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.     

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.     

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.     

Hydrogen solubility in a titanium aluminide alloy

The dissolution of hydrogen in a titanum aluminide alloy based on Ti₃Al, Ti-24 Al-11 Nb, has been measured as a function of temperature and hydrogen pressure. Both the terminal solubility and the overall solubility, or total uptake of hydrogen, were determined. The terminal solubility of hydrogen in the α₂ matric phase increases with temperature, while the overall solubility decreases with increasing temperature in the high-temperature region. The partial molar heat of solution in the α₂ phase was determined to be−(21.97 ± 1.46) kJ/mol [H], while the partial molar heat of formation of the hydride phase was −(70.29 ± 2.34) kJ/mol [H]. The standard partial molar free energy of hydrogen in the α₂ phase in equilibrium with the hydride was −(74,060 ± 2000) + (87.8 ± 2.1) T J/mol [H].     

The initiation and growth of fatigue cracks in a titanium aluminide alloy

The micromechanics of small, naturally initiated fatigue cracks and large through-thickness fatigue cracks have been studied in the titanium aluminide alloy Super Alpha 2. The microstructure investigated had equal volume fractions ofα ₂ and Β phases. Crack growth rates were higher than through α-Β titanium alloys. Initiation of small cracks was found always to occur in theα ₂ phase, and small cracks grew belowΔK _th, the minimum cyclic stress intensity required for growth of large fatigue cracks. A method previously proposed for reconciling the growth rates of large and small cracks is applied to these results.     

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.     

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.     

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.     

Recrystallization and grain growth in metastable beta III titanium alloy

The progress of recrystallization and subsequent grain growth has been systematically investigated in a metastable beta titanium alloy (Ti-11.5 Mo-6 Zr-4.5 Sn). Quantitative evaluation of the kinetics of these processes over a wide range of temperature, deformation, and initial grain sizes has been performed. For a given deformation, the average grain boundary velocity, decreasing with the reciprocal of annealing time, suggests the occurrences of recovery with second order kinetics concurrent with the recrystallization. The amount of deformation, varying from 20 to 80 pct cold reduction and proportional to the stored energy of deformation in the alloy, increases the average grain boundary migration rate during recrystallization by three orders of magnitude. The temperature dependence of the recrystallization rate, however, remains unaffected by the amount of deformation at 83 kcal/mole (347 kJ/mole). The isothermal grain growth kinetics follow the power law such that the time exponent of the process remains at a value of 0.35 at most annealing temperatures. The excellent agreement between the driving force exponent of recrystallization and the time exponent of grain growth based on a model which relates the driving force dependence of the rates of both processes, clearly suggests that the kinetics of these processes are controlled by a single mechanism,i.e. impurity dependent boundary migration. This paper is based on a presentation made at a symposium on “Recovery, Recrystallization and Grain Growth in Materials” held at the Chicago meeting of The Metallurgical Society of AIME, October 1977, under the sponsorship of the Physical Metallurgy Committee.     

High-temperature fracture and fatigue-crack growth behavior of an XD gamma-based titanium aluminide intermetallic alloy

A study has been made of the effect of temperature (between 25 °C and 800 °C) on fracture toughness and fatigue-crack propagation behavior in an XD-processed, γ-based titanium aluminide intermetallic alloy, reinforced with a fine dispersion of ∼1 vol pct TiB₂ particles. It was found that, whereas crack-initiation toughness increased with increasing temperature, the crack-growth toughness on the resistance curve was highest just below the ductile-to-brittle transition temperature (DBTT) at 600 °C; indeed, above the DBTT, at 800 °C, no rising resistance curve was seen. Such behavior is attributed to the ease of microcrack nucleation above and below the DBTT, which, in turn, governs the extent of uncracked ligament bridging in the crack wake as the primary toughening mechanism. The corresponding fatigue-crack growth behavior was also found to vary inconsistently with temperature. The fastest crack growth rates (and lowest fatigue thresholds) were seen at 600 °C, while the slowest crack growth rates (and highest thresholds) were seen at 800 °C; the behavior at 25 °C was intermediate. Previous explanations for this “anomalous temperature effect” in γ-TiAl alloys have focused on the existence of some unspecified environmental embrittlement at intermediate temperatures or on the development of excessive crack closure at 800 °C; no evidence supporting these explanations could be found. The effect is now explained in terms of the mutual competition of two processes, namely, the intrinsic microstructural damage/crack-advance mechanism, which promotes crack growth, and the propensity for crack-tip blunting, which impedes crack growth, both of which are markedly enhanced by increasing temperature.     

Grain size and grain growth in an equiaxed alpha-beta titanium alloy

Methods of revealing grain size in a two-phase α-β titanium alloy have been examined and observations on beta grain growth in the presence of alpha have been carried out. The technique proposed by Greenfield and Margolin¹ for revealing β matrix grain sizes has been shown not to produce grain growth. However, for grain sizes of about 10 μm the G.M. technique does not reveal all the grains because of the similarity in orientation in neighboring grains. These clusters of similarly oriented grains are shown to persist as grain growth takes place but the misorientation between grains within a cluster decreases. Both the beta grain growth and alpha particle coarsening follow the same time dependency from which it is shown that a linear relationship exists between α particle size and β grain size. It is proposed that α particles must dissolve from theβ grain edges for β grain growth to occur. The linear dependency between beta grain size,D _β, and alpha particle size,d _α, can be rationalized either on the basis of geometrical or surface tension considerations. Formerly with New York University. Formerly Graduate Student with New York University.     

Deformation and microstructure development during hot-pack rolling of a near-gamma titanium aluminide alloy

Deformation behavior and microstructure development during hot pack rolling of the near-gamma titanium aluminide alloy Ti-45.5Al-2Cr-2Nb (atomic percent) were established. Deformation behavior was investigated through rolling at various nominal furnace temperatures and parallel modeling studies using a finite difference approach to predict temperature transients during workpiece transfer from the furnace and during the rolling operation itself. Agreement between measured rolling pressures and predictions based on a rule-of-mixtures (ROM) average of the flow stresses of the pack components (at the predicted temperatures and strain rates within the roll gap) was excellent. As-rolled microstructures were interpreted in terms of the Ti-xAl-2Cr-2Nb pseudobinary phase diagram, predicted temperature transients during rolling, and the static (no deformation) phase-transformation behavior of the program material. These results demonstrated the strong influence of furnace preheat temperature on microstructure development, as well as the tendency for temperature transients due to radiation heat losses and roll chilling to suppress phase transformations.     

Fracture and toughening mechanisms in an α2 titanium aluminide alloy

The deformation and fracture behaviors of the Ti-24Al-11Nb alloy with an equiaxed α₂ + β microstructure have been characterized as a function of temperature by performing uniaxial tension andJ _IC fracture toughness tests. The micromechanisms of crack initiation and growth have been studied bypost mortem fractographic and metallographic examinations of fractured specimens, as well as byin situ observation of the fracture events in a scanning electron microscope (SEM) equipped with a high-temperature loading stage. The results indicate that quasistatic crack growth in the Ti-24Al-11Nb alloy occurs by nucleation and linkage of the microcracks with the main crack, with the latter frequently bridged by ductile β ligaments. Three microcrack initiation mechanisms have been identified: (1) decohesion of planar slipbands in the α₂ matrix, (2) formation of voids and microcracks in β, and (3) cracking at or near the α₂ + β interface due to strain incompatibility resulting from impinging planar slip originated in α₂. The sources of fracture toughness in the 25 °C to 450 °C range have been attributed to crack tip blunting, crack deflection, and a bridging mechanism provided by the ductile β phase. At 600 °C, a change of toughening mechanisms leads to a lowering of the initiation toughness (theK _IC value) but a drastic increase in the crack growth toughness and the tearing modulus.     

Developing hydrogentolerant microstructures for an α2 titanium aluminide alloy

The feasibility of developing hydrogen-tolerant microstructures for α₂ titanium aluminide alloys by heat treatment has been investigated. In particular, a variety of microstructures for the Ti-24Al-11Nb (in atomic percent) alloy was developed by manipulating the heat-treatment conditions. After screening by the Vicker hardness tests, three microstructures were evaluated for their resistance to hydrogen embrittlement by performing sustained load creep tests in a gaseous hydrogen environment at an elevated temperature, followed by post-creep, slow-rate tensile tests at room temperature. Tensile tests of hydrogen-exposed specimens without prior creep exposure were also performed. The results indicate that one particular microcstructure of the Ti-24Al-11Nb alloy is resistant to hydrogen embrittlement under the test conditions and hydrogen contents investigated, providing evidence that heat-treatment techniques can be used to develop hydrogentolerant microstructures for α₂ titanium aluminide alloys.     

Deoxidation of titanium aluminide by Ca-Al alloy under controlled aluminum activity

Removal of oxygen in titanium aluminide (TiAl) by chemically active calcium-aluminum (Ca-AI) alloy was carried out around 1373 K with the purpose of obtaining extra-low-oxygen TiAl. The deoxidation experiments were preceded by an investigation of the phase equilibria of the system Ti-Al-Ca at 1273 and 1373 K. The compositions of the Ca-AI alloy deoxidant, which equilibrates with TiAl, and the experimental conditions suitable for the deoxidation were of particular interest. In experiments in which Ti-Al samples were submerged in liquid Ca-AI alloys at 1373 K, the surfaces of the samples severely deteriorated and became nodular. When TiAl powders were mixed with CaO and the deoxidant was supplied in vapor form, powders which initially contained 510, 1100, and 4200 ppm O were deoxidized to about 160, 490, and 670 ppm O after deoxidation at 1373 K in 86.4 ks (1 day). Among many conditions tested, the use of TiAl powders mixed with CaCl₂ was most effective for deoxidation at 1373 K. CaCl₂ was used as a flux to facilitate the deoxidation by decreasing the activity of the deoxidation product CaO. In the case that TiAl powders mixed with CaCl₂ and reacted with Ca-AI vapor at 1373 K for 86.4 ks, the powders initially containing 510, 1100, and 4200 mass ppm O were deoxidized to a level of 62, 140, and 190 mass ppm O, respectively. No significant change in morphology of the particle after deoxidation was observed. The titanium and nitrogen concentrations in the powders remained constant, whereas calcium, which was present only in trace amounts initially, increased up to 160 mass ppm after the deoxidation treatment.     

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.