Microstructural evolution and mechanical properties of a forged gamma titanium aluminide alloy

A two-phase gamma titanium aluminide alloy, Ti-47Al-1Cr-1V-2.5Nb (in at.%), was studied under forged and various subsequent heat treatment conditions, to investigate the microstructural evolution and the effect of microstructure on room temperature (RT) tensile properties and fracture toughness behavior. Four classes of microstructure and three types of lamellar formation were identified, and their formation mechanisms were analyzed using various analytical techniques including metallography, electron optics, differential thermal analysis (DTA), and crystallography. It was found that both tensile and toughness behavior were profoundly affected by the microstructural variations.     

Compressive creep behavior of spray-formed gamma titanium aluminide

The creep behavior of spray-formed γ-TiAl with a fine, equiaxed fully lamellar (FL) microstructure was studied in a temperature-stress regime of 780 °C to 850 °C and 180 to 320 MPa. An apparent stress exponent of 4.3 and an activation energy of 342 kJ/mol were observed in the high-temperature high-stress regime. Compared with the FL γ-TiAl which was obtained through conventional casting+heat treatment processes, the spray-formed γ-TiAl exhibited higher creep resistance. The higher creep resistance observed in the present study was discussed in light of the interstitial level, the chemical composition, the grain size, and the interlocking of lamellae at the grain boundary, which in turn may be a function of interlamellar spacing and the step height of the serrated grain boundaries. It was suggested that the small interlamellar spacing and possibly larger step height may contribute to the higher creep resistance observed in the present study.     

Plastic-flow and microstructure evolution during hot deformation of a gamma titanium aluminide alloy

The hot workability of a near gamma titanium aluminide alloy, Ti-49.5Al-2.5Nb-1.1Mn, was assessed in both the cast and the wrought conditions through a series of tension tests conducted over a wide range of strain rates (10⁻⁴ to 10⁰ s⁻¹) and temperatures (850 °C to 1377 °C). Tensile flow curves for both materials exhibited sharp peaks at low strain levels followed by pronounced necking and flow localization at high strain levels. A phenomenological analysis of the strain rate and temperature dependence of the peak stress data yielded an average value of the strain rate sensitivity equal to 0.21 and an apparent activation energy of ∼411 kJ/mol. At low strain rates, the tensile ductility displayed a maximum at ∼ 1050 °C to 1150 °C, whereas at high strain rates, a sharp transition from a brittle behavior at low temperatures to a ductile behavior at high temperatures was noticed. Dynamic recrystallization of the gamma phase was the major softening mechanism controlling the growth and coalescence of cavities and wedge cracks in specimens deformed at strain rates of 10⁻⁴ to 10⁻² s⁻¹ and temperatures varying from 950 °C to 1250 °C. The dynamically recrystallized grain size followed a power-law relationship with the Zener-Hollomon parameter. Deformation at temperatures higher than 1270 °C led to the formation of randomly oriented alpha laths within the gamma grains at low strain levels followed by their reorientation and evolution into fibrous structures containing γ + α phases, resulting in excellent ductility even at high strain rates.     

High-temperature low-cycle fatigue of a gamma titanium aluminide alloy Ti-46Al-2Nb-2Cr

The low-cycle fatigue (LCF) behavior of a gamma titanium aluminide alloy Ti-46Al-2Nb-2Cr in fully lamellar (FL) and nearly lamellar (NL) microstructural conditions is studied at 650 °C and 800 °C, with and without hold times. At 650 °C and 800 °C, the alloy in either condition exhibits cyclic stability at all strain levels studied, excepting the NL structure which shows slight cyclic hardening at higher strain levels at 650 °C. Fracture in the FL condition occurs by a mixed mode comprising delamination, translamellar fracture, and stepwise fracture. On the other hand, fracture occurs mostly by translamellar mode in the NL condition. At both test temperatures, the alloy in the FL condition obeys the well-known Manson-Coffin behavior. The fatigue resistance of the alloy at 650 °C in the FL condition is very much comparable to, while in the NL condition it is superior to, that of Ti-24Al-llNb alloy. At 650 °C, a 100-second peak tensile strain hold doubles the fatigue life of the alloy in the FL condition, while a 100-second hold at compressive peak strain or at both tensile and compressive peak strain degrades fatigue life. The observed hold time effects can primarily be attributed to mean stress. Irrespective of the nature of the test, the hysteretic energy (total as well as tensile) per cycle remains nearly constant during the majority of its life. The total and tensile hysteretic energy to fracture, at both test temperatures, increase with cycles to failure, and the variation follows a power-law relationship. Formerly NRC Senior Resident Associate, Wright Laboratory.     

Hydrogen effects on brittle fracture of the titanium aluminide alloy Ti-24Al-11Nb

Varying amounts of hydrogen were dissolved in the titanium aluminide alloy Ti-24Al-llNb (atomic percent). Virtually all of this hydrogen probably precipitated as hydride on cooling because the terminal solubility in the dominant Ti₃Al phase is very low at room temperature. Although the yield strength (YS) increased, the ultimate tensile strength (UTS), ductility, fracture stress in notched bend bars, and fracture toughness decreased with increasing amounts of hydride. The strength and fracture properties, for all hydride contents, did not change with testing speed below about 5 to 50 mm/min but decreased steeply for speeds greater than that. The presence of hydride decreased the critical value of testing speed by about an order of magnitude. Brittle cracks in bluntly notched bend bars, with or without hydride, nucleated at the notch root or at a distance below the root which was less than one fifth of the distance to the peak stress location. This result suggests that the cleavagelike cracking in this material is not controlled by normal stress alone but has some dependence on the applied strain. The fracture surfaces of notched or precracked specimens, with or without hydride, consisted entirely of cleavagelike fracture, but these cracks exhibited stable crack propagation. This permitted both the measurement of crack resistance or R curves and also observation of the initiation and propagation of the crack with increasing K_I. The results showed that cracks initiated discontinuously at characteristic sites within the plastic zone and along the slip bands when the plastic deformation ahead of the precrack developed to a particular and reproducible extent. Literature cleavage models were compared to results for the present tests. WU-YANG CHU, Formerly Visiting Professor, Carnegie Mellon University,     

Phase transformation behavior of gamma titanium aluminide alloys during supertransus heat treatment

Microstructure evolution in a wrought near-gamma titanium alloy, Ti-45Al-2Cr-2Nb, was investigated by a series of heat treatments comprised of initial heating high in the alpha-plus-gamma phase field followed by short-time heating in the single-phase alpha field. The initial heating step led to a dispersion of gamma particles which pinned the alpha grain boundaries. The kinetics of the gamma grain dissolution during subsequent heating in the single-phase field were interpreted in terms of models for both interface reaction-controlled and diffusion-controlled processes. The model for diffusion-controlled dissolution yielded predictions comparable to the observed times, whereas the model for interface reaction-controlled behavior predicted dissolution kinetics over an order of magnitude slower than observed. The growth of the alpha grains, both before and after the dissolution of the gamma phase, was also modeled. Section size limitations to the ability to use supertransus heating to obtain uniform and moderately fine alpha grain sizes were examined using the transformation models and a simple heat transfer analysis approach. The results were validated through the heat treatment of subscale and full-scale forgings. 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.     

Auger electron analysis of the initial oxidation of titanium aluminides based on Ti-48Al

The surface of a Ti-48 at. Pct Al alloy was examined by Auger electron microscopy to study oxidation at room temperature. On exposure to air at room temperature, both Al and Ti oxides were observed together with an abundance of C. The amount of C was always larger in the two-phase α₂ + γ region compared to the single-phase γ region. The Ti oxides formed on the surface of they grains were primarily Ti₂O₃ rather than TiO₂. On depth profiling with Ar⁺ ion sputtering, lower oxide states of Ti were found. This was attributed to either the Ar⁺ ion sputtering or the fact that the inner layers of oxide represented oxides of Ti in their lower valence states. The A1₂O₃ was stable and did not exhibit any transient oxidation states. The dominant oxidation product on the surface of sputtered single-phase γ grains after an 84-hour exposure in the ultrahigh vacuum Auger chamber at room temperature is A1₂O₃. A depletion of C and O occurred beneath the oxide surface in some γ grains. The chemical shift between the Al L_2,3MM and A1₂O₃ L_2,3(A1)M_(O)M_(O) peaks in the Auger spectrum of A1₂O₃ formed on the γ phase in TiAl was found to be 11 eV. Y.T. Peng, Graduate Student, Formerly with the Materials Science and Engineering Program, University of Texas at Arlington, Arlington, TX 76019,     

High-temperature deformation behavior of a gamma TiAl alloy—Microstructural evolution and mechanisms

The present investigation was carried out in the context of the internal-variable theory of inelastic deformation and the dynamic-materials model (DMM), to shed light on the high-temperature deformation mechanisms in TiAl. A series of load-relaxation tests and tensile tests were conducted on a fine-grained duplex gamma TiAl alloy at temperatures ranging from 800 °C to 1050 °C. Results of the load-relaxation tests, in which the deformation took place at an infinitesimal level (ε ≅ 0.05), showed that the deformation behavior of the alloy was well described by the sum of dislocation-glide and dislocation-climb processes. To investigate the deformation behavior of the fine-grained duplex gamma TiAl alloy at a finite strain level, processing maps were constructed on the basis of a DMM. For this purpose, compression tests were carried out at temperatures ranging from 800 °C to 1250 °C using strain rates ranging from 10 to 10⁻⁴/s. Two domains were identified and characterized in the processing maps obtained at finite strain levels (0.2 and 0.6). One domain was found in the region of 980 °C and 10⁻³/s with a peak efficiency (maximum efficiency of power dissipation) of 48 pct and was identified as a domain of dynamic recrystallization (DRx) from microstructural observations. Another domain with a peak efficiency of 64 pct was located in the region of 1250 °C and 10⁻⁴/s and was considered to be a domain of superplasticity. 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.     

Microstructural evolution and superplastic deformation behavior of fine grain 5083Al

The microstructural evolution during superplastic deformation of a fine grain Al-4.7 pct Mg alloy (5083Al) has been studied quantitatively. Starting from an average grain size of 7 μm, grain growth was monitored in this alloy both under static annealing and with concurrent superplastic deformation at a high test temperature of 550°C. Grain size was averaged from measurements taken in longitudinal, transverse, and thickness directions and was found to grow faster during concurrent superplastic deformation than for static annealing. A grain growth law based on an additive nature between time-based and strain-based growth behavior was used to quantify the dynamics of concurrent grain growth. The extent of void formation during deformation was quantified as the area fraction of voids on L-S planes. This void fraction, referred to as the cavity area percent, was recorded at several levels of strain for specimens deformed at two different strain rates. A constitutive equation incorporating this grain growth data into the stress-strain rate data, determined during the early part of deformation, was generated and utilized to model the superplastic tensile behavior. This model was used in an effort to predict the stress-strain curves in uniaxial tension under constant and variable strain rate conditions. Particular attention was paid to the effects of a rapid prestrain rate on the overall superplastic response and hardening characteristics of this alloy.     

Influence of β grain size on tensile behavior and ductile fracture toughness of titanium alloy Ti-10V-2Fe-3Al

The β grain size of the alloy Ti-10V-2Fe-3Al was varied by heat treatment, and the tensile behavior and fracture toughness were evaluated as a function of β grain size at room temperature. The alloy showed stress-induced martensitic transformation, and the triggering stress for this transformation varied with grain size. The 0.2 pct yield stress exhibited a Hall-Petch relationship with grain size. The ductile fracture toughness was found to increase with decrease in grain size, and it was also shown to follow a Hall-Petch kind of relationship. The grain boundary and the stress-induced martensitic contribution to fracture toughness were separated out.     

Analysis of grain growth in a two-phase gamma titanium aluminide alloy

Microstructure evolution during annealing of a wrought near-gamma titanium aluminide alloy, Ti-45.5Al-2Nb-2Cr (at. pct), in the temperature range 1200 °C to 1320 °C was investigated. The mean grain size of the alpha phase as well as the volume fraction and size of the gamma particles were evaluated as a function of annealing temperature and time. Isothermal annealing at temperatures above the alpha transus, T _α=1300 °C, led to rapid grain growth of the alpha phase, the kinetics of which could be described by a simple power-law type expression with a grain growth exponent p=2.3. Alpha grain growth was significantly retarded during annealing at subtransus temperatures (1200 °C≤T≤1300 °C) by the pinning influence of gamma-phase particles. Limiting grain size values predicted by computer simulation models applicable for high-volume fractions of precipitates/particles were in good agreement with experimental findings. The kinetics of alpha grain growth in the presence of gamma particles were analyzed, and the results showed that a grain growth exponent of p≈2.6 could satisfactorily account for the experimental results.     

热处理对SiCp/Al复合材料拉伸变形行为的影响

采用压力浸渗法制备SiC体积分数为55%的SiCp/Al-10Si-0.7Mg复合材料.通过拉伸试验和扫描电镜研究不同热处理工艺对该复合材料拉伸变形行为及显微组织的影响.结果表明,热处理对高体积分数SiCp/Al-10Si-0.7Mg复合材料的弹性模量影响很小,但T6热处理可显著提高其在拉伸时的弹性极限,而冷热循环处理可以在T6热处理的基础上进一步提高拉伸弹性极限.高体积分数SiCp/Al-10Si-0.7Mg复合材料的拉伸弹性极限的提高主要是热处理过程中基体铝合金中的固溶和时效强化、残余内应力的消除、位错强化和加工硬化综合作用的结果.     

