Creep deformation in near-γ TiAl: Part 1. the influence of microstructure on creep deformation in Ti-49Al-1V

The influence of microstructure on creep deformation was examined in the near-y TiAl alloy Ti-49A1-1V. Specifically, microstructures with varying volume fractions of lamellar constituent were produced through thermomechanical processing. Creep studies were conducted on these various microstructures under constant load in air at temperatures between 760 °C and 870 °C and at stresses ranging from 50 to 200 MPa. Microstructure significantly influences the creep behavior of this alloy, with a fully lamellar microstructure yielding the highest creep resistance of the microstructures examined. Creep resistance is dependent on the volume fraction of lamellar constituent, with the lowest creep resistance observed at intermediate lamellar volume fractions. Examination of the creep deformation structure revealed planar slip of dislocations in the equiaxed y microstructure, while subboundary formation was observed in the duplex microstructure. The decrease in creep resistance of the duplex microstructure, compared with the equiaxed y microstructure, is attributed to an increase in dislocation mobility within the equiaxedy constituent, that results from partitioning of oxygen from the γ phase to the α₂ phase. Dislocation motion in the fully lamellar microstructure was confined to the individual lamellae, with no evidence of shearing of γ/γ or γ/α₂ interfaces. This suggests that the high creep resistance of the fully lamellar microstructure is a result of the fine spacing of the lamellar structure, which results in a decreased effective slip length for dislocation motion over that found in the duplex and equiaxed y microstructures. BRIAN D. WORTH, formerly with the Department of Materials Science and Engineering, The University of Michigan     

Transformations in α 2+γ titanium aluminide alloys containing molybdenum: Part II. Heat treatment

The response of as-cast structures of 12 alloys in the Ti-Al-Mo system containing 44 to 50 at. pct Al and 2 to 6 at. pct Mo to simple single step heat treatments in the temperature range 1373 to 1673 K is described. The microsegregation patterns present in the cast structure persist to a large extent after heat treatment, especially below 1673 K. However, tentative conclusions regarding phase equilibria in this temperature and composition range are drawn from the results. High-temperature equilibria are dominated by the β, α+β, and α+γ phase fields, while the β+γ phase field dominates equilibrium below 1473 K. Three major types of transformation behavior are observed: a massive α to γ transformation, which occurs within the α phase on quenching from 1673 and 1573 K in alloys centered around the 48 pct Al composition; a eutectoid transformation from α to B2+γ mixtures, which occurs at 1473 K and below in alloys centered around the 48Al-4Mo and 46Al-6Mo compositions; direct γ precipitation in β, which occurs primarily in the 44Al-6Mo composition at 1273 K and below; and finally growth of γ lamellae in α+γ lamellar structures with B2 precipitation on lamellar interfaces, which occurs over a broad range of alloy compositions and temperatures.     

Phase transformation and microstructural evolution in Ti-44Al-4Nb-4Zr alloy during heat treatment

Phase transformation and microstructural evolution have been studied in Ti-44Al-4Nb-4Zr-0.2Si-0.1B alloys that were cooled from theα +β phase region with various cooling rates. It has been shown that the cooling rates have different influence on the morphology of the transformation products for the three phase transformations studied,α →α ₂, B2 →ω, andα →γ. Under slow cooling, all three transformations can be fulfilled. Under rapid cooling, B2 →ω is partially detained and a diffuseω phase forms as metastable phase, butα →γ is almost completely suppressed, which supports that theγ lamellae formation is diffusion controlled.     

