包晶TiAl合金非平衡凝固特征及其组织演化

发布时间：2018-05-03 08:12

本文选题：包晶TiAl基合金 + 深过冷凝固　；参考：《西北工业大学》2016年博士论文

【摘要】：TiAl基合金是重要的包晶合金之一,其凝固组织对Al含量具有高度的敏感性,特别是较窄Al含量的包晶TiAl基合金。对于以铸造为首选工艺的TiAl基合金而言,Al含量的变化严重影响了铸锭和铸件的凝固组织特征。通常铸造属于近平衡凝固的范畴,薄壁铸件或铸锭的边缘由于液相到固相的相变过程进行的非常快,偏离了近平衡凝固,更接近于非平衡凝固。目前,对于包晶TiAl基合金非平衡凝固的研究工作主要集中于二元和三元以及少数多元TiAl基合金,而对高Al含量的多元包晶TiAl基合金非平衡凝固过程中的相选择和组织演化规律的研究相对缺乏和有限。基于此,本文系统地研究了多元Ti-xAl-2Cr-2Nb(x=46,47,48,49,50,51,52)(at.%)合金在近平衡凝固条件下初生β/α相凝固的Al含量临界值、相组成以及凝固组织演化规律;并选定工业化应用的Ti-48Al-2Cr-2Nb(at.%)合金和Al含量临界值(初生β/α相凝固)附近的Ti-50Al-2Cr-2Nb(at.%)合金为研究对象,研究了过冷度和冷却速率对这两种合金非平衡凝固过程中的相选择和组织演化规律的影响,探究了深过冷凝固过程中亚结构和魏氏组织、羽毛组织以及块状组织等亚稳组织形成和影响因素,探讨了过冷和急冷耦合条件下,亚稳相的析出和长大,获得了以下主要研究结果。对Ti-xAl-2Cr-2Nb(x=46,47,48,49,50,51,52)(at.%)合金近平衡凝固和非平衡凝固组织研究的结果表明:在近平衡凝固条件下,当Al含量(CAl)≤50at.%时,合金以β相为初生相进行包晶凝固,相组成主要为α_2相和γ相以及少量的B2相;当CAl50at.%时,合金以α相为初生相进行凝固,相组成主要为α_2和γ两相。初生β/α相凝固的Al含量临界值为50at.%;随着CAl的增加,γ相的正方畸变度(c/a)随之增加,α_2相的c/a值呈下降趋势;同时随着CAl的增加,初生相的结晶温度范围先减小后增大,一次枝晶间距随之变化。当CAl=48at.%时,前期为初生β相形核且未充分长大,后期为包晶α相以β相为核心形核长大的过包晶逐层凝固。棒状试样的凝固组织明显细化,二次枝晶臂生长受到抑制,一定程度也抑制了α→(α_2+γ)固态相变,并未改变初生β/α相凝固的Al含量临界值。采用电磁悬浮熔炼的深过冷凝固技术实现了Ti-48Al-2Cr-2Nb(at.%)合金和Ti-50Al-2Cr-2Nb(at.%)合金的深过冷,获得的最大过冷度分别为370K和290K。对于前者,在所获得的过冷度范围内,β相均能作为初生相优先形核,这表明该合金凝固组织具有较高的稳定性;而对于后者,在较小过冷度下(ΔT120K),β相均能作为初生相优先形核。一旦超过临界过冷度ΔT*=120K,亚稳α相取代初生β相成为领先相。这表明Al含量临界值(初生β/α相凝固)附近的合金更容易发生凝固模式的改变。此外,运用经典形核理论分析了Ti-48Al-2Cr-2Nb(at.%)合金中初生相和亚稳相的形核与过冷度的关系,合理地解释了各相的竞争形核。运用BCT模型分析了各相过冷度的分布;结合实验结果,较好地阐述了过冷凝固组织的演化过程。在较小过冷度下,由于已凝固枝晶重熔,导致晶粒细化,而在大过冷度下出现二次细化现象是由枝晶碎断造成的,与过冷凝固过程中的内应力密切相关。在非平衡凝固组织中观察到了丰富的亚结构,主要由变形孪晶、高密度位错以及少量层错组成,这表明过冷凝固过程中存在巨大的内应力。随着过冷度增大,微观应变呈增大趋势。TiAl合金的深过冷凝固过程相当于塑性变形过程,其内应力释放机制为孪生剪切变形,层片结构中的γ相发生了塑性变形,形成大量的γ/γ孪晶(P型孪晶)和γ相层片内部的孪晶(Q型孪晶),甚至出现了孪晶交割。在中等过冷度条件下,观察到有魏氏组织(γ_w)的形成;在大过冷度条件下,能够观察到由羽毛组织(γ_f)、块状组织(γ_m)与片层耦合的组织形貌。γ_w和γ_f以及γ_m不是从过冷液相中直接凝固形成而是通过一系列固态相变形成。同时,高密度位错与层错以及固态相变的过冷都为其在低冷却速率下的形核提供了足够的驱动力,它们的形成与片层团尺寸、过冷度、合金成分以及冷却速率密切相关。采用电磁悬浮和铜模急冷铸造相结合的方法制备了Ti-48Al-2Cr-2Nb(at.%)合金和Ti-50Al-2Cr-2Nb(at.%)合金急冷快速凝固锥形试样,通过ANSYS软件模拟估算出两种合金的冷却速率分别在2.9×103 K/s至2.6×104 K/s和3.6×103 K/s至2.6×104 K/s之间。通过线性拟合获得一次枝晶间距λ_1和冷却速率T的函数关系分别为:λ_1=323.40(T|-)~(-0.32)和λ_1=2.70×10~3(T|-)~(-0.53)。对于前者,随着冷却速率的增加至2.1×104 K/s时,包晶转变和共析转变受到抑制;随着冷却速率的进一步增加至2.6×10~4 K/s时,出现了块状γ相,强急冷诱发大的起始过冷度和高密度位错有利于块状γ相的形成;对于后者,如果冷却速率一旦超过临界值(4.0×10~3 K/s),能够获得单一的包晶α相;在低的冷却速率下(4.0×10~3 K/s),仅能在糊状区观察到初生β相的形核;在高的冷却速率下(4.0×10~3 K/s),β相形核和包晶反应受到抑制而亚稳α相从非平衡液相中直接形核并长大,最终获得细化均匀的胞状α相凝固组织。实验结果和动力学分析表明:冷却速率和Al含量的变化能够引起初生β相和包晶α相的相选择,可以获得预期的相组成和显微组织,包晶α相的直接形核和长大受高冷却速率下溶质富集和形核过冷的控制;深过冷凝固和急冷凝固的耦合作用,有利于亚稳α相的形核与长大。
[Abstract]:TiAl based alloy is one of the most important peritectic alloys. Its solidification structure is highly sensitive to the content of Al, especially the peritectic TiAl based alloy with narrow Al content. For the TiAl based alloy with the preferred casting process, the change of Al content seriously affects the solidification structure of the ingot and the casting. In the category, the edge of the thin-walled castings or ingot is very fast due to the phase to solid phase transformation process, which deviates from the near equilibrium solidification and is closer to the non equilibrium solidification. At present, the research work on the non equilibrium solidification of the peritectic TiAl based alloys is mainly concentrated on two yuan and three yuan and a few multivariate TiAl based alloys, and the multiple packages of high Al content The study of phase selection and microstructure evolution of amorphous TiAl based alloy is relatively deficient and limited. Based on this, the critical value, phase composition and solidification structure of Al content of primary beta / alpha phase solidification of multiple Ti-xAl-2Cr-2Nb (x=46,47,48,49,50,51,52) (at.