高Nb-TiAl热变形过程中的晶界特征和显微组织演化

本文研究了Ti-45Al-8.5Nb-(W,B,Y)合金热变形过程中的晶界特征和显微组织演化规律。采用等离子冷床炉熔炼制备的Ti-45Al-8.5Nb-(W,B,Y)合金具有典型的近片层组织并在晶界有高温β/B2相残留,富Nb、W、Ti等、贫Al的晶界β/B2相主要是熔炼过程中因冷速较快、β稳定元素的低扩散系数以及合金中各元素的分配系数差异导致β → α相变不能完全进行。合金中的 B 和Y等微量元素分别以硼化物和Y2O3的形式存在。晶界β/B2相的形态、尺寸、成分和稳定性等受后续热变形影响显著,高温和应力作用会使β相发生破碎细化并促进合金中的元素扩散会引起晶界β/B2相的成分变化。当合金在 (α + γ) 两相区进行热压缩变形时,会有部分β相向α相转变（β/B2 → α2）,主要通过β相中二次α的析出和β相被相邻α相的蚕食等方式进行,热压缩变形会促进β/B2相向更为密排结构的α2相转变。     

Flow characteristics and constitutive modeling for elevated temperature deformation of a high Nb containing TiAl alloy

The hot deformation behavior of a high Nb containing TiAl alloy with a nominal composition of Ti–42Al–8Nb–(W, B, Y) was investigated at temperatures ranging from 1000 °C to 1150 °C and strain rates from 10⁻³ s⁻¹ to 0.5 s⁻¹ on a Gleeble thermo-simulation machine. The work hardening regime and flow softening behavior of the alloy were analyzed in detail. The results revealed that the onset of dynamic recrystallization (DRX) was quite easy for the present alloy, whereas the dynamic recovery (DRV) was impeded during the hot deformation. The DRX kinetics was studied by Avrami-type equation. The low Avrami exponents of the proposed equation indicate a lower recrystallization rate compared to ordinary metals and alloys. Based on the classical hyperbolic-sine law and the kinematics of DRX, the constitutive equations of the work hardening-recovery period (i.e. flow stress before the peak) and flow softening process (i.e. flow curve after the peak stress) were established for the present alloy, respectively. Comparisons between the experimental and calculated results revealed that except the severely cracked specimens, the stress–strain curves predicted by the established model are in good agreement with experimental results.     

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。     

全片层高Nb含量TiAl合金长期热循环后的显微组织不稳定性

对全片层Ti-45Al-8.5Nb-（W,B,Y）合金在900℃下进行长期热循环（500次和1000次）实验,采用扫描电镜（SEM）及透射电镜（TEM）研究该合金长期热循环后的显微组织不稳定性。结果表明：合金经热循环后主要产生两种类型的组织不稳定性：1）长期热循环特别是1000次热循环后,在Al偏析处易产生因晶界迁移引起的不连续粗化,随着循环次数的增加,元素扩散致使Al偏析逐渐减少;2）1000次热循环后,α2片层变细且发生断裂,这是由α2→γ相变导致的α2片层溶解所致。同时,γ晶粒在α2片层或（α2＋γ）片层内部以任意方向形核。     

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.     

Reaction mechanism in high Nb containing TiAl alloy by elemental powder metallurgy

1 Introduction High Nb containing TiAl alloys have attracted much attention owing to their low densities and potential applications at high-temperature environments[1,2]. It has been found that Nb is the essential and effective element improving their me…     

TiAl基合金的高温塑性变形行为

采用Gleeble-1500热模拟机在变形温度为1 000～1 150 ℃、应变速率为10~(-3)～10~0 s~(-1)的变形条件下,研究Ti-47Al-2Cr-0.2Mo(摩尔分数,%)合金的热变形行为.利用光学显微镜和扫描电子显微镜分析合金在不同变形条件下的组织演化规律.结果表明:流变应力随着应变速率提高和变形温度降低而增大;在变形过程中,流变应力随着变形量增大而增大,当流变应力达到峰值后趋于平稳,表明合金在变形过程中发生了动态再结晶;热变形过程的流变应力可采用双曲正弦本构关系来描述,平均激活能为337.75 kJ/mol;从合金的组织演化过程中可以看出,合金中不均匀的原始组织得到明显均匀化,变形后的组织是由α_2/γ层片晶团和γ晶粒组成的双态组织,在α_2/γ层片晶团和γ晶粒的晶界交界处发现分布均匀的B_2相,并且随着变形温度升高B_2相数量逐渐减少.     