Influence of temperature transients on the hot workability of a two-phase gamma titanium aluminide alloy

The hot deformation behavior, microstructure development, and fracture characteristics of a wrought two-phase γ-titanium aluminide alloy Ti-45.5Al-2Nb-2Cr containing a fine, equiaxed microstructure were investigated with special reference to the influence of temperature transients immediately pre-ceding plastic deformation. Specimens were soaked at 1321 °C or 1260 °C, cooled directly to test temperatures of 1177 °C and 1093 °C, and upset under conditions of constant strain rate and tem-perature. Plastic flow behavior and microstructure evolution occurring in tests involving prior tem-perature transients were compared with those occurring in specimens which were directly heated to the test temperature and upset under identical deformation conditions. Flow curves associated with prior exposure at 1321 °C exhibited very sharp peaks and strong flow softening trends compared to those obtained under isothermal conditions,i.e., involving no temperature transients. During cooling from 1321 °C, the metastable α phase undergoes limited or complete decomposition into α/α₂ + γ lamellae, depending on the final temperature (1177 °C/1093 °C). Subsequent hot deformation leads to partial globularization of the lamellae together with extensive kinking and reorientation of lamellae. In contrast, isothermal deformation at 1177 °C/1093 °C preserves the fine, equiaxed microstructure, through dynamic recrystallization of the γ grains. Cracking observed in specimens deformed at 1093 °C and 1.0 s⁻¹ after exposure at 1321 °C has been attributed to the low rate of globularization as well as the occurrence of shear localization. Plastic flow behavior observed in this work is compared with that observed in several single-phase and two-phase gamma titanium aluminide alloys in order to identify mechanism(s) responsible for flow softening.     

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.     

On the creep behavior of the Ti3Al titanium aluminide Ti25Al10Nb3V1Mo

The temperature and stress dependence of the steady-state creep behavior of the Ti₃Al alloy Ti25Al10Nb3V1Mo (at.%) has been evaluated. Two microstructural conditions were evaluated as follows: As processed (rolled) consisting of the fine grained (approx. 6–10 μm) β plus ordered α₂ phase and beta heat treated consisting of coarse grained (approx. 150 μm) retained ordered B2 phase with a fine Widmanstatten structure within the grain interiors. The steady-state creep behavior of both microstructural conditions was studied over the temperature range of 650–815°C. The apparent creep activation energies and stress exponents were measured for both microstructural conditions. The temperature and stress dependence of the steady-state creep rate of both microstructures can be described well by the power law creep equation suggesting dislocation motion as the operative deformation mechanism. Over the temperature-stress regime of the present study, the creep deformation of the fine grained microstructure possibly breaks down into a low temperature (dislocation core diffusion controlled) regime and a high temperature (bulk diffusion controlled) regime within the power law creep region as indicated by the apparent creep activation energies measured. Upon β heat treatment, creep deformation is found to be governed by a single rate limiting process. At temperatures and stress levels where a direct comparison can be made, the steady-state creep rates of the β heat treated Ti-25-10-3-1 exhibit an order of magnitude decrease over those of the processed material. This suggests the possibility of some mechanism other than power law creep controlling within the regime corresponding to the low apparent activation energy of the fine grained microstructure.     

Plastic-flow behavior and microstructural development in a cast alpha-two titanium aluminide

Plastic-flow behavior and microstructural development were investigated for a cast α₂ titanium aluminide, Ti-24Al-11Nb (atomic percent), using the isothermal hot-compression test. Regimes of warm- and hot-working behavior were inferred from flow curves adjusted for deformation heating effects. Plots of flow stress as a function of inverse temperature and estimates of the strain-rate-sensitivity index confirmed the transition from warm to hot-working conditions over a rather narrow temperature range. Hot working in theα ₂ +β phase field was also marked by a rather high activation energy (viz., ∼1080 kJ/mole) for the controlling deformation process, which appeared to consist of dynamic globularization of the ordered-α ₂ phase. A sharp decrease in the activation energy was noticed when the deformation temperature was increased above the β-transus. Microstructural observations also indicated development of an unrecrystallized structure during warm working, with cavities and wedge cracks being found near the bulged free surfaces of the upset specimens. The plastic-flow phenomenology exhibited a number of similarities to that found in the wrought version of the Ti-24Al-11Nb alloy. Formerly Senior Research Scientist, Metalworking Group, Battelle Memorial Institute, Columbus, OH 43201