Creep Behaviour in Ti-based Alloys
S.K. Sahay", S.K. Singh, B.Goswami and A.K. Ray*
National Institute of Technology, Jamshedpur, India, *National Metallurgical Laboratory, Jamshedpur, India
(Received April 26, 2005: final form June 15, 2005)
ABSTRACT is a continuing time dependent deformation process at a constant stress in which the amount and rate of straining Simulation of different mechanical properties in are established by the material itself under imposed titanium base alloys is described in the review to study stress and temperature conditions. The creep rate stems the relative competencies with service exposure at high from a balance of simultaneous strain hardening and temperature. Precipitation and solution hardening of thermally activated recovery processes such as cross matrix, as well as solutes that increase activation energy slip at low temperatures and dislocation climb at for self diffusion and that increase twinning propensity, temperatures above 14 TM (TM —> absolute melting increase creep life. A reduction of creep properties temperature). Normally creep becomes of engineering appears on strain induced plate formation or relative significance at a homologous temperature (TH) greater instability of interfaces of lamella. An increase in ß- than 0.5. A number of metals and alloys above phase in microstructure by fast cooling reduces fracture homologous temperature behave in many respects like toughness properties, whereas a transverse texture of viscoplastic materials. Andrade's pioneering work on thermomechanical treatment reduces impact properties creep laid the foundation of our understanding of the as compared to solution treated and annealed specimen. subject 111. Coarsening of phases at high temperature degradation The amount of deformation expected from Ti-based appears to be rectified in many cases by Zener pinning alloys under constant stress as a function of time and from either indigenous or exogenous particulates. temperature is of great practical importance. Hence, a Microstructural and orientation sensitivity rankings better understanding of the mechanism of high have marked material performance under shear plugging temperature deformation is essential to evaluate the during ballistic limit test. performance of Ti-based alloys in different kinds of high temperature service. High temperature deformation Key words: Titanium, Aluminides, Intermetallics, in Ti-based alloys is characterized by extreme Creep, Diffusion, Composites, Degradation, Prevention. inhomogeneity. The principal deformation processes at elevated temperature are slip, sub-grain formation and grain boundary sliding. A number of secondary 1. INTRODUCTION deformation processes such as multiple slip, formation of extremely coarse slip bands, kink bands, fold Interest in studies of creep behavior in Ti-based formation at grain boundaries and grain boundary alloys has been triggered by scientific impulses and migration contribute to creep. New deformation enormous practical consequences. Creep is a mechanisms come into play at elevated temperatures. progressive deformation of material at constant stress. It The importance of thermally activated diffusion
* Corresponding author: Dr. S.K. Sahay (E-mail: [email protected]). Department of Metallurgy & Materials Science, National Institute of Technology, Jamshedpur-831014, India
323 Vol. 24, No. 5, 2005 Creep Behaviour in Ti-based Alloys
controlled process is implicit in the characteristic of deformation of hard oriented polysysthetically twinned creep. It is strongly suggested that the thermally crystal with lamellar plates oriented parallel to activated diffusion controlled process become active on compression axis at 1150K shows α2/γ interface to be exposure at elevated temperatures. One of the most more stable than γ/γ interface and pseudo twin interface spectacular aspects of creep at elevated temperatures is and 120 deg. rotational fault to be less thermally stable the grain boundary sliding. However, there remains than true twin interface for γ/γ interface. Significant much to be done to identify the mechanisms of creep in differences in yield strength and shear strength appear Ti-based alloys. While preparing the article, efforts have due to variation in microstructure and grain size during been made to update our knowledge by introducing new shock loading. Fracture toughness of Ti-4.5Al-3V-2Mo- concepts, which have emerged in recent years as a result 2Fe alloy improves with the decrease of cooling rate. of excellent experimental verifications. Emphasis has Increasing α-phase content within ß-phase been placed on the provision of theoretical descriptions microstructure increases fracture toughness of alloy. of various phenomena. A systematic approach has been Thick material and thin sheet of IMI 834 on super- made to disentangle the separate combinations of plastic forming with combinations introduce an various mechanisms of creep in Ti-based alloys. interesting joining process by diffusion bonding. After Traditional disputes and the ambiguity on the subject post-bond heat treatment at 1173-1223K the product have been handled with clarity of thought. exhibits properties similar to the parent metal. Ion bean Global consumption strategies of titanium alloys in sputter sectioning of γ-TiAl single crystal at 1133 Κ to different standard multimetallic forms stem from their 1307K and 1053K to 1278K respectively has been excellent light weight and creep properties. Interrupted investigated using tracer diffusion. Ti is more creep tests of fine grained Ti3SiC2 in the 1273-1473K anisotropic than Ni because of defect structure between temperature range show volume conserving plastic mutually perpendicular directions, whereas nickel deformation mode in secondary creep regime whereas diffuses randomly in the sub lattices. cavities and microcrack formation appear in tertiary stage in response to extreme plastic anisotropy.
Excellent creep resistance has been attributed to ζ-Τϊ58ί3 2. CREEP PROPERTIES OF TITANIUM precipitate and B2 precipitate at the interfaces of (α +γ) ALLOYS AND PREVENTION MEASURES lamellae of Ti-47Al-2W-0.5Si alloy respectively.
