ON THE ORDERING TRANSFORMATIONS IN Ti_3AL-Nb ALLOY

Study was made of the behaviour of ordering transformation in Ti_3AI-Nb alloy,including the ordering at high temperatures,the transformation of high temperature β-phase during cooling,and the decomposition of metastable β-phase during aging.The results show that the ordered primary α_2 and high temperature β in alloy form at 1060℃.The transformation of high temperature β-phase proceed by β→α_2+ω type during cooling,and the decomposition of metastable β and ω type proceeded by(β+ω)_(metustabte)→(α_2+β)_(stable)during aging at 700℃.     

一种新型奥氏体Fe-Mn-Al耐热钢铝化物涂层相变的研究

对一种新型奥氏体Fe—Mn—Al耐热钢铝化物涂层的显微结构、组成、相变进行了详细地研究。渗层共分两层,外层为有序超结构固溶体FeAl,内层由FeAl、β-Mn,V_4C_3组成。当这种涂层在静态空气下,在450—750℃温度范围内氧化50—1000小时时,这两层经历着不同的相变规律。外层是:FeAl(→Fe_3Al)→α-Fe→α-Fe+γ→γ。内层是:FeAl+β-Mn(→Fe_3Al+β-Mn)→α-Fe+β-Mn→α-Fe+β-Mn+γ→α-Fe+γ→γ。涂层的相变和涂层的退化紧密相连。贫Al的FeAl相的形成,意味着退化开始。γ的出现,称之促使退化加剧。涂层完全转变成γ,退化即告终。     

Influence of Al content on cast microstructures of Ti–Al intermetallic compounds

Cast microstructures of Ti–(35–58) at% Al alloys have been studied using optical microscope, scanning electron microscope and transmission electron microscope. The critical compositions of transition from primary β to α phase and from α to γ phase were determined to be about 49.5 at% Al and 55.5 at% Al respectively. The peritectic compositions of α_p and γ_p were measured to be about 47 and 54 at% Al respectively. A minimum dendrite cell size was observed at 46.5 at% Al, which is believed to arise primarily from the variation behavior of the partition coefficient and liquidus slope with Al content. The mode of solid state transformation β→α during solidification at a given cooling rate varied from the massive transformation to the precipitation of Widmanstätten α plates and of α grains with Al content. The mode of transformation α→γ, on the other hand, varied from the precipitation to the massive transformation with Al content. This variation of transformation modes at a given cooling rate was explained in terms of the variation of the driving force and mobility with Al content.     

THERMODYNAMICS OF MARTENSITIC TRANSFORMATION IN Fe-Mn-C AND Fe-Ni-C ALLOYS

A Central Atom Model is introduced and the LFG and Hsu models are modified in order toevaluate the driving force for the martensitic transformation in Fe-Mn-C and Fe-Ni-C al-loys.The results show that the relationship between the driving force and the yield strength ofaustenite at Ms temperature,σ_(0.2)~γ/M_s,fits Hsu's formula;ΔG~=2.1σ_(0.2)~γ/M_s+907J/mol.The M_s temperatures of Fe-Mn-C and Fe-Ni-C alloys are also calculated.Thecalculated results are in good agreement with experimental values.     

Ti-22Al-25Nb与TC11异种钛合金的线性摩擦焊接

通过Ti-22Al-25Nb(Ti2AlNb基合金)与TC11((α+β)钛合金)的线性摩擦焊接,研究焊接工艺参数对接头外貌及界面结合率的影响,观察接头附近显微组织,并测试了其显微硬度。结果表明,随着焊接工艺参数,如摩擦时间、摩擦频率和摩擦压力的提高,接头的结合率显著提高;在焊接和随后的冷却过程中,TC11合金侧的热影响区域发生了α→β→α′相变,形成的大量针状马氏体α′相使焊缝区的显微硬度值显著增大;Ti-22Al-25Nb合金侧的热影响区域主要发生了(O,α2)→B2/β相变,随着O相和α2相的减少,该区域金属的显微硬度值显著降低。在合适的工艺条件下,线性摩擦焊接Ti-22Al-25Nb合金与TC11合金能够形成质量完好的焊接接头。     

Phase transformation in the β phase of super α2 Ti3Al base alloys during static annealing and superplastic deformation at 700–1000 °C

