high- refinement of grain structures and the accommodation strain-rate superplasticity is critically controlled by the process to relax the stress concentration by the presence accommodationhelper to relax the stress concentration of the helpers, such as a soft phase, amorphousphase resulting from grain boundary sliding. Optimumsuper- or liquid phase, as summarizedin Fig. l0.23) It is impor- plasticity is obtained at a temperature close to the partial tant to note that the current modelsproposed for super- melting point or solidus. Control of the distribution of plastic deformation are all considered to be based on a liquid phase is the most important to limit decohesion solid state of the materials. If the accommodationproc- al.,64) at a liquid phase in a tensile stress field. Geeet ess in the solid state by diffusional or plastic flow is not showedthat the effective viscosity in very thin liquid fully adjusted, the high concentration in local stress 105 films can be times the bulk value and molecular forms cavities around the interfaces of the particles or relaxation times can be 1010 times slower. It is therefore at triple junctions of the grain boundaries. The results suggested that decohesion at a liquid phase is limited in the present review reveal that the presence of a liquid even in a tensile stress field whenthe liquid phase is very phase at grain boundaries or liquid boundaries at high thin. It is important for attaining high-strain-rate su- temperatures is one of the possible accommodation perplasticity to note that a liquid phase should be less processes to relieve the stress constraints by grain bound- than l*/* in volume fraction, Iess than 30nmin thick- ary sliding, and this leads to a decrease in the level and ness and discontinuously distributed. A very thin and the growth rate of cavities. Therefore, a newmodel of discontinuous liquid phase is required both to relax the the special process promoted by an accommodation stress concentrations and to limit the decohesion. helper such as an isolated liquid phase is proposed as Thepresence of the liquid phaseat interfaces and grain the accommodationprocess for grain boundary sliding boundaries not only enhances the strain rate but also in the high-strain-rate superplastic deformation mecha- changes the deformation mechanism, in addition It nism. Thenewfinding of the accommodationhelper such reduces the local stresses and thus decreases both the as a liquid phase in superplastic materials will totally cavity nucleation and growth rates. As shownin Fig. 9, change the traditional understanding of the super- whenthe liquid phase is isolated only at triple junctions (a) or the thickness of the liquid phase along the Grain BoundarySliding + Accommodation boundaries is very thin (b), atomsin the solid state across two adjacent grains can still experience a traction force, SOlid State

Accnmmodationby difnlsionaj Qr plastic tlow Is not fu]ty ach:eved r~~~~]~Lo~~~~~~~~J}r?~~ AIarge vetumetraction ct r[ner secondPhaseparttclG5 High strain rnte Pinning gratn growth L8rge etongatiQn

High concontration At partieies or lu :ocal stress by trlple Junctions sRding nt bQundar! and in~erface

Excessive c8vlty nuclcation Premature rai]ure o :~hhancdd rel~~aiion in stress concent}a'~id'ns (a) (b) :*=iPIPre ,h,.?~ t.~~t b.! diffY~ipnal or plas,jF fi?V

Fig. 9. Schematic representation of grain structure in the Fig. lO. Aschematic explanation for superplasticdeformation presence of grain boundary liquid phases.38) mechanismsof GBSand accommodation.23)