Nd203, a stable high temperature oxide, develops Creep properties have been studied in the Ll2- indigenously and increases oxidation resistance of TiAl ordered (cubic) titanium trialuminides derived from alloy by reduction of oxygen in γ-phases through tetragonal D022-ordered Al3Ti by alloying with fourth- internal oxidation of Nd, whereas trace nickel content in period transition elements such as Cr, Μη, Fe, Co, Ni, Ti-6Al-2Sn-4Zr-2Mo:I increases creep rate by Cu and Zn. This system exhibits low fracture toughness accelerated crystal lattice self diffusion and increasing and low yield strength. The drawback is reevaluated by rate of dislocation climb. Stress induced plate (SIP) increasing Ti concentration and Β doping in the cubic formation increases creep strain in a metastable alloy of titanium trialuminides stabilized with Mn. The fracture ßTi-14.8V, whereas reduction in grain size reduces SIP toughness and compressive yield strength rose to higher formation and creep rate. The effect of elements on values after increase in Ti concentration. alloying elements on dislocation climb controlled M. Heilmair et al. have studied the creep behavior of deformation process at high temperature has been Mn stabilized boron doped dual phase Ti rich Ll2 studied. Phase transformation, recrystallisation and trialuminides at elevated temperature (1373-1473K) microstructural coarsening are controlled by alloying under compression 121. Apparent activation energy for elements. Alloying TiAl with Nb increases creep life by creep is estimated from the- temperature compensated increase of activation energy of diffusion and twinning strain rate vs. shear modulus normalized stress analysis. propensity, whereas alloying with C increases life of The reported value matches with the activation energies TiAl base alloys by precipitation hardening. Creep reported for creep and Ti/Al diffusion to γ-ΤϊΑΙ/α2-
324 S.K. Sahay et al. High Temperature Materials and Processes
Ti3Al type of intermetallics. The analysis also done using the first-principle density functional theory demonstrates that the present system can be used at a code VASP. To obtain estimated interface energy for marginal · increment of temperature by 200K for semi coherent interfaces, a correction has been applied structural applications with reference to a γ-TiAl alloy to take into account the effect of misfit to the favoured and single phase Fe and Mn stabilized titanium interface configurations. P-type nucleates homo- trialuminides. The unusual combination of properties geneously and hence possesses lower interface energy exhibited by ternary carbides (Ti3SiC2) has been high than H-type, which nucleates inhomogeneously as stiffness, ease of machinability, good conductivity, reported by Benedek et al. IM. thermal shock resistance, and oxidation and fatigue Orthorhombic-based alloys or O-base alloys have resistance. Between 1373K and 1473K, carbides fail in been developed as potential high temperature and light compression, flexure and tension with a brittle to plastic weight for application in aircraft engines. Often the transition. Mobile dislocations in such systems multiply microstructure contains an Ο (orthorhombic) phase and during deformation, arrange themselves in either arrays forms β (or β0) phase when an O-based alloy is cooled on identical slip planes or in dislocation walls forming from processing temperature. There was significant low angle grain boundaries normal to the basal plane. strengthening to large primary transient strain, and Perfect and mixed basal plain dislocations or the two better room temperature ductility was obtained in a two independent slip systems operate during deformation. phase structure of Ο and β than in single phase Ο alloys. Plasticity in such cases is possible by a combination of Creep lifetime has been determined in the primary creep deformations, kink band formation of individual grains domain. A large fraction of ß-phase stabilizer can be as well as shear band formation. Fine grained Ti3SiC2 dissolved into solid solutions of O-phase. sample creeps under tension at 1273-1473K by a In order to enhance high temperature mechanical combination of plastic deformation, crack formation and properties Tang et al. 151 created a new alloy of cavitations. Plastic deformation operates in the quasi- quaternary systems, where the fourth element was either steady state creep regime, where the minimum creep Mo, W or V. A low creep rate and a small transient rate obeys power law. Acceleration of tertiary stage creep strain were exhibited by a system of ΤΪ-22Α1- creep rate leads to intergranular cracking initiated by 20Nb-2W alloy. W-modified (ß+O) two phase ΤΪ-22Α1- large internal stresses. It has been confirmed by high 20Nb-2W alloy has shown the activation energy of stress creep test, where tertiary creep occurs before creep to be 368 KJ/mol, where the creep process is steady state creep. The competing processes are rate of dominated by dislocation climb. Comparably smaller accumulation and relaxation of internal stresses /3/. In primary transient strain and significantly lower order to improve high temperature mechanical minimum creep rate have been shown by this alloy as properties and corrosion resistance numerous dopants compared to (ß+O) two phase alloys. The apparent have been added to two phase TiAl alloys. activation energy of creep has been enhanced by The effect of B, C, Υ and Si on precipitation during addition of W for a low minimum creep strain. aging at elevated temperature has been studied. Table 1 shows different creep properties to describe Improvement of high temperature strength, creep creep as a dislocation controlled process by climb and resistance and contribution to refinement of lamellar independence of prior ß-grain size. Ti-47Al-2W-0.5Si, a structure has been made possible by carbon. C-doped γ-TiAl alloy, has been developed recently with better
alloys from perovskite structure Ti3AlC, referred to as creep resistance. An important role is played in such a
P-type, hexagonal phase Ti2AlC, referred to as H-type system by W and Si, where Si forms ζ-Τί58ί3 particles at
and empirical ternary hexagonal phase Ti3AlC2. Except the interface of (α + γ) lamellar structure and W for the stacking sequenc,e both types of hexagonal provides solid solution strengthening and B2-particle
phases are similar. Ti3AlC2 has been observed as a formation at the lamellar interfaces. Blocky B2-phase precipitate in γ-TiAl for higher C concentrations and at has always been formed in the alloy by ß-phase
higher annealing temperatures. For P-type and H-type stabilization effect of W. Ti5Si3-particles are often precipitates, calculations of interface energies have been formed in the B2-phase. So formation of B2-phase and
325 Vol. 24, No. 5, 2005 Creep Behaviour in Ti-based Alloys
Table 1 Creep behavior, 310 MPa creep in air of Ti-22AI-20Nb-2W 151.
