Creep deformation of fully lamellar TiAl controlled by the viscous glide of interfacial dislocations

Creep mechanisms of fully lamellar TiAl with a refined microstructure (γ lamellae: 100–300 nm thick, α₂ lamellae: 10–50 nm thick) have been investigated. A nearly linear creep behavior (i.e. the steady-state creep rate is nearly proportional to the applied stress) was observed when the alloy was creep deformed at low applied stresses (<400 MPa) and intermediate temperatures (650–810°C). Since the operation and multiplication of lattice dislocations within both γ and α₂ lamellae are very limited in a low stress level as a result of the refined lamellar microstructure, creep mechanisms based upon glide and/or climb of lattice dislocations become insignificant. Instead, the motion of interfacial dislocation arrays on γ/α₂ and γ/γ interfaces (i.e. interface sliding) has found to be a predominant deformation mechanism. According to the observed interfacial substructure caused by interface sliding and the measured activation energy for creep, it is proposed that creep deformation of the refined lamellar TiAl in the intermediate-temperature and low-stress regime is primarily controlled by the viscous glide of interfacial dislocations.     

Primary creep deformation behaviors related with lamellar interface in TiAl alloy

Constant tensile stress creep tests under the condition of 760~816°C/172~276 MPa in an air environment are conducted, and the microstructural evolution during primary creep deformation at the creep condition of 816°C/172 MPa was observed by transmission electron microscopy (TEM) for the lamellar structured Ti-45. 5Al-2Cr-2.6Nb-0.17W-0.lB-0.2C-0.15Si (at.%) alloy. The amount of creep strain deformed during primary creep stage is considered to be the summation of the strains occurred by gliding of initial dislocations and of newly generated dislocations. Creep rate controlling process within the primary stage seems to be shifting from the initial dislocation climb controlled to the generation of the new dislocations by the phase transformation of 2 to as creep strain increases.     

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.     

C含量对铸造TiAl合金组织和力学性能的影响

在Ti-47.5Al-3.7(Cr,V,Zr)合金中添加0.05%~0.2%C(原子分数,下同),采用冷坩埚悬浮熔炼方法制备出了层片组织TiAl合金铸棒,通过组织观察、室温拉伸和蠕变性能测试研究了C含量对TiAl合金组织和力学性能的影响。结果表明,添加0.05%~0.2%C后,合金仍可获得择优取向层片组织。随C含量增加α2层片体积分数略有增加,层片间距呈细化趋势。当C含量超过0.1%时,在α2和γ层片内和层片界面上有细小的Ti2AlC型碳化物析出,碳化物析出相的尺寸和数量随C含量增加有所增加。添加0.05%~0.2%C后提高了合金室温的抗拉强度和屈服强度,且随C含量增加提升幅度逐渐增大,当C含量为0.2%时,分别将抗拉强度和屈服强度提升了101和123 MPa。添加C元素后显著改善了合金的蠕变性能,当C含量为0.1%时蠕变性能最佳,与不含C的合金相比,其塑性蠕变应变降低了一半、相同应变时的蠕变速率降低了1个数量级以上。添加0.1%C提升合金蠕变抗力的机制主要是通过抑制合金在蠕变初期的位错萌生和增殖过程;在γ层片中形成割阶和位错碎片阻碍位错继续运动,使得合金在蠕变第一阶段的应变硬化程度迅速增加;此外,析出的Ti2AlC型碳化物进一步强化层片界面和基体,与层片间距细化共同提高了穿层片滑移位错的运动阻力。     

Creep behaviour of a cast TiAl-based alloy for industrial applications

The creep behaviour of a cast TiAl-based alloy with nominal chemical composition Ti–46Al–2W–0.5Si (at.%) was investigated. Constant load tensile creep tests were performed in the temperature range 973–1073 K and at applied stresses ranging from 200 to 390 MPa. The minimum creep rate is found to depend strongly on the applied stress and temperature. The power law stress exponent n is determined to be 7.3 and true activation energy for creep Q is calculated to be 405 kJ/mol. The initial microstructure of the alloy is unstable during creep exposure. The transformation of the α₂(Ti₃Al)-phase to the γ(TiAl)-phase, needle-like B2 particles and fine Ti₅Si₃ precipitates and particle coarsening are observed. Ordinary dislocations in the γ-matrix dominate the deformation microstructures at creep strains lower than 1.5%. The dislocations are elongated in the screw orientation and form local cusps, which are frequently associated with the jogs on the screw segments of dislocations. Fine B2 and Ti₅Si₃ precipitates act as effective obstacles to dislocation motion. The kinetics of the creep deformation within the studied temperature range and applied stresses is proposed to be controlled by non-conservative motion of dislocations.     

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.     

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.     

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.     