Ti₃Al based alloys have admirable superplasticity with elongation greater than 1000% at temperatures between 950 and 1000 °C. In this study, the phase evolution and mechanical behavior of a super α₂ alloy Ti–25%Al–10%Nb–2%V–1%Mo (in at.%) at lower temperatures of 700–900 °C are further examined, and the reasons for the reduced elongations are examined. The initial grain size was around 2 μm. Although the super α₂ alloy exhibited superior superplastic elongations of 1500% at 960 °C, the elongation dropped to 600% at 900 °C, 330% at 850 °C and 140% at 750 °C. The β→α₂^′ transformation occurred in accordance with the equilibrium phase diagram during static annealing, but the transformation was enhanced during dynamic straining at temperatures from 750 to 900 °C. The β→α₂^′ transformation in the two-phase region is shown to be diffusion controlled, and not a massive or martensitic transformation. With the fine acicular α₂^′ plates inside the β grains, the grain boundary sliding accommodation process across the BCC β grains was impeded, leading to lower tensile elongations.     

热轧态TC4合金不同热处理后的组织变化及硬度

通过热膨胀法测定TC4合金的(α+β)/β相变温度,研究不同温度、不同冷却方式下的热处理工艺对热轧态TC4合金的显微组织及硬度的影响。结果表明,(α+β)/β相变温度范围在970～990℃之间;(α+β)两相区温度范围内退火,随着温度的升高,α→β的转变程度增大,得到由等轴α和转变态β构成的双态组织;相变点以上温度退火,得到明显的魏氏组织;高温退火、冷速过快时,得到马氏体组织;高温退火对合金硬度的影响较大。     

Experimental Study of Phase Equilibria in the System Cu-Al-Sn

Phase equilibria in the Cu-rich corner of the ternary system Cu-Al-Sn have been re-investigated. Final equilibrium microstructures of 20 ternary alloy compositions near Cu₃Al were used to refine the ternary phase diagram. The microstructures were characterized using optical microscopy (OM), x-ray diffraction (XRD), electron probe microanalysis and transmission electron microscopy. Isothermal sections at 853, 845, 833, 818, 808, 803 and 773 K have been composed. Vertical sections have been drawn at 2 and 3 at% Sn, showing β₁ as a stable phase. Three-phase fields (α + β + β₁) and (β + β₁ + γ₁) result from β → α + β₁ eutectoid and β + γ₁ → β₁ peritectoid reactions forming metastable β₁ in the binary Cu-Al. With the lowering of temperature from 853 to 818 K, these three-phase fields are shifted to lower Sn concentrations, with simultaneous shrinkage and shifting of (β + β₁) two-phase field. The three-phase field (α + β + γ₁) resulting from the binary reaction β → α + γ₁ shifts to higher Sn contents, with associated shrinkage of the β field, with decreasing temperature. With further reduction of temperature, a new ternary invariant reaction β + β₁ → α + γ₁ is observed at ~813 K. The β disappears completely at 803 K, giving rise to the three-phase field (α + β₁ + γ₁). Some general guidelines on the role of ternary additions (M) on the stability of the ordered β₁ phase are obtained by comparing the results of this study with data in the literature on other systems in the systems group Cu-Al-M.     

Zn—Al合金先共晶相的形核与生长特性

采用定向凝固－液淬技术和金相定量分析,研究了铝的质量分数为3．0％－28．4％和Zn-Al二元合金先共晶固溶体的析出规律。结果表明,共晶富锌相和先共晶富铝相都具有瞬时形核特点,但生长规律不同。先共晶富锌η相在形核后生长速度较快,随后逐渐减慢;共共晶富铝β相或α相在形核后生长较慢,达到一定过冷度后迅速长大。β相除了通过包晶反应L＋α→β依附于α相析出外,还从剩余液相中直接形核。     

Elasticity-based model of the variant selection observed in the β to α phase transformation of a Zircalloy-4 sample

The β→α texture inheritance of a Zircalloy-4 sample has been investigated after an α→β→α transformation cycle. The final inherited α texture has been determined from a crystal orientation map determined by electron back-scattering diffraction, whereas the texture of the high temperature β phase has been reconstructed by a method analysing the orientations and misorientations of α variants. The comparison of the α texture calculated from the parent β texture without variant selection with the experimental sharp α texture shows differences due to a strong variant selection mechanism occurring during the phase transformation at cooling.A model of a variant selection mechanism based on the elastic anisotropy of the parent β phase leads to a simulated inherited α texture with the main characteristics of the experimental texture.     