Cooling rate Test Transient Transient Minm creep 1% creep Failure life- after β temp. strain, (%) time, (h) rate, (%/h) life-time, (h) time, (h) treatment, (K) (K/min) 276 923 0.30 620 4.85x1ο*4 1302 >2694 273.3 0.49 134 2.01 xlO"3 247 1602 276 973 0.30 99 4.12xl0"3 175 792 303 0.58 107 4.98xl0*3 98 570 273.3 0.39 15 2.86xl0"2 21 141 276 1023 0.37 8 5.33xl0*2 12 62 303 0.36 9 5.52xl0*2 12 82
Table 2 Tensile properties and creep rupture properties of TiAl alloys /!/.
Alloy (at.-%) 298K Tensile property 1173Κ Tensile property 1173K/240MPa σ σο.2, Ob, δ5, (%) 0.2, δ5, (%) Life, δ, (%) (MPa) (MPa) (MPa) (MPa) (hr) Ti-48A1 379 424 1.6 252 362 31.2 25.5 36.8 Ti-47.5Al-0.25Nd 449 489 1.2 337 446 21.2 92.5 26.8
Ti5Si3-particles within lamellar interfaces is interrelated. hinders the slip of dislocations during deformation at
The orientation relationships between Ti5Si3 precipitates room temperature. The deformed regions near Nd203 and B2-phases were investigated by R. Yu et al. under particle exhibited a very high dislocation density. Trace TEM. Studies of orientation relation (OR) revealed that elements have a profound influence on elevated the two 3-fold axes of Ti5Si3 and B2-phase are parallel temperature mechanical behavior of Ti alloys. and attributed to the largest overlapping volume of Substantial creep strain accumulation reductions have
Ti5Si3 and B2-phase in reciprocal space of B2 and Ti5Si3 been found when trace of Si is present in the alloy of Ti- precipitate along lamellar interfaces upon dissolution of 6Al-2Sn-4Zr-2Mo (Ti-6-2-4-2). Creep resistance a2 plates, where Ti5Si3 particles are directly formed increases on addition of trace levels of Bi and S. On the from preliminary B2 particle instead of from γ/α2 matrix contrary dramatic reduction of creep resistance can be 161. seen by trace of iron in the system of Ti-6-2-4-2. Fu-Sheng Sun et al. introduced rare earth R.W. Hayes et al. have investigated intermediate neodymium (Nd) as an effective third element to TiAl temperature creep behavior of Ti-6Al-2Sn-4Zr-2Mo system for improving mechanical properties /7/. alloy in the presence of trace element Ni /8/. Results of Addition of Nd has resulted in reduction of oxygen in γ- creep investigation over wide stress limits and within phase by forming Nd203 through internal oxidation and temperature limits of 783-838K have been studied. rendering the oxide as a stable component at high Increasing nickel content increases primary creep temperature. Addition of Nd increases the creep rupture strains, steady state strain rates and apparent stress. properties of alloys, as given in Table 2. Nd203 phase High nickel lowers the value of apparent activation
326 S.K. Sahay et al. High Temperature Materials and Processes
energy at different stress levels, but, for low nickel mechanism varies with ß-phase stability. A higher content, apparent activation energy is similar to that of stable Ti-13%Mn alloy exhibits 3 times higher creep life intrinsic lattice self diffusion in alpha titanium. Fast than a lower stable Ti-14.8V alloy at 95% of YS. It has diffusing impurities like Ni dominate microstructure and been reported that stable alloys deform by slip only creep behavior. Creep in the high stress regime is while lesser stable alloys deform by slip and twinning controlled by dislocation motion in alpha phase. The (SIP) 191. As summarized in Fig.l, a flow chart with a rate of lattice self diffusion increases the rate of brief summary of diversity of property combinations for dislocation climb, leading to a reduction in creep a lamellar topography, the high temperature resistance of α-phase as suggested by Hayes et al. /8/. performance appears to be a promising feature from The aerospace, chemical and biomedical industries lamellar type Ti-alloys /10-17/. are the main consumers of ß-titanium alloys. Time dependent slip and twinning have been the mode of creep deformation at low temperatures. However, no 3. CREEP PROPERTIES OF significant ambient temperature creep has been studied TITANIUM COMPOSITES
on a ß-Ti 13 wt.-% Mn. The concept of metastable ßc has been introduced. Retention of metastable ß-phase up For applications in a wide variety of technological to quenching without martensitic transformation with field, the properties of titanium alloys are always within the help of a critical amount of ß-phase stabilizer has expectations of further improvement in terms of low