1431 C 1996 ISIJ ISIJ International, Vol. 36 (1996), No. 12 plasticity mechanism.Thepresence of the helpers to relax ramics are significantly finer, usually less than Ium stress concentrations resulting from the sliding of the as shown in Table 5, whereas superplastic behavior grain boundaries and/or interfaces at very high strain can be observed for grain sizes typically 1-lOptm in rates is very powerful. Oneof the most important and metalllc systems including intermetallics. The optimum potentially exciting research areas in the near future is superplastic strain rate for each superplastic material is to design and achieve the optimum distribution of the clearly different from each other at a given constant grain helpers as well as a refinement of the grain size. size, as shownin Fig. 11, although it is a commonfeature that the superplastic strain rate range increases with the refinement of grain for all of the metallic alloys, metal 4. Potential for High-strain-rate Superplasticity in matrix composltes, intermetallics and ceramics. The Intermetallics and Ceramic Materials superplastic strain rate range at a given constant grain The typical potential for superplasticlty in inter- size in both metallic alloys and metal matrix composites metallics and ceramics is summarizedin Tables 4and 5, is muchhlgher than in intermetallics and ceramics. It is respectively. There is muchin commonin the description aiso understood that the grain size of ceramics is one of superplasticity among metallic materials, inter- order finer than that of metalllc materials at a given metallics and ceramics. The high mvalue of 0.5 or over, constant superplastic strain rate. For example, at a strain suggesting that grain boundary sliding maybe one of rate of l0~4 s~ I In Fig. 11, the grain size of ceramics is the dominant deformation mechanismsin superplastic about I~mwhereas the corresponding grain size of ceramics as well as metallic materlals, indicates the metallic materials is about 10 ~m. assurance of neck stability under tensile loading but The difference In the optimumsuperplastic straln rate these values alone are not reliable indlcators for obtain- amongthe superplastic metallic materials, intermetallics ing large ductllity. The basic structural requirement and ceramics is associated with the scaling law for high plasticity.94) remains the sarne, i.e., a fine, equiaxed grain size that is temperature From a comparison of the reasonably stable during high temperature deforma- diffusivity betweenaluminum-basedmatrix and Y-TZP tion and the presence of high angle grain boundaries. at 0.75 of their respectlve melting points, it was deter- The phenomenaof grain boundary sliding, generally mined that the grain boundary diffusivity of aluminum equiaxed deformed grains, strain-enhanced grain growth was ten times that of Zr4+ jon in 16molo/o Y203- and intergranular cavitation are commonto all the stabilized Zr02' Thls reported ratio is approximately in superplastic materials including metallic materials, the sameratio of the typical - 10 ,am grain size of meta]s intermetallics and ceramics. The incidence of grain to the typical - I ~mgrain size for ceramics in super- boundary sliding remains an important topological plasticity. Therefore, the lower superplastic strain rates characteristlc in superplastlcity. in the ceramics mayresult from the lower diffusivity in The corresponding grain sizes for superplastic ce- the accommodationmechanismsto relieve the stress

Table 4. Typical potential for superplastic behrrvior in intermetallics.

Materiai Etongation, strainraLe, stress, m Tem.* Grain References % S'I MPa value K size, m Fe~(Al.Si) lOO l04 95 0.3 ll23 IOO Hanadaet al. (65) Ni,Al+B 160 5x IO-s 23O o.45 973 l .6 Kimet al. (66) Ni3Al+Cr 1OO l0'3 20 0.5 1273 lO Wright et al. (67) Ni,Al+Cr+Zr 280 10'3 1273 lO Wright et ai. (67) Ni3Si 650 l0'3 o.5 1353 15 Nieh et al. (68) Ni,Al 640 9xl04 1373 6 Valiev et al. (69) Ni,(Si.Ti) lOO 6xIO's 65 1123 5.5 Takasugi et al. (70) TiAl 250 8xI04 165 0.4 1298 5 Imayevet al. (7 1) Ti,Al+Nb,V.Mo 570 2xI04 26 o.5 1253 Yanget al. (72) TiAVri,Al 405 l04 6 0.6 1273 1.2 Tokizane et al. (73) TiAVTi,Al 275 3xI04 50 0.5 1323 5 Chenget aL (74) TiNi 440 6xI04 o.45 1273 l .4 Ezaki et al. (75) TiAVTi,Al 400 3xIOJ 20 0.44~.7 1323 2 Wajata et al. (76) TiAVTi Al 550 6xI0'3 15 05 l323 0.85 Ameamaetal. 77

