In order to shed a new light on the complex microstructural evolution in the Ti-Al-Mo system, we employ ab initio calculations to study bcc-fcc structural transformations of ordered o-TiAl(+Mo) and disordered -TiAl(+Mo) to ordered -TiAl(+Mo) and hypothetically assumed disordered disTiAl(+Mo) alloys, respectively. In particular, tetragonal (Bain’s path) and trigonal transformations are combined with the concept of special quasi-random structures (SQS) and examined. Our calculations of the ordered phases show that the o tetragonal transformation of TiAl is barrierless, i.e., proceeds spontaneously, reflecting the genuine structural instability of the o phase. Upon alloying of 7.4 at. energy minima is formed. Yet a higher Mo content of 9 at. opposite-direction barrierless transformation o , i.e., fully stabilizing the o phase. Considering the disordered phases, the -Ti0.5Al0.5-xMox and dis-Ti0.5Al0.5-xMox are energetically very close. Importantly, for all here considered compositions up to 11 at. a small energy barrier separates -TiAl(+Mo) and dis(+Mo) energy minima. Finally, a trigonal path was studied as an alternative transformation connecting disordered and dis TiAl phases but it turns out that it exhibits an energy barrier over 60 meV/at. which, in comparison to the Bains’s path with 9 meV/at. barrier, effectively disqualifies the trigonal transformation for the TiAl system.

中文翻译：

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化学障碍的从头算起是基于bcc的TiAl（+ Mo）的有效稳定机制

为了给Ti-Al-Mo系统中复杂的微观结构演变提供新的思路，我们使用从头算计算来研究有序o-TiAl（+ Mo）和无序-TiAl（+ Mo）的bcc-fcc结构转变分别订购有序的-TiAl（+ Mo）和假设的无序DisTiAl（+ Mo）合金。特别地，将四边形（贝恩路径）和三角变换与特殊的拟随机结构（SQS）的概念相结合并进行了研究。我们对有序相的计算表明，TiAl的o四方转变是无障碍的，即自发进行，反映了o相的真正结构不稳定性。经7.4at合金化​​。形成能量极小值。更高的钼含量为9 at。反向无障碍转换o，即完全稳定o相。考虑到无序相，-Ti0.5Al0.5-xMox和dis-Ti0.5Al0.5-xMox在能量上非常接近。重要的是，对于所有此处考虑的成分，最高可达11 at。一个小的能垒将-TiAl（+ Mo）和dis（+ Mo）能量最小值分开。最后，研究了三角形路径作为连接无序相和disAlAl相的替代转换，但事实证明，它在60 meV / at以上具有能垒。与贝恩斯的9 meV / at路径相比。障碍，有效地取消了TiAl系统的三角变换的资格。研究了三角路径作为连接无序和DisTiAl相的替代转换，但事实证明，它在60 meV / at以上具有能垒。与贝恩斯的9 meV / at路径相比。障碍，有效地取消了TiAl系统的三角变换的资格。研究了三角形路径作为连接无序相和DisAl相的替代变换，但事实证明，它在60 meV / at的范围内表现出能垒。与贝恩斯的9 meV / at路径相比。障碍，有效地取消了TiAl系统的三角变换的资格。