Microstructural evolution and creep deformation behavior of novel Ti−22Al−25Nb−1Mo−1V−1Zr−0.2Si (at.%) orthorhombic alloy

The microstructural evolution and creep deformation behavior which were adjusted and controlled by age treatment of a novel Ti?22Al?25Nb?1Mo?1V?1Zr?0.2Si (mole fraction, %) alloy, were investigated. The microstructures were obtained at different heat treatment temperatures and analyzed by SEM and TEM techniques. The creep behavior of the alloy was studied at 650 °C, 150 MPa for 100 h in air. The results showed that the initial microstructure mainly contained lath-like α₂, B2, and O phases. The precipitated O phase was sensitive to aging temperature. With the aging temperature increasing, the thickness of the precipitated O phase was also increased, and the length was shortened. The creep resistance of this alloy was relevant to the morphology and volume faction of the lamellar O phase. The increase of lamellar O phase in thickness was the main reason for the improved creep properties.     

Microstructural stability of a cast Ti–45.2Al–2W–0.6Si–0.7B alloy at temperatures 973–1073 K

Microstructural stability of a cast intermetallic Ti–45.2Al–2W–0.6Si–0.7B (at.%) alloy after ageing in the temperature range from 973 to 1073 K up to 10,000 h was studied. The microstructure of the alloy consists of lamellar and TiAl-rich regions. Lamellar regions consist of fine lamellae of γ(TiAl) and α₂(Ti₃Al) phases, fine B2 and Ti₅Si₃ precipitates on the lamellar interfaces. Fine α₂ lamellae are partially decomposed before ageing. Small volume fraction of coarse B2 particles and few ribbon-like borides were found within lamellar regions. TiAl-rich regions contain coarse α₂ lamellae, rod-like B2 particles, small volume fraction of coarse Ti₅Si₃ precipitates and ribbon-like borides. The as-received microstructure of the alloy is unstable during long-term ageing at temperatures ranging from 973 to 1073 K. The α₂ phase in the lamellar and TiAl-rich regions transforms to the γ phase and fine needle-like B2 precipitates. The microstructural instabilities lead to softening of the alloy. The microhardness decrease is measured to be faster in the lamellar regions compared to that in the TiAl-rich regions.     

High-temperature deformation in a Ta–W alloy

High-temperature deformation was investigated in a Ta–2.5 wt% W commercially available tantalum alloy, at temperatures in the range of 1523–1723 K and at a stress range extending from 35 MPa to 210 MPa. The experimental data, which cover several orders of magnitude of strain rate, show that the stress dependence of creep rate is high and that the temperature dependence of creep rate is higher than that for self-diffusion in tantalum. An analysis of the experimental data indicates that a threshold stress for creep exists, and that the temperature dependence of the threshold stress is much stronger than that attributable to the shear modulus. By considering the effect of the threshold stress and its temperature dependence on creep plots, it is demonstrated that the true creep characteristics of Ta–2.5 wt% W are consistent with those reported for solid-solution alloys at high stresses. In particular, the creep behavior of the alloy exhibits a transition from a region controlled by viscous glide to a high-stress region related to the breakaway of dislocations from solute-atom atmospheres. An examination of creep substructure in Ta–2.5 wt% W reveals the presence of interaction between moving dislocations and dispersion particles. It is suggested that such an interaction provides the most likely source of the threshold stress for creep in the alloy.     

Effects of thermal exposure on the microstructure and properties of a γ-TiAl based alloy containing 44Al–4Nb–4Zr–0.2Si–0.3B

The microstructural stability after long-term aging at 700°C of a TiAl-based alloy containing 44Al–4Nb–4Zr–0.2Si–0.3B (in at%) has been studied. The microstructural observations are consistent with those established previously, i.e. even as low as 700°C the microstructure is unstable in that after prolonged aging, decomposition of the α₂ lamellae and increased volume fraction of gamma phase have been observed. Despite this, it has been shown that, with a combination of slow cooling from the annealing temperature and a ‘stabilisation’ treatment at an intermediate temperature, the mechanical properties of this alloy can be retained. This increase in thermal stability has been attributed to the large α₂ lamellae which result from slow cooling and are less susceptible to complete dissolution during aging, and also to the formation of gamma grains which are embedded in and constrained by the ω phase between the lamellar colonies. ©     

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.     

Effects of discontinuour coarsening of lamellae on creep strength of fully lamellar TiAl alloys

High temperature creep of a binary Ti-42mol%Al alloy with fully lamellar structure was studied to examine effects of lamellar spacing on creep strength. Strain hardening is more significant in a finer lamellar material, resulting in higher creep strength at high stresses. Discontinuous coarsening of lamellae takes place during creep, and is more substantial in the finer lamellar material at low stresses. Because of the microstructural degradation, the strengthening by fine lamellae diminishes at low stresses. Some specimens were annealed at high temperatures to finish the discontinuous coarsening prior to creep testing. In these specimens, the strengthening by fine lamellae becomes effective even at low stresses.     