Table 5. Typical potential for superplastic behavior in ceramics. Material E]ongation, Strain rate, Stress, m Tem., Grain References % MPa value K size, m U02 69 -104 1 1623 2 Chunget al. (78) MgO > 80 2xI0'5 11O 0.8 1327 0.1-1.4 Cramponet a]. (79) Si02/ Al203 300~}OO 8xlO's 1 1298 1 Si02/ Al20a 135 2xI04 1 1423 1 Wanget al. (80) Y-TZP > 120 0.5 1723 0.3 Wakai et al. (81) Al203 > 45 10~ 30 1693 1.6 TZP/209~,, Al20a > 200 10~ 16 0.5 1723 0.5 Wakai et al. (83) Y-TZP 35O 3xl0~ 10 O.33 1823 0.3 Nieh et al. (84) TZP/80%Al203 120 3xl0~ 16 1823 1 Wakai(85) Y-TZP 99 l04 1 1600 0.53 Duclos et a]. (86) Si3NdSiC 150 4xI0-5 30 0.5 1873 0.2 Wakai(87) Y-TZP 800 8xIO's 9 0.5 1823 0.3 Nieh et al. (88) TZP/20%AJ203 625 4xI04 6 0.67 1923 O.S Nieh el al. (89) Ti 02 >60 10-3 0.33 1073 0.04 Hahnet al. (90) Y-Zr02 >150 10~ 8 0.33 1423 0.08 Ma)'o (91) Y-TZP+SI02 l038 10~ 12 1673 0.26 Kajihara et al. (92) Y-TZP 320 3xIO's 0.3 1803 0.5 Maet al. 93

~~ 1996 ISIJ 1432 ISIJ International, Vol. 36 (1 996), No. 12

105

Wiftlcl,t 4 liquid GBph(Ise 1O o Metauic Alloys l Metauic Composites Impurity cootrolled l Superpbsticity: 3 1 creep: l 1O l c,, n*3, p'l, D*~ n*2, p* 2* D=Dgb 102 ,s o I dl ca I a: iol t:1 C: D '~ 100 D] o d2

(/)o 10'1 *V gt 1- u),:f 10~2 ll- l * 10~3 A 2~ e 'V ~C: D~ e e 5 10~4 l' e co A A e 1O~5 e da > dl iO~6 A IntermetalUcs O Ceramics tog 0~7 ' a 1 ~2 O~1 Oo O1 02 1O 1 1 I 1 Fig. 12. schematic nlustration ofthe logarithmic variation of tnverse Crain Size, um-1 strain rate with stress for Y-TZPceramics without a hquid grain phase, that the Fig. Il. The variation in superplastic strain rate with refine- boundary assuming superplasticity is ment of grain size for metallic alloys, metal matrix mechanism simnar to the behavior composites, intermetallics and ceramics. in metals at high stresses but is impededat low stress ievels by an impurity-controned process.100) concentration by sliding of grain boundaries or inter- in the sample. It is nowwell established that Al203 and faces. Si02 maysegregate to the grain boundaries in Y-TZP Grain boundary sliding itself could be the rate con- ceramics and form an amorphousphase which becomes trolling mechanismif the rate of the accommodation liquid at the elevated temperatures used for tensile processes is faster than for grain boundary sliding. testing. There have been several reports establishing the Alternatively, if grain boundary sliding is intrinsically presence of a grain boundary amorphousphase in these rapid, the rate controlling accommodationprocess would materials. For example, Hermanssonet al.,96) examined determine the characteristics of superplastic flow stress. 3Y-TZPcontaining 0.092wto/o Al203 and 0.011 wtolo rat~- To date, the accommodationprocess seemsto be Si02 and they reported the presence of an intergranular controlling, therefore the proposed various theoretical glassy phase having a thickness of - 12nm. Similarly, models for superplasticity differ from each other pri- Stoto et al.,97) reported the presence of an amorphous marily in the details of accommodatingthe stress con- phase in 3Y-TZP containing 0.065wto/o Al203 and centrations that arise due to grain boundary sliding. 