been referred to ßc-phase. Based on the amount of ß- density, high modulus, and high strength. Ceramic phase, stabilizer classifications have been done in terms reinforcement into titanium matrix has been tried in this of lower or higher stable alloys. For example alloys with context, where continuous fiber reinforced titanium lower Mn content stabilize ß-phase alloy more than matrix composites (SCS-6 fiber reinforcement) have higher Mn content. Creep behavior is affected been accounted for by an anisotropy in property, significantly by stability of alloy. The presence of complex in fabrication, expensive and unstable. Z.Y. intermittent metastable phases like ω-phase leads to Ma et al. have studied the effect of particle (TiC) twinning and stress induced transformations in less reinforcement in Ti-6A1-4V alloy matrix /18Λ Varying stable alloys. stress in the range of 823-923K changes stress exponent Ramesh and Ankem 19/ have analyzed the ambient apparently in TiCp/Ti-6Al-4V composite. Creep data temperature creep behavior of a Ti-14.8V alloy in terms for medium stress region has been consistent with that of deformation modes, extent of deformation and grain for α-Ti, which is governed by activation energy of size. Stress induced plate (SIP) formation and slip in all dislocation climb. In the absence of a threshold stress cases have been mode of deformation during tensile and for creep, the composite creeps by lattice diffusion creep deformation. Decreasing grain size decreases the controlled dislocation climb process in a-Ti. extent of SIP formation and ambient temperature creep Incorporation of threshold stress into creep data strain or limits creep strain by constricted SIP produces consistent rationalization comparable to that of formation. SIP grows in thickness as well as in length α-Ti when medium stress is applied at lower with time as revealed by interrupted creep test on a temperature, whereas at lower stress values similar specific size grain of Ti-14.8 alloy. There has been consistency has appeared by including threshold stress similar observation of time dependent twinning of a-Ti- data due to grain boundary sliding mechanism in α-Ti. 0.4wt.-% Mn alloy due to diffusion of oxygen The creep rate of TiCp/Ti-6Al-4V composite has been interstitials. Decreasing grain size decreases creep 1 -3 orders of magnitude lower than those of α-Ti in the exponent of empirical equation and leads to increasing low and medium stress regions. Power creep law fails difficulty of SIP formation. Expressions in terms of when associated with abnormally high stress exponents. grain size sensitivity denoted by power law parameter Powder and liquid metallurgy route introduce particles have been developed from grain size dependency of directly to form a composite. Comparatively low creep exponent. Control and extent of deformation production cost raised interest in an in-situ process such
327 Vol. 24, No. 5, 2005 Creep Behaviour in Ti-based Alloys
Formation Multi-domain lamellar structure
formation (α2'->·α2 + γ) • Coherency strain accommodation • Twin or pseudo-twin relations * Strain induced nucleation of twins Features • Elastic strain energy relations
DS γ -TiAl and a-Ti3Al • Interfacial and bulk chemical free lamellae energy /11/ * Thickness • Isotropic domains within lamellae * Grain size • Curved domain boundaries • Spacing Grain refining • Volume fraction Heat-treatment • Orientation /10/ • α-phase solution treatment
• (α2 + γ) aging • High TS of ultra-fine lamellae Stability • Instability at high Hard-oriented ΤΪ-46Α1 PST crystal lamellae temperature parallel to compression axis • Poor creep resistance • Six oriented variants of γ/γ-interfaces /12/ • Coarsening by elimination of γ/γ- interfaces • Increase of true twin interfaces • Migration of γ/γ-interfaces and ledge Nitriding formation Effects of ambient conditions • Disappearance of interface • Decrease of lamellar thickness upto * Formation of equiax grains 0.3 at.-% Ν • Stability of a2fy-interfaces /13/ • Increase of lamellar thickness above 0.3 at.-% Ν
• Formation of Ti2AlN nitrides " Change in aluminium level in matrix Creep properties /14/ S Thinning and dissolution of a2 lamellae S Coarsening of a2 lamellae S Segregation of micro-alloy at ledge interface S Segregant misfit/mis-orientation Crack Formation S Reduction of climb by large segregant Lamellar orientations (Nb, Ta, W) • In-plane kink angle S Acceleration of climb by small • Through thickness twist angle segregant (Si) • Crack propagation S Relation of amount of segregant with ο Minimum resistance through grains stress concentration and lattice distortion ο Maximum resistance through S Improvement of creep resistance by boundaries segregant related stabilization of a2-phase • Arrest /15, 16/ ο Multiple nucleation ο Ligament formation ο Ligament failure /17/
Fig. 1: Flow chart for features for lamellar microstructure/10-17/.
