DEVELOPMENT OF HIGH PERFORMANCE TiAl ALLOYS USING RAPIDLY SOLIDIFIED POWDER WITH MARTENSITIC STRUCTURE.

研究了等离子旋转电极法（PREP）制备的快速凝固富钛TiAl合金粉末的初始组织对高温压制性的影响。就微观结构而言，存在两种粉末。第一种是由孪晶相关的α_2（Ti_3Al）板条片晶组成的马氏体相的表面浮凸，称为M粉。第二种粉末为树枝状结构，由α_2单相组成，除晶界外没有其他显微组织，称为D粉末。Ti-40at%Al合金M粉和D粉经1273 K热处理后分别形成α 2 + γ（TiAl）微双相和α 2/γ片层组织。显微双相组织是由γ沉淀物在孪晶相关的α_2基体中以通常的取向关系组成的。在1273 K、50 MPa、14.4ks的热压条件下，D粉末很难实现致密化。另一方面，在相同的条件下，M粉末易于通过热压压制。在热压过程中，与孪晶相关的α_2和α_2晶界变为随机晶界，α_2和γ相失去了通常的取向关系。换句话说，低能量边界被改变为适合于晶界滑动的高能量边界。在α_2和γ晶粒内几乎没有观察到位错。结果表明，在热压过程中，M粉的变形方式以晶界滑动为主。Ti-45和47 at%Al合金中的D和M粉末在热压过程中表现出与Ti-40 at%Al合金相同的趋势。从实用角度出发，探讨了PREP工艺条件的优化，以提高M粉的数量分数。

Effect of initial microstructure on the compactibility at elevated temperature in rapidly solidified Ti-rich TiAl alloy powders produced by the plasma rotating electrode process (PREP) has been investigated. There are two kinds of powders with respect to the microstructure. The first one has a surface relief of a martensitic phase and consists of twin related alpha_2 (Ti_3Al) lath platelets, which is referred to as M powder. The second one has a dendritic structure and consists a single phase of alpha_2 with no microstructure other than grain boundary, which is referred to as D powder. alpha_2+gamma (TiAl) microduplex and alpha_2/gamma lamellar structures are formed in M and D powders of the Ti-40 at%Al alloy by heat treatment at 1273 K,respectively. The microduplex structure consist of gamma precipitate in the twin related alpha_2 matrix with the usual orientation relationship. It is difficult to compact the D powder by hot pressing at 1273 K under 50 MPa for 14.4 ks. On the other hand, the M powder is compacted easily by hot pressing with the same condition. The twin related alpha_2 and alpha_2 boundary changes to random ones and the alpha_2 and gamma phases are lost the usual orientation relationship in the duplex structure during the hot pressing. In other words, the low energy boundaries are changed to the high energy ones suitable for grain boundary sliding. Dislocations are scarcely observed inside of both the alpha_2 and gamma crystal grains. It is concluded that the grain boundary sliding is a predominant deformation mode in the M powder during the hot pressing. D and M powders in Ti-45 and 47 at%Al alloys show the same tendency as those in Ti-40 at%Al alloy during hot pressing. From the practical viewpoint the optimization of PREP conditions is also discussed to increase the number fraction of M powder.