0.002wto/o Si02' Onthe other hand, Nieh et al.,88,98) Especially for ceramics a contribution by a dislocation were unable to detect the presence of an amorphous slip related accommodationprocess should be expected phase in 3YTZPspecimenscontaining 0.005 wto/o Al203 to be less than that by diffusional stress accommodation and 0.002wto/o Si02' An extensive microstructural because of the intrinsically strong strength and the investigation of 3YTZPsamples containing 0.005 wto/o directionality in covalent bonding of the crystal bonds. Al203 and 0.002 wto/o Si02 has shownthat the crystalline This predicts that extensively high dislocation activity lattices of adjacent grains are continuous up to their should be an insufficient accommodationprocess during commonboundaries.99) The observation in Y-TZP superplastic in this flow ceramics, and situation leads to ceramics with total Al203 and Si02 contents at or below a need for the presence of accommodationhelpers such -0.007 wto/o showsthat there is no amorphousphase at as liquid or glassy phases. the grain boundaries at least downto the fringe spacing The superplastic ceramics have been addressed to of 2.5A. From these observations and the consistent have two types of boundary depending on whether or reports docurnenting amorphous phases in Y-TZP there is liquid not any at the boundary at the testing ceramics where the Al203 and Si02 contents are higher, temperatures. In fact both superplastic ceramics of it becomesapparent that it is necessary to examine ionically-bonded and covalently-bonded ceramics, e.g., separately the characteristics of superpiasticity in Y-TZP zirconia polycrystals, Y-TZP95) and Si3N41SiC com- samples without and with an amorphousphase at the posite,87) often contain a very small amountof a liquid interfaces. The deformation mechanismsin these two or glassy phase in the grain interface. The presence of types of material have been discussed in somedetailloo) a thin film of liquid or glassy phases at the grain inter- and are reviewed in the following sections. face can drastically enhancediffusion through the inter- Figure 12 depicts schematically the predicted loga- face and can have a dramatic effect on the superplastic rithmic variation of strain rate with stress for two dif- characteristics as well. In practice, the precise role of ferent grain sizes, dl and d2 (where dl d2), of YTZP grain boundary processes becomesof special significance ceramics without a liquid grain boundary phase, when in Y-TZPceramics because the nature of the grain it is assumedthat the superplasticity mechanismis similar boundaries dependscritically uponthe level of impurities to the behavior in metals at high stresses but it is irnpeded

