无包套近等温锻造TiAl合金的显微组织和拉伸性能(英文)

以真空自耗电弧熔炼技术熔炼名义成分为Ti-47Al-2Nb-2Cr-0.4(W,Mo)(摩尔百分数)的TiAl合金铸锭,并以该熔炼铸锭进行无包套的近等温锻造实验,研究该TiAl合金铸锭的高温可锻性、显微组织及拉伸性能。结果表明:在无包套的近等温锻造工艺中,该熔炼铸锭显示出较好的高温可锻性,经涂覆玻璃粉浆保护,铸锭在经过60%锻造变形后其锻饼表面无明显裂纹。TiAl合金的铸造组织由细小、均匀的层片状晶团(α2+γ)和少量存在于片层团界的等轴γ晶粒构成;经近等温锻造后,锻饼组织则主要由平均晶粒尺寸为20μm的等轴γ晶粒和一些破碎的片层组织构成,在一些难变形区域,依然存在弯曲变形的片层组织。室温拉伸性能检测表明,由于晶粒细化效应,锻饼的平均抗拉强度由铸锭的433MPa提高到573MPa。     

Cyclic-oxidation behavior of TiAl and of TiAl alloys

The cyclic-oxidation behavior of (in w/o) Ti-36Al, Ti-35Al-0.1C, Ti-35Al-1.4V-0.1C and Ti-35Al-5Nb-0.1C was studied between 800 and 1000° C in air. A few experiments were also performed in oxygen. Scale spallation after oxidation in air occurs during cooling on TiAl, TiAl-C, and TiAl-V at or close to the metal/scale interface when a critical scale thickness has been achieved. This process repeats and can lead to a stratified scale. These three materials form scales composed of an inward-growing fine-grain mixture of TiO₂-Al₂O₃ and an outward-growing coarse-grain TiO₂ layer or TiO₂+Al₂O₃ mixture. The TiAl-Nb alloy had a significantly different behavior. The scale on this material grew very slowly because a protective Al₂O₃ layer formed at the metal/scale interface. This behavior resulted in much better resistance to spallation because the critical scale thickness was reached only after a much longer time, and is different from the behavior of the other three alloys. Oxidation in air leads to slight nitridation of the subsurface zone beneath the scale. In comparison to oxidation in air, oxidation in oxygen improves the cyclicoxidation behavior. Whereas the scale formed in air was uniformly thick over the entire surface, the scale grown in oxygen varied locally in structure and thickness. A large fraction of the surface was covered with a thin Al₂O₃ layer, while the remaining part formed a two-layer scale similar to that formed in air. The results are discussed briefly in the light of a recently published model for scale spallation under compressive stress, however, quantitative estimations are not possible due to a lack of relevant data.     

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.     

Fatigue properties of TiAl alloys

The fatigue properties of TiAl alloys, namely fatigue life, cyclic stress–strain behaviour and fatigue crack growth resistance are reviewed in the present paper. The influence of different parameters (microstructure, defects, temperature and environment) on these properties is examined. Finally, some considerations on the fatigue reliability of TiAl components are proposed.     

Crystal growth of TiAl alloys

The growth of TiAl alloys over a range of Al concentrations has been considered for ingots processed by the floating zone technique. For Al-rich alloys, single crystals of γ-TiAl cannot be grown for compositions below Ti-54 at% Al since a banded microstructure forms due to the limitations imposed by the L + → γ peritectic reaction. However, near stoichemetric TiAl crystals can be grown by the traveling solvent method when using a TiAl---Co flux, although optimum processing conditions have not yet been realized. For Ti-rich alloys, evidence of a growth morphology consisting of β-phase dendrites embedded within a continuous matrix of the peritectic -phase is found for ingots processed up to 200 mm h⁻¹. The final lamellar orientation of the ingot is then determined by the orientation of the peritectic -phase and not by that of the leading β dendrites.     

Oxidation-induced embrittlement of TiAl alloys

The ductility of oxidised TiAl-based alloys is reduced even when the oxygen-rich region is of the order 100 nm thickness; this loss in ductility is smallest in lamellar samples. Removal of this oxidised region restores ductility. Acoustic events are observed during tensile tests at stresses above 300 MPa and cracks at about 250 MPa. In-situ tensile tests on samples, with part of the oxygen-enriched region removed, have shown that cracks are formed only in regions where the oxygen-rich region is present. X-ray diffraction measurements have shown that the oxygen-rich surface generates a tensile stress in the top 1 or 2 μm of the alloy of about 250 MPa corresponding to a compressive stress in the oxygen-rich layer of 2000 MPa. It is concluded that embrittlement is caused by (i) the tensile stress induced by the oxygen-rich region and (ii) the corresponding ease of crack nucleation in this region. Subsequent propagation is controlled by the fracture toughness.     

