Development of Directionally Solidified TiAl/Ti3Al Alloys

定向凝固锭只包含一个层状取向的完整锭可以生产。不幸的是，这些铸锭中的片层组织是垂直于生长方向的。然而，由于只观察到一个取向，因此可以通过在定向凝固过程中使用晶种来实现微观结构控制。如果要用晶种来控制片层微观结构的取向，必须满足两个要求。第一个是液相必须先成核。基本思想是在成核时控制α相的取向。由于层状组织的形成与(111)γ // (0001) α取向关系有关，一旦确定了α相的取向，层状组织的取向也就确定了。其次，在加热或冷却时，α相或α -2相必须是热力学稳定的。也就是说，在加热或冷却时，合金不能通过γ单相区，因为可能会发生再结晶。这两个要求在ti - si系统中都得到了满足。在40-50 at%Al的组成范围内，向TiAl中添加Si使初级α区向Al含量低得多的方向移动。在Ti-43Al-35Si组合物中，α和α -2在熔融温度到室温范围内都是稳定的，并且可以使用种子材料控制层状微观结构的取向。通过拉伸试验和三点弯曲试验确定了室温力学性能。结果表明，Ti-43Al-3Si合金的断裂韧性优于相同取向的TiAl-PST晶体。

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.