多元TiAl合金定向全片层组织生长稳定性及热稳定性的机理与控制

TiAl alloy with aligned lamellar microstructure possesses excellent mechanical properties. In particular, the shortcoming of low room temperature ductility has been completely overcomed. Its room temperature elongation reaches to 4~10%, which breaks through the important obstacle of the application. However, the size of the ingot should be large in the actual application. Furthermore, how to solve the growth instability of alpha phase, which could form aligned lamellar microstructure during the process of the subsequent cooling, controlled by the alpha seed with specific crystal orientation and maintain the thermal stability of aligned lamellar microstructure are very critical to make the large size ingot be used in practice. The key scientific problems and targets in this project are shown as follows:. The variation characteristics of flow field at the solidification interface and its effects on the horizontal and longitudinal segregations of the solute field with different diameters of specimen are investigated. The mechanism of growth instability of the alpha phase controlled by quasi-alpha seed is studied and the method of regulating flow model is proposed to control solute field, expecting the stable and continue growth of alpha phase in the multicomponent alloy ingot with large size and aligned lamellar microstructure. The mechanisms of competitive growth of multiphase microstructure and selection growth of alpha phase are researched and then the reasonable controlling parameters are given to achieve a stable growth of alpha phase and obtain aligned lamellar microstructure in alloys with different compositions. In addition, the mechanism why the unstable disintegration of aligned lamellar microstructure occurs under the conditions of high temperature and high temperature loading should be made clear, then reasonable heat treatment principle and method are put forward to improve its thermal stability at high temperature.

TiAl合金定向全片层组织有很好的综合性能，彻底解决了室温塑性低的问题，室温延伸率4-10%，突破了该材料进入应用的重要障碍。但实际应用中需要大尺寸的棒锭，解决大尺寸中被特定晶向的a籽晶所引晶的a 相（在随后冷却中形成定向全片层组织）生长不稳定以及保持高温下定向全片层组织具有良好热稳定是进入应用的另一关键所在。本项目研究的关键科学问题及目标：研究随直径变化，凝固界面处流场变化特征及其对溶质场沿横向和纵向偏析的作用规律；探明被引晶的a相生长不稳定的机理，给出调控流动模式来控制溶质场的方案，在大尺寸多元合金铸锭中实现a相的稳定连续生长，获得定向全片层组织；探明准籽晶引晶过程中多相组织的竞争生长及a相超出的选择机理，给出合理的控制参数，在多种成分中实现a相的稳定生长，获得定向全片层组织；弄清定向全片层组织在高温及高温受力条件下不稳定蜕变机制，提出合理的热处理原理和方法，提高其在高温下的热稳定性.

电磁约束定向凝固技术，作为一种先进的定向凝固技术，特别适用于高熔点、易氧化、与模壳材料反应强烈的金属材料的无污染制备。与光悬浮区熔定向凝固技术比，其最大的特点在于约束熔体的力主要是电磁力而不是表面张力。通过合理调整电磁力的分布和大小，既能制备大尺寸圆棒试样，又能制备不同宽厚比的矩形试样。目前，利用该技术已经成功制备直径16~20mmTiAl合金定向全片层组织，但是关于直径30mmTiAl合金试样的制备则还没有深入展开。因此，本项目主要研究直径30mmTiAl合金试样的电磁约束定向凝固，针对其中籽晶制备、定向晶生长、过渡区溶质分布与相的选择、径向温度梯度、杂晶形成机理、初始引晶和定向段稳定生长等问题进行了系统研究：首先，采用实验与模拟结合的方法研究不同直径、不同抽拉速率、过渡区溶质分布、径向温度梯度、径向过冷度和合金化元素等关键参数对大尺寸TiAl合金电磁约束定向凝固过程的影响。建立了一套籽晶法电磁成形制备TiAl合金定向全片层组织中杂晶形成的过冷度判定模型，定量判定任何参数条件下定向凝固过程中凝固界面附近杂晶形成的倾向。其次，发现大尺寸TiAl合金径向温度梯度差异较大，固液界面呈多种复杂生长形态，从边缘到心部依次为枝-胞-枝生长形貌，杂晶易从心部、表面形成。本研究通过凝固理论和数值模拟分析杂晶形成机制，改进工艺参数，有效抑制了杂晶形成，实现了大尺寸TiAl合金成功引晶。建立了数值模拟-工艺参数-组织控制关系，成功实现初始引晶和近稳态区生长的稳定性，使得定向率大幅提高，定向全片层组织宽度达24mm，为TiAl合金定向全片层组织进入实际叶片应用提供了依据。最后，研究了低温长时和高温短时处理对定向全片层稳定性的影响，结果表明经过热处理后，定向全片层组织中部分比较薄的α2片层会局部分解，而γ片层增厚，且α2片层内部和α2/γ界面有弥散相析出，但无块状γ相形成，定向片层组织表现出良好的热稳定性。