Numerical models of creep and boundary sliding mechanisms in single-phase, dual-phase, and fully lamellar titanium aluminide

Finite element simulations of the high-temperature behavior of single-phase γ, dual-phase α₂+γ, and fully lamellar (FL) α₂+γTiAl intermetallic alloy microstructures have been performed. Nonlinear viscous primary creep deformation is modeled in each phase based on published creep data. Models were also developed that incorporate grain boundary and lath boundary sliding in addition to the dislocation creep flow within each phase. Overall strain rates are compared to gain an understanding of the relative influence each of these localized deformation mechanisms has on the creep strength of the microstructures considered. Facet stress enhancement factors were also determined for the transverse grain facets in each model to examine the relative susceptibility to creep damage. The results indicate that a mechanism for unrestricted sliding of γ lath boundaries theorized by Hazzledine and co-workers leads to unrealistically high strain rates. However, the results also suggest that the greater creep strength observed experimentally for the lamellar microstructure is primarily due to inhibited former grain boundary sliding (GBS) in this microstructure compared to relatively unimpeded GBS in the equiaxed microstructures. The serrated nature of the former grain boundaries generally observed for lamellar TiAl alloys is consistent with this finding.     

Characterization and modeling for forging deformation of Ti-6Ai-2Sn-4Zr-2Mo-0.1 Si

The deformation behavior during upset forging has been determined for Ti-6242 in both the (α + β) and β starting microstructures. For (α + β), flow softening attributed to deformation heating was observed. Deformation heating accounted for only a fraction of the extensive flow softening of the β microstructure. The dependence of log σ on 1/T was linear for (α + β) and bilinear for the β microstructure, with an approximate transition temperature of 930 °C. The two temperature regimes for β corresponded to distinct deformed microstructures which were manifestations of different softening mechanisms, all promoting the flow-induced transformation of metastable β microstructure to the equilibrium (α + β) microstructure. Based on the experimental data, flow stress equations for both microstructures, and empirical equations describing the flow softening behavior of β have been developed. WithT and ġe as the only input variables, these equations can accurately predict the σ - ε relationships for process modeling of this alloy.     

Effect of the lamellar grain size on plastic flow behavior and microstructure evolution during hot working of a gamma titanium aluminide alloy

The kinetics of dynamic spheroidization of the lamellar microstructure and the associated flow-softening behavior during isothermal, constant-strain-rate deformation of a gamma titanium aluminide alloy were investigated, with special emphasis on the role of the prior-alpha grain/colony size. For this purpose, fully lamellar microstructures with prior-alpha grain sizes between 80 and 900 μm were developed in a Ti-45.5Al-2Nb-2Cr alloy using a special forging and heat-treatment schedule. Isothermal hot compression tests were conducted at 1093 °C and strain rates of 0.001, 0.1, and 1.0 s⁻¹ on specimens with different grain sizes. The flow curves from these tests showed a very strong dependence of peak flow stress and flow-softening rate on grain size; both parameters increased with alpha grain/colony size. Microstructures of the upset test specimens revealed the presence of fine, equiaxed grains of γ + α ₂ + β phases resulting from the dynamic spheroidization process that initiated at and proceeded inward from the prior-alpha grain/colony boundaries. The grain interiors displayed evidence of microkinking of the lamellae. The frequency and severity of kinking increased with strain, but were also strongly dependent on the local orientation of lamellae with respect to the compression axis. The kinetics of dynamic spheroidization were found to increase as the strain rate decreased for a given alpha grain size and to decrease with increasing alpha grain size at a given strain rate. The breakdown of the lamellar structure during hot deformation occurred through a combination of events, including shear localization along grain/colony boundaries, microbuckling of the lamellae, and the formation of equiaxed particles of γ + β ₂ + α ₂ on grain/colony boundaries and in zones of localized high deformation within the microbuckled regions.     

Excimer laser surface processing of Ti-6Al-4V

We have examined the effect of surface processing in air, using excimer laser light at 248 nm wavelength, on the oxygen content, microstructure, and surface hardness of Ti-6Al-4V. Processing with a single pulse results in the transformation of theα +β material toα′ martensite. Multiple pulse processing results in rapid incorporation of oxygen in the material. Oxygen initially dissolves in the material in the liquid phase. As the concentration exceeds the solid solubility limit during solidification, TiO particles precipitate. In contrast to equilibrium oxidation processes in Ti, only TiO is observed as an oxidation product; further processing results in increased oxygen incorporation and an increased volume fraction of TiO but no other oxides of Ti. The TiO particle size is a function of the oxygen concentration and the number of pulses, with some grain growth occurring after many pulses. The effects of solution hardening by dissolved oxygen and precipitation hardening by the TiO are identifiable as functions of oxygen concentration and mean free path between particles, respectively. A maximum surface hardness almost twice that of electropolished Ti-6Al-4V is observed.     