低Al含量Ti-Al合金相变行为的分子动力学模拟

利用分子动力学模拟研究了Ti-5Al和Ti-10Al两种合金的β→α相变过程。比较分析了不同Al含量下相变过程的体系内能、径向分布函数、不同晶体结构相对含量的变化以及晶体结构的演化。结果表明:Ti-10Al较Ti-5Al更快发生α相形核析出,体现出实际相变过程中Al作为α相稳定元素的作用;β→α的结构转变通过{110}β面原子层间的相互滑移发生,并伴随一定畸变,新相与母相间晶体学关系符合{0001}α//{110}β;新相中易形成层错、孪晶等晶体缺陷,以消除部分相变过程中畸变引起的应力。     

MICROSTRUCTURE AND PROPERTIES OF Mn-CONTAINING TiAI ALLOYS

The microstructure of Ti-33wt-％ Al-(1.6-4.5)wt-％ Mn alloys are composed ofγ+α_2 phases,but the volume of x_2-phase is a little.Mn alloying decreases the latticeparameters a and c of γ-phase and the c/a ratio becomes nearly to one,because the atomicradius of Mn is less than the atomic radius of Ti or Al.Mn alloying promotes the formation oftwins in γ-phase and increases the ability of plastic deformation of TiAl alloys at room tem-perature.     

THEEFFECTS OF SOLUTION TREATMENT ON NEAR ALPHA HIGH TEMPERATURE TITANIUM ALLOYS

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Microsegregation in high Nb containing TiAl alloy ingots beyond laboratory scale

Microsegregation in big ingots of Ti–45Al–(8–9)Nb–(W, B, Y) alloy had been studied. The composition and microstructural morphology of the large ingot exhibited significant microinhomogeneity. Three types of microsegregation were observed in as-cast microstructure of the large ingot. First is the solidification segregation (S-segregation) at interdendritic area, in which the composition is characterized by higher Al, B (boride), and Y (oxide) contents and lower Nb and W contents. Second is the β-segregation at the boundary and triple junctions among α grain due to the phase transformation of β → α. The composition at the segregation area is characterized by higher Nb and W additions that lead to the formation of β particles and γ phase. Third is the α-segregation that forms local lamellar structure composed of β, γ and α plates due to phase transformation of α → α₂ + β + γ. The microsegregation for the PAM ingot is lower than that for SM ingot in terms of the volume fraction of β phase. The reason is that the PAM melting can offer better control of pouring temperature and rather fast cooling rate by water-cooled copper crucible.     

Strain-induced α-to-β phase transformation during hot compression in Ti−5Al−5Mo−5V−1Cr−1Fe alloy

The dynamic phase transformation of Ti?5Al?5Mo?5V?1Cr?1Fe alloy during hot compression below the β transus temperature was investigated. Strain-induced α-to-β transformation is observed in the samples compressed at 0?100 K below the β transus temperature. The deformation stored energy by compression provides a significant driving force for the α-to-β phase transformation. The re-distribution of the solute elements induced by defects during deformation promotes the occurrence of dynamic transformation. Orientation dependence for the α-to-β phase transformation promotion is observed between {100}-orientated grains and {111}-orientated grains. Incomplete recovery in {111}-orientated grains would create a large amount of diffusion channels, which is in favor of the α-to-β transformation. The effects of reduction ratio and strain rate on the dynamic phase transformation were also investigated.     

Effects of Ni Content and Solution-Aging Treatment on Multi-Stage Transformations of TiNi Shape Memory Alloys

Effects of Ni content and solution-aging treatment on transformation type and transformation temperature of Ti100-xNix (x=33.3-75) alloys were investigated by differential scanning calorimeter (DSC). The results show that one-stage transformation B2→B19′ of the solution-quenched Ti100-xNix alloys occurs. The martensitic transformation temperature TM is constant at x = 40-49, decreases sharply at x = 49-52, increases gradually at x = 52-56, and is constant again at x = 56-70. The alloys after aging at 773 K for 3.6 ks and 36 ks will have the occurrence of one-stage transformation B2→B19′ at x = 40-50.5, and the TM is constant first and then decreases suddenly with increasing of x; when x = 50.5-52, the alloys have the occurrence of two-stage transformation B2→R→B19′ and the TM decreases gradually with increasing of x; when x=53-70, the alloys have the occurrence of three-stage transformation and the TM1 and TM2 all increase first and then keep constant with increasing of x. All alloys after aging at 773 K for 360 ks have the occurrence of one-stage transformation B2→B19′ except for Ti49Ni51 and Ti47Ni53 alloy, which will have the occurrence of two-stage transformation B2→R→B19′ and three-stage transformation, respectively     

