Microstructural study of pre-yielding and pre-yield cracking in TiAl-based alloys

Samples of Ti44Al8Nb1B (at.%), Ti46Al8Nb (700 ppm oxygen) and of the alloy K5 (Ti–46Al–2Cr–3Nb–0.2W–0.15B–0.4C (800 ppm oxygen) which have been tested in tension at stresses below their macroscopic yield stresses, have been examined using transmission electron microscopy. In lamellar samples it has been shown that dislocation multiplication takes place at stresses from about 400 MPa, well below their 0.2% proof stress. In samples with a duplex microstructure no dislocation activity is observed until the applied stress exceeds the macroscopic yield stress. Deformation twinning is initially observed in lamellar samples at stresses just below the 0.2% proof stress. No acoustic emission events are observed corresponding to the twinning seen in the fully lamellar samples. These observations are discussed in terms of the different pre-yielding behaviour of lamellar and duplex samples and in terms of acoustic emission signals, pre-yield cracking and pre-yield twinning. It is concluded that pre-yield cracking is caused by slip in gamma grains in near fully lamellar samples and by slip in lamellae longer than about 70 μm in fully lamellar samples. It is further concluded that all pre-yield acoustic emission is caused by cracking.     

Pre-yielding and pre-yield cracking in TiAl-based alloys

Samples of Ti44Al8Nb1B (at.%) and Ti46Al8Nb (700 wt ppm oxygen) and of the alloy K5 (Ti-46.5Al-2Cr-3Nb-0.2W-0.15B-0.4C (800 wt ppm oxygen) have been heat treated to produce duplex, near fully lamellar and fully lamellar microstructures. These samples have been tested in tension, using detectors to detect acoustic events during pre-yielding. Many acoustic emission events and corresponding cracking, occur well below the 0.2% proof stress in the near fully lamellar samples of Ti44Al8Nb1B, in fully lamellar Ti46Al8Nb and fully lamellar K5. In fully lamellar Ti44Al8Nb1B alloy some samples show no acoustic events before yield and others show only single acoustic emission events. Duplex samples generate no signals until the applied stress exceeds the macroscopic yield point. The stress–strain curves from Ti448Nb1B, Ti46Al8Nb and from K5 show that in near fully lamellar and fully lamellar samples significant pre-yielding occurs, but none is obvious in the duplex samples. These observations are discussed in terms of the factors contributing to pre-yield plasticity and to pre-yield cracking in these alloys and with reference to earlier acoustic emission work where the reported behaviour is very different. It is concluded that yield, within sufficiently long lamellae or within gamma grains, can give rise to pre-yield cracking and thus to acoustic emission in lamellar samples.     

The significance of acoustic emission during stressing of TiAl-based alloys. Part II: Influence of cracks induced by pre-stressing on the fatigue life

Fully lamellar samples of Ti44Al8Nb1B have been tested in tension–tension fatigue, after being pre-stressed, in order to investigate the effect on fatigue life of the cracks which are introduced by pre-stressing. It has been found that pre-stressing samples at 0.95 of the 0.2% proof stress leads to early failure when they are fatigue-tested at stress levels of about 0.65 of the 0.2% proof stress. Samples tested in fatigue at this same stress level, but which have not been pre-stressed, can show significantly longer fatigue lives, although the fatigue lives show a very large scatter. Tinting experiments, carried out after pre-stressing, have shown conclusively that the cracks initiated by pre-stressing act as failure initiation sites during the subsequent fatigue tests. Even in the absence of pre-stressing, samples of Ti44Al8Nb1B fail in fatigue at stress levels of about 0.65 of the 0.2% proof stress in contrast to the fatigue limit of low strength TiAl alloys of about 0.9 of their 0.2% proof stress. The significance of these observations on the use of grain-refined higher strength TiAl alloys under fatigue conditions is discussed.     

Characterization of microplasticity in TiAl-based alloys

Three different characteristic microstructures of Ti–44Al–8Nb–1B (at.%) have been studied by in situ loading, with acoustic emission and image correlation and ex situ by electron backscatter diffraction and transmission electron microscopy. Microyielding and pre-yield cracking occur at different applied stress levels; nearly fully lamellar material yielding at the lowest and duplex material at the highest stress. The early microyielding in the lamellar microstructures is explained by strain heterogeneity seen in the early stages during loading; the microyielding is higher in lamellae at 45° than in lamellae parallel or perpendicular to the loading direction. Duplex microstructures show no detectable strain heterogeneity until macroscopic yield occurs. The lower bound of the internal stress present in undeformed near lamellar and fully lamellar samples was estimated by observations of dislocation loops at alpha-2/gamma interfaces. The observations are discussed in terms of the influence of internal stress on the tensile and fatigue properties of the different microstructures.     