328 S.K. Sahay et al. High Temperature Materials and Processes as self propagating high temperature synthesis (SHS), degradation and development of alloying concepts with mechanical alloying and rapid solidification technique respect to enhanced high temperature capability have to produce composite. SHS reactions between Ti, C and been studied in depth. Dislocation climb controls
B4C form (TiB + TiC) Ti composite. Improvement in deformation in the range of the intended operating creep rupture life has been made possible by stability of temperature. Observations have been made on complex interfacial phases. Deformation is incompatible at the processes of phase transformation such as interface of TiB-Ti matrix. Along the loading direction recrystallisation, microstructural coarsening and dislocations are tangled at the end of TiB, which give microstructural degradation and their effects on the long rise to initiation of voids, subsequent growth, term service. Nb addition increases activation energy of coalescence and eventual fracture of composite. diffusion and the propensity of twinning at ambient Strengthening effect of TiB is limited by stacking fault temperature. Both room and high temperature strength aided mass diffusion. Therefore TiB strengthening is property improve by alloying with high contents of Nb. less effective than TiC on the creep rupture life. Creep Microstructural stability increases and dislocation rupture life is improved by graphite addition to the motion is interrupted by precipitates like carbides. So composite because it generates more TiC /19/. microalloying with carbon enhances high temperature capability /21/. At 1150K under the applied stress of 158-316 MPa, deformation studies have been done on 4. COMPARATIVE CREEP BEHAVIOR hard-oriented polysynthetically twinned (PST) crystal AND DEGRADATION with the lamellar plates oriented parallel to the compression axis. Quantitative examination of the PST Steady state creep behavior has been investigated in crystal during creep deformation shows microstructural
Ti-47A1—2Mn, Ti-47Al-2Zr and ΤΪ-48Α1 (at.-%). Cast changes. The respective proportions of α2/γ, true twin, and heat treated alloys consisting of a fully lamellar pseudotwin and 120 deg. rotational fault interfaces in (FL) structure have been investigated. Kim et al. have the as-grown PST crystal have been determined. Creep investigated steady state creep behavior as a function of deformation leads to lamellar coarsening by dissolution 2 stress between 70-300MN/m in the temperature range of a2-lamellae and migration of γ/γ interfaces. Lamellar 973-1173K based on the effects of alloy chemistry and coarsening and destruction of lamellar structure during microstructure. Creep resistance is higher for FL creep deformation result in accelerated creep rate after microstructure than for duplex one. DTBT of TiAIMn the minimum strain rate in the creep curve. After 4 pet. ternary system affects the steady state creep rate. Power of creep strain at 1423K, about 32 pet. of α2/γ law creep equation describes temperature and stress interfaces, 51 pet. of true twin interfaces, 74 pet. of dependence of the steady state creep rate in both pseudotwin interfaces and 80 pet. of 120 deg. rotational microstructures, which gives rise to the suggestion that faults disappear. During creep deformation, the true the dislocation motion dominates deformation twin interfaces are more thermally stable than 120 deg. mechanism. Activation energy of cast intermetallics is rotational fault and pseudotwin interface /22, 23/. higher and involves a more complicated process than simple diffusion. Steady state creep resistance of cast alloy has been three orders of magnitude higher than Ti- 5. OTHER MECHANICAL PROPERTIES 6Al-2Sn-4Zr-2Mo-0.1Si (mass-%) alloy and two orders of magnitude higher than for a Ti-25Al-10Nb-3V-lMo Intensive studies of TiAl based alloys have been (wt.-%) alloy as reported by Kim et al. 1201. The done in order to make them suitable for use in turbine titanium aluminidc alloys as compared to other blades, radial diffuser and transition ducts and structural materials offer better creep strength in turbocharger wheels and exhaust valves. Often this addition to oxidation and corrosion resistance for high system performs better. Physical properties of a TiAl temperature applications. base alloy (CTI-8) exhibit lower thermal expansion, Mechanisms of creep deformation, microstructural lower electrical resistivity, higher specific stiffness and
329 Vol. 24, No. 5, 2005 Creep Behaviour in Ti-based Alloys
higher .specific heat relative to currently used materials. introduce lattice distortion in a systematic way without However CTI-8 is higher in thermal conductivity than extensive atomic rearrangement. The extents of mutual that of Ti-6A1-4V alloy as reported by Zhang et al. UM. displacements between lattice site and interstitial site act The Debye model offers a simple but very effective as scattering centers and contribute to resistivity. method to describe the phonon contribution to the Gibbs Precipitation processes in Ti-N alloys have been energy of crystalline phases. Difficulty arises in monitored by Sundararaman et al. by changes in extrapolation of data for metastable state of crystals. It resistivity 1261. The presence of coherent interfaces is suggested to construct Gibbs energy from its physical between α and α'-phases shows the occurrences of peak components such as ground state energy, lattice resistivity. Coherency strains leading to incoherent vibration contribution, the electronic excitation and the interfaces attribute to a break away leading to decay in electronic spin ordering etc. to improve the reliability of resistivity maximum. The reverse case has been the result. Metastable structures are a major concern in