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.

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.