1433 O 1996 ISIJ ISIJ Internationai, Vol, 36 (1996), No. 12 at low stress levels because of an impurity-controlled process. Although experimentai data are limited, in- Wtih liquid GBphase spection is consistent with the reported trends.lol - I03) loterface reaction i solutiofl- p,ecipitation colltrolled creep: I creep: In the absence of amorphousphase at the grain I any n'2, p'l, D¥Oad I n'l, p¥3, D¥Diq boundaries, the interfaces in the very high purity 3Y- I l TZPremain crystalline and it is reasonable to assume I in I that, since tests are conducted ceramics under con- dt ditions where the equilibrium subgrain size is larger than the grain size, superplastic deformation will occur by a ¥u, similar in superplastic metals. noted mechanismas As =~7 d2 earlier, grain boundary sliding is the dominant process in the superplastic region for metals and sliding takes ~~]l i place by the movementof dislocations along the grain boundaries.47) This process, whensuitably modeled to incorporate accommodationthrough intragranular slip, da > dl leads to Eq. (1) with n=2, p=2and D=Dgb. it the limited data at available, log Based on present ¥ ~ that the true superplastic mechanismis realized appears Fig. 13. Schematic illustration of the logarithmic variation of only at the higher stress ievels in ceramics where Y-TZP strain rate with stress for Y-TZPceramics with a the experimental value of n is close to 2. In this region, liquid grain boundary phase, assuming solution- al.,103) there are reports by Chokshil02) and Ye et of precipitation creep is impededat low stress levels by interface reaction I oo) values of p of - 3and - 2.1, respectively. The behavior an process. at the lower stress levels, however, is not consistent with the trends generally observed in superplastic metals. that and Dliq' where Dliq is the diffusion n = I, p=3 D= Although both metals and Y-TZPceramics exhibit an coefficient in the liquid. As the stress is reduced, interface increase in the value of n at the lower stress levels, the reaction creep becomesrate-controlling and the precise value of p for metals remains essentially the samein variation of strain rate on stress dependsuponthe nature regions I and 11 whereasin YTZPceramics the evidence of this reaction but, typically, n = 2, p= I and D= D*d, in Fig. 12 suggests that p decreases to - I at the lower where D*d is the diffusion coefficient in the adsorption stresses.102) The stress exponent of 3at low stresses is layer between the liquid film and the crystalline matrix. consistent with an impurity-controlled type of creep such Figure 13 illustrates schematically the predicted loga- as the accommodationof grain boundary sliding by a rithmic variation of strain rate with stress for Y-TZP viscous drag process. Since a stress exponent of 3 is a ceramics containing an extensive liquid phase for two characteristic of dislocations movingbyviscous glide, I 04) grain sizes, dl and d2 (where dl d2)' This predicted the accommodationof sliding by a drag process will lead trend is consistent with experimental data for 2Y- directly to n = 3. In addition, because the total strain is TZP.l09) accumulated by sliding at the boundaries and intra- Theinterpretation of superplastic-like flow in Y-TZP granular slip serves only as an accommodationprocess, ceramics is madedifficult by the paucity of experimental it will lead also to Finally, it is anticipated that data well-documented microstructures and by the p= I . on D=Dat low stresses, whereDis the diffusion coefficient clear demonstration that, depending upon the purity for the appropriate solute within the crystalline lattice. level and especially the total content of Al203 and Si02 It has been reported that yttria fails to segregate to the in undoped Y-TZPsamples, the nature of the grain grain boundaries in 3Y-TZPin the absenceof an amor- boundaries may change from fully crystalline to a phous intergranular phase97) but this is in disagreement situation where there is a very extensive amorphous with direct experimental evidence for the segregation of phase. In addition, experiments on 3Y-TZPceramics someyttria to the grain boundary regions in the very show that the morphology of the amorphousphase high purity 3Y-TZPsamples where there is no amor- changes with the sintering temperature.97) This vari- phous phase.88,98,99) Thus, it is reasonable to conclude ability in the nature of the interfaces leads to difficulties that Y3+ cations are available and they mayimpose a in predicting the high temperature mechanical properties drag effect on the movinglattice dislocations which serve of any selected samples, and in this respect Figs. 12 to accommodatethe grain boundary sliding. and 13 are two possible situations related to materials Themeasuredstress exponent dependscritically upon where a liquid phase is either absent or very extensive, the purity level of the material becauseof the propensity respectively. A further complication arises because, in for Al203 and Si02 to segregate at the interfaces to form YTZPsamples where glass is added as a dopant, the an amorphousgrain boundary phase. The influence of mechanical behavior depends also upon the viscosity purity level is clearly evident in specimens of 2Y-TZP of the glassllo,lll) and upon the testing temperature with low purity where n is significantly lower.l05) In relative to the melting temperature of the grain boundary YTZPceramics containing an extensive amorphous phase, I 12) grain boundary phase, it is probable that superplas- Using Figs, 12 and 13, Table 6 summarizesthe pre- tic deformation occurs primarily through a diffusion- dicted characteristics in the low and high stress regions controlled solution-precipitation mechanisml06-I08) so for Y-TZPceramics without and with an amorphous

