Bainitic Transformations in Extremely Lowcarbon Steels

ISIJ International, Vol. 35 (1995), No. 8, pp. 962-968

Bainitic Transformations in Extremely LowCarbon Steels

YasuyaOHMORI

Department of Materials Science & Engineering, Faculty of Engineering. Ehime University, Bunkyo-cho, Matsuyama, Ehime-ken, 790 Japan. (Received on March8. 1995, accepted in final form on May26. 7995)

The models so far proposed in order to explain the mechanismof bainitic transformations have been reviewed and the problems arising from themhave been discussed. Themorphologyand the crystallography of bainitic structures are quite similar to those of martensite which forms via diffusionless processes. The kinetic properties of bainite, however, exhibit typical of diffusional (reconstructive) natures such as C-curve formations in TTTdiagrams. If bainitic transformations occurred in a displacive mechanismwhere at least the substitutional atoms are assumedto displace diffusionlessly, bainitic reaction would disappear in the interstitial-free steels, the Bs-temperatures, being equal to the Ms-temperatures. It has, however, been confirmed that bainitic reactions really occur in such extremely low carbon steels with their ownC-curves. ln order to get acomprehensiveunderstanding for these conflicting results, someexperiments using extremely low carbon steels have been carried out. It has been demonstrated that the results can be comprehensively explained by the model where the lattice changeand the relaxation of elastic stress due to it are assumed to occur via individual atomic jumps and a lattice invariant shear, respectively. KEYWORDS:phasetransformation; bainite; Widmanstatten structure; Iow carbon steel; surface relief; alloy partition.

stitial-free alloys be It also 1. Introduction would necessary. seems difficult to explain the formation of C-curves in TTT The mechanismof bainitic transformation has been diagrams by a displacive model. These features so far the subjects of numerousinvestigations and three distinct obtained favour a diffusional (reconstructive) transfor- models have beenproposed so far, i.e., a displacive,1 ~ 5) mation mechanism.In the case of diffusional (reconstruc- diffusional (reconstructive)6~9) tive) a and a coupled diffu- transformation, crystallographic aspects such as sional/displacive models. Io) orientation relationship, habit plane and surface reliefs In the displacive model for the transformations from should also be explained consistently. It has been report- austenite to ferrite,1 1'12) it is assumedthat, although the ed that, even in the diffusional (reconstructive) model, product ferrite nucleates with carbon partitioning, its one-to-one atomic site correspondence at the coherent full growth involves a carbon supersaturation as in the parent/product interfaces as expected in the case of ledge case of martensite, the alloy partitioning occurring mostly mechanismcan produce surface reliefs similar to those after its 18) growth. Thusthe bainitic structures should grow in displacive transformation.16~ In this case, especial- at temperatures below To where the Gibbs free energy ly in the case of austenite to ferrite or ferrite to austen- of the parent phase is equal to that of the product phase ite transformation, however, the reason why the trans- of the samechemical compositions. If it were the case, formation can proceed even under the significantly large bainitic structures in interstitial-free steels would be elastic strain, which will be produced by the reconstruc- martensite and cementite particles wo,uld precipitate in tive lattice change at the coherent interphase boundary a similar fashion to those in a tempered martensite in with the lattice-site correspondence, should be clarified. the case of carbon containing steels. On these points, Theother modelin an intermediate case is the coupled there are still manyarguments. diffusional/displacive transformation mechanism.10) In al.,13-15) this Wilson et however, demonstrated by tem- case, as far as substitutional atoms are concerned, perature measurementsduring rapid cooling from aus- bainitic transformation is assumedto be in a displacive tenite that very low carbon Fe-Cr and Fe-Ni alloys fashion but the partition of interstitial atoms is thought transform into bainitic ferrite at temperatures much to occur at the advancing product/parent interfaces. This higher than Ms. Although the partition of small amounts implies that bainitic reaction in interstitial-free steels of interstitial atoms involved might control the nuclea- should be the sameas that of martensite. tion events, the difference in the formation temperature In order to specify the mechanismreally operative, betweenbainitic ferrite and martensite is quite large and further experimental evidences will be needed. In the further detailed examination using completely inter- present report, therefore, the austenite to ferrite and the