The controlling creep processes in TiAl alloys at low and high stresses

The creep response of a nearly-lamellar Ti–47Al–4(W, Nb, B) alloy is studied at 760 °C in a wide stress range 100–500 MPa. The alloy exhibits excellent creep resistance with a minimum creep rate of 1.2×10⁻¹⁰/s at 100 MPa and the time to 0.5% creep strain of 1132 h at 140 MPa. The controlling creep process is probed by analysis of the post-creep dislocation structure and by observation of incubation period during stress reduction test. The results indicate that creep is controlled by dislocation climb at low stresses (Class II type) and by jog-dragged dislocation glide at high stresses (Class I type). The transition from Class II to Class I type creep occurs at about 180 MPa. The excellent creep resistance of the studied alloy compared to other W containing TiAl alloys is attributed to its highly stable lamellar microstructure consisting eventually of coarse gamma laths.     

Effects of alloying elements on creep of TiAl alloys with a fine lamellar structure

Creep experiments were conducted on five powder-metallurgy TiAl alloys with fine grains (65–80 μm), fine lamellar spacings (0.1–0.16 μm), and different compositions [Ti–47Al (+Cr, Nb, Ta, W, Si)] at temperatures of 760°C and 815°C and stresses from 35 to 723 MPa. Results show that at a given lamellar spacing, replacing 1% Nb (atomic percent) with 1% Ta and replacing 0.2% Ta with 0.2% W induced little effect, but addition of 0.3% Si decreased the creep resistance by a factor of 3–4 under otherwise identical conditions. Field emission TEM was used to characterize the changes of microstructure and alloy element distribution before and after creep. It was found that thinning and dissolution of α₂ lamellae and continuous coarsening of γ lamellae were the main creep processes and the microalloying elements tended to segregate at lamellar interfaces, especially at ledges during creep. The effects of different alloying elements are interpreted in terms of the interaction of alloy segregants with misfit and/or misorientation dislocations at the lamellar interface. That is, the interaction retards the climb of interfacial dislocations and thus the creep process in the case of large segregants (Nb, Ta, W), but facilitates the climb and creep in the case of small segregants (Si).     

In-situ tensile deformation behavior of powder metallurgy Ti6Al4V alloys

In this study, in-situ tensile deformation behavior of powder metallurgy (PM) Ti6Al4V alloys was investigated to analyze the crack initiation and propagation. Accordingly, the fracture mechanisms of the as-sintered and forged PM alloys were summarized. At the initial stage of plastic deformation, cracks appeared in the stress concentration area of pores in the as-sintered Ti6Al4V alloy, and the crack propagation direction was along the phase boundary. Due to the existence of pores, early fracture was obtained, resulting in low elongation of 6.3%. After forging, the crack initiation occurred between α lamellar structure, and the propagation direction was along the lamellar direction. The fine lamellar structure in different directions in the forged PM Ti6Al4V alloy can hinder the crack propagation, thus improving the plasticity. As a result, better comprehensive mechanical performance was obtained in the forged sample, with UTS of 960 MPa, YS of 850 MPa, and EL of 16%.     

Microstructure and creep behavior of directionally solidified TiAl-base alloys

Tensile creep tests were conducted on directionally solidified TiAl alloys to discern the effect of alloying and lamellar orientation. A seeding technique was used to align the TiAl/Ti₃Al lamellar structure parallel to the growth direction for alloys of Ti–47Al, Ti–46Al–0.5Si–0.5X (X=Re, W, Mo, and Cr), and Ti–46Al–1.5Mo–0.2C (at.%). Tensile creep tests were performed at 750 °C using applied stresses of 210 and 240 MPa. Aligning the lamellar microstructure greatly enhances the creep resistance which can further be improved by additional alloying.     

