Influence Of Heat Treatment On Microstructure And Creep Properties Of TiAl-Nb Alloy

TiAl alloys had been considered as the boradest application prospects as the lightweight structural material due to having a lot of properties such as the superior lightweight, better hot resistant, corrosion resistant and high specific strength etc at high temperatur. By means of heat treatment at different regimes, creep properties measurement and microstructure observation, the creep behavoirs of the as-cast TