Fatigue and fracture behavior of a fine-grained lamellar TiAl alloy

The fatigue and fracture resistance of a TiAl alloy, Ti-47Al-2Nb-2Cr, with 0.2 at. pct boron addition was studied by performing tensile, fracture toughness, and fatigue crack growth tests. The material was heat treated to exhibit a fine-grained, fully lamellar microstructure with approximately 150-μm grain size and 1-μm lamellae spacing. Conventional tensile tests were conducted as a function of temperature to define the brittle-to-ductile transition temperature (BDTT), while fracture and fatigue tests were performed at 25 °C and 815 °C. Fracture toughness tests were performed inside a scanning electron microscope (SEM) equipped with a high-temperature loading stage, as well as using ASTM standard techniques. Fatigue crack growth of large and small cracks was studied in air using conventional methods and by testing inside the SEM. Fatigue and fracture mechanisms in the fine-grained, fully lamellar microstructure were identified and correlated with the corresponding properties. The results showed that the lamellar TiAl alloy exhibited moderate fracture toughness and fatigue crack growth resistance, despite low tensile ductility. The sources of ductility, fracture toughness, and fatigue resistance were identified and related to pertinent microstructural variables.     

The role of grain size and selected microstructural parameters in strengthening fully lamellar TiAl alloys

More than 5 years ago, wrought processing was first used to produce fully lamellar (FL) microstructures in TiAl alloys having grain sizes less than ≈400 μm. These alloys exhibit an improvement in overall balance of properties, especially at high temperatures. More recently, such microstructural forms led to exceptional yield strengths (500 to 1000 MPa at low temperatures) while maintaining attractive high-temperature properties. The improvements appeared to be related to an unusually high apparent sensitivity of strength to grain size. Studies reported an apparent value for the slope of the Hall-Petch (HP) plot approaching 5 MPa√m for FL gamma alloys, while that for single-phase or duplex microstructures is near unity. The present investigations examine the slope of the HP plot for FL microstructures, paying particular attention to the lamellar microstructural variables. Results show that the α ₂ lamellar thickness and spacing and the γ lamellar thickness can vary over more than two orders of magnitude with typical process methods. These spacings influence the value of k _y in the HP (grain size) relationship. Since they often change concomitantly with grain size in processing, they can give rise to a large scatter in the HP plot. The investigations also examine the flow behavior, glide barriers, and slip multiplicity for polysynthetically twinned (PST) crystals (the single-grain analogue of FL material), and then map this behavior into an explanation of the yield behavior of high-strength FL gamma alloys. 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 initial microstructure on microstructural instability and creep resistance of XD TiAl alloys

A number of lamellar structures were produced in XD TiAl alloys (Ti-45 at. pct and 47 at. pct Al-2 at. pct Nb-2 at. pct Mn+0.8 vol pct TiB₂) by selected heat treatments. During creep deformation, microstructural degradation of the lamellar structure was characterized by coarsening and spheroidization, resulting in the formation of fine globular structures at the grain boundaries. Grain boundary sliding (GBS) was thought to occur in local grains with a fine grain size, further accelerating the microstructural degradation and increasing the creep rate. The initial microstructural features had a great effect on microstructural instability and creep resistance. Large amounts of equiaxed γ grains hastened dynamic recrystallization, and the presence of fine lamellae increased the susceptibility to deformation-induced spheroidization. However, the coarsening and spheroidization were suppressed by stabilization treatments, resulting in better creep resistance than the microstructures without these treatments. Furthermore, well-interlocked grain boundaries with lamellar incursions were effective in restraining the onset of GBS and microstructural degradation. In the microstructures with smooth grain boundaries, a fine lamellar spacing significantly lowered the minimum creep rate but rapidly increased the tertiary creep rate for the 45 XD alloy. For the 47 XD alloy, well-interlocked grain boundaries dramatically improved the creep resistance of nearly and fully lamellar (FL) structures, in spite of the presence of coarse lamellar spacing or equiaxed γ grains. However, it may not be feasible to produce a microstructure with both a fine lamellar spacing and well-interlocked grain boundaries. If that is the case, it is suggested that the latter feature is more beneficial for creep resistance in XD TiAl alloys with relatively fine grains.     

High-temperature strength and ductility increases of Ti-Mo-Al alloys by step aging

Step-aging programs, based on principles of particle-dislocation interactions, were developed systematically to obtain increases in the high-temperature strength and ductility properties of Ti-7 at. pct Mo-Al alloys. A triple-step aging program applied to Ti-7 Mo-16 Al produced a yield stress σ_0.2 = 1,500 MN/m², elongation to fracture ε _F = 4 pct at room temperature, and σ_0.2 = 900 MN/m², ε _F = 12 pct at 600°C. A two-step aging program resulted in σ_0.2 = 1,350 MN/m², ε _F = 5 pct at room temperature; σ_0.2 = 800 MN/m², ε _F = 20 pct at 600°C.     

High-temperature strength and ductility increases of Ti-Mo-Al alloys by step aging

Step-aging programs, based on principles of particle-dislocation interactions, were developed systematically to obtain increases in the high-temperature strength and ductility properties of Ti-7 at. pct Mo-Al alloys. A triple-step aging program applied to Ti-7 Mo-16 Al produced a yield stress σ_0.2 = 1,500 MN/m², elongation to fracture ε _F = 4 pct at room temperature, and σ_0.2 = 900 MN/m², ε _F = 12 pct at 600°C. A two-step aging program resulted in σ_0.2 = 1,350 MN/m², ε _F = 5 pct at room temperature; σ_0.2 = 800 MN/m², ε _F = 20 pct at 600°C. Formerly Assistant Research Professor, Materials Research Laboratory, Rutgers University     

应变速率对全层状TiAl基合金室温拉伸性能的影响

以Ti-47Al-2Cr(摩尔分数,％)合金为对象,研究了应变速率对不同晶团尺寸的全层状TiAl基合金室温拉伸性能的影响.结果表明,全层状TiAl基合金的室温强度随应变速率的加快而提高,低延性全层状TiAl基合金的室温延伸率对应变速率不敏感,而高延性全层状TiAl基合金的室温延伸率对应变速率敏感,并随应变速率的加快而提高.     

共析钢中片层状渗碳体冷轧后的形态变化

The pearlitic transformation and the deformation behavior of lamellar cementite after cold rolling in eutectoid steels Fe-0. 76%C-0. 137%Mn (mass fraction) were studied by means of Formastor-F (Full Automatic Transformation Testing Instrument) and field emission scanning electronic microscopy (FESEM) observation. Fine and coarse pearlite were obtained in the eutectoid steels austenitized at 900℃ for 15min, then hold at 620℃ for 90 s and 690℃ for 7 h, respectively. Thedeformation behavior of cold rolled lamellar cementite could be classified as: cleavage fracture, inhomogeneous slip, fragmentation, thinning or necking, and homogeneous bending. The cementite lamellae with the thickness of more than 100 um could be deformed plastically.     

Fatigue deformation of TiAl base alloys

The fatigue behavior of Ti-36.3 wt pct Al and Ti-36.2 wt pct Al-4.65 wt pct Nb alloys was studied in the temperature range room temperature to 900°C. The microstructures of the alloys tested consisted predominantly of γ phase (TiAl) with a small volume fraction of γ phase (Ti₃Al) distributed in lamellar form. The alloys were tested to failure in alternate tension-compression fatigue at several constant load amplitudes with zero mean stress. Fracture modes and substructural changes resulting from fatigue deformation were studied by scanning electron microscopy and transmission electron miscroscopy respectively. The ratio of fatigue strength (at 10⁶ cycles) to ultimate tensile strength was found to be in the range 0.5 to 0.8 over the range of temperatures tested. The predominant mode of fracture changed from cleavage type at room temperature to intergranular type at temperatures above 600°C. The fatigue microstructure at low temperatures consisted of a high density of a/3 [111] faults and dislocation debris of predominantly a/2 [110] and a/2 [110] Burger's vectors with no preferential alignment of dislocations. At high temperatures, a dislocation braid structure consisting of all 〈110〉 slip vectors was observed. The changes in fracture behavior with temperature correlated well with changes in dislocation substructure developed during fatigue deformation. S. M. L. SASTRY was formerly NRC Research Associate in the Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH     

Ti-6Al-4V在步进轧制中的显微组织变化

对步进轧制在940℃α β区轧制Ti 6Al 4V合金棒材的锥形体取变形量不同的部位,利用光学显微镜、透射电镜观察其组织形貌及位错组态,分析了其变形机制,探讨了材料显微组织随变形量变化的规律及其形变硬化、回复、再结晶的演变过程.实验表明利用步进轧机加工,所得Ti 6Al 4V合金棒材芯部和边部的组织较均匀.     

The stability of lamellar structures

The thermal stability of lamellar structures under creep or hot-working conditions is considered. It is suggested that the microstructural stability of the two-phase alloy can be altered by the introduction of boundaries formed by deformation and recovery processes. Expressions are derived for the rate of growth at triple points formed in this way. These expressions are related to the stability of unidirectionally grown eutectics under creep conditions, and spheroidization observed under hot working conditions. Under certain limiting conditions the results reduced to those of Mullins for “grooving” at grain boundary-free surface junctions by volume and surface diffusion.     

Microstructural characterization of multicomponent Nb-Ti-Si-Cr-Al-X alloys

Recently, alloys based on the Nb-Ti-Si system have become of interest for high-temperature structural applications. In the present work, the microstructure of multicomponent Nb-30Ti-8Si-10Cr-10Al-X (in at. pct) alloys in the as-cast and heat-treated conditions was studied using X-ray diffraction, electron probe microanalysis, scanning electron microscopy and transmission electron microscopy. The effect of temperature and time on phase evolution was examined in detail. The as-cast microstructure was found to be composed of three phases:β (bcc), silicides (M₅Si₃ type), and a Cr-rich Laves phase. Theβ phase was found to display B2-type ordering. The silicides in these alloys were generally quite stable during heat treatment, whereas the Cr-rich Laves phase was observed to dissolve on solutionization at temperatures above 1300 °C. Aging of the solutionized materials between 900 °C and 1100 °C led to the precipitation of fine particles of another Laves phase in theβ matrix. In addition, theβ matrix revealed a tendency toward phase separation into Nb-rich (β ₁) and Ti-rich (β ₂) regions. The volume percentage and chemical composition of each phase has been determined as a function of time and temperature and the changes in microstructure have been rationalized in terms of the distribution of elements in various phases. The role of different alloying elements on the formation of these phases has also been critically examined.     

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     

Microstructural characterization of rapidly solidified Al-Ta alloys

Two Al-rich Al-Ta alloys containing by weight 3 and 6 pct Ta have been rapidly solidified from the melt using the ‘gun’ technique. The microstructures and the crystal structures of the phases in the as-solidified as well as those formed on subsequent decomposition of the supersaturated solid solution have been characterized. A supersaturated solid solution was obtained in both the alloys in the as-solidified condition indicating a solid solubility extension of Ta in Al to almost 6 wt pct. The supersaturated solid solutions formed in both the alloys have been found to be quite stable up to 673 K (for 1 hour). Annealing at higher temperatures resulted in the formation of rod-shaped precipitates inside the grains and massive precipitates along grain boundaries. The rod-shaped precipitates arranged in a regular pattern constitute a new metastable intermediate phase Al₇Ta having an ordered structure. The massive precipitates which form along grain boundaries constitute the equilibrium Al₃Ta phase with a tetragonal crystal structure. The transformation behavior and the morphology of the transformation products are detailed in this paper.     

热等静压保温保压时间对TiAl合金有关性能的影响

研究了热等静压时间对TiAl合金有关特性的影响.在其它条件不变的情况下,10 min保温保压后,TiAl合金的密度已经达到3.46g/cm~3.时间从10 min逐渐增加到70min,所得TiAl合金的密度有所增加,但增加不明显,所得TiAl合金是一种非稳定状态,时间的延长对物相的影响不大.同时随着时间的延长,TiAl合金内部产生了微裂纹.1380℃,保温1h热处理后,合金内部的微裂纹消失,物相组成也转变成稳定的TiAl相.     

New aspects on the superplasticity of fine-grained 7475 aluminum alloys

Thermomechanical processes were developed which give fine grain sizes of 6 and 8 μm in the 7475 Al alloy. Superplastic properties of this material were evaluated in the temperature range of 400 °C to 545 °C over the strain-rate range of 2.8 x 10^-4 to 2.8 X 10^-2 s^-1. The maximum ductility exhibited by the alloy was approximately 2000 pct, and optimum superplasticity was achieved at a strain rate of 2.8 X 10^-3 s^-1 which is higher by an order of magnitude than other 7475 Al alloys. This result is attributed to the presence of fine dispersoids which maintain the fine grain size at high homologous temperatures. The flow stress and strain-rate sensitivity strongly depend on the grain size. The superplastic 7475 Al alloy has strain-rate sensitivities of 0.67 (6 μm) and 0.5 (13 μm) and an activation energy which is similar to the one for grain boundary diffusion of aluminum. Microstructural investigation after superplastic tests revealed zones free of dispersoid particles at grain boundaries primarily normal to the tensile direction. These dispersoidfree zones (DFZs) appear even after 100 pct elongation and are occasionally as large as 5 μm across. This result demonstrates the importance of diffusional flow in superplastic deformation of the fine-grained 7475 Al alloy especially at low elongations.     

Creep behavior of TiAl alloys with enhanced high-temperature capability

For high-temperature applications, creep strength is of major concern, in addition to oxidation and corrosion resistance, and determines the application range of titanium aluminide alloys in competition with other structural materials. Thus, this work was aimed at identifying mechanisms of creep deformation and microstructural degradation and at developing alloying concepts with respect to an enhanced high-temperature capability. The analysis shows that dislocation climb controls deformation in the range of the intended operation temperatures. Further, complex processes of phase transformations, recrystallization, and microstructural coarsening were observed, which contribute to microstructural degradation and limit component life in long-term service. By alloying with high contents of Nb, both room- and high-temperature strength properties can be improved as Nb increases the activation energy of diffusion and increases the propensity for twinning at ambient temperature. For alloys with enhanced high-temperature capability, microalloying with carbon is also of particular use, because carbide precipitates effectively hinder dislocation motion and are thought to increase microstructural stability. 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 development during solidification of stainless steel alloys

The microstructures that develop during the solidification of stainless steel alloys are related to the solidification conditions and the specific alloy composition. The solidification conditions are determined by the processing method,i.e., casting, welding, or rapid solidification, and by parametric variations within each of these techniques. One variable that has been used to characterize the effects of different processing conditions is the cooling rate. This factor and the chemical composition of the alloy both influence (1) the primary mode of solidification, (2) solute redistribution and second-phase formation during solidification, and (3) the nucleation and growth behavior of the ferrite-to-austenite phase transformation during cooling. Consequently, the residual ferrite content and the microstructural morphology depend on the cooling rate and are governed by the solidification process. This paper investigates the influence of cooling rate on the microstructure of stainless steel alloys and describes the conditions that lead to the many microstructural morphologies that develop during solidification. Experiments were performed on a series of seven high-purity Fe-Ni-Cr alloys that spanned the line of twofold saturation along the 59 wt pct Fe isopleth of the ternary alloy system. High-speed electron-beam surface-glazing was used to melt and resolidify these alloys at scan speeds up to 5 m/s. The resulting cooling rates were shown to vary from 7°C/s to 7.5×10⁶°C/s, and the resolidified melts were analyzed by optical metallographic methods. Five primary modes of solidification and 12 microstructural morphologies were characterized in the resolidified alloys, and these features appear to be a complete “set” of the possible microstructures for 300-series stainless steel alloys. The results of this study were used to create electron-beam scan speedvs composition diagrams, which can be used to predict the primary mode of solidification and the microstructural morphology for different processing conditions. Furthermore, changes in the primary solidification mode were observed in alloys that lie on the chromium-rich side of the line of twofold saturation when they are cooled at high rates. These changes were explained by the presence of metastable austenite, which grows epitaxially and can dominate the solidification microstructure throughout the resolidified zone at high cooling rates. J. W. ELMER, formerly Graduate Student at the Massachusetts Institute of Technology