Microstructural development and creep deformation in equiaxed γ,γ + α2 and γ + α2 + B2 titanium aluminides

The development of microstructure and its influence on creep properties have been studied for structures including equiaxed γ, duplex, and other structures of varying α₂ morphology in two Ti-48Al-2Cr-2Nb alloys. Heat treatment at 1125°C have been utilized to produce equiaxed γ microstructures in alloys with or without Mo additions. The γ→α transformation produces α₂ plates with several orientation variants with γ grains during subsequent annealing of the equiaxed γ microstructures below the α transus. Formation of this α₂ morphology results from rapid up-quenching (UQ), and this structure persists through annealing, cooling, and creep testing. Differences in minimum creep rates for several microstructures, containing varying amounts of multi-or single variant γ/α₂ grains are shown to be minimal. The presence of Mo has also resulted in improved creep resistance in equiaxed γ and γ + α₂ + B2 structures, as compared to similar microstructures in the Ti-48Al-2Cr-2Nb alloy. Deformation during creep at 760 °C at stresses between 200 and 400 MPa occurs by a combination of twinning and dislocation glide without recrystallization, resulting in power-law stress exponents in the range of 6 to 9. Only minimal strain path dependence of the minimum creep rate is detected in a comparison of creep rates in stress jump, stress drop, and single stress tests. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlandom, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees.     

Decomposition and dissolution kinetics ofδ′ precipitation in Al-Li binary alloys

Aside from its technological importance, the Al-Li alloy system also exhibits interesting phase transformations involving both equilibrium and metastable states. Recent theoretical studies have shown that a supersaturated solid solution could take different transformation paths when it is quenched into theα +γ′ field. Suggestions were made that a rapidly quenched solution phase should first undergo a congruent ordering transformation before it decomposes into a two-phase mixture by either a secondary spinodal decomposition or the classical nucleation and growth process. Moreover, a metastable miscibility gap was predicted at lower temperatures. The objective of this research is to study the transformation paths and dynamics in Al-Li binary alloys of three compositions (5.2, 7.0, and 12.0 at. pct Li). This investigation emphasizes thein situ small-angle X-ray scattering (SAXS) observations on specimens subjected to various aging conditions. Special attention is paid to the early stages of the transformation in an attempt to characterize the various possible modes of phase separation on one hand and to study the dynamics of the precipitation process on the other. The following results are obtained: the congruent ordering precedes decomposition at low temperatures; the metastableγ′ solvus curve is reconfirmed; but the predicted metastable miscibility gap is not found. Guinier radii measurements of the particles showed Ostwald ripening is quickly reached upon heating to the aging temperatures. Slowing down behavior is seen at aging temperatures close to the solvus boundary. Activation energies for Li diffusion were obtained using the modified Lifshitz, Slyozov, and Wagner (MLSW) model. A test of dynamical scaling behavior is carried out for the Al-12.0 at. pct Li alloy. Formerly Visiting Scientists at the University of Illinois     

