Nb-Si基超高温合金的定向凝固研究进展

主要论述水冷铜坩埚内的Czochralski定向凝固、电子束定向凝固、光悬浮定向凝固、整体定向凝固和电磁冷坩埚定向凝固5种定向凝固的基本原理,优缺点以及研究定向凝固Nb-Si基超高温合金所取得的进展。现阶段,基本不用Czochralski定向凝固和电子束定向凝固研究Nb-Si基超高温合金了;到目前为止,光悬浮定向凝固是制备研究Nb-Si基超高温合金的主要手段;整体定向凝固制备的Nb-Si基超高温合金的断裂韧性已达20MPa·m1/2左右;电磁冷坩埚定向凝固制备的Nb-Si基超高温合金的高温拉伸强度已达200MPa(1250℃)。     

先进定向凝固技术

简要回顾了传统定向凝固技术及其存在的问题，概述了西北工业大学凝固技术国家重点实验室近年来在定向凝固技术领域所取得的重要成果，其中包括超高梯度定向凝固，电子束区熔定向凝固，深过冷定向凝固，电磁约束成形定向凝固，单晶连铸，经熔体热处理的定向凝固等先进定向凝固技术。提出了外场作用下定向凝固的新构想。     

定向凝固技术的研究现状及发展趋势

定向凝固技术可获得定向及单晶组织结构,大大改善材料的力学和物理性能。阐述了定向凝固理论及定向凝固技术的研究现状,定向凝固技术的发展过程就是温度梯度不断提高、冷却速率不断加快的过程。简述了几种新型定向凝固技术,并展望了定向凝固技术的未来发展趋势。     

单晶水平定向凝固法

水平定向凝固法是制备低位错单晶体的重要方法,论述了水平定向凝固法的优点和缺点,并报道了水平定向凝固法的特点,温度分布,以及水平定向凝固法的数字模拟的方面的工作.     

定向凝固技术的研究进展与应用

定向凝固技术是制备具有单一取向要求的凝固组织和高性能材料的重要方法,是研究凝固理论和新型材料的重要手段。在介绍定向凝固技术原理的基础上,评述了传统定向凝固技术的发展及存在的弊端,简述了几种新型定向凝固技术,以及它们在制备新材料中的应用。     

高温合金定向凝固技术研究进展

首先回顾了定向凝固的发展历史,重点分析了液态金属冷却定向凝固的技术特点。总结了高温度梯度下制备的定向凝固法单晶高温合金在组织和性能方面的研究现状,结合作者在本领域的研究,着重分析了定向凝固温度梯度、凝固速率、晶体取向、熔体超温处理、熔体对流控制对组织和性能的作用规律和机制,认为高温度梯度定向凝固是细化组织、减少缺陷、提高合金性能的重要途径。最后展望了高温合金定向凝固的发展趋势。     

新型真空熔化定向凝固装置原理研究

对真空感应熔炼与双区加热结合在一起的定向凝固装置进行了可行性研究，并研制出了一套新型真空熔化定向凝固装置，该装置兼有熔炼和定向凝固的功能，熔炼好的合金液能自动底注到定向炉内的型壳中，定向凝固部分采用了先进了双区加热技术，定向凝固结晶生长速率能在很大范围内调节。     

定向凝固TiAl合金成分−组织−性能研究进展

TiAl合金由于其低密度和高比强度,在航空材料中展现出良好的应用前景。通过定向凝固控制TiAl合金晶体取向,有助于大幅提升合金高温性能和服役温度,促进TiAl合金在新一代航空发动机上的应用。综述了近年来TiAl合金定向凝固的研究方法和成分?组织?性能关系的研究进展,总结了国内外定向凝固TiAl合金的主要研究单位及研究主题,简要介绍了定向凝固方法与模壳材料的应用情况。从合金成分角度,分析并总结了α、β相稳定元素和其他常见元素对定向凝固组织和性能的作用;从力学性能角度,介绍了定向凝固高Nb?TiAl合金在高温拉伸、蠕变、高周疲劳性能上的优势及相关机理;从定向凝固工艺角度,归纳了生长速率和温度梯度对合金凝固路径、片层取向及宏观、微观偏析的影响。展望了定向凝固TiAl合金的未来发展方向。     

定向凝固超细柱晶组织及其形成条件

本文通过对凝固材料的组织、性能与凝固参数的依存关系的分析,提出了定向凝固超细柱晶组织的概念,并利用最新研制的ZMLMC超高梯度定向凝固装置获得了Ni-5(wt)％Cu合金和钴基高温合金K10的定向凝固超细柱晶组织。     

