Development of fundamental processes of directional solidification of TiAl-base Alloy

TiAl基合金片层组织的力学性能相对于片层取向具有极大的各向异性。然而，如果片层组织能够平行于生长方向排列，所得到的材料应该具有良好的力学性能组合。开发了晶种和定向凝固(DS)技术，以使片层边界平行于生长方向。利用这些技术可以控制多种TiAl基合金的片层结构。这些合金的DS铸锭具有良好的室温拉伸性能和高温强度，特别是蠕变强度。为了更大规模地生产TiAl铸件，人们试图通过播种和定向凝固来提高DS铸锭的尺寸和生长速度。为了了解DS钢锭在多组分应力状态下的力学行为，在垂直于加载轴方向施加约束的情况下，对不同片层取向的DS钢锭进行了压缩试验。研究发现，这种约束显著增加了具有中间片层取向的DS钢锭的屈服应力。对约束条件下的滑移系统进行了分析，结果与基于各向异性弹性连续体描述的变形分析结果吻合较好。

Mechanical properties of the lamellar microstructure of TiAl-base alloys are extremely anisotropic with respect to the lamellar orientation. However, if the lamellar microstructure can be aligned parallel to the growth direction, the resulting material should possess a good combination of mechanical properties. The seeding and directional solidification (DS) techniques were developed for the lamellar boundaries to be aligned parallel to the growth direction. The lamellar structure of a wide variety of TiAl-base alloys can be controlled using the techniques. DS ingots of these alloys exhibit an excellent combination of room temperature tensile properties and high temperature strength, in particular, creep strength. Attempts were made to increase the size and growth rate of DS ingots for larger-scale production of TiAl castings through seeding and directional solidification. In order to obtain information on the mechanical behavior of DS ingots under the state of stress with multi-components, compression tests of DS ingots with various lamellar orientations were made under the constraint imposed along a direction perpendicular to the loading axis. Such constraint was found to significantly increase the yield stress of DS ingots with intermediate lamellar orientations. Slip systems which operate under the constraint were examined and the results were found to be in good agreement with those of the deformation analysis based on the description of anisotropic elastic continuum.