Effects of Nb and Al on the microstructures and mechanical properties of high Nb containing TiAl base alloys

The influence of Nb and Al contents on the microstructure and yield strength of high Nb containing TiAl base alloys was investigated. The experimental results show that the yield strength at 900 °C of the alloys with the same type of microstructure, such as fully lamellar (FL), nearly lamellar (NL) and degraded fully lamellar (DFL), increases with increasing Nb content and decreasing Al content in the composition range of 0–10 at.% Nb and 44–49 at.% Al. DFL is the degraded form of FL microstructure after exposure at 1050 °C for 30 h. It is shown that the Nb addition in the alloys increases the value of the σ₀ term in the Hall–Petch relation of yield stress vs. lamellar spacing. This result has been related to TEM observations of dislocation structure in deformed specimens. The observations indicated that high level of Nb solute in the γ-TiAl matrix leads to a high critical resolved shear stress (CRSS) of dislocation loops. High Nb addition also reduces the degradation rate of FL microstructure after exposure at 1050 °C for 30 h. Both effects of high Nb addition are related to the change of the directionality of Ti–Ti (Nb) and Nb–Al bonds in the lattice. The decrease in Al content results in an increase in the volume fraction of α₂ phase, which leads to a decrease in the lamellar spacing of the lamellar structure. The high temperature strength of the alloys is determined by the lamellar spacing λ through the Hall–Petch equation k_λλ^−1/2.     

Analysis of local microstructure after shear creep deformation of a fine-grained duplex γ-TiAl alloy

The present work characterizes the microstructure of a hot-extruded Ti–45Al–5Nb–0.2B–0.2C (at.%) alloy with a fine-grained duplex microstructure after shear creep deformation (temperature 1023 K; shear stress 175 MPa; shear deformation 20%). Diffraction contrast transmission electron microscopy (TEM) was performed to identify ordinary dislocations, superdislocations and twins. The microstructure observed in TEM is interpreted taking into account the contribution of the applied stress and coherency stresses to the overall local stress state. Two specific locations in the lamellar part of the microstructure were analyzed, where either twins or superdislocations provided c-component deformation in the L1₀ lattice of the γ phase. Lamellar γ grains can be in soft and hard orientations with respect to the resolved shear stress provided by the external load. The presence of twins can be rationalized by the superposition of the applied stress and local coherency stresses. The presence of superdislocations in hard γ grains represents indirect evidence for additional contributions to the local stress state associated with stress redistribution during creep.     

Kinetics of high temperature deformation of lamellar Ti48Al–2Nb–2Cr

Based on microstructural observations, the deformation resistance of lamellar Ti48Al–2Nb–2Cr in the temperature range between 1000 and 1200 K is expressed by treating the material as a composite of regions with lamellar and globular structure deforming independently of each other. For stresses below 1000 MPa the dislocation velocity is lower in the lamellar regions compared to the globular ones due to a larger athermal hardening component caused by interfaces (lamella boundaries in the lamellar structure and (sub)grain boundaries in the globular structure). Combining deformation kinetics and structural evolution allows the modelling of the maximum deformation resistance and the subsequent softening resulting from the increase in globular volume fraction. The model is applied to predict the creep life.     

Fatigue threshold and crack propagation in γ-TiAl sheets

The fatigue crack propagation behaviour of two different microstructures — a coarse-grained designed fully lamellar (DFL), and a fine-grained near γ (FG) — of a Ti–46.5 at.% Al–4 at.% (Cr, Nb, Ta, B) alloy was studied. Both the threshold of stress intensity range and standard long crack growth behavior were determined. A special technique was applied to separate the different mechanisms — intrinsic and extrinsic effects — and their changes with crack length. The fatigue crack propagation rate of long cracks is much smaller in the DFL microstructure than in the FG microstructure at the same stress intensity range. The effective threshold of stress intensity range of both microstructures is about 1.7 MPa√m. The threshold of stress intensity range shows a strong R-curve behavior. In other words the propagation–non-propagation conditions of cracks are significantly influenced by the crack extension. The long crack thresholds of stress intensity range at the stress ratio 0.1 are relatively large; they are about 4.5 and 8 MPa√m in the DFL and the FG microstructure, respectively. The differences in the crack growth behavior between the two microstructures are mainly induced by extrinsic resistance mechanisms.     