Recent advances of wrought TiAl alloys

1　ＩＮＴＲＯＤＵＣＴＩＯＮＴｗｏ ｐｈａｓｅ ｇａｍｍａＴｉＡｌａｌｌｏｙｓｈａｖｅｇｏｏｄｓｔｒｅｎｇｔｈｒｅｔｅｎｔｉｏｎａｂｉｌｉｔｙａｔｈｉｇｈｔｅｍｐｅｒａｔｕｒｅｓｗｈｉｃｈ ,ｉｎｃｏｍｂｉｎａｔｉｏｎｗｉｔｈｌｏｗｄｅｎｓｉｔｙ ,ｍａｋｅｓｔ     

Diffusion creep of intermetallic TiAl alloys

The creep behaviour of γ-TiAl with L1₀ structure without second phases, γ-TiAl with precipitated particles of α₂-Ti₃Al with D0₁₉ structure, and γ-TiAl with the H-phase Ti₂AlC has been studied at low stresses in the temperature range 900–1200°C. The obtained data allow the construction of creep deformation mechanism maps for the studied alloys which may be used for an extrapolation of the observed creep behaviour. At higher stresses dislocation creep occurs in all alloys, which is well described by the Dorn equation with stress exponents in the range 3–5. Extended Coble creep with threshold stress was observed only for the studied two-phase alloys. A strong temperature dependence of the threshold stress for Coble creep was found for the TiAl alloy with carbide particles.     

Heat treatment restores ductility to forged titanium alloys

This paper describes heat-treatment studies of forged compressor wheels carried out to determine whether wheels rejected due to low ductility could be salvaged by heat treatment. Adequate improvement in ductility was developed, by annealing about 100°F below the β transus, cooling at a low rate to the stabilizing temperature (1200°F), and stabilizing for a short time. The accompanying decrease in strength was slight.     

以Cu基非晶为中间层的TiAl合金瞬时液相扩散连接界面组织及力学性能

采用Cu₄₆Zr₄₆Al₈非晶薄片作中间层材料研究了不同温度与时间下TiAl合金的瞬时液相扩散连接(TLP),利用扫描电镜(SEM)、能谱仪(EDS)和X射线衍射仪(XRD)对连接层组织与成分分布进行了分析,并用电子万能试验机对连接试样的力学性能进行测试。结果表明,以Cu₄₆Zr₄₆Al₈非晶薄片作中间层,能得到连接紧密、无气孔缺陷裂痕等的良好接头。在连接过程中,中间层与基体发生剧烈的扩散反应,在连接层中形成三层组织,生成复杂的反应产物。在900 ℃×30 min下得到的焊接件,其剪切强度可达80.4 MPa。     

Mechanism of isothermal oxidation of the intel-metallic TiAl and of TiAl alloys

The oxidation behavior of Ti36Al, Ti35Al-0.1C, Ti35Al-1.4V-0.1C, and Ti35 Al-5Nb-0.1C (mass-%) in air and oxygen has been studied between 700 and 1000°C with the major emphasis at 900°C. Generally an oxide scale consisting of two layers, an outward- and an inward-growing layer, formed. The outward-growing part of the scale consisted mainly of TiO₂ (rutile), while the inward-growing part is composed of a mixture of TiO₂ and -Al₂O₃. A barrier layer of Al₂O₃ on TiAl between the inner and the outer part of the scale was visible for up to 300 hr. Under certain conditions, the Al₂O₃ barrier dissolved and re-precipitated in the outer TiO₂ layer. This shift leads to an effect similar to breakaway oxidation. Only the alloy containing Nb formed a longlasting, protective Al₂O₃ layer, which was established at the metal/scale interface after an incubation period of 80–100 hr. During this time, Nb was enriched in the subsurface zone up to approximately 20 w/o. The growth of the oxide scale on TiAl-V obeyed a parabolic law, because no Al₂O₃ barrier layer formed; large Al₂O₃ particles were part of the outward-growing layer. A brittle 2-Ti₃Al-layer rich in O formed beneath the oxide scale as a result of preferential Al oxidation particularly when oxidized in oxygen. Oxidation in air can lead also to formation of nitrides beneath the oxide scale. The nitridation can vary between the formation of isolated nitride particles and of a metal/Ti₂AlN/ TiN/oxide, scale-layer system. Under certain conditions, nitride-layer formation seemed to favor protective Al₂O₂₃ formation at the metal/scale interface, however, in general nitridation was detrimental with the consequence that oxidation was generally more rapid in air than in oxygen.     

Strength and ductility in TiAl alloys

Tensile behavior of two-phase TiAl alloys at room temperature (RT) is analyzed for duplex and lamellar microstructural forms. The Hall-Petch relationship with high constants in fully-lamellar material is explained as a combined function of grain-size and deformation-anisotropy. The low ductility and its inverse relationship with grain size are explained using the anisotropic tensile properties of lamellar structures and assuming that the fracture is controlled by the crack nucleation process involving the pile-ups of dislocations under shear stress. The crack initiation toughness and associated strains near the crack tip are used to explain the inverse relationship between ductility and toughness.     