observed in metastable conditions of precipitation in the this context as their thermo-physical properties are not system under longer aging time. Accurate modeling of experimentally accessible. Calculations of a Debye mature gamma titanium aluminide alloys should be temperature have been made possible by using frozen continued to evaluate intermetallics under service load phonon approach, linear response theory or ab initio conditions. Young's modulus measurements of γ-TiAl force constant method. In the original derivation the alloys, consisting of polysynthetically twinned (PST) Debye temperature is related to sound velocity and can materials data, show significant differences based on alloy chemistry, lamellar spacing and lamellar be calculated from elastic constants. The difference in orientation. A consistent set of orthotropic elastic behavior of cubic and hexagonal systems is similar to properties for individual grains have been determined the trends found for their elastic constant and in turn by a microstructure based three dimensional finite their densities of states. Data are available for high element analyses. Similar expectations of elastic temperature entropy-Debye temperature in accordance modulus in those alloys that contain lamellar structure with empiricism. are difficult 1211. Chen and Sundman /25/ propose high temperature entropy-Debye temperature data for those structures Light weight, superior biocompatibility and bio that are dynamically unstable at low temperatures, from corrosion resistance, good mechanical properties and calculations of isotropic bulk modulus using a scaling low elastic modulus titanium and its alloys are referred factor of 0.5. Pseudo-potential plane-wave calculation for classes of biomedical implant materials. Ti-6A1-4V plots the binding energy curves, derivation of bulk ELI, an implant alloy, contains an extra low interstitial module from universal equation of state, and calculation release Al and V ions to treat long term health problems of Poisson's ratio for adjustment of scaling factor 125/. in addition to large variation of Young's modulus with Microstructural changes such as defects, precipitation respect to surrounding bones. These have been replaced reaction, short and long range ordering phenomenon and by a new class of ß-phase or metastable ß-phase type phase separation sequences during phase transformation alloy. The aim is to develop a non toxic, greater have been investigated by resistivity measurement. biocompatible, low modulus and better processable Occurrences of a peak resistivity on aging and alloy. Near ß-type titanium alloy contains non toxic subsequent decay process can be explained as follows. elements such as Nb, Ta and Zr (Ti-13Nb-Zr), whereas Increased electron scattering from clusters or a non- the metastable ß-type alloys are defined by Ti-35Nb- random distribution of atoms results in maximum 5Ta-7Zr and Ti-29Nb-13Ta-4.6Zr. A number of resistivity corresponding to the state of GP zones. metastable compositions of ß-stabilized titanium alloys Combined effects of cluster size and shape, cluster/ at elevated temperature form a' and a"-martensite phases of hexagonal and orthorhombic structures matrix interface structure and separation of inter-cluster respectively upon quenching. A slow quench or affect the peak resistivity values. However these isothermal aging at intermediate temperature may form findings are applicable for only substitution alloys and metastable ω phases. In this system of ω-phase not interstitial solid solutions. Interstitial solutes
330 S.K. Sahay et al. High Temperature Materials and Processes
containing (α +ß), the Young's modulus appears to be highest in ω-phase and lowest in ß-phase. Investigating the binary Ti-Mo system has shown lowest YM in a"- martensite within the system of a, a', a" and ß-phases. A decrease in YM and an increase in internal friction of alloys quenched from different solution temperature are related to the increasing volume fraction of oc"- martensite in the matrix of (α +ß) phases. Controversy also exists as to interpretation of the low modulus of (β +α") as compared to α-phases. Strain induced transformations of a"-martensite from ß-phase increase YM. The existence of the lowest YM of ß-phase and comparatively lower modulus for a" than α-phase has been reported. The influence of solution treatment on Grain size, μιη the formation of a"-martensite in the metastable ß- Fig. 2: Comparison of measured yield strength of the phase type Ti-29Nb-13Ta-4.6Zr alloy has been studied. alloy (solid line) and the yield stress of the β a"-phase transformation rate during cooling varies with phase estimated with the Hall-Petch equation variation in solution treatment temperature, time of (dotted line) /28/. treatment and quenchant. Treatment parameters control a"-martensite formation through ß-phase stability and grain size. Martensite transformation is suppressed Ti-22Al-27Nb alloy is controlled by primary effect of within limits of small and large grain dimensions of ß- spherical a2-particles dispersed in the B2-phase matrix phase. Within critical sizes the a"-martensite has lower during hot rolling and annealing in the (B2 + a2) two- strength and hardness but better ductility with respect to phase region. ß-phase matrix. The yield strength of the ß-phase at Lamellar structure within smaller B2-grains formed different grain size as computed from the Hall-Petch upon slow cooling from annealing temperature produces equation is compared with the yield measured strength improvement in room temperature tensile strength and as shown in Figure 2. Formation of a"-martensite elongation (see Fig. 3) /29/. Investigations using high results in reduction of the yield strength of the alloy rate loading have been prompted by aerospace /28/. Good high strength to weight ratio and good industries for applications in the field of ballistics and workability have rendered TiiAlNb (O), the ordered amour development, crash test worthiness, and high rate orthorhombic phase, as a suitable candidate for high forming and machining and satellite protection. temperature application. The optimum combination of Responses of materials to impact events such as bird strength, creep and fracture toughness properties has strike, foreign object damage and blade contaminant been provided by the primary constituent of an (0+B2) have been studied in areas of jet turbine engines. two-phase microstructure of Ti-22Al-27Nb (mol.