铸造高铌TiAl合金的热变形能力研究

在Gleeble-1500热模拟试验机上对高铌TiAl合金进行等温热压缩试验,研究其热压缩变形的流变应力行为.压缩试验的形变温度为900～1250℃,形变速率为5×10-4～1s-1,工程压缩应变为50%和80%.用Zenner-Hollomon参数的指数函数能较好地描述该合金高温变形时的流变应力行为;所获得的峰值应力热变形方程为Op=197.61n{(Z/9.59×1015)0.285+[(Z/9.59×1015)0.570+1]1/2},其变形激活能为(497±49)kJ/mol.本研究还给出了铸造高铌TiAl合金热变形抗力图.     

Effect of trace tungsten on the microstructure of TiAl alloys containing Nb and B

New TiAl alloys, containing 45 at.% A1, 7 at.% Nb, x at.% W, and 0.15 at.% B （x = 0, 0.2, 0.4, and 0.7） were prepared by arc melting and drop casting consequently. Using optical microscopy, scanning electron microscopy （SEM）, and electron superprobe technologies, the effects of tungsten on the microstructural evolution of the TiA1 alloys, including the colony size and lamellar spacing, were analyzed. It was found that cellular structures and dendrites were formed in the as-cast TiA1 alloys, and heavy metals, such as niobium and tungsten, tend to segregate strongly at the interface of the cellular structures and dendrites. Trace tungsten can effectively impede the grain growth and narrow the interlamellar spacing. 0.4 at.% tungsten is more effective in refuting the microstructure of the TiAI alloys.     

热锻开坯对高铌TiAl合金微观组织的影响

采用热等静压技术处理了高铌TiAl合金铸锭,对直径为115 mm的高铌TiAl合金铸锭在1150～1300℃进行了热锻开坯.结果表明:热等静压处理后的高铌TiAl合金由大量粗大的α2/γ片层晶粒和少量的γ等轴晶、B2相等组成,晶粒度约为80～150μm;通过变形量为75%的2次锻造成形后锻饼表面和内部质量较为完好,没有出现裂纹和太多的氧化现象;铸态粗大片层组织在变形后基本破碎,晶粒得到明显细化.     

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.     

Microstructure control and high temperature properties of TiAl base alloys

An equiaxed fine grain structure, a γ grain structure with the precipitated ₂ laths, and a fully lamellar structure were obtained by the microstructure control using thermomechanical processing and heat treatment. The key to obtaining the equiaxed fine grain structure using isothermal forging is to decompose the lamellar structure and then produce the fine grain microstructure through dynamic recrystallization. TiAl base alloys consisting of fine equiaxed grains, in particular, Ti-39Al-9V consisting of the γ and B2 phases exhibited superplastic elongation of more than 600% at 1423 K. Creep rupture properties of TiAl binary alloys with various microstructures were studied in purified He in the temperature range from 1073 to 1373 K. Above 1173 K the precipitated ₂ phase improved the steady state creep rate and creep life. At 1023 K, the ₂ phase did not improve the creep rate, although the steady state creep rate decreased and the creep life increased as the γ grain size increased.     

Microstructural evolution in high Nb containing TiAl alloys by reactive hot pressing

High Nb containing TiAl alloys with nominal composition of Ti-45Al-9（Nb, W, B） were fabricated at different hot pressing temperatures. The experimental results show that the microstructure of alloys hot pressed at 1300 ℃ is inhomogeneous, for lots of particulate substances containing Ti3Al phase and Nb powders dispersed in TiAl phase matrix. At 1 350 ℃, only a few lamellar colonies form and the diffusion of Nb element is incomplete. With the increase of hot pressing temperature, the mierostrueture will be more homogeneous. However, borides appear at higher sintering temperature. Meanwhile, the microstructure becomes coarse at 1 500 ℃. 1 400 ℃ will be an appropriate hot pressing temperature for reaction synthesis of high Nb containing TiAl alloys consisting of fully lamellar （FL） microstructure with the colony size of 30-80 μm. Nb element dissolves into the Ti-Al matrix by diffusion. Pore nests form in situ after Nb powders diffusion.     

合金化对TiAl合金高温变形行为的影响

TiAl合金的热加工（锻造、热挤压、板材轧制等）窗口窄,高温变形能力差,室温脆性大等成为限制其应用的关键因素,本文主要综述了合金成分对TiAl合金热变形加工以及超塑性成形影响等方面的研究现状,从TiAl合金的晶体结构、β相含量、显微组织细化、热变形激活能四个方面探讨了合金成分对TiAl合金热变形加工的影响,并指出合金化方式提高TiAl合金热变形加工能力应该遵循的几点原则,以及TiAl合金热变形加工的未来发展趋势。     

Homogenization treatment of high Nb containing TiAl alloys with as-cast and as-forged microstructures