The phase evolution,mechanical behavior,and microstructural instability of a fully-lamellar Ti–46Al(at.%) alloy

A Ti–46Al(at.%) polysynthetically twinned alloy, which contained a characteristic lamellar spacing of λ=1.3 μm, was converted to a polycrystalline fully-lamellar microstructure with λ=19.9 nm using an α-phase solution treatment followed by α₂+γ aging. Tensile experiments, performed on microsamples extracted from within single grains of the polycrystalline material, illustrated that ultrafine lamellae led to exceptional tensile strength. However, the ultrafine lamellae were not stable at elevated temperatures and the ultrafine material was found to have relatively poor creep resistance.     

Tensile creep of Mo–Si–B alloys

Multiphase Mo–Si–B alloys containing a Mo solid solution matrix and brittle Mo₃Si and Mo₅SiB₂ (T2) intermetallic phases are candidates for ultra-high-temperature applications_. The elevated temperature uniaxial tensile response at a nominal strain rate of 10^?4 s^–1 and the tensile creep response at constant load between 1000 °C and 1300 °C of a (i) single phase solid solution (Mo–3.0Si–1.3B in at.%), (ii) two-phase alloy containing ～35 vol.% T2 phase (Mo–6Si–8B in at.%) and (iii) three-phase alloy with ～50 vol.% T2 + Mo₃Si phases (Mo–8.6Si–8.7B in at.%) were evaluated. The results confirm that Si in solid solution significantly enhances both the yield strength and the creep resistance of these materials. A Larson–Miller plot of the creep data showed improved creep resistance of the two- and three-phase alloys in comparison with Ni-based superalloys. The extent of Si dissolved in the solid solution phase varied in these three alloys and Si appeared to segregate to dislocations and grain boundaries. A stress exponent of ～5 for the solid solution alloy and ～7 at 1200 °C for the two multiphase alloys suggested dislocation climb to be the controlling mechanism. Grain boundary precipitation of the T2 phase during creep deformation was observed and the precipitation kinetics appear to be affected by the test temperature and applied stress.     

Recovery,recrystallization and phase transformations during thermomechanical processing and treatment of TiAl-based alloys

The conversion of the cast microstructure of a Ti–45Al–10Nb (at.%) ingot due to extrusion and subsequent heat treatments was investigated using scanning and transmission electron microscopy. The starting lamellar microstructure was broken down by dynamic recrystallization into a fine-grained duplex microstructure after extrusion, however, a small amount of remnant lamellae was retained. It was found that the fine substructures in the remnant lamellae varied from one area to another. Recrystallization and phase boundary bulging were found to be the major mechanisms responsible for lamellar globularization. The existence of remnant lamellae was ascribed to the anisotropic flow behavior of the starting lamellar colonies. Microstructural and process variables that influence the scale of remnant lamellae will be elucidated. Annealing in the (α+γ) two-phase field cannot eliminate the remnant lamellar structures. However, the detrimental effect of remnant lamellae can be minimized after alpha treatments provided that the remnant lamellae are smaller than about 80 μm.     

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.     

Strengthening behavior of beta phase in lamellar microstructure of TiAl alloys

β phase can be introduced to TiAl alloys by the additions of β stabilizing elements such as Cr, Nb, W, and Mo. The β phase has a body-centered cubic lattice structure and is softer than the α₂ and γ phases in TiAl alloys at elevated temperatures, and hence is thought to have a detrimental effect on creep strength. However, fine β precipitates can be formed at lamellar interfaces by proper heat treatment conditions and the β interfacial precipitate improves the creep resistance of fully lamellar TiAl alloys, since the phase interface of γ/β retards the motion of dislocations during creep. This paper reviews recent research on high-temperature strengthening behavior of the β phase in fully lamellar TiAl alloys.     

Effect of stresses on the structural changes in high-chromium steel upon creep

The effect of stresses on the microstructure and dispersed particles in a heating-performance Fe?0.12C–0.06Si–0.04Ni–0.2Mn–9.5Cr–3.2Co–0.45Mo–3.1W–0.2V–0.06Nb–0.005B–0.05N (wt %) steel has been studied under long-term strength tests at Т = 650°C under initial applied stresses ranging from 220 to 100 MPa with a step of 20 MPa. Under an applied stress of 160 MPa, which corresponds to a time to fracture of 1703 h, a transfer from short- to long-term creep takes place. It has been shown that alloying with 3% Co and an increase in W content to 3% significantly increase the short-term creep resistance and slightly increase the long-term strength upon tests by more than 10⁴ h. The transfer from short- to the long-term creep is accompanied by substantial changes in the microstructure of the steel. Under long-term creep, the solid solution became depleted of tungsten and of molybdenum down to the thermodynamically equilibrium content of these elements in the solid solution, which leads to the precipitation of a large amount of fine particles of the Laves phase at the boundaries of laths and prior austenitic grains. At a time to fracture of more than 4 × 10³ h, the coalescence of the M₂₃С₆ carbides and Laves-phase particles occurs, which causes the transformation of the structure of fine tempered martensite lath structure into a subgrained structure.