C 1995 ISIJ 962 ISIJ International. Vol. 35 (1995), No. 8 ferrite to austenite transformatiou processes in extremely 700 low carbon ferritic and ferrite/austenite duplex phase _.Oo/o fE____ ---,500/0-- /~-. . stainless steels ~ 100o/o have beenexamined, and the mechanisms 600 //pF+w!'-~-, - of bainitic transformations have been discussed by Ms~~ comparingthe present results with the previous studies. (a) P 500 ~1100 2. Kinetic Properties of Widmanstatten Structures and :~ ~~~ Bainite a,~ 1000 WA E To avoid terminological confusion, Widmanstatten a, H 900 structures and bainite are defined as follows and both of / will be referred to "baimtic structures" hereafter: them as WA+cr (1) Both Widmanstatten structures and bainite are 800 in plate- or lathlike transformation products forming (b) the between the diffusional trans- temperature range 1 1O 1OO 1OOO IOOOO formation without accompanying surface reliefs and Time (s) martensitic transformation with sharp and straight surface reliefs. Fig. 1. TTTdiagrams for extremely low carbon steels.26,28) (a) 0.0041 o/*C-30/0Mn-1 o/.Cr steel (2) Thedifference betweenthemexists in the fact¥that and (b) 8/y duplex phase stainless steel. Widmanstatten structures nucleate on the grain boundary allotriomorphs of the product phase, while bainite lath or plates nucleate directly on the parent grain _ 500 boundaries. o the phases to (D The decompositions from parent both = as Widmanstatten structures and bainite always occur after 400 cl~ Bainitic ferrite incubation times with typical C-curves. This implies E some ~~ ee that the reconstructive. Since the grain processes are = o e ee in o e boundary allotriomorphs of the product phase form 300 o Lath martensite a specific orientation relationship with one of the ~ _ooc,_oo H~o o o boundary,19'20) u' o parent grains separated by the the ac- c o 'r' o tivation for the nucleation of o energy Widmanstatten H 200,0 structures on these semicoherent interphase boundaries 5000 i OOOO 15000 20000 'c will be muchreduced as comparedwith that for the Cooling rate ( /s) direct nucleation of bainite on the grain boundaries of Fig. 2. Variation oftransformation temperature with coohng the parent phase because of the epitaxy. Therefore, rate in an Fe-15'/.Ni anoy (arter Wnsonel al.13)). Widmanstatten structures and bainite have their may Ms-points in these steels. Wilson et al.i3- 15) examined individual C-curves. In fact, it shownin the previous was the variation of transformation temperature with cool- study21) that the C-curve for Widmanstatten ferrite is ing rate in low carbon Fe alloys and confirmed that separated from that of bainite in low carbon low alloy very the temperature corresponding to the rate of high strength steels. However, if Widmanstatten ferrite maximum bainitic transformation is higher than tem- laths nucleate subsequently after the grain boundary much Ms peratures for lath martensite formation. example of ferrite allotriomorph formation, the C-curve of the An their resultsl3) for low carbon Fe-150/0Ni alloy Widmanstatten ferrite merely showthe incubation a very may is shownin Fig. 2. With increasing cooling rate, time for the grain boundary ferritb nucleation. Even two separate plateaux of the transformation temperatures are in such a circumstance, it must be noted that the recognized, i.e., 350'C for bainitic ferrite and 260'C for Widmanstatten structures can definitely be separated lath martensite. The fact that in low carbon from martensite in view of kinetics. even very steels the bainitic reaction can be separated from that of If the displacive or the coupled diffusional/displacive martensite suggests the possibility that the formation of mechanism operative, then Bs temperature would were bainitic C-curves does arise from the reconstructive approach point with reducing interstitial contents. Ms of substitutional atoms rather than the Figures l(a) and 1(b) show the TTTdiagrams for the processes diffusion of interstitial atoms. Thus, it must be extremely low carbon ferritic steel austenitized at 1100'C very important to examinewhether the bainitic C-curves are and the ~/y duplex phase stainless steel solutionized at formed in completely interstitial-free materials not. l 300'C, respectively.22,23) The chemical compositions or of them are 0.0041 oloC-O. 190/.Si-2.98"/oMn1 .Oo/oCr- 3. Alloy Partition 0.0 1Oo/oNb~).OIOo/oTi~).O 18010Al and 0.0 170/.C~).48 o/¥Si -O o/o 46 "/. o/o Ni-7 o/o olo It has been well established that carbon n-O . . Cr-O. are . 85 M . Cu-6 82 7 79 atoms M0~).28'/¥W0.1430/,N, respectively. Widmanstatten enriched in the untransformed austenite during the structures in both the alloys form after certain incuba- progress of bainite transformation in steels22,23) as can tion times at temperatures muchhigher than Mstemper- be seen in Fig. 3.23) The stage where such partition of ature. Martensite laths form in athermal fashion below carbon atomsoccurred has beenthe subject for numerous