Structure of Quenched Ti–Ru Alloys

Quenched titanium–ruthenium alloys containing 0.25–4 at % ruthenium have been studied using X-ray diffraction analysis, optical metallography, transmission electron microscopy, and microhardness measurements. It has been found that, during the quenching of the alloys containing 0.25, 0.5, and 1 at % ruthenium, a polymorphic β → α transformation occurs with the formation of a two-phase (α + β) structure. In Ti–1.5 at % Ru and Ti–2 at % Ru alloys, a martensitic β → α″ transformation occurs. The quenched Ti–3 at % Ru alloy has a β + ω structure. The complete stabilization of the β phase takes place in the alloy with 4 at % ruthenium. In the electron-diffraction patterns of alloy containing 4 at % ruthenium, diffuse scattering that indicates the formation of ω-phase-related displacements in the locations of atoms has been observed.     

Quasi-peritectic solidification reactions in 6xxx series wrought Al alloys

Secondary intermetallic phase formation during directional solidification of two 6xxx series wrought Al alloys at low growth velocities of 5–30 mm/min has been investigated using differential scanning calorimetry, transmission electron microscopy and scanning transmission electron microscopy. Thermodynamic calculations predict that a quasi-peritectic reaction, L+Al₁₃Fe₄ → α-Al+α-AlFeSi, should occur during equilibrium solidification of the alloys. However, no composite Al₁₃Fe₄/α-AlFeSi particles, but composite Al₁₃Fe₄/β-AlFeSi particles and triple phase junctions have been observed for the first time, indicating a divorced metastable β-AlFeSi quasi-peritectic reaction, L+Al₁₃Fe₄ → α-Al+β-AlFeSi. More detailed analysis suggests that the metastable β-AlFeSi quasi-peritectic reaction is more favourable both at nucleation and during growth. No unique orientation relationship was found between primary Al₁₃Fe₄ and peritectic β-AlFeSi. The nucleation and growth of peritectic phases and the morphology evolution of the two intermetallic phases, Al₁₃Fe₄ and β-AlFeSi, are discussed.     

TC21合金时效过程中的相变研究

采用XRD、SEM、TEM及显微硬度测试等手段,系统研究了TC21合金固溶处理后的相变以及合金在550～850℃时а"相在时效过程中的分解机制及组织演变规律,结果表明:1000℃固溶30 min淬火后,TC21合金形成а"马氏体,且合金中存在少量β及O相(Ti2AlNb);随时效温度的升高,а"相逐步发生а"→а+а"_高→а+β_(亚稳)→а+β,а"+а'+β_(亚稳)→а+β,а"→а+β等分解过程;TC21合金的显微硬度依赖于时效温度和时效时间,时效时间延长,合金显微硬度先迅速增大,达到最大值后再逐渐减小.时效温度升高时,合金显微硬度达到最大值的时间缩短,且合金最终的显微硬度随时效温度的升高而降低.     

TC21两相钛合金中斜方马氏体的时效分解

研究了TC21两相钛合金淬火后马氏体在时效过程中的组织结构变化及其引起的强化效应。结果表明:合金淬火后得到交错排列的针状斜方马氏体组织,在300-700℃之间时效4h,α″相的分解次序遵循α″→α″+α→α+β规律。低温时效时首先形核析出针状的初生α相,随着时效温度的升高,初生α相在长大的同时其片层之间析出十几纳米宽、几个微米长的细小次生α相,且β相呈10～50nm大小的颗粒状弥散分布在α相之间,随后的时效过程中次生α相和β相迅速长大,最终斜方马氏体完全分解为α+β混合相。显微硬度分析表明,利用斜方马氏体的逆转变,通过在时效过程中均匀地析出细小的次生α相和纳米级弥散分布的β相可使合金具有明显的时效强化效果,500℃时效4h后,TC21合金的显微硬度比淬火态提高了35%。