High-temperature deformation and failure of an orthorhombic titanium aluminide sheet material

The high-temperature deformation and failure behavior of an orthorhombic titanium aluminide sheet alloy (fabricated by diffusion bonding of six thin foils) was established by conducting uniaxial tension and plane-strain compression tests at 980 °C and strain rates between 10⁻⁴ and 10⁻² s⁻¹. The stress-strain response was characterized by a peak stress at low strains followed by moderate flow softening. Values of the strain-rate sensitivity index (m) were between 0.10 and 0.32, and the plastic anisotropy parameter (R) was of the order of 0.6 to 1.0. Cavity nucleation and growth were observed during tensile deformation at strain rates of 10⁻³ s⁻¹ and higher. However, the combined effects of lowm, low cavity growth rateη, and flow softening were deduced to be the source of failure controlled by necking and flow localization rather than cavitation-induced fracture prior to necking.     

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.     

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.     

Effects of temperature on the fatigue crack growth behavior of cast gamma-based titanium aluminides

This article reports the results of an experimental study of the effects of temperature (25 °C, 450 °C, and 700 °C) on the fatigue crack growth behavior of three near-commercial cast gamma titanium aluminide alloys (Ti-48Al-2Cr-2Nb, Ti-47Al-2Mn-2Nb+0.8 pct TiB₂, and Ti-45Al-2Mn-2Nb+0.8 pct TiB₂). The trends in the fatigue crack growth rate data are explained by considering the combined effects of crack-tip deformation mechanisms and oxide-induced crack closure. Faster fatigue crack growth rates at 450 °C are attributed to the high incidence of irreversible deformation-induced twinning, while slower crack growth rates at 700 °C are due to increased deformation by slip and the effects of oxide-induced crack closure.     

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.     

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.     

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.     

Fatigue crack growth and fracture toughness behavior of an Al-Li-Cu alloy

Slip behavior, fracture toughness, and fatigue thresholds of a high purity Al-Li-Cu alloy with Zr as a dispersoid forming element have been studied as a function of aging time. The fracture toughness variation with aging time has been related to the changes in slip planarity,i.e., slip band spacing and width. Although the current alloy exhibits planar slip for all aging conditions examined, the crack initiation toughness,Klc, compares favorably with those of 2XXX and 7XXX aluminum alloys. Near threshold fatigue crack growth results in air and vacuum suggest that irregularities in the crack profile and the fracture surfaces and slip reversibility are some of the major contributing factors to the crack growth resistance of this alloy.     

An investigation of the effect of texture on the high-temperature flow behavior of an orthorhombic titanium aluminide alloy

The effect of mechanical and crystallographic texture on the flow properties of a Ti-21Al-22Nb (at. pct) sheet alloy was determined by conducting uniaxial tension and plane-strain compression tests at temperatures between 900°C and 1060°C and strain rates between 10⁻⁴ and 10⁻² s⁻². Despite the presence of noticeable initial texture, all of the mechanical properties for a given test temperatur and strain rate (i.e., peak stress, total elongation to failure, strain-rate sensitivity, and normal plastic anisotropy), were essentially identical irrespective of test direction relative to the rolling direction of the sheet. The absence of an effect of Mechanical texture on properties such as ductility was explained by the following: (1) the initially elongated second-phase particles break up during tension tests parallel to the rolling direction of the sheet, thereby producing a globular morphology similar to that noted in samples taken transverse to the rolling direction; and (2) failure was flow localization, rather than fracture, controlled. Similarly, the absence of an effect of mechanical texture on strain-rate sensitivity (m values), normal plastic anisotropy (r values), and the ratio of the plane strain to uniaxial flow stresses was rationalized on the basis of the dominance of matrix (dislocation) slip processes within the ordered beta phase (B2) as opposed to grain boundary sliding. Aggregate theory predictions supported this conclusion inasmuch as the crystallo graphic texture components determined for the B2 phase ((001) [100] and (−112) [110]) would each produce identical r values and uniaxial and plane-strain flow stresses in the rolling and transverse directions.     

An investigation of the effect of texture on the high-temperature flow behavior of an orthorhombic titanium aluminide alloy

The effect of mechanical and crystallographic texture on the flow properties of a Ti-21Al-22Nb (at. pct) sheet alloy was determined by conducting uniaxial tension and plane-strain compression tests at temperatures between 900 °C and 1060 °C and strain rates between 10⁻⁴ and 10⁻² s⁻¹. Despite the presence of noticeable initial texture, all of the mechanical properties for a given test temperature and strain rate (i.e., peak stress, total elongation to failure, strain-rate sensitivity, and normal plastic anisotropy) were essentially identical irrespective of test direction relative to the rolling direction of the sheet. The absence of an effect of mechanical texture on properties such as ductility was explained by the following: (1) the initially elongated second-phase particles break up during tension tests parallel to the rolling direction of the sheet, thereby producing a globular morphology similar to that noted in samples taken transverse to the rolling direction; and (2) failure was flow localization, rather than fracture, controlled. Similarly, the absence of an effect of mechanical texture on strain-rate sensitivity (m values), normal plastic anisotropy (r values), and the ratio of the plane strain to uniaxial flow stresses was rationalized on the basis of the dominance of matrix (dislocation) slip processes within the ordered beta phase (B2) as opposed to grain boundary sliding. Aggregate theory predictions supported this conclusion inasmuch as the crystallographic texture components determined for the B2 phase ((001) [100] and ( 12) [110]) would each produce identical r values and uniaxial and plane-strain flow stresses in the rolling and transverse directions.     

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,     

High-temperature creep of the intermetallic alloy Ni3Al

Constant stress creep tests have been conducted on Ni₃Al (Hf, B) single crystals in an attempt to characterize the high-temperature creep behavior of this alloy. In contrast to intermediate temperature creep behavior, steady-state creep was observed at 1273 K. This extended steady-state region was formed in less than 1 pct creep strain and lasted for the duration of the creep tests. Primary creep was, however, observed to be limited in nature and consistent with inversetype creep behavior. These observations, preliminary transmission electron microscopy (TEM) observations, and the measured values for the stress exponent(n = 4.3 ± 0.1) and activation energy (Q _c = 398 ± 41 kJ/mole) all suggest that high-temperature creep involves both dislocation mobility and the recovery of dislocation substructure. Attempts to identify a single dislocation mechanism for high-temperature creep were unsuccessful, and it was concluded that a number of slip systems were active at the high temperatures used in these experiments. Formerly Graduate Student, Department of Materials Science and Engineering, Stanford University     

Influence of microstructure on intrinsic and extrinsic toughening in an alpha-two titanium aluminide alloy

The toughening mechanisms in the Ti-24A1-11Nb (Ti-24-11) alloy have been identified previously to include crack-tip blunting, bridging, and deflection by the ductileβ phase, while the fracture mechanisms involve the nucleation, growth, and linkage of microcracks with the main crack. By performing appropriate theoretical analyses and critical experiments, the relative contributions of intrinsic and extrinsic toughening mechanisms, including microcrack shielding, crack-tip blunting, bridging, and deflection by theβ phase, to the initiation and crack growth toughness values of the Ti-24-11 alloy have been studied for three microstructures. The results indicate that the microstructure affects not only the amount of toughness enhancement, but also the type of toughening mechanisms present in the Ti-24-11 alloy. The initiation toughness in Ti-24-11 arises from the matrix toughness, crack-tip blunting, and, occasionally, from crack deflection by the ductile phase. As a result, theK _IC values increase with the volume fraction of the ductile phase. In contrast, the resistance curve behavior originates from (1) a change of crack-tip singularity, which occurs when the blunted crack extends into the plastic zone, (2) crack bridging by ductile phase and shear ligaments, and (3) microcrack shielding, which occurs mostly at elevated temperatures.     

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.