-%) Explosive loading or common plate impact has been alloy. Superior creep properties are exhibited by (0+B2) used for shock wave testing. To study the impact of two-phase lamellar structures that have formed on equal flat plate on a plate target, a flat and parallel flayer cooling from a high temperature and B2-single phase. plate has been accelerated down a smooth bore gun by Developments of both high temperature creep and either a powder propellant breach or compressed gas. room temperature ductility have been enabled by grain Millett et al. /30/ have investigated the behavior of two refinement of high temperature B2-phases. Transfor- different alloys based on the intermetallic phase TiAl mation during cooling produces fine grained columnar with different microstructure and grain size after shock structure of (0+B2). Grain refining is difficult and ß- loading /30/. Observations have shown differences in phases always maintain a tendency to coarsen. In the equation of state (stress-shock velocity-particle pre-alloyed powder metallurgy route B2-grain size of velocity), one-dimensional yield strength and shear
331 Vol. 24, No. 5, 2005 Creep Behaviour in Ti-based Alloys
B2 grain size (μηι) Evans /31/ have studied the stress-corrosion and 1200 200 50 20 10 corrosion fatigue effects in a saline environment on fatigue crack growth of a conventional Ti-6A1-4V alloy. Mill annealed bimodal and fully transformed lamellar 1100 microstructures have been assessed. Growth of cracks shorter than 1mm is microstructurally sensitive. It has D.1? 1000 been demonstrated by the lamellar variant through a Έ corner crack specimen geometry containing a part crack % ¥00 with a maximum length of 2.5 mm. The growth rates are s sensitive to both environment and mean stress response. 55 Mill annealed and bimodal microstructures are insensitivity to salt solution condition. In order to W improve the lower fracture toughness air cooling has to be avoided in case of ß-type Ti-4.5Al-3V-2Mo-2Fe 700 alloy annealed at the temperature of 1123K. A two step cooling (TSC) after solution treatment at that 0.1 0.2 0.3 0,4 600 1/2 temperature has been proposed. Comparisons have been Reciprocal square root of B2 grain size (μιη" ) carried out on the alloy between solution treated plus Fig. 3: Effect of prior B2-grain size on the room aged (STA) and slow furnace cooling (SFC) treatment. temperature tensile strength /29/. TSC treatment by slow cooling the alloy at a cooling rate of 0.075 K/s from a temperature of 1123K to 723K and subsequently, water quenching to room temperature strength. Microstructural comparisons clearly reveal that produces relatively higher fracture toughness. Fracture a duplex small grain microstructure is stronger than toughness of alloy appears to be similar between a two- coarse grain fully lamellar microstructure. Increasing step cooling route and an alloy annealed in two steps. shock stress increases shear strength markedly. This Fracture toughness of a two-step cooled alloy is lower effect has been significantly greater in intermetallic than STA route. Slow furnace cooled alloys show alloys than simple alloys like Ti-6A1-4V. This is a relatively lower fracture toughness. Decreasing cooling manifestation of the high work hardening rates that are rate decreases fracture toughness of slow-furnace- characteristic to intermetallic alloys. cooled alloys monotonously. Factors responsible for Ti alloys offer great potential to the aerospace increasing fracture toughness in two-step cooled and offshore, biomedical and chemical processing solution treated and aged alloys have been suggested to industries. A variety of alloys has been developed to be plate like α-phase and secondary α-phase, meet specific requirements, for example, extra low respectively, that appear in the ß-phases during cooling interstitial (ELI) variants of the Ti-6A1-4V system have and aging. An increase in fracture toughness stems from been recommended for impact materials by ASTM. the presence of such kinds of transformation products Improved hot corrosion resistances for geothermal (Figure 4a to 4c)/32-34/. applications through minor additions of ruthenium or palladium have been derived from (α + β) alloys. Resistance to reducing acids increases by incorporating 6. DIFFUSION MECHANISM AT the same elements into beta alloys. The inherently high HIGH TEMPERATURES content of molybdenum used for beta stabilization retards the corrosion effect that develops due to Physical and mechanical properties differ between decomposition of hydraulic fluids in aircraft assemblies. grain boundary and bulk because of differences in However, comparable mechanical performances have to atomic structure. Diffusion at grain boundary has been be accompanied by the corrosion resistance. Bache and several orders of magnitude higher than in the bulk
332 S.K. Sahay et al. High Temperature Materials and Processes
Fig. 4: (a) Schematic drawings of phase diagram of Ti-alloys /32/. (b) Heat-treatment schedules of preliminary study to obtain the coarsest secondary phase /32/. (c) Core study to obtain improved mechanical properties of the alloy I3>2I.