Decomposition and dissolution kinetics ofδ′ precipitation in Al-Li binary alloys

Aside from its technological importance, the Al-Li alloy system also exhibits interesting phase transformations involving both equilibrium and metastable states. Recent theoretical studies have shown that a supersaturated solid solution could take different transformation paths when it is quenched into theα +γ′ field. Suggestions were made that a rapidly quenched solution phase should first undergo a congruent ordering transformation before it decomposes into a two-phase mixture by either a secondary spinodal decomposition or the classical nucleation and growth process. Moreover, a metastable miscibility gap was predicted at lower temperatures. The objective of this research is to study the transformation paths and dynamics in Al-Li binary alloys of three compositions (5.2, 7.0, and 12.0 at. pct Li). This investigation emphasizes thein situ small-angle X-ray scattering (SAXS) observations on specimens subjected to various aging conditions. Special attention is paid to the early stages of the transformation in an attempt to characterize the various possible modes of phase separation on one hand and to study the dynamics of the precipitation process on the other. The following results are obtained: the congruent ordering precedes decomposition at low temperatures; the metastableγ′ solvus curve is reconfirmed; but the predicted metastable miscibility gap is not found. Guinier radii measurements of the particles showed Ostwald ripening is quickly reached upon heating to the aging temperatures. Slowing down behavior is seen at aging temperatures close to the solvus boundary. Activation energies for Li diffusion were obtained using the modified Lifshitz, Slyozov, and Wagner (MLSW) model. A test of dynamical scaling behavior is carried out for the Al-12.0 at. pct Li alloy. Formerly Visiting Scientists at the University of Illinois     

Microstructural stability of fine-grained fully lamellar XD TiAl alloys by step aging

XD TiAl alloys (Ti-45 and 47Al-2Nb-2Mn+0.8 vol pct TiB₂) (at. pct) were oil quenched to produce fine-grained fully lamellar (FGFL) structures, and aging treatments at different temperatures for different durations were carried out to stabilize the FGFL structures. Microstructural examinations show that the aging treatments cause phase transformation of α ₂ to γ, resulting in stabilization of the lamellar structure, as indicated by a significant decrease in α ₂ volume fraction. However, several degradation processes are also introduced. After aging, within lamellar colonies, the α ₂ lamellae become finer due to dissolution, whereas most of the γ lamellae coarsen. The dissolution of α ₂ involves longitudinal dissolution and lateral dissolution. In addition, at lamellar colony boundaries, lamellar termination migration, nucleation and growth of γ grains, and discontinuous coarsening occur. With the exception of longitudinal dissolution, all the other transformation modes are considered as degradation processes as they result in a reduction in α ₂/γ interfaces. Different phase transformation modes are present to varying degrees in the aged FGFL structures, depending on aging conditions and Al content. A multiple step aging reduces the drive force for phase transformation at high temperature by promoting phase transformation via longitudinal dissolution at low temperatures. As a result, this aging procedure effectively stabilizes the lamellar structure and suppresses other degradation processes. Therefore, the multiple step aging is suggested to be an optimal aging condition for stabilizing FGFL XD TiAl alloys.     

The mechanism of formation of a fine duplex microstructure in Ti-48Al-2Mn-2Nb alloys

The mechanism of formation of the fine duplex microstructure resulting from the α → γ transformation in water-quenched Ti-48Al-2Mn-2Nb alloys was studied using transmission and analytical electron microscopy. As-cast Ti-48Al-2Mn-2Nb alloys were heat treated in the α phase field and water quenched to room temperature. The resulting microstructure (referred to as a fine duplex microstructure) consisted of equiaxed grains and abutting lath colonies. Both the colonies and the grains were composed of the γ phase, twinned γ laths, and α₂ laths. It was found that the transformation from α to γ in the fine duplex microstructure took place through long range diffusional processes, and compctitive growth between the equiaxed and lath morphology occurred. Nucleation of they phase from the α matrix can occur through nucleation on stacking faults, followed by growth through the sympathetic nucleation and growth of new γ laths on a substrate lath. The observed misorientations and the interfacial structures between the laths were found to be consistent with such a mechanism. Compctition between such nucleation and growth mechanisms for the equiaxed and lath morphologies of γ leads to the formation of lath colonies (of γ and α₂) interspersed with equiaxed grains in these alloys. Formerly Visiting Scientist, Metals and Ceramics Division, Oak Ridge National Laboratory This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

Microstructural development and creep deformation in equiaxed γ, γ+α 2, and γ+α 2+B2 titanium aluminides