Effect of boron addition on tensile ductility in lamellar TiAl alloys

Various cast or wrought fully lamellar TiAl-based alloys with and without boron addition have been assessed. It has been found that titanium boride precipitates are the predominant factor influencing the room temperature tensile ductility. Large sized titanium boride precipitates often observed in high-alloyed TiAl alloys (such as Ti–44Al–8Nb–1B) cause premature failure in as-cast samples through promoting crack propagation via debonding between boride-matrix interfaces or cracking through boride precipitates themselves, giving rise to a typical tensile ductility of 0.3%. Refinement in titanium boride precipitates, via hot working or fast cooling during casting, will significantly improve the tensile ductility. In low-alloyed alloys (such as Ti–48Al–2Cr–2Nb–1B) the effect of boride precipitates is not as significant as it is in the high-alloyed alloys mainly because of their small sizes.     

Microstructure and mechanical properties of beta TiAl alloys elaborated by spark plasma sintering

The microstructure evolution and room temperature mechanical property of beta containing Ti–44Al–3Nb–1Mo–1V–0.2Y alloy consolidated by spark plasma sintering was studied. Pre-alloyed powders were sintered for 2 min in the range 900–1250 °C under 100 MPa. It was found that duplex and lamellar microstructures were obtained depending on the SPS temperature. The duplex microstructure formed at 1150 °C and 1175 °C, and the lamellar structure was achieved above 1200 °C. However, coarsening of lamellar colonies occurred with further increasing of the sintering temperature. The specimen with fine lamellar colonies exhibited a relatively high compressive strength, whereas the one with duplex microstructure showed a superior final strain.     

Effects of heat shock at 800 °C on mechanical properties of γ-TiAl alloys

Effect of heat shock on the mechanical properties and the microstructure of TiAl alloys (Ti–47Al–2Cr–2Nb and Ti–45.3Al–2Cr–2Nb–0.1W–0.15B) with near fully lamellar structure were investigated. After heat shock process from room temperature to 800 °C for 500 cycles, the microstructure demonstrated that lamellar microstructure has been destructed by the presentation of some γ and α₂ block phases in lamellar structure, resulting in the ductility of as-polished Ti–47Al–2Cr–2Nb alloy decreased by about 70% and the ultimate tensile strength (UTS) reduced by about 25%, and the ductility of as-unpolished Ti–47Al–2Cr–2Nb alloy decreased by more than 70% and the ultimate tensile strength (UTS) was reduced by about 35%. The ductility of Ti–45.3Al–2Cr–2Nb–0.1W–0.15B alloy decreased by about 60% and the ultimate tensile strength (UTS) reduced by about 18% after heat shock, which was resulted from the appearance of small α₂ block phase at interfaces of lamellar colonies and microcracks at the interfaces of ribbon boride and lamellar structure.     

Interface microstructure evolution and bonding strength of TC11/γ-TiAl bi-materials fabricated by laser powder deposition

Laser powder deposition was applied to fabricate the Ti–6.5Al–3.5Mo–1.5Zr–0.3Si (wt%)/Ti–47Al–2Cr–2Nb–0.2W–0.15B (at%) bi-material system. The as-deposited TC11 alloy shows a basket-wave-like morphology while the as-deposited γ-TiAl alloy consists of fully α₂/γ lamellar microstructures. Regarding the thermal mismatch between TC11 and γ-TiAl during processing, the interface microstructure evolution was concerned. The transformation pathway was illustrated. It is found that the content changes of Al elements and β-stabilizers Mo, Cr, and Nb are responsible for the evolution of microstructures at the interface. The fracture surfaces are located at the γ-TiAl side. The bi-material shows a brittle-fracture manner, with the ultimate tensile strength of 560 MPa.     

Microstructure and mechanical properties of a spark plasma sinteredTi–45Al–8.5Nb–0.2W–0.2B–0.1Y alloy

A high Nb containing TiAl alloy from pre-alloyed powder of Ti–45Al–8.5Nb–0.2B–0.2W–0.1Y was processed by spark plasma sintering (SPS). The effects of sintering temperature on the microstructure and mechanical properties were studied. The optimized conditions yield high densities and uniform microstructure. Specimens sintered at 1100 °C are characterized by fine duplex microstructure, leading to superior room temperature mechanical properties with a tensile strength of 1024 MPa and an elongation of 1.16%. Specimens sintered at 1200 °C are of fully lamellar microstructure with a tensile strength of 964 MPa and an elongation of 0.88%. The main fracture mode in the duplex microstructure was transgranular in the equiaxed γ grains and interlamellar in the lamellar colonies. For the fully lamellar structure, the fracture mode was dominated by interlamellar, translamellar and stepwise failure.     

