铸态TiAl-Nb合金的组织结构与蠕变性能

Microstructure and Creep Behavior of As-cast TiAl-Nb Alloy

通过蠕变性能测试和组织形貌观察,研究了铸态TiAl-Nb合金在近890910℃温度区间的蠕变行为。结果表明,铸态TiAl-Nb合金的组织结构主要由层片状γ/α2两相组成,不同取向γ/α2两相层片状组织之间存在不规则锯齿状形态的晶界,该锯齿状非层片晶界由单一γ相组成。在高温蠕变期间,合金具有较好的蠕变抗力和较长的蠕变寿命;合金在蠕变期间的变形机制是大量位错以位错列的形式剪切层片状γ/α2两相,其中,大量位错在基体中滑移,发生反应可形成位错网,可促进位错的攀移,减缓应力集中,改善合金的蠕变抗力。与α2-Τi3Al相比,γ-TiAl相有较弱的强度。因此,蠕变期间合金中的裂纹易于在与应力轴呈45°角、且与层状结构相平行的晶界处萌生与扩展,直至蠕变断裂是合金在蠕变期间的断裂机制;其中,与层状结构相平行的断口呈光滑表面,而与层状结构呈一定角度的断裂表面存在撕裂棱,为较高强度的α2-Ti3Al相阻碍裂纹扩展所致。

The creep behaviors and deformation features of the as-cast TiAl-Nb alloy were investigated at temperature 890900 oC by creep properties measurement and microstructure observation. Results show that the microstructure of as-cast TiAl-Nb alloy consists of γ/α2phases with the lamellar feature, and the boundaries with irregular serrated configuration consist of single γ phase, and locate between the lamellar γ/α2phases with different orientations. The as-cast TiAl-Nb alloy displays a better creep resistance and a longer creep life at high temperature. The deformation mechanism of the alloy during creep is significant amount of dislocations shearing into the lamellar γ/α2phases in the form of dislocation rows. Moreover, a large number of dislocations slipping in the matrix may react to form the dislocation networks, which may promote the climbing of dislocations to retard the stress concentration and to improve the creep resistance of the alloy. Compared to α2-Ti3 Al phase, the γ-phase possesses a weaker strength; therefore, the crack is easily initiated along the boundaries with single γ phase which is at about 45° angles relative to the stress axis, and propagate along the boundaries being parallel to the orientation of the lamellar γ/α2 phases up to the occurrence of creep fracture, which is thought to be the fracture mechanism of the alloy during creep. Thereinto, the tearing edges formed on the surface of the fracture are inclined with the lamellar γ/α2 phases, which is attributed to the α2-Ti3 Al phase with better strength hindering the crack propagation during creep.