The development of microstructure and its influence on creep properties have been studied for structures including equiaxed γ, duplex, and other structures of varying α ₂ morphology in two Ti-48Al-2Cr-2Nb alloys. Heat treatments at 1125 °C have been utilized to produce equiaxed γ microstructures in alloys with or without Mo additions. The γ → α transformation produces α ₂ plates with several orientation variants within γ grains during subsequent annealing of the equiaxed γ microstructures below the α transus. Formation of this α ₂ morphology results from rapid up-quenching (UQ), and this structure persists through annealing, cooling, and creep testing. Differences in minimum creep rates for several microstructures containing varying amounts of multi- or single variant γ/α ₂ grains are shown to be minimal. The presence of Mo has also resulted in improved creep resistance in equiaxed γ and γ+α ₂+B2 structures, as compared to similar microstructures in the Ti-48Al-2Cr-2Nb alloy. Deformation during creep at 760 °C at stresses between 200 and 400 MPa occurs by a combination of twinning and dislocation glide without recrystallization, resulting in power-law stress exponents in the range of 6 to 9. Only minimal strain path dependence of the minimum creep rate is detected in a comparison of creep rates in stress jump, stress drop, and single stress tests. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations Committees.     

Microstructure evolution through the α→γ phase transformation in a Ti-48 At. pct Al alloy

The α→γ phase transformation during rapid quenching and subsequent isothermal aging has been investigated in a Ti-48 at pct Al alloy. The microstructure changes from a completely massively transformed γ-grain structure to a mixed microstructure of the massively transformed γ grains and the untransformed (meaning massively untransformed) fine α ₂/γ lamellae with an increase in the cooling rate from the high-temperature α phase field. Fine γ grains are generated from these fine α ₂/γ lamellae by subsequent again at 1323 K. The fine γ grains contain many defects, such as dislocations, microtwins (or stacking faults), domain boundaries, and variants, which are frequently observed in the massive γ grains. This result suggests that the formation mechanism of the fine γ grains during aging is similar to that of the massive γ grains. When the fine γ/γ lamellar sample, which is formed by preliminary aging at a lower temperature (1173 K), is aged at a higher temperature (1323 K), apparent changes in microstructure could not be recognized. This result indicates that the fine γ-grain formation is closely related to the α ₂ → γ phase transformation in the fine α ₂/γ lamellae. This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformation Committees.     

Room-temperature deformation behavior of directionally solidified multiphase Ni-Fe-Al alloys

Directionally solidified (DS) β + (γ + γ′) Ni-Fe-Al alloys have been used to investigate the effect of a ductile second phase on the room-temperature mechanical behavior of a brittle 〈001〉-oriented β (B2) phase. The ductile phase in the composite consisted of a fine distribution of ordered γ′ precipitates in a γ (fcc) matrix. Three microstructures were studied: 100 pct lamellar/rod, lamellar + proeutectic β, and discontinuous γ. The β matrix in the latter two microstructures contained fine-scale bcc precipitates formed due to spinodal decomposition. Room-temperature tensile ductilities as high as 12 pct and fracture toughness (K _Q ) of 30.4 MPa √m were observed in the 100 pct lamellar/rod microstructure. Observations of slip traces and dislocation substructures indicated that a substantial portion of the ductility was a result of slip transfer from the ductile phase to the brittle matrix. This slip transfer was facilitated by the Kurdjumov-Sachs (KS) orientation relationship between the two phases and the strong interphase interface which showed no decohesion during deformation. In microstructures which show higher values of tensile ductility and fracture toughness, 〈100〉 slip was seen in the β phase, whereas 〈111〉 slip was seen in the β phase in the microstructure which showed limited ductility. The high ductility and toughness are explained in terms of increased mobile dislocation density afforded by interface constraint. The effect of extrinsic toughening mechanisms on enhancing the ductility or toughness is secondary to that of slip transfer.     