The effect of heat treatment on the microstructure evolution of a high Nb containing TiAl alloy has been studied. The results indicate that β-segregation, β-segregation and S-segregation in the as-cast and as-forged alloys can be effectively eliminated at the temperature above Tα （1350-1400℃） for long holding time （12-24 h） and the full lamellar （FL） microstructure is gained. For the two alloys, the lamellar colony sizes are 120 μm and 2000 μm, respectively after heat treatment at 1400℃ for 12 h. Meanwhile, the sizes are 210 μm and 3000 μm, respectively at 1350℃ for 24 h. To get a fine homogenous microstructure, the primary as-cast alloy is first subjected to preheat treatment for eliminating the segregations. After the preheat treatment, the ailoy is processed by the multi-step canned forging to attain the microstructure with fine grain size.     

Nucleation and thermal stability of carbide precipitates in high Nb containing TiAl alloys

This work monitors the nucleation, growth and coarsening of carbides in powder metallurgically processed, high Nb containing TiAl alloys. The effects of carbon content, annealing conditions and internal defects on the precipitation and stability of carbides were systematically investigated by high energy X-ray diffraction and transmission electron microscopy. In general, at 800 °C the carbide microstructure can still change significantly up to 1000 h of annealing. It is found that a higher carbon concentration promotes the carbide precipitation process and increases the thermal stability of carbides. Internal interfaces and other crystallographic defects act not only as heterogeneous nucleation sites for perovskite Ti₃AlC carbides but also as carbon sinks. This retards the carbide nucleation in the interior of γ-TiAl grains. By homogenising the carbon distribution through solution heat treatment the nucleation of carbides in the γ-TiAl matrix is significantly accelerated as an effect of higher matrix carbon content.     

基于分子动力学的3D打印TiAl纳米多晶合金变形机制及其温度相关性分析

借助透射电镜观察和分子动力学计算,对3D打印Ti-6Al-4V合金的变形行为及其温度相关性进行了系统研究。结果表明,温度在TiAl纳米多晶体变形机制的竞争中起关键作用。当温度低于800 K,平均晶粒尺寸低于8.3 nm的单相TiAl纳米多晶合金首先出现位错运动,且层错保留在晶粒中并形成交错结构。同时,大尺寸晶粒(≥8.3 nm)为位错运动提供了足够的空间,很少在晶粒中形成层错。在双相TiAl+Ti₃Al纳米多晶合金中,层错的交割是低应变(ε<18.0%)TiAl晶粒的主要变形机制,并且Ti₃Al晶粒保持其初始结构。当ε≥18.0%时,Ti₃Al晶粒中的位错开始运动并形成层错交割。当温度高于800 K时,Ti和Al原子处于高能状态,主要的变形机制与具有非晶结构的滑移边界有关。非晶滑移边界及再结晶结构是双相TiAl+Ti₃Al纳米多晶合金组织变形的最重要特征。     

Effects of contents of Nb and C on hot deformation behaviors of high Nb X80 pipeline steels

The behavior of the flow deformation and the effects of contents of Nb and C on deformation behaviors of high Nb X80 pipeline steels during hot compression deformation were studied by thermal simulation test. The content of solid solution Nb was quantificationally studied during the reheating and hot deformation process, and the effects of change of solid solution Nb in steels on hot deformation behaviors were discussed. The results show that the contents of Nb and C have great effects on the flow stress behaviors of high Nb X80 pipeline steels. When the C content in steels is constant, the recrystallization activation energy increases from 387 to 481 kJ/mol with increasing the Nb from 0.082% to 0.13% (mass fraction). However, the effect of Nb is correlative to the C content, i.e. w(Nb)/w(C). When w(Nb)/w(C) decreases from 3.61 to 2.18, the recrystallization activation energy decreases from 481 to 434 kJ/mol.     

Effect of Cr, Nb, Mn, V, W and Si on high temperature oxidation of TiAl alloys

Alloys of Ti−(47,51)Al, Ti47Al−4Cr, Ti48Al−2Cr2Nb, Ti47Al−1Mn, Ti39.4Al−10V, Ti48.4Al−1.9W, and Ti43Al−2W0.1Si were oxidized at temperatures between 800 and 1000 °C in air to determine the effect of each alloying element on oxidation behavior. Among the alloys tested, the Ti48.4Al-1.9W alloy displayed the best oxidation resistance, due to the beneficial effects of W, whereas the Ti39.4Al-10V alloy displayed the worst oxidation resistance, due to the formation of volatile V-oxides. Cr was harmful while Nb was beneficial. The oxidation rate of each alloy was not strictly proportional to temperature, because each alloying element had different temperature sensitivity with respect to oxidation rate.