TiAl基合金冷坩埚定向凝固研究现状与发展趋势

冷坩埚定向凝固技术基于材料电磁加工原理,将合金连续熔化、电磁约束成形和连续凝固过程统一,避免模壳法定向凝固造成的合金熔体污染,是TiAl等高活性合金高纯净定向凝固的重要方法之一。报告了TiAl基合金冷坩埚定向凝固技术的研究结果,建立了电磁场、温度场与电磁冷坩埚系统主要参数之间的规律性关系。通过改变冷坩埚的横截面形状,初步制备出具有定向凝固组织的TiAl基合金圆、方和扁锭,研究结果表明该方法不但适合于Ti-Al二元合金,而且施与多元多相TiAl基合金也是可行的。通过对成分和组织的控制,所研究的TiAl基合金定向凝固试样的室温拉伸塑性最高达到1.5%左右。最后指出将该方法应用于耐高温的轻质高Nb含量的γ-TiAlNb合金的定向凝固是未来的发展趋势。     

非稳态过程与凝固界面形态选择

以透明模型合金丁二腈－乙醇系为实验系统，直接观察其单相凝固过程中界面形态的非稳态演化过程，从三个方面：①平胞转变②胞枝转变③枝晶一次间距，系统地考察了凝固界面形态选择的时间相关性和历史相关性。结果表明：①导出了凝固初始过渡区溶质再分配的一个新公式；②提出了确定平界面稳定性临界条件的可测判据和实验方法；③从实验上和理论上解释了胞枝转变行为；④揭示了枝晶一次间距的历史相关性；⑤发现凝固形态选择的历史相     

强化换热对CdZnTe晶体生长过程的影响

为了优化CdZnTe晶体生长过程的工艺参数,利用数值模拟方法研究了强化换热对晶体生长过程固液界面凹陷、溶质组分偏析的影响.结果表明:当坩埚轴向散热强度大幅度增加时,固液界面前沿的对流显著增强;随着凝固过程的进行,固液界面凹陷深度先是显著减小,随后显著增加;晶体起始段溶质组分的径向偏析明显减小,溶质组分轴向等浓度区增长.当坩埚侧面径向散热强度增加时,固液界面前沿的对流和界面凹陷深度先是有所减弱,随后又有较大增加.当坩埚内壁碳膜厚度增加时,界面前沿的对流强度显著减弱,而固液界面凹陷深度明显增加.径向散热和碳膜厚度的增加皆不能明显影响晶体内溶质组分分布.     

Numerical model for nucleation of peritectic alloy during unidirectional solidification

Based on transient nucleation theory, a numerical model has been constructed to describe the nucleation process of a new phase in front of the liquid–solid interface of a prior steady-growth phase in peritectic alloy with the combination of the concentration field calculated by a self-consistent numerical model for cellular/dendritic growth. The results show that the nucleation incubation time of a new phase varies with the solidification rate during unidirectional solidification. During unidirectional solidification of the Zn–4.0 wt.% Cu alloy, the incubation time changes very slightly when the solidification rate increases from 50 to 500 μm/s, but it increases significantly when the solidification rate exceeds 500 μm/s. The calculated results show a reasonable agreement with the experimental ones. This model reveals that nucleation of a new phase is time-dependent and reasonably explains the effect of the solidification velocity on the behaviors of nucleation and growth of ɛ dendrites in the matrix of the η phase in unidirectional solidification of Zn rich Zn–Cu alloys.     

Effect of magnesium content on thermal and structural parameters of Al–Mg alloys directionally solidified

In this study, the influence of magnesium content on thermal and structural parameters during the unsteady-state unidirectional solidification of Al–Mg alloys is analyzed. Using a special device, Al–Mg alloys containing 5, 10, and 15 wt% Mg were submitted to unidirectional solidification. Using a data acquisition system, the temperature variations along the casting during solidification were measured. From these results, the variations of solidification parameters as growth rate of dendrite tips, thermal gradient, cooling rate, and local solidification time were determined. The variation of global heat transfer coefficient at metal/mould interface was estimated through the adjustment of experimental temperature variation close to the interface and numerical predictions. Primary and secondary dendrite arms spacing variations during solidification were measured by optical microscopy. From these results, comparative analysis were developed to determine the influence of magnesium content.     