Deformation characteristics of polysynthetically twinned (PST) crystals during creep at 1150 K

The creep deformation characteristics of a lamellar polysynthetically twinned (PST) crystal of the composition Ti–48 mol% Al was investigated as a function of the lamellar orientation with respect to the compression axis and the applied stress. The creep resistance of hard PST orientation with the lamellar plates parallel or perpendicular to the compression axis was substantially higher than that of soft orientations with their lamellar plates oriented at intermediate angles to the compression axis. This fact could be associated with the predominant deformation of the hard orientations by deformation modes with the slip plane inclined to the lamellar interfaces in contrast to the predominant deformation of the soft orientations by deformation modes with the slip plane parallel to the lamellar plates. In the soft orientations, mainly straight ordinary dislocations with the Burgers vector b=1/2[1-10] aligned parallel to the lamellar interfaces were encountered in domains with a high resolved shear stress as well as in domains with no resolved shear stress. In the hard orientations, ordinary dislocations b=1/2[110] and superdislocations with a Burgers vector of the type b=1/2〈112] were observed. Despite a high resolved shear stress in certain oriented domains, superdislocations of the type b=〈101] were not found to play a considerable role during creep deformation under the investigated conditions. Cross twinning contributed to the deformation in favourably oriented variants, but twinning parallel to the lamellar interfaces was much more pronounced in the soft orientations leading to a substantial lamellar refinement. This microstructural hardening during creep results in the observed stress exponents of the soft orientations near unity at high stresses. A change in stress exponent from near unity at high stresses to about 9 at low stresses occurs at about 200 MPa. A critical stress of about 200 MPa for parallel twinning is proposed as a reason for the change in stress exponent.     

On the fatigue behavior of γ-based titanium aluminides: role of small cracks

Gamma-TiAl based alloys have recently received attention for potential elevated temperature applications in gas-turbine engines. However, although expected critical crack sizes for some targeted applications (e.g. gas-turbine engine blades) may be less than ∼500 μm, most fatigue-crack growth studies to date have focused on the behavior of large (on the order of a few millimeters) through-thickness cracks. Since successful implementation of damage-tolerant life-prediction methodologies requires that the fatigue properties be understood for crack sizes representative of those seen in service conditions, the present work is focused on characterizing the initiation and growth behavior of small (a∼25–300 μm) fatigue cracks in a γ-TiAl based alloy, of composition Ti–47Al–2Nb–2Cr–0.2B (at.%), with both duplex (average grain size of ∼17 μm) and refined lamellar (average colony size of ∼145 μm) microstructures. Results are compared to the behavior of large (a>5 mm), through-thickness cracks from a previous study. Superior crack initiation resistance is observed in the duplex microstructure, with no cracks nucleating after up to 500 000 cycles at maximum stress levels (R=0.1) in excess of the monotonic yield stress, σ_y. Comparatively, in the lamellar microstructure cracks nucleated readily at applied maximum stresses below the yield stress (85% σ_y) after as few as 500 cycles. In terms of crack growth, measurements for small fatigue cracks in the duplex and lamellar microstructures showed that both microstructures have comparable intrinsic fatigue-crack growth resistance in the presence of small flaws. This observation contrasts previous comparisons of large-crack data, where the lamellar structure showed far superior fatigue-crack growth resistance than the duplex structure. Such “small-crack effects” are examined both in terms of similitude (i.e. crack tip shielding) and continuum (i.e. biased microstructural sampling) limitations of traditional linear elastic fracture mechanics.     

放电等离子烧结原位制备TiC/Ti_2AlC复合材料及室温力学性能评价

以Al_4C_3、Ti和石墨粉为原料(Ti、Al、C的摩尔比为6:1:3),利用放电等离子烧结(SPS)技术通过原位反应制备出TiC/Ti_2AlC的复合材料.结果表明,基体相TiC的晶粒尺寸在2～5 μm左右,反应生成的Ti_2AlC颗粒尺度纵向长度为4～10 mm,横向宽度为1～2 mm,且弥散均匀分布在基体中.三元层状相Ti_2AlC的引入大大提高了复合材料的力学性能,复合材料的维氏硬度HV为11 GPa,断裂韧性K_(IC)为5.3 MPa·m~(1/2),抗弯强度sf为(470±50) MPa,弹性模量E为(228±30) GPa.通过压痕法观察了裂纹扩展路径,讨论了材料的断裂机制和增韧机制.材料以沿晶断裂为主,伴随少量穿晶断裂.