%) alloys under near equilibrium solidification conditions are systematically studied. The Ti-48Al-2Cr-2Nb (at.%) alloy and the Ti-50Al-2Cr-2Nb (at.%) alloy near the Al content (primary beta / alpha phase solidification) were selected as the research object. The effects of the supercooling and cooling rate on the phase selection and the microstructure evolution of the two alloys during non equilibrium solidification were studied. The formation and influence factors of the metastable tissues such as the Cheng Zhongya structure and the wechis structure, the feather tissue and the massive tissue are discussed. The precipitation and growth of the metastable phase are discussed under the conditions of supercooling and quench cooling. The following main research results are obtained. Study on the near equilibrium solidification and non equilibrium solidification structure of Ti-xAl-2Cr-2Nb (x=46,47,48,49,50,51,52) (at.%) alloy The results show that, under the condition of near equilibrium solidification, when the content of Al (CAl) is less than 50at.%, the alloy uses beta phase as the primary phase to solidify the peritectic phase. The phase composition is mainly alpha _2 phase and gamma phase and a small amount of B2 phase. When CAl50at.%, the alloy solidified with alpha phase as primary phase, and the phase composition is mainly alpha _2 and gamma phase. The critical Al content of primary beta / alpha phase solidification is critical. The value of the value is 50at.%, with the increase of CAl, the square aberration (c/a) of the gamma phase increases and the c/a value of the alpha _2 phase decreases. At the same time, with the increase of CAl, the crystallization temperature range of the primary phase decreases first and then increases, and then the primary dendrite spacing changes. When CAl=48at.%, the initial phase of the primary beta phase nucleus is not fully grown, and the later phase of the peritectic alpha phase is beta. The peritectic crystals of the core nucleation are solidified by layer by layer. The solidification structure of the bar like specimen is obviously refined, the growth of the two dendrite arm is restrained, the solid phase transformation of alpha to (alpha _2+) is suppressed to a certain extent, and the critical value of the Al content of the primary beta / alpha phase solidification is not changed. Ti-48Al-2Cr-2Nb is realized by the deep supercooling solidification technology of electromagnetic suspension melting. (at.%) the maximum supercooling degree of alloy and Ti-50Al-2Cr-2Nb (at.%) alloy is 370K and 290K. for the former. In the overcooling range, the beta phase can be used as the primary phase of the primary phase, which indicates that the solidification structure of the alloy has high stability, and for the latter, the beta phase is equal to the lower supercooling degree (delta T120K). As a primary nucleation, once the critical subcooling degree of T*=120K is exceeded, the metastable alpha phase is replaced by the primary beta phase as the leading phase. This indicates that the alloy near the critical value of the Al content (primary beta / alpha phase solidification) is more likely to change the solidification mode. In addition, the classical nucleation theory is used to analyze the primary phase and metastable phase in the Ti-48Al-2Cr-2Nb (at.