963 C 1995 iSIJ ISIJ International, Vol. 35 (1995), No. 8

1.8 ~S h 1:: a) ,4 c: 1 tu a,

~: 426 'c ::: ,o (D 1 371 'C = 316 'c o 266 'c f:

J:,o O~: 0.6 i~ ~> 40 l (c) o 20 40 SO 80 1OO Fragmented austenite laths Bainite (olo)

Fig. 3. Enrichment of carbon content in austenite by bainitic Cr transformation in an 0.6~/*C-20/.Si steel (after Matas 30 a/.23)). el ~;~ ~_

:: .2 ca 20 c(D 20 /tm c Oo 10 Ni

/v\, Mo o 40 Distance (c) Fig. 5. EPMAanalyses for Cr, Ni, Moacross Widmanstatten austenite plates in the duplex phase stainless steel transformed at 900"C for 15 min.26) (a) surface reliefs, 30 (b) the corresponding microstructure and (c) the results of EPMAanalyses. ~~ ~_ cr C t Similar results Ti alloys have also been obtained by :;:e Austen i te Iath on CQ on eu t Enomotoet al.25) Therefore, it seems reasonable to C Grain boundary 20 ,:, pm that the bainitic transformations proceed via C' austen i te assume a C reconstructive process,24,2 5) i.e., individual substitutional 8 I Austen i te lat h atomic jumps across the phase boundaries. 10 I Ni Further interesting phenomenawas observed in the 6/y duplex phase stainless steel.26) Figures 4and 5show the results of the EPMAanalyses for the Widmanst~tten Mo austenite laths formed at I OOOand 900'C. Here (a), o Distance (b) and (c) are the surface reliefs, the corresponding microstructures and the results of analyses, Fig. 4. analyses for Cr, Ni, across Widmanstatten EPMA EPMA Mo respectively. reliefs austenite plates in the duplex phasestainless steel trans- In both cases, surface are quite sharp formed at IOOO'Cfor 15min.26) (a) surface reliefs, and straight as if they form via a displacive mechanism (b) the corresponding microstructure and (c) the results (see Figs. 4(a) and 5(a)). The corresponding micro- of EPMAanalyses. structure of the laths formed at I OOO'Cexhibits exactly the samemorphologyas the surface reliefs (compareFig. arguments. In the diffusional (reconstructive)6~9) and 4(a) with 4(b)). Asignificantly large alloy partition in Ni models,10) the coupled diffusional/displacive the parti- andCr are also recognized (Fig. 4(c)). Onthe other hand, tion is expected to occur during the progress of bainitic the austenite laths formed at 900'C were decomposed 5) transformation, whereasin the displacive mechanisml~ into either small fragments or the laths with heavily martensitic structure supersaturated in interstitial atoms distorted interfaces as in Fig. 5(b). In this case, the alloy forms first at temperatures below To and then carbon partition is muchlarger than that at I OOO'C. If it is atoms are depleted from martensite. Thus the alloy assumed that the laths form via a reconstructiv~ partition should be examinedin the very early stage of mechanismand the equilibrium compositions between transformation. In a Cu-Al-Zn alloy, Tadaki et al.24) two phases do not vary significantly in the temperature confirmed that even the growing tip of bainite nucleus range between I OOOand 900'C, the partition of alloy- is of almost the equilibrium chemical compositions. ing elements is reduced with lowering transformation