Microstructure development in the directionally solidified Al–4.0 wt% Cu alloy system

The effect of convection on microstructure formation is examined experimentally and theoretically for the vertically upwards-directional solidification of Al–4.0 wt% Cu alloys. In this alloy system, the rejected solute is heavier than the solvent so that fluid flow occurs due to the presence of radial gradients in temperature and composition. A numerical model is presented which shows that convection effects cause the composition to vary along the interface such that the composition increases from the center to the periphery of the sample. This composition variation causes the macroscopic interface to become convex, and give rise to a systematic variation in microstructure along the interface. Critical experiments have been carried out to examine planar to cellular (and cellular to dendritic) transition in a given sample due to the increase in concentration along the interface, and the experimental results are analyzed through the measurements of interface composition and thermal gradient. In addition, the variation in local primary spacing with interface composition is also characterized and compared with the results of the numerical model. It is shown that microstructure transitions and microstructural scales can be correlated quantitatively with the theoretical results based on interface composition and on temperature and solute gradients at the interface.     

Microstructure development in the directionally solidified Al–4.0 wt% Cu alloy system

The effect of convection on microstructure formation is examined experimentally and theoretically for the vertically upwards-directional solidification of Al–4.0 wt% Cu alloys. In this alloy system, the rejected solute is heavier than the solvent so that fluid flow occurs due to the presence of radial gradients in temperature and composition. A numerical model is presented which shows that convection effects cause the composition to vary along the interface such that the composition increases from the center to the periphery of the sample. This composition variation causes the macroscopic interface to become convex, and give rise to a systematic variation in microstructure along the interface. Critical experiments have been carried out to examine planar to cellular (and cellular to dendritic) transition in a given sample due to the increase in concentration along the interface, and the experimental results are analyzed through the measurements of interface composition and thermal gradient. In addition, the variation in local primary spacing with interface composition is also characterized and compared with the results of the numerical model. It is shown that microstructure transitions and microstructural scales can be correlated quantitatively with the theoretical results based on interface composition and on temperature and solute gradients at the interface.     

Solute distribution at the solid-liquid interface during steady-state peritectic solidification

The solute distributions at the solid-liquid interface during steady-state directional solidification of two contrasting peritectic systems were examined in detail by electron probe microanalysis. In the Al-Al₃Ti system the peritectic transformation plays a negligible part in the solidification reaction whereas in the Cd-Cd₃Ag system the peritectic transformation dominates. Knowing this, the solid-liquid interface profile during solidification can be predicted with reasonable accuracy on the assumption of equilibrium at all points on the solid-liquid interface. In the Cd-Cd₃Ag system the composition of the peritectic product is rapidly equilibrated by solid-state diffusion. A diffusion coefficient is estimated.     

Solidification Microstructures of a Single-crystal Superalloy under Ultra-high Temperature Gradient Conditions

The solidification microstructures and solute segregation of a newly developed hot corrosionresistant single-crystal Ni-base superalloy were investigated with a zone-melting and ultra-highthermal gradient unidirectional solidification apparatus.Compared with the microstructures solidifiedat conventional low thermal gradient conditions,the dendrite arm spacings,the interdendriticmicroporosity and γ/γ' eutectic,and the severity of solute segregation of the single-crystalsuperalloy solidified at ultra-high thermal gradient conditions were considerably reduced.It wasshown that the microstructure solidified under ultra-high thermal gradient condition is ideal for thefull exploitation of the excellent property potentials of single-crystal superalloys.     

Numerical model for microsegregation in ductile iron

Abstract

A numerical, non-steady state microsolute redistribution model is presented for ductile iron. The model takes into account solute diffusion in the solid and liquid phases, interface movement, a non-linear growth rate for the austenite phase and total solute conservation in the microvolume sphere. Preliminary calculations show that interface movement can be ignored and a linear austenite growth rate can be used for solidification conditions occurring during directional solidification experiments and keel block solidification. The numerical calculations of the solute distribution in the liquid and solid phases show reasonable agreement with the available experimental measurements.     

Simulation of dendrite growth of Fe–C alloy using phase field method

采用相场法模拟了Fe--0.5%C合金等温凝固过程中单个枝晶和多个枝晶的生长, 研究了过冷度、各向异性、界面厚度、晶体取向以及扰动对枝晶形貌的影响, 获得了具有二次分枝的枝晶形貌, 再现了枝晶生长过程及枝晶臂之间的竞争生长. 模拟结果表明: 凝固过程中存在溶质富集和枝晶偏析, 枝晶主干溶质浓度最低, 枝晶臂之间的液相浓度最高. 随着过冷度的增大, 枝晶生长加快且分枝发达; 界面厚度直接影响枝晶的生长速度; 各向异性影响枝晶的形态; 晶体取向与坐标轴方向一致时枝晶优先生长;扰动的加入导致枝晶分枝的形成.