@1996 ISIJ 1434 ISIJ International, Vol. 36 (1996), No. 12

sliding in the high-strain-rate superplastic deformation Table 6. Deformatron mechamsmsmY TZPceranucs mechanism.A slmilar contribution of the accommoda- Mechanism tion helpers to relax stress concentrations at very high Nature of GB LowStress High stress strain rates can be expected for the accommodation No amorphous phase Impurity~ontrolled Superplasticity: process in ceramlcs. Oneof the most important research creep: in future high-strain-rate _ areas the near to achieve n s 3tp ::: 1,D = D n = 2,p = 2,D = Dsb superplasticity in ceramics is to design the optimum E,ctensive amorphous Interface reaction Solution-precipltation phase controlled crcopf crcep: distribution of helpers. l,p 3,D ::5 Da~ n = 2,p = 1,D =Ddd n = = 5. Summaryand Perspectives Table 7. Experimentaldata for 2YTZPreported by Lakki 3) of the speakers and other attendees at et a!.1 1 The consensus the first workshop on high-straln-rate sLrperplasticity Nature of GB Lowstress High stress he]d at Lawrence Livermore National Laboratory on 2.4 3.0 5, 1994, that high-strain-rate superplasticity Noamorphous phase n :s - n>l March1 was (AlaO, wt %, p = l p=3 represents an area which has not beenstudied extensively Si02 wt %) = 620 kJ mol-i Q= 420 kJ mcl-l Q and many aspects of this phenomenonneed further Extensivc amorphous n = 2.4 ~0.4 n::1 exploration. Drs. AndyCrowsonand Wilbur Simmons phase (Ala03 O.066wt 1 p = mol-i list of items in the discussion session which %, Si02 0.091 wt %) Q= 580 kJ presented a summarizedwell what was presented at the workshop. In the following, these and other items are headlined and grain boundary phase. For comparison, Table 7 docu- assessed.32) ments the experimental results reported by Lakki et 5.1. of Material Behavior/Mechanisms a/.,113) from tests conducted on two sets of samples Understanding of 2Y-TZPwhere, from the chemical analyses of the Threshold Stress: It would appear that a threshold powders, it is anticipated that there is either no amor- stress is observed in fine-grained materials that exhibit phousgrain boundary phase or an extensive amorphous high-strain-rate superplasticlty. It is generally believed phase, respectively: the values of Qin Table 7 are the that this threshold stress is assoclated with impurities, measured activation energies for flow. A comparison and with the fine second phases, in the fine grained shows reasonable agreement between the experlmental material. In some currently inexplicable way, these data in Table 7 and the trends predicted in Table 6, particles apparently inhibit grain boundary sliding. The including the activation energy at high stresses in the threshold stress is seen to increase with a decrease in material where there is no amorphousphase (420 kJ temperature and with a decrease in grain size. Noade- mol~1) since the superplasticity mechanismpredicts a quate theories or mechanismshave been developed to lower activation energy under these conditions due to explain the possible origin of a threshold stress. control by grain boundary diffusion. Grain Boundary Sliding and AccommodationProc- The Incidence of grain boundary sliding remains an esses: Evidence is very strong that grain boundary important topological characteristic in superplasticity. sliding is the principal deformation mechanlsmduring The various theoretical models for superplasticity differ high-strain-rate superplastic flow. The accommodation from each other primarily in the details of accommo- processes that must take place to allow continued grain dating the stress concentration that arises due to grain boundary sliding, however, are not well understood. boundary sliding. Dueto the intrinsically strong covalent Experiments to develop correct models for understand- bond strength in ceramics and also because of the ing the plastic flow mechanismsthat lead to high-strain- directionality of the bonds(and consequently high Peierls rate sensitivlty are very muchin order. stress), it is anticipated that a dislocation slip motion Liquid PhaseContributions: High-strain-rate super- related accommodationprocess will contribute less in plasticity is generally observed at temperatures near to ceramics than diffusional stress accommodation. This the melting temperature of the fine-gralned materials suggests that extensive dislocation activity and slip- studied. Experiments have revealed that partial melting is believed related texture formation should be minimal in super- at grain boundaries often occurs, which to plasticity of ceramics. Therefore, the precise details of accelerate the grain boundary sliding process. Melting at interphase boundaries also play role. The in the grain or phase boundary structure, including liquid may a way the liquid phase contributes to high-strain-rate- or glassy (amorphous grain boundary) phases, are which needs expected to be extremely important in ceramics. The superplasticity remains an unexplained area and results in the early part of the present review for metallic additional evaluation with critical experiments and materials reveal that the presence of a liquid phase at critical material systems. Cavitation: fine grained materials cavity after grain boundaries or liquid boundaries at high tempera- Some extensive superplastic deformation and do not. It tures is one of the possible accommodationprocesses to some cavitation relieve the stress constraints by grain boundary sliding, is important to understand howto minimize deterioration of mechanical and this leads to a decrease in the level and the growth which can lead to severe is properties in the superplastically formed part. rate of cavities. A new model proposed where an Lattlce Diffusion: Although accommodationhelper such as an Isolated liquid phase Grain Boundary and diffusion is generally believed to control the acts as the accommodationprocess for grain boundary vacancy

