Development of Directionally Solidified TiAl/Ti3Al Alloys

可以生产对于整个铸锭仅包含一个层状取向的定向凝固铸锭。不幸的是，在这些锭片层显微组织是垂直于生长方向。然而，由于仅观察到一个取向，因此可以通过在定向凝固期间使用晶种来实现微观结构控制。如果要通过使用籽晶来控制层状微结构的取向，则必须满足两个要求。首先，α相必须首先从液体中成核。基本思想是控制成核时α相的取向。由于层状显微组织以（111）γ//（0001）α取向关系形成，因此一旦确定了α相的取向，层状显微组织的取向也被确定。其次，在加热或冷却时，α相或α-2相必须是热稳定的。也就是说，在加热或冷却时，合金必须不通过γ单相区，因为可能发生再结晶。这两个要求在TiAl-Si系统中得到满足。在40-50原子%Al的组成范围内，向TiAl中添加Si使主要α区域向低得多的Al含量移动。在Ti-43 Al-35 Si的组成下，α或α-2从熔化温度到室温是稳定的，并且可以使用晶种材料来控制层状微观结构的取向。通过拉伸试验和三点弯曲试验测定了其室温力学性能。从弯曲试样取向与缺口平行的层状显微组织，Ti-43 Al-3Si合金被发现有一个更大的断裂韧性比TiAl PST晶体相同的取向。

Directionally solidified ingots containing only one lamellar orientation for the complete ingot can be produced. Unfortunately, the lamellar microstructure in these ingots was perpendicular to the growth direction. However, since only one orientation was observed, microstructural control may be achievable by using a seed crystal during directional solidification. There are two requirements that must be met if the orientation of the lamellar microstructure is to be controlled by using a seed crystal. The first is that the alpha phase must nucleate first from the liquid. The basic idea is to control the orientation of the alpha phase upon nucleation. Since the lamellar microstructure forms with the (111) gamma// (0001) alpha orientation relationship, once the orientation of the alpha phase is determined, the orientation of the lamellar microstructure is also determined. Secondly, upon heating or cooling, either the alpha phase or the alpha-2 phase must be thermodynamically stable. That is, upon heating or cooling, the alloy must not pass through the gamma single-phase region since recrystallization may occur. These two requirements are met in the TiAl-Si system. Within the composition range of 40-50 at%Al, the addition of Si to TiAl shifts the primary alpha region towards much lower Al content. At a composition of Ti-43Al-35Si, either alpha or alpha-2 is stable from the melting temperature to room temperature and the orientation of the lamellar microstructure can be controlled using a seed material. The room temperature mechanical properties were determined by tensile tests and by three point bend tests. From bend specimens oriented with the notch parallel to the lamellar microstructure, the Ti-43Al-3Si alloy was found to have a greater fracture toughness than a TiAl-PST crystal of the same orientation.