The orthorhombic O phase precipitation within the D019-α2 phase has attracted increasing attention recently in high Nb containing TiAl (high Nb-TiAl) alloys since the precipitation temperature is close to the expected service temperature of the alloys. In this study, in-situ synchrotron high energy X-ray diffraction (HEXRD) reveals that the O phase precipitates at 550 °C while it dissolves into the α2 phase at 750 °C during heat treatments. However, under external stress the O phase unexpectedly precipitates from α2 phase at 800 °C and even 900 °C. The O phase formation proceeds further in the presence of a critical stress promoted by internal stress accumulation in the α2 phase, whereas the reverse O→α2 phase transformation takes place when the internal stresses are relaxed. Additionally, it has been revealed that the O phase preferentially precipitates from specifically oriented α2 grains with one of their <112¯0> directions aligned perpendicular and their 〈0001〉 directions rotated by an angle of 120° out of the external load axis. This α2 phase orientation facilitates the α2→O crystal transition during uniaxial compression. Transmission electron microscopy (TEM) study shows that stress-induced α2→O transformation is governed by small atomic shifts in the α2 lattice. In addition, the selective growth of certain O variants via shuffling along an [112¯0]α2 direction is found to accommodate the external strain component in this direction.

中文翻译：

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TiAl 合金中的应力诱导正交 O 相


由于析出温度接近合金的预期使用温度，D019-α2 相内的正交 O 相析出近年来在高 Nb 含量的 TiAl（高 Nb-TiAl）合金中引起了越来越多的关注。在本研究中，原位同步加速器高能 X 射线衍射 （HEXRD） 揭示了在热处理过程中，O 相在 550 °C 时析出，而在 750 °C 时溶解到 α2 相中。然而，在外部应力下，O 相在 800 °C 甚至 900 °C 时意外地从 α2 相析出。 O 相形成在 α2 相中内应力积累促进的临界应力下进一步进行，而当内应力松弛时发生反向 O→α2 相变。此外，已经揭示 O 相优先从特定取向的 α2 晶粒析出，它们的 <112 ̄0> 方向之一垂直对齐，并且它们的 〈0001〉 方向与外部载荷轴成 120° 角旋转。这种 α2 相取向有助于在单轴压缩过程中实现 α2→O 晶体转变。透射电子显微镜 （TEM） 研究表明，应力诱导的 α2→O 转变受 α2 晶格中小原子位移的控制。此外，发现某些 O 变体通过沿 [112 ̄0] α2 方向的洗牌选择性生长以适应该方向的外部应变分量。