because of non-uniformity of the grain boundary microstructure. Considerable importance has been given structure, a type of point defect involved in the mass to superplastic forming (SPF) within temperature range transport process. Several metallurgical processes at of 1213-1253K and diffusion bonding. A gas turbine usual working temperatures, like creep, corrosion, solid- engine exhaust system requires joining of metals with state transformation etc., are controlled by diffusion minimum damage to the base metal. short-circuit character of grain boundary. Literature Interfaces between super alpha 2 and Ti-6A1-4V about grain boundary structure of hep crystal lattice of formed within temperature of 1193-1213K at a pressure Ti is scarce. Calculations have been done using of 6-10 MPa for one hour have been studied (Table 3). energies, entropies and relaxation volumes associated The isostatic diffusion bond interfaces are (1) free from with different vacancy locations and vacancy jumps. On porosity, (2) located mainly on Ti-6A1-4V side of bond the basis of the most probable paths, rough and (3) contain fine α-platelet structure in place of approximations have been made about boundary equiax α-phase that is present before bonding and diffusion. retained ß-phase /36/. Ti alloys sheet structures are For twin grain boundaries in α-Zr and a-Ti, analyses manufactured with high structural efficiency at low cost have been done on the minimum energy structure, by superplastic forming and diffusion bonding (DB). defect properties and diffusion. Some studies of Joints with excellent mechanical properties have been structure and energy of four GBs in hep lattice have produced successfully by use of DB for thick section Ti been done. Twins and non symmetric GBs form a alloys. The effects of DB parameters on IMI 834 have periodic array of well matched, followed by distorted regions. Twins contain less energetic dislocations than Table 3 nonsymmetrical GBs. Atomic interactions can be Shear strength of super alpha-2/Ti-6Al-4V bonds for represented by central force potentials constructed given bonding conditions /36/. within the embedded atom method. The mechanism of diffusion in twin boundary and Bonding Pressure Bonding Shear the grain boundary for hep metals has been explained by temperature (MPa) time strength Fernandez et al. /35/. Intermetallics are poor in ductility (K) (h) (MNm'1) and formability. Alloying and microstructural control 1213 10 1 421±11 recover the problem by developing α2 - β phase system. 1193 10 1 336±l4 Nb and Mo facilitate super a2-phase formation through ingol metallurgy route to give fine and duplex 1193 6 1 28S±20
333 Vol. 24, No. 5, 2005 Creep Behaviour in Ti-based Alloys
been studied. Tensile, shear and impact properties have the diffusion mechanism. Tracer analysis established by been studied in diffusion bonded joints in IMI 834. Ikeda et al. illustrates Ti diffusion along both plane and Interfacial voids and poor properties in bonded joints stack directions, where the former is faster than the have appeared to be completely removed with an latter /38/. Correlation with vacancy jump explains additional increase in mechanical properties after post- differences in point defect formation energies. On the DB heat-treatment at 1293K for 6 hours, where DB other hand, Ni diffusion has been less anisotropic and { temperature has been 1223K (Table 4) /37/. Diffusion suggests similar probability to occupy two sorts of appears to be a basic process that plays an important sublattice sites. role in fabrication and practical usage at high temperature. A vacancy mechanism controls the diffusion of elements. This has been measured by tracer 7. CONCLUSION elements using polycrystals of TiAl. TiAl exists as an Llo type ordered structure up to th'e melting point of Titanium alloys exist in an intermetallic form of 1753K. Ti atoms arrange in planes and Al atom stacks. phase compositions. Often these systems produce Diffusion anisotropy in this case has been measured by directional anisotropies. 44 <>:, To improve high temperature stability and creep life Ti and Ni in a Ti4(VAls4 single crystal. Correlation exists between defect structure and vacancy jump with of TiAl systems, the following steps are recommended:
Table 4 Tensile, shear and impact properties of IMI 843 and BD joints produced under various conditions /37/.
Material Bulk Tensile properties Shear Impact condition/bonding deformation 0.2% PS UTSj Elongation strength energy 2 2 2 parameters (MNm" ) (MNm" ) (%) (MNm" ) (J) 1. As received 0 946 1083 14.3 — 4.2 2. Thermal cycle 0 870 1029 16 663 5.4 1263K, 0.5h
3. Thermal cycle 0 873 1015 19.5 — 5.0 1263K, 2h 4. DB 1223Κ (0.5h), 0.2 928 1022 2.3 284-483 2.2 5 MNm'2 5. DB 1263Κ (0.5h), 0.4 910 1056 17.7 703 3.2 l5MNm"2 6. DB 1263 Κ (2h), 0.5 895 1003 19.8 675 2.6 2.5 MNm'2 7. DB 1263Κ (0.5h), 1.0 884 1039 17.1 675 4.9 5 MNm'2
8. DB as 4 + 1293K 0.2 913 1032 16.5 - 4.5 (6h) + 973Κ (2h)
9. DB as 5 + 1293K 0.4 922 1028 16.8 — 4.3 ^2h) + 973K (2h)
10. DB as 6+ 1293K 0.5 915 1022 16.9 — 4.6
(2h)+ 973Κ(2h) -
11. DB as 7+ 1293K 1.0 917 1029 13.5 — 4.4 (2h) + 973K (2h)
334 S.K. Sahay et al. High Temperature Materials and Processes
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