Nb含量对铸造高铝TiAl合金高温强度和室温塑性的影响

研究了Nb含量对铸造高铝Ti Al合金高温强度和室温塑性的影响。结果表明,合金900℃的抗拉强度随Nb含量的增加先升高后降低,Nb含量在4%~9%(原子分数)之间的高铝合金表现出优异的高温强度水平,其中7%Nb合金最高,达587 MPa。分析认为,在高Nb合金化固溶强化的基础上,Al/Ti反位缺陷造成的强化效应、层片组织处于拉伸硬取向以及应变诱发的形变孪晶强化也有可能是高Nb含量高铝铸造Ti Al合金900℃具有优异强度水平的重要机制。合金室温塑性随Nb含量的增加而呈线性下降,由2%Nb合金的1.2%降为9%Nb合金的0.3%,这可能是由于B2相随Nb含量的增加而显著增加所致。     

Microstructural evolution of a forged TiAl based alloy during heat treatment at subtransus temperature

1　ＩＮＴＲＯＤＵＣＴＩＯＮＴｈｅＴｉＡｌ ｂａｓｅｄａｌｌｏｙｈａｓｂｅｅｎｋｎｏｗｎｔｏｂｅｏｎｅｏｆｔｈｅｍｏｓｔｐｒｏｍｉｓｉｎｇｃａｎｄｉｄａｔｅｓｆｏｒｈｉｇｈｔｅｍｐｅｒａｔｕｒｅｓｔｒｕｃｔｕｒａｌａｐｐｌｉｃａｔｉｏｎｓ[1].Ｔｈｅｐｒｏｐｅｒｔｉｅｓｏｆｔｈｅａｌｌｏｙｓｓｔｒｏｎｇｌｙｄｅｐｅｎｄｏｎｔｈｅｉｒｍｉｃｒｏｓｔｒｕｃｔｕｒｅｓ[2 ].ＴｈｅＴｉＡｌｂａｓｅｄａｌｌｏｙｓｗｉｔｈａｆｕｌｌｙｌａｍｅｌｌａｒｍｉｃｒｏｓｔｒｕｃｔｕｒｅｓｇｅｎｅｒａｌｌｙｄｉｓｐｌａｙ…     

First-principles study of shear deformation in TiAl alloys

The stress-induced phase-transition γ-TiAl ↔ ₂-Ti₃Al in TiAl alloys was systemically studied by density functional theory (DFT) calculations. We found that the key role in such phase-transition is the alloy composition. Firstly through studying the relationship between the formation energy, structure (fct and hcp) and alloy composition (Ti:Al = 1–3), we acquired that the stability of fct or hcp phases depends on the Ti:Al ratios. When the ratio meets about 1.5, the formation energies of fct and hcp phases are very close, which means both structures have the similar stability. Further shear deformation calculations on such alloys with different structures and compositions show that phases with Ti:Al value close to 1.5 have smaller dislocation nucleation energies and lower maximum shear strengths than those of the stoichiometric TiAl and Ti₃Al phases. Such results not only reveal the composition's effect on the reversible phase transition, but also exhibit the intrinsic controlling role on the alloying properties.     

Solidification of high Nb containing TiAl based alloys

Abstract

Casting of titanium aluminides is an attractive processing route for production of near net shape components: turbocharger wheels, valves and aero-engine components are presently at the heart of casting developments. Among the casting alloys under consideration are a number of niobium rich TiAl based alloys that contain low boron additions for grain refinement and minor additions of other elements to enhance creep resistance. An essential condition that must be met to achieve grain refinement is a solidification pathway competed via β-(Ti), e.g. a pathway that avoids peritectic growth of α-Ti. In this contribution we describe the microsegregation analysis of a unidirectionally solidified sample from the ternary alloy Ti–45Al–8Nb. The corresponding solidification path is discussed on the basis of thermodynamic calculations and is shown to closely follow Scheil predictions with some amount of back-diffusion for aluminium. The analysis indicates that the nucleation undercooling for peritectic α (Ti) in the deep mushy zone is significant.     

等离子旋转电极法制备钛铝粉末性能表征

采用等离子束旋转电极法制备Ti-47Al-2Cr-2Nb球形粉末,粉末的平均粒径d50为85 μm, 松装密度为2.65 g/cm3。截线法测得粉末的球形度为99.6%;粉末的流动性为10.40 s/50 g;粉末的平均氧、氮含量分别为0.05%和0.004%。XRD分析表明,粉末的相组成为α2相和γ相,主相为富钛α2相;粉末经热等静压后主相为γ相,伴随少量α2相。SEM背散射照片表明,粉末颗粒的晶粒为胞状组织,晶粒平均大小约为2 μm;对不同粒径的颗粒进行EPMA分析表明,颗粒内部化学成分与预合金棒接近,颗粒表面有部分Al元素挥发,约为2at%     

Review of alloy and process development of TiAl alloys

The improved understanding of the factors that control microstructure and properties of TiAl alloys is reviewed together with current work aimed at developing both wrought and cast products. It is suggested that the choice of alloy composition is perhaps far simpler than the complex literature would suggest and the factors that underlie alloy choice will be explained. These factors include the processability of the alloy as well as the properties and examples will be given where this dual approach of defining both processability and properties is central to the successful application. In addition other aspects of processing that will be discussed include cost-effective processing, accuracy of compositional control and control of processing conditions appropriate for the specific alloy. Some current applications of TiAl components are summarised before considering some of the challenges still remaining for TiAl-based alloys.