Microstructural refinement and mechanical properties improvement of elemental powder metallurgy processed Ti-46.6Al-1.4Mn-2Mo alloy by carbon addition

Strengthening of a gamma TiAl alloy was sought by a chemical modification of the composition with carbon. Up to 0.6 at. pct of carbon was added to the Ti-46.6Al-1.4Mn-2Mo alloy processed by elemental powder metallurgy. Carbon addition resulted in considerable microstructural changes such as refinement, by a factor of about 2, of the lamellar microstructure and carbide precipitation. The cause of the lamellar structure refinement is twofold, increased heterogeneous nucleation rate and decreased γ platelet growth rate, the net result of which was a retarded diffusional transformation kinetics of α to α/γ lamellae. As a consequence of the microstructural changes, the high-temperature tensile properties and the creep properties of the alloy were significantly improved. Anomalous hardening was also observed at 800 °C, resulting in a tensile yield strength of 700 MPa. The strengthening effect of carbon was realized by the microstructural refinement and by precipitation hardening of intergranular as well as interlamellar Ti₃AlC. In terms of the tensile properties and the creep properties, the optimum amount of carbon addition was 0.3 at. pct.     

Thermoelastic martensite and shape memory effect in B2 Base Ni-Al-Fe alloy with enhanced ductility

A ductile shape memory alloy of the Ni-AI-Fe system has been developed using principles through the control of microstructure. Addition of Fe to the binary Ni-Al shape memory alloy allows the introduction of a ductile face-centered cubic (fcc) phase in an otherwise extremely brittle β phase alloy, leading to an improvement in its ductility while retaining its ability to exhibit shape memory arising from the martensitic transformation of theβ phase to Ll₀ structure. It is shown that the transformation temperature in the ternary Ni-AI-Fe alloy can be easily controlled by the preannealing in the β+ γ region. Experimental results on the effect of different annealing treatments on the microstructures and the shape memory behavior in this alloy are presented and discussed.     

Effects of tensile stress on microstructural change of eutectoid Zn-AI alloy

Microstructural changes and phase transformations of eutectoid Zn-Al-based alloy ZnA120.2Cul.8 (wt pct) were studied under tensile stress by using X-ray diffraction and scanning electron microscopy (SEM) techniques. It was found that the lamellar microstructure of the heat-treated eutectoid Zn-Albased alloy changed partially into a spheroidized structure at the rupture part of the specimen after tensile testing, while the lamellar structure at the bulk part of the specimen remained stable in the original state. The X-ray diffraction identification results showed that two phase transformations,i.e., decomposition of metastable phase η′_T and a four-phase transformation, α+ ε → T + η, occurred during tensile testing. It was concluded that the tensile stress affected not only microstructural change but also phase transformation of the alloy. The SEM observation on the etched specimen showed clearly the morphology of the microstructural change.     

Influence of oxygen on phase transformations in a Ti-48 At. pct Al alloy

The composition and structure of Ti-48 at. pct Al alloys with various oxygen contents, quenched from a homogeneous α state, have been studied by means of one-dimensional atom-probe (1DAP) and transmission electron microscopy (TEM) analysis. Two regimes are observed. The change from one regime to the other depends on the global oxygen content. If the oxygen content is lower than 1.2 at. pct, the α→γ _m massive transformation is involved during the quench. The alloys, hence, exhibit massive γ _m-structure regions and regions having a two-phase (α ₂+γ) ultrafine lamellar structure. Very thin α ₂ plates, saturated with oxygen, are observed in γ _m regions. The precipitation of these α ₂ plates is promoted by excess oxygen in the γ _m structure. Within ultra-fine lamellar-structure regions, oxygen is concentrated in α ₂ lamellae (not saturated with oxygen) and is found to be responsible for the high volume fraction of α ₂ phase. When the oxygen content is larger than 1.2 at. pct, the massive transformation is suppressed and the ultrafine lamellar structure is only observed in quenched samples. Analysis of the α → α ₂ chemical ordering in the classical lamellar structure, formed within the (α+γ) dual-phase field, shows that high oxygen contents favor the chemical-ordering reaction of α phase at high temperatures (e.g., 1423 and 1523 K). It has, hence, been inferred that, above 1.2 at. pct O, the α → γ _m massive transformation is suppressed and replaced by the α → α ₂+y transformation paths. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.