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deformation process in high-strain-rate superplasticity, Powderprocessing, mechanical alloying, foil processing, the is exact contribution not precisely established. Ato- rapid solidification, nanopowder materials, represent mic mobility in the lattice, in the grain boundary, at the novel procedures and systems for obtaining unique interface of two phases, and along dislocations are likely fine-structured products. to piay important roles. Electric and Acoustic Field Effects: It is knownthat Adiabatic Heating: Whena sample is deformed to electric and acoustic field effects can alter the mechanical large strains at high strain rates, significant adiabatic properties and especially the cavitation characteristics will heating occur. This contribution can assist achieving of materials. The exact mechanismthat leads to the high-strain-rate superplasticity. Virtually no quantitative softening of metallic materials by the presence of such studies have been pursued to determine the influence fields is not understood. Studies of the defonTlation of adiabatic heating on the tensile ductility and the behavior of fine-grained materials at elevated tempera- stress-strain characteristics of fine-grained materials at ture under the influence of electric and acoustic fields high strain rates. are worthy of investigation. Dynamic Recrystallization: It is believed that re- 5.3. Computational Modeling crystallization during straining of fine grained materials Constitutive mayoccur in somesystems. The driving force for such Equations/Relations: The flow char- acteristlcs "dynamic recrystallization" is not well understood and of traditional fine-grained materials are its possible importance in optimizing high-strain-rate well-defined by relations that involve the grain size, the superplasticity is unknown. stress and the temperature. These relations, however, need modification in their application high-strain-rate Materlals Forces: High velocity forces on a sample to result in unexpected improvementsin formability charac- superplastlc materials. This inabi]ity to use the same constitutive relations teristics. Application of this knowledge to high-strain- appears to be associated with the rate superplasticity awaits further experimentation. uncertainty of the grain boundary structure, the presence of impurities and ultra-fine second phases, and the type of interphase boundaries. 5.2. Materials Developmentand Processing Grain Size. Liquid Phase, Interface/Interphase Con- SubmicronGrains: It is generally agreed that the best tributions: Thesefactors needto be introduced in order approach to achieving high-strain-rate superplasticity is to develop correct micro-mechanical model to to create materials with submicron grains containing a assess the specific flow behavior of high-strain-rate superplastic high angle boundaries. Even ceramic materials take on materials. truly metallic-1ike superplastic characteristics whenthey Improved Data Base: The present data base of are made ultra-fine grained and tested at very high materials that exhibit high-strain-rate superplastlcity is homologoustemperatures. In order to understand better the overall charac- Solid Solution Alloys: Most of the commerclal meager. teristics expected from high-strain-rate superplastlc alurninum alloys that have been madeby superplastic materials, metal systems other than aluminum-base sheet forming are based solid solution alloy of on a should be investigated. Systems based titanium, magnesiumin aluminum (5083). Such an alloy exhibits on nickel and magnesiumare ripe for investlgation. In a high, but not ideal, strain-rate sensitivity exponent. addition, the true stress-true strain generated Typically, the exponent, is about O.33 rather than 0.5 curves m, should be evaluated for their strain-hardening and greater. Apparently, grain size is not important or an strain-softening properties. variable. Such ailoys should be examined for their Deformation Maps: Constitutive relations for grain potential as high-strain-rate superplastic materials. boundary sliding, for dislocation creep and for dif- Alloying additions which can reduce the flow stress and fusional creep, permit the creation of deformation mech- simultaneously maintain a high-strain-rate sensitivity are anism The useful of these those worthy of extensive studies. maps. most mapsare that based plots of grain size strain Phase Transformations: Bulk superplastic forming are on versus rate at fixed homologous Deformation of materials by taking advantage of simultaneous phase a temperature. maps for high-strain-rate superplastic materials require transformations (internal stress superplasticlty) has an introduction of threshold stresses and micro- attractive possibilities. Materials be prepared in new must structural features. such a that the transformation time coincides with way Microstructure Microstructural Evolution: the forming time. and There is great need for establishing relations between Heterophase Boundaries: Design of materials with a stress, strain-rate sensitivity and the mlcrostructural dissimilar phases, containing special interfaces, can be a evolution that with strain. Theserelations become potentially powerful method of maintaining fine occurs a indispensable guides in optimizing the superplastic structure. It is possible that multi-phases lead to can forming of materials into product shapes. greater stability of grains than the typical two-phase systems that are commonlystudied for their superplastic Acknowledgments characteristics. This work was performed in part under the financial Thermal-mechanical Treatments: Novel procedures support of the Ministry of Education, Science and to achieve ultra-fine structures, procedures that and Culture of Japan as a Grant-in-Aid. Oneauthor (K. are economical, are important objectives in obtaining Higashi) gratefully acknowledges support from the U. materials that exhibit high-strain-rate superplasticity. S. Army Research Office (Grant No. DAAH04-94-G- @1996 ISIJ 1436 ISIJ International, Vol. 36 (1 996). No. 12

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