%) alloy. The relationship between the nucleation of the phase and the supercooling degree is explained reasonably. The distribution of the supercooling degree of each phase is analyzed by the BCT model, and the evolution process of the supercooling solidification structure is explained well by the experimental results. The grain refinement is caused by the remelting of the solidified dendrite under the small supercooling, and two times under the large supercooling degree. The phenomenon is caused by the breakage of dendrite, which is closely related to the internal stress in the process of supercooling solidification. A rich substructure is observed in the non-equilibrium solidification structure, mainly composed of deformation twins, high density dislocation and a small amount of stacking faults. This indicates that there is a huge internal stress in the process of supercooling solidification. With the increase of supercooling, the microstrain shows the microstrain. The deep supercooling solidification process of the increasing trend of.TiAl alloy is equivalent to the plastic deformation process. The internal stress release mechanism is twin shear deformation. The plastic deformation occurs in the gamma phase in the layer structure, forming a large number of gamma / gamma twins (P twins) and the twins in the gamma phase layer (Q type twins), even the twin intertwinning. Below, the formation of Wechsler tissue (gamma _w) is observed; under the condition of excessive cooling, the microstructure of the coupling between the feather tissue (gamma _f) and the massive tissue (gamma _m) can be observed. Gamma and gamma _f and gamma _f and gamma _m are not directly formed from the supercooled liquid phase but are formed by a series of solid phase transitions. At the same time, high density dislocation and stacking fault and solid state are formed. The supercooling of the state phase transition provides sufficient driving force for its nucleation at low cooling rate. Their formation is closely related to lamellar size, supercooling, alloy composition and cooling rate. The Ti-48Al-2Cr-2Nb (at.%) alloy and Ti-50Al-2Cr-2Nb (at.%) alloy are prepared by the combination of electromagnetic suspension and copper die cold casting. By ANSYS software simulation, the cooling rates of two kinds of alloys are estimated from 2.9 x 103 K/s to 2.6 x 104 K/s and 3.6 x 103 K/s to 2.6 x 104 K/s, respectively. 53). For the former, with the increase of cooling rate to 2.1 x 104 K/s, the peritectic transformation and eutectoid transition are suppressed. As the cooling rate increases to 2.6 * 10~4 K/s, massive gamma phase appears. Strong chilling induced large initial undercooling and high density dislocation are beneficial to the formation of massive gamma phase; for the latter, if the cooling rate is one. A single peritectic alpha phase can be obtained at the above critical value (4 x 10~3 K/s); at a low cooling rate (4 x 10~3 K/s), only the nucleation of primary beta phase can be observed in the paste region; at a high cooling rate (4 x 10~3 K/s), the beta phase nucleus and the peritectic reaction are suppressed and the metastable alpha phase is nucleated directly from the nonequilibrium liquid phase and eventually obtained. The experimental results and kinetic analysis show that the change of cooling rate and Al content can lead to the phase selection of primary beta phase and peritectic alpha phase, and the expected phase composition and microstructure can be obtained. The direct nucleation of the peritectic alpha phase and the control of solute enrichment and nucleation supercooling under high cooling rate can be obtained. The coupling effect of deep undercooling and rapid solidification is beneficial to nucleation and growth of metastable alpha phase.

【学位授予单位】：西北工业大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG244.3

【相似文献】