C 1995 ISIJ 964 ISIJ International. Vol. 35 (1995), No. 8 temperature, the degree of supersaturation increasing. rather wavy, implying that the diffusional migration of Thus, the driving force for the migration of interphase the interphase boundaries occurred after the Widman- boundary via interdiffusion will increase at lower statten ferrite formation as in the case of the temperatures. Onthe other hand, since the partition of microstructural change of Widmanstatten austenite at alloying elements during the phase transformation is 900'C. This implies that wavy interfaces themselves significantly large at higher temperatures, I OOO'C,the would not provide the evidence of a reconstructive driving force for the migration of interphase boundary formation of laths and would be produced as the will be reduced, much larger alloy partition being secondary effect after the phase transformation.28) Thus expected at lower temperature decomposition. This somemechanismother than the complete diffusional one interpretation is in keeping with the present results. It where the elastic strain arising from the coherent is, thus, nec~ssary to examinethese processes in addition interphase boundary migration with atomic site corre- to the precise determination of the equilibrium phase spondencel8) is relaxed by diffusion of substitutional diagrams with using muchsimpler systems such as high atoms should be considered. purity ternary FeNi-Cr alloys. As described in the preceding section, Widmanst~tten austenite laths formed at 90(~800'C in the duplex phase 4. Surface Reliefs and the Morphology of Bainitic stainless steel decomposedinto either the laths with Structures heavily distorted interfaces or the fragments with a significant large alloy partition during the isothermal Theformation of surface reliefs has often beenreferred holding after transformation. Anexampleof fragmented to as an evidence of displacive transformation.1) austenite particles observed by meansof transmission Aaronson et al.,6) however, argued that surface reliefs electron microscopy is shownin Fig. 7.26) Parallelogram can also be formed by the diffusional ledge mechanism. cross sections of them can often be observed. Detailed In the latter case, if the elastic strain due to the phase dislocation structures on these interfaces shown in change by the ledge mechanismwere built up, the Frg. 826) are quite interesting. Parallel array of misfit progress of phasetransition would be ceasedby the strain dislocations densely spaced as well as those of lattice energy which consumesthe driving force in the early dislocations can be seen as has already been reported by stage of transformation, producing rather small surface et al.29) Theyare parallel to I0>v direction. reliefs. While, if such elastic strain relaxed also Ameyama a I an were Thus, in order to disscuss the transformation mechanism, by diffusional process, very sharp surface reliefs at a close attention must be paid to confirm if such an higher temperatures would not be expected and the austenite particle is really the original lath or not. surface rumpling due to the volume change would be The morphologies of Widmanstatten ferrite and produced. Thesurface reliefs induced by transformation bainite in plain carbon steels are also of interest for at both OOOand 900'C in the duplex phase stainless I considering the mechanismof phase transformation. steel,26) however, are very sharp and are often tentlike Both Widmanstatten structures and upper bainite in as reported previously.27) Figures 6(a) and 6(b) illustrate steels lathlike. Lower bainite in ferritic steels, example of surface reliefs of Widmanstatten ferrite are an however, is platelike.30) Temperature separating formed the 0.00410/.C-30/0Mn-10/¥Cr steel surface upper on and lower bainite in plain carbon steels is about 350'C and the corresponding microstructure revealed by light and almost the with temperature separating lath etching, respectively.28) The surface reliefs formed at same and plate martensites.31) This does not seemto be an 600'C are quite sharp and straight, but interphase accidental coincidence, and be related to the boundaries of the corresponding microstructure are may mechanismof transformation as in the case of martensite. In the extremely low carbon ferritic steels, however, since

Frg. 6. Surface reliefs and the corresponding microstructure in the 0.00410/0C30/0Mn-loloCr steel transformed at 600'C for 10min.28) (a) surface reliefs, and (b) the Fig. 7. Fragmentedaustenite laths in the duplex phase stain- corresponding microstructure. less steel transformed at 900'C for 15 min.26)

965 C 1995 ISIJ ISIJ International, Vol. 35 (1995), No. 8

s\'1p~~s\~

;, ~/~/i~//~~of;\'~~~'\1ee~\ Direction of xl Ji shape strain ~~"; ';)' Ji ,, ,,:;'t ;~~;:; Ferrite T (a) ~~i subunit:d///~r~/ (cross section f/////~~ of a needle) l; l;

/~:;:)i'~/~""/)~~;~//~//l; / Dfslocations /1 v (b) Pl

i\1ee~\ox\ ~~:(\e ~\~e~~~4i~

Fig. 8. Dislocation configurations on the interphase boundary of the fragmented austenite laths for formed at 900*C (c) l5min.26) Ferrite lath

Fig. lO. Amodel of upper bainite formation.37) The growth direction of either the bainite subunits or the bainitic lath which lies close to the invariant line is normal to this figure. Thus, these figures show the cross sections normal to the growth direction for either the bainite subunits or the laths. (a) nucleation of bainite subunit, (b) autocatalytic nucleation of ferrite subunits and (c) the lath formation by the coalescence of them.

ed in lath martensite.31) Such a situation is difficult to explain by a direct application of the theory of martensite crystallography34 ~ 36) but can be explained by considering the relaxation of the shapestrain within the austenite matrix.37) Here it is assumedthat the lattice invariant shear must be chosen to makethe direction of the shape strain as parallel as possible to the slip direction in the Fig. 9. Recovered bainite formed in the o,olO'/,c-3"/.Mn- matrix as well as the condition minimizing the magnitude 1"/.Cr steel transformed at 500'C for 20min.'*) of shapestrain. In addition to these conditions, a bainitic lath is be Mstemperature is muchhigher than 350'C, only assumedto formed by the coalescence of the upper needlelike ferrite bainite and Widmanstatten ferrite form. Although subunits lying parallel to the invariant can line initially close to I0>y// >. within slip band almost the formed bainitic structures are highly 1 11 a parallel 1}y//{ 10}.. dislocated, the dislocation density be muchreduced to {I1 1 Similar approachcan be made may also for bainite illustrated in by the recovery during the isothermal holding after upper as Fig. lO. Figure 10(a) showsthe nucleation of needlelike ferrite subunit, the phase transformation as in Fig. 9.28) Here, an a the growth direction close to the invariant line being 0.010"loC-3 o/oMn-1oloCr steel wastransformed at 500'C normal to this figure. Dislocations for 20minand the recovered ferrite laths with relatively are generated also in the parent austenite at the side-edge of it wherethe largest 10w dislocation density can be seen. It should also be elastic stress is concentrated by the shape deformation. pointed out that an individual Widmanstatten or bainite Catalytic effects of dislocations will lath is formed by the coalescence of the needlelike ferrite induce manynee- subunits dlelike bainite subunits within this slip band shown aligning parallel on a specific plane in a similar as in Fig, lO(b). The coalescence of will fashion such that a martensite lath forms.32) them produce a lathlike bainite lying almost parallel to the slip band as in Fig. 10(c). It 5. Crystallography of Bainitic Structures should be noted that the growth direction of a lath is normal to the figure and only the sidewise orientation relationship The between the parent growth direction can be recognized in Fig. lO(c). austenite and upper bainite (or Widmanstatten ferrite) As to lower bainite in high carbon steels, the is always close that to of KurdjumovSachs.The habit crystallography of martensite34~36) have been applied plane and the growth direction of each lath paral- successfully.38,39) is, are It however, interesting to note that lel to {223}y//{451}. v// and as reported the introduction of the dilatation parameter ~ 0.97-0,99 previously.32,33) This is also .quite similar = to that observ- is necessary to explain the habit plane.38,39) This implies

C 1995 ISIJ 966 iSIJ International, Vol. 35 (1995). No. 8

that the lattice parameter of the parent austenite in the model corresponds to the plane which involves the vicinity of bainite should be expanded somehow, horizontal line and is normal to this figure, and the plane suggesting the enrichment of carbon atoms in austenite comprising the riser involves the vertical line. For at the interphase boundary. Therefore the partition of simplicity, although the atomic exchangesoccur via the carbon atoms is likely to occur during the phase migration of vacancies in an actual situation, the transformation. illustration of them wasneglected in this figure. Within the parent ferrite, the atomic exchange between iron 6. AModel for Bainitic Transformation atomsand alloying elements mayoccur quite frequently by thermal fluctuation. If such exchangeoccurs in the The experimental evidences so far obtained indicate an parent phase in contact with the product phase into the that the kinetic properties and the partition of alloying direction favourable for the phase transformation, the elements in bainitic structures arise definitely out of a interphase boundary will migrate via small atomic reconstructive phase change and are difficult to be a displacement less than atomic distance in Fig. 1(b), explained by displacive mechanism. The crystallo- an as 1 a the alloying elements being partitioned partially without graphic properties, however, are mucheasily understood loosing the interface coherency. The repetition of these in terms of a displacive theory. In order to interpret these processes will lead to the successive migration of the observations comprehensively, a modelwhere the lattice interphase boundary (see Figs. 1(b) and 1(c)). That is, changefrom the parent to the product phase occurs via 1 1 the mutual exchange of atomic sites prior to the a reconstructive mechanismand then the elastic strain interphase boundary migration, which leads to the alloy due to the lattice changeis partially relaxed by shear has partition, and the individual atomic displacements at the been devised.40) As an example for the reconstructive interphase boundary will result in the lattice changewith phase change from the parent ferrite to the product keeping the interface coherency. Exactly similar austenite with Kurdjumov-Sachs orientation relation- mechan- ism has also been proposed by Howel8) for fcc to hcp ship, the migration of a coherent interphase boundary transformation. In the fcc to hcp transformation, is illustrated in Fig. 11, schematically. Theplane onwhich case however, the close-packed planes in both the phases the atomic sites of substitutional elements are projected are parallel and remain the invariant plane without is the (1 12)a//(1 2)v' Theatomson successive three atom- as 1 rotation during the phase transformation, only ic layers in ferrite and the corresponding two layers any the stacking sequence of them being changed. Thus, in austenite are projected on one plane. The open and the direction of the shape strain should be parallel to the solid symbols are iron and substitutional alloying this plane. In the of fcc bcc bcc fcc elements, respectively. The broken line shown in Fig. case to or to transformation, the situation is not simple in the l 1(a) indicates an interphase boundary. Comparingthis so as of fcc to hcp transformation because the figure with that illustrated by Rigsbee and Aaronson,8) case coherent interphase boundary migration produces large elastic the terrace of the migration interphase boundary in their a strain and either considerably large atomic diffusion or it. ll shear [ill] [110] deformation should take place to reduce If the hr~.A~~~~AJLs elastic strain is relaxed by diffusion, the surface reliefs AA~~ similar to those of martensite will not be produced and ~ "~ 6 AAAA only surface rumpling due to the volume changewill be =* AJ'~~O~~~O" _ ~OOO~ A OeOO~ A ~ ~ o0060eOOA A A ~ observed. Whereasif the elastic strain is partly relaxed l~ 60 O~>r~~~;,~~:~~O ~~ l O O LO_ by a lattice invariant shear with forming an invariant ~ -----_ . I~ r~___ -- = -~LAiJr~r~Q l A ~ ~ A ~ plane, the phenomenological theory of martensite34~36) A~~~~IOILO 10 O, O O O O O O O O O O O also be applied to bainitic transformation. In the r ~> ~~~~~~ r ~, can ~ ~ latter case, of course, the elastic strain due to the phase ~ ~A~AA,e~~~ site (a)OA~AAAA~~O O O O O O O (b)O O O O O O O O change with atomic correspondence can not be completely relaxed, and the residual strain is recognized as the shape strain which is 0.2~).3 in magnitude in the A JL A A ~ A Ic~ JL O atomlcslteonaplane case of ferrous martensite and be observed as surface 60 atomicsiteonalayerbelowit can O O O O e O O A reliefs. These in JL,ro---O~~~'O~~~O1, A ~ ~ processes can be formulated a manner it ~ ~ ~> ~:-~Le O atomic site on a layer above similar to those in martensite: ~ ~ ~ ~ ~~l~~ atomicexchange A A r P1=8RBP2 atomicdisplacement r OOOOOOOO~ ~ ~ ~ T where, P1' 8. RBandP2are the shape strain, a dilatation A ~ A A ~ A ~ A ------interphaseboundary parameter, the lattice deformation and the lattice (c) O O O O O O O O invariant shear, respectively. Although in the martensite open mark:solventatom crystallography the dilatation is unity, that in (il2)6 Il parameter (112~ solid mark : soiute atom bainitic transformation should be changed in the case Fig. Diffusional migration ofan interphase boundarywith u. where alloy partition occurs. This model of transforma- keeping coherency at the interface. (a) exchange of tion has been referred to "Shear-Assisted Diffusional lattice sites, (b) migration of interphase boundaryand as model"40) slight displacements of atomic sites, and (c) further Transformation and seemsto be compatible migration of the interface and the enrichment of with the most features of the experimental results so far alloying elements in the vicinity of the interface. obtained. It should be mentioned here a similar concept

967 C 1995 ISIJ ISIJ International, Vol. 35 (1 995), No. 8 has also beensuggested by Tadaki et a/.24) for the bainite cussions during the course of this research. The author in a Cu-Zn-AI alloy. In order to confirm the generality is also grateful to the membersof the Advanced of this model, further positive evidences showing that Instrumentation Center for Chemical Analysis, Ehime the lattice deformation occurs definitely by diffusional University, for the provision of a JEM-2000EX mechanismand the lattice invariant shear takes place transmission electron microscope. after the diffusional lattice change should be obtained. REFERENCES 7. Conclusions l) T. Koand S. A. Cottrell: J. Iron Steel Inst., 172 (1952), 307. 2) K. Shimizu and Z. Nishiyama: Trans. Jpn. Inst. Met,, (1964), In the present various aspects of low carbon 5 paper, 225. bainitic structures have been reviewed and some ex- 3) J. M. Oblak and R, F. Hehemann: Transformation and perimental results obtained for extremely low carbon Hardenability in Steels, Climax MolybdenumCo., (1967), steels have been described. Summaryof the results are 15. 4) H. K. D. H. Bhadeshia V. Metall., as follows: and D. Edmonds:Acta 28 (1980), 1265. (1) Widmanstatten structures and bainite form with 5) H. K. D. H. Bhadeshia and J. W. Christian: Metall. Trans. A, C-curves in almost interstitial-free ferritic steels even and 21A (1990), 767. steel. a low carbon 6/y duplex phase stainless 6) K. R. Kinsman and H. I. Aaronson: Transformation and (2) Thesebainitic structures accompanywell defined Hardenability in Steels, Climax MolybdenumCo., (1967), 33* surface reliefs in all the cases observed. Thecorrespond- 7) E. P. Simonen. H. I. Aaronsonand R. Trevedi: Metall. Trans., ing microstructures revealed by light etching, however, 4(1973), 1239. exhibit the relatively large interphase boundary migra- 8) J. M. Rigsbee and H. I. Aaronson: Acta Metall., 27 (1979), 365. tion after the lath formation. This indicates that the 9) G. J, Shiflet and H. I. Aaronson: Acta Metal/., 27 (1979), 377. transformation mechanismshould be argued based on lO) G. B. Olson, H. K. D. H. Bhadeshiaand M. Cohen:Acta Metal/., (1989), the structures of the initially formed laths. 37 381. l) H. K. D, H. Bhadeshia: Acta Metall., 29 (1981), 17. (3) In duplex phase stainless steel, Widmanst~tten l I1 a 12) H. K. D. H. Bhadeshia: Met. Sci., 16 (1982), 159. laths significant formed at higher ternperatures with alloy 13) E. A. Wilson, S. P. Allen andJ. Butler: Met. Sci., 16 (1 982), 539. partition did not changethe lathlike morphologyduring 14) E. A. Wilson: Met. Sci., 18 (1984), 471. the isothermal holding for considerably long time after l5) E. A. Wilson: Mater. Sci, Technol., 7(1991), 1089, the transformation. 16) Y. C. Liu and H. I. Aaronson: Acta Metall., 18 (1970), 845. 7) U. Dahmen:Sc,'. Metall., 21 (1987), 1029. Thoseformed at temperatures below 900'C l accompa- l 8) J. M. Howe:Metall. Trans. A, 25A(1994), 1917. also the well defined surface reliefs. Light etching ny of l9) P. L. Ryder and W. Pitsch: Acta Metall., 14 (1966), 1437. them, however, revealed that such laths decomposedinto 20) Ph. Maitrepierre, D. Thivellier and R. Tricot: Metall. Trans., 6 either the laths with heavily distorted interphase (1975), 287. 21) Y. Ohmori, H. Ohtani and T. Kunitake: Trans. Iron Steel Inst. boundaries or small fragments after the lath formation. Jpn., Il (1971), 250. These distorted laths exhibited quite large alloy a 22) S. J. Matas, R. F. Hehemannand H. I. Aaronson: Metall. Trans., partition, arise which probably from the interdiffusion 2(1972), 1077. across the interphase boundary after the initial lath 23) S. J. Matas and R. F. Hehemann:Trans. Metal/. Soc., AIME, formation. 221 (1961), 179. (4) The crystallographic properties of Widman- 24) T. Tadaki. C. J. Qjang and K. Shimizu: Mater. Trans. JIM, 32 (1991), 757. statten structures and bainite are almost similar upper 25) M. Enoinoto andM. Fujita: Metall. T,'ans, A, 21A(1990), 547. lath 1 to those of martensite. 26) Y. Ohmori, K. Nakai, H. Ohtsubo and Y. Isshiki: ISIJ Int., 35 (5) Theexperimental results obtained are compatible (1995), 969. with the model40) where the lattice change occurs via a 27) K. R. Kinsman. E. Eichen and H. I. Aaronson: Metall. Trans. A, 6A (1975), 303. reconstructive mechanismwith the lattice-site corre- 28) Y. C. Jung, K. Nakai, H. Ohtusboand Y. Ohmori: ISIJ Int., 34 spondenceat coherent interphase boundary and the a (1994), I160. large elastic strain this lattice is due to change partially 29) K. Ameyama,G. C. Weatherly and K. T. Aust: Acta Metall. relaxed by lattice invariant shear with forming an Mater., 40 (1992), 1835. invariant plane (habit plane). In this case, the residual 30) Y. Ohmori and R. W. K. Honeycombe:Proc. ICSTIS, Suppl. to Trans. Iron Steel Inst. Jpn., 12 (1972), 160. strain is the shape strain which is recognized as surface 1 31) A. R. and G. Krauss: Trans. (1967), 651. reliefs. Mader ASM,60 32) H. Ohtani, S. Okaguchi, Y. Fujishiro and Y. Ohmori: Metall. Acknowledgments Trans. A, 21A (1990), 250. 33) J. D. WatsonandP. G. McDougall: Acta Metall., 21 (1973), 961 research carried the finan- . The present was out under 34) J. S. Bowles and J. K. Mackenzie: Acta Metall., 2(1954), 129. cial support by the Basic Research Committee of The 35) J. K, Mackenzie and J. S. Bowles: Acta Metall., 2(1954), 138. lron and Steel Institute of Japanand someof the materi- 36) J. S. Bowles and J. K. Mackenzie: Acta Metall., 2(1954), 224. 37) Y. Ohmori: Philos. Mag. A, 57 (1988), 337. als were supplied by the Bainite ResearchCommitteeof 38) G. R. Srinivasan andC. M.Wayman:Acta Metall., 16 (1968), 621 Thelron andSteel Institute of Japanfor round robin test. . a 39) Y. Ohmori: Mater. Trans. JIM, 30 (1989), 487. The author should like to express his sincere thanks for 40) Y. Ohmori, K. Nakai and H. Ohtsubo: Solid-Solid Phase them to The lron and Steel Institute of Japan. Thanks Transformations, ed. by W. C. Johnson, J. M. Howe, D. E. are also due to his colleagues, Prof. K. Nakai and Mr. Laughlin and W. A. Sofa, TMS,(1994), 905. H. Ohtsubo, for their stimulating but constructive dis-

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