Room-temperature tensile deformation of polysynthetically twinned (PST) crystals of TiAl

Tensile deformation behavior of polysynthetically twinned (PST) crystals of TiAl with a nearly stoichiometric composition was investigated as a function of the angle ø between the lamellar boundaries and tensile axis at room temperature. Tensile elongation to fracture strongly depends on the angle ø but its dependence on the angle ø is not symmetrical with respect to ø = 45°. A tensile elongation as large as 20%, which is far larger than any other reported values on TiAl-based compounds, has been obtained for ø = 31°. The yield stress also strongly depends on the angle ø but any particularly significant tension-compression asymmetry in yield stress has not been observed. Fracture has been found to occur in a brittle manner without showing any local contraction even after deformation to more than 10%. When the tensile axis is perpendicular or inclined to the lamellar boundaries, fracture occurs in a cleavage-like mode with a habit plane parallel to the lamellar boundaries while fracture occurs in a zigzag across the lamellar boundaries when the tensile axis is parallel to the lamellar boundaries.     

Effect of deformation temperature on fatigue and fracture behavior in TiAl polysynthetically twinned crystals

The temperature and orientation dependence of cyclic deformation, fatigue life, and fracture behavior in TiAl polysynthetically twinned (PST) crystals were investigated, focusing on the change of plastic strain energy and deformation mode in the γ domains. Stress-controlled fatigue tests were performed at 1 or 10 Hz using the same stress amplitude in tension and compression (R=−1) over a temperature range from −196 °C to 700 °C. The fatigue strength at ϕ=45 deg (ϕ being the angle between the loading axis and lamellar planes) decreased monotonically with increasing temperature. At ϕ=0 deg, the fatigue strength was high up to 500 °C, but the fatigue life decreased rapidly above 600 °C because of dynamic recovery and interlamellar separation. The plastic strain energy-stress amplitude curves in specimens fatigued with ϕ=45 deg increased monotonically with stress amplitude for all temperatures and for higher temperatures with ϕ=0 deg. At 25 °C and −196 °C with ϕ=0 deg, three regions in the plastic strain energy-stress amplitude curves were observed. This anomalous change in the plastic strain energy at lower temperatures was due to a transition in primary deformation mode between twinning and slip by ordinary dislocations in some domain orientations. 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.     

Effect of deformation temperature on fatigue and fracture behavior in TiAl polysynthetically twinned crystals

The temperature and orientation dependence of cyclic deformation, fatigue life, and fracture behavior in TiAl polysynthetically twinned (PST) crystals were investigated, focusing on the change of plastic strain energy and deformation mode in the γ domains. Stress-controlled fatigue tests were performed at 1 or 10 Hz using the same stress amplitude in tension and compression (R=−1) over a temperature range from −196 °C to 700 °C. The fatigue strength at ϕ=45 deg (ϕ being the angle between the loading axis and lamellar planes) decreased monotonically with increasing temperature. At ϕ=0 deg, the fatigue strength was high up to 500 °C, but the fatigue life decreased rapidly above 600°C because of dynamic recovery and interlamellar separation. The plastic strain energy—stress amplitude curves in specimens fatigued with ϕ=45 deg increased monotonically with stress amplitude for all temperatures and for higher temperatures with ϕ=0 deg. At 25 °C and −196 °C with ϕ=0 deg, three regions in the plastic strain energy—stress amplitude curves were observed. This anomalous change in the plastic strain energy at lower temperatures was due to a transition in primary deformation mode between twinning and slip by ordinary dislocations in some domain orientations. 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.     

Recovery and recrystallization of cold-rolled polysynthetically twinned (PST) crystals of TiAl

The recovery and recrystallization behavior of cold-rolled polysynthetically twinned (PST) crystals of TiAl with a nearly stoichiometric composition was investigated as a function of reduction in thickness and subsequent annealing time at 900 and 1000°C. The change in microhardness was found to occur in two stages regardless of the amount of reduction; the first stage associated with the decrease in dislocation density and the second stage related with the annealing-out of deformation twins introduced in TiAl lamallae during rolling. However, the recrystallization mode was found to vary depending on the amount of reduction in thickness. In specimens rolled to less than 20% reduction in thickness, the full recovery in hardness occurs preserving the lamellar structure. In contrast, in specimens rolled to more than 40%, a structure composed of equiaxed grains of TiAl is obtained after recrystallization. A mechanism is proposed to interpret the unusual recrystallization behavior of PST crystals of TiAl cold-rolled to less than 20%.     

Environmental effects on the room temperature ductility of polysynthetically twinned (PST) crystals of TiAl

Environmental effects on the room temperature ductility of polysnthetically twinned (PST) crystals of TiAl with two different lamellar orientations have been investigated through tensile tests conducted in four different atmospheres at strain rates in the range of 2.0 × 10⁻⁴ to 1.0 × 10⁻¹ s⁻¹. The tensile ductility of TiAl PST crystals is sensitive to test environment. It is higher when tested in vacuum or in dry air than in air or in hydrogen gas. Such environmental loss in ductility decreases with increasing strain rate. These results suggest that the environmental embrittlement of TiAl PST crystals can be interpreted in terms of hydrogen embrittlement. The fracture mode of TiAl PST crystals whose lamellar boundaries are inclined to the tensile axis also depends on test environment and strain rate. On the basis of the results obtained, the mechanism of the environmental embrittlement of TiAl PST crystals is discussed, with particular emphasis on the interaction of the lamellar boundaries with hydrogen atoms.     

Ti-Al-Nb-W基合金高温变形过程中的组织演化

在Thermecmastor-Z动态热模拟试验机上对Ti-43Al-4Nb-1.4W和Ti-43Al-4Nb-1.4W-0.6B-0.2Y合金进行高温压缩变形实验(实验温度范围为1 050~1 150℃,应变速率范围为0.001~1 s 1),对其热变形组织进行显微分析,并利用热加工Z参数(Zener-Hollomon参数)综合描述变形温度及应变速率对材料热变形行为的影响。结果表明:2种合金在不同高温压缩变形条件下均发生不同程度的动态再结晶;随Z参数值的降低,β相逐渐由不规则形状转变为球形,且长大明显,同时,动态再结晶晶粒的体积含量也随之增加;Ti-43Al-4Nb-1.4W基合金的高温变形机制与Z参数值密切相关;在低Z值条件下,其主要变形机制为动态再结晶和β相的球化、长大;在高Z值条件下,其主要变形机制为片层的扭折、重新取向和局部动态再结晶;加入微量B和Y后,动态再结晶程度增大,这主要与颗粒诱发动态再结晶形核有关。     

Microstructural evolution during creep of single-phase gamma TiAl

Mechanical experiments and transmission electron microscope (TEM) observations indicate that single-phase γ-TiAl does exhibit primary, secondary, and inverse creep, but not steady-state creep. Constant stress creep tests of γ-Ti-51Al-2Mn conducted at 550 °C, 597 °C, and 703 °C have been interrupted at different stages in the creep process. The TEM observations of these specimens were used to document the microstructural evolution that occurs during creep. Superdislocation motion was activated and subsequently exhausted during primary creep. Ordinary dislocations were observed to be pinned during primary creep, but with time, these dislocations began to bow past their pinning points. The extended region of inverse creep has been related to the bowing and multiplication of these ordinary dislocations. Quantitative measurements of dislocation density were performed, and while the density of superdislocations remained constant, the density of ordinary dislocations increased by an order of magnitude during the life of a creep test. The acceleration in the creep rate has been related to this increase in the density of ordinary dislocations, but the change in dislocation density was not high enough to account for the increase in the creep rate. This suggests that both the mobility and density of ordinary dislocations increase as creep progresses. 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.     

Constitution modeling and deformation behavior of yttrium bearing TiAl alloy

The deformation flow behaviors of Ti-45Al-5.4V-3.6Nb-0.3Y alloy at different temperatures and strain rates were studied by isothermal compressing simulation test.The apparent activation energy of deformation was calculated to be 402.096 kJ/mol and constitutive equation was established to describe the flow behavior.Microstructure and flow softening observations exhibited that Ti-45Al-5.4V3.6Nb-0.3Y alloy had bad hot workability at low temperature (lower than 1 100 oC) and high strain rate (higher than 0.5 s–...     

Plastic instability during creep deformation of a NiAl-Hf single-crystal alloy—A case study

Tensile samples from NiAl-Hf single crystals, having the same nominal composition and heat treated and creep tested under identical conditions at 1144 K, were found to exhibit very different rupture lives and creep ductilities. A case study was conducted on two samples with creep rupture lives of 343.6 and 37.0 hours (with corresponding creep ductilities of 12.3 and 39.9 pct, respectively) in order to find the causes of such a large variation in creep properties. Detailed microstructural analyses using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) indicated that the sample with higher rupture life and lower ductility had deformed uniformly along the length of the gage section, whereas the sample with lower rupture life and higher ductility (sample L) deformed by localized plastic deformation resulting in shear failure. This shear failure was due to a plastic instability in sample L which was caused by the presence of a high density of large Hf-rich interdendritic particles that were formed during casting of the single-crystal ingot but did not go into solution during the homogenization heat treatment. The role of these particles in causing nonuniform deformation, which led to strain localization and a premature failure in sample L, has been described in detail.     

高Nb-TiAl合金的高温循环变形机制

在750℃下对近片层Ti-45Al-8Nb-0.2W-0.2B-0.1Y合金进行了静拉伸和循环变形,观察和分析变形后试样的微观组织.合金在750℃时的循环应力-应变曲线位于静拉伸应力-应变曲线之上,显示出明显的循环硬化特征;在循环变形过程中呈现先硬化后稳定.透射电镜观察显示,在750℃下循环变形和拉伸的合金试样中均发现有大量的位错钉扎、塞积及缠结存在,而形变孪晶仅在循环变形后的合金试样中存在.合金在750℃下的循环变形中孪生起重要作用.     

铸造TiAl基合金的热变形行为及加工图

通过高温压缩模拟试验结果建立TiAl基合金的热加工图,结合扫描电镜、透射电镜等试验手段,研究铸造TiAl基合金在温度为1 000~1 150℃、应变速率为0.001~1 s 1范围内的热变形行为。结果表明:铸造TiAl基合金是温度、应变速率敏感材料,其流变应力随温度升高和应变速率降低而降低。铸造TiAl基合金的高温变形机制以层片晶团的扭折、弯曲及动态再结晶过程为主。在高温(1 150℃),低应变速率(≤0.01 s 1)下变形后,铸态组织中β相含量明显减少直至消除。在变形温度1 150℃、应变速率0.001 s 1下变形时,铸造TiAl基合金未发生超塑性变形;此时由于动态再结晶晶粒异常长大导致加工图上该区域功率耗散值未达到最大,而是有减小的趋势。     

The effect of thermal exposure on microstructural stability and creep resistance of a two-Phase TiAl/Ti3Al lamellar alloy

During annealing of a two-phase TiAl/Ti₃Al lamellar alloy at 1273 and 1323 K, the lamellar microstructure evolves into a coarse, globular microstructure. For short annealing times (less than about 1000 hours), microstructural evolution occurs predominantly by intrapacket termination migration coarsening. For longer annealing times, cylinderization and conventional Ostwald ripening coarsening mechanisms are observed. The activation energy for the rate-controlling diffusion process governing intrapacket termination migration coarsening of the lamellar microstructure was determined to be 215 kJ/mol. Compression creep tests reveal that the minimum creep rate and primary creep strain of the lamellar alloy increase with increasing prior annealing time. Furthermore, in contrast to the lamellar microstructure, the globular microstructure is not susceptible to deformation-induced spheroidization during compression creep testing. Modeling demonstrates that the increase of the minimum creep rate and primary creep strain as a consequence of annealing of the lamellar alloy can be accounted for by consideration of two factors: the decrease in the work-hardening rate of the lamellar alloy in response to the overall decrease in interphase interfacial area and the decreased mechanical strengthening effect associated with transformation from a lamellar to a globular microstructure. Formerly Graduate Student, Department of Materials Science and Engineering, University of Virginia     

强变冷形单晶铜线退火过程组织性能演变研究

经强冷变形后的单晶铜线会产生明显的亚结构。在退火过程中该亚结构将发生转变．以中拉单晶铜线为对象,通过金相、力学性能测试法、电阻测试等手段研究了经强冷变形后的单晶铜线在不同退火工艺制度下的组织性能变化和再结晶过程．研究结果表明：加工态单晶铜线材的再结晶温度开始在250℃左右,比相同冷变形率下的SCR连铸纯铜杆的再结晶温度高约50℃．强冷变形单晶铜线再结晶形核的孕育期随温度升高而缩短．500。C时的孕育期不足2min．单晶铜在退火的回复阶段导电性能得到改善,但温度较高发生再结晶时,由于晶界数量的不断增加,有抑制电阻率减小的作用．强冷变形后的单晶铜线要想恢复足够的塑性,则难以避免成为多晶,如果既要恢复单晶铜线的塑性和导电性,又要维持单晶的组织形态,进行高温超短时间退火将有助于解决这一问题．     

Microstructure development during high-velocity deformation

An austenitic stainless steel was deformed at high (10³ s⁻¹) strain rates at two levels of strain by electromagnetic forces. Transmission electron microscopy (TEM) studies, X-ray diffraction analysis, and superconducting quantum-interference device (SQUID) measurements show that high strain rates induce the formation of stacking faults and twin structures, enhance the tendency for ɛ-martensite formation, and suppress the amount of α′-martensite. The increased presence of stacking faults and twin structures at high strain rates can be explained by an easy nucleation of partial dislocations at high strain rates and a superior aptitude for partial dislocations to react to high strain rates due to their jump frequency. The suppression of α′-martensite can be explained by the adiabatic heating produced during electromagnetic forming.     

TiAl合金建筑材料焊后热处理组织及性能研究

对建筑用TiAl合金电子束焊接接头进行了两种热处理试验研究,借助金相显微镜(OM)分析了接头不同区域的显微组织,并对焊接接头进行显微硬度测试,分析了两种热处理方式对建筑TiAl合金焊接接头组织及硬度带来的变化.研究表明,TiAl合金电子束焊接后焊缝组织主要为α2相,B相与O相.焊接接头局部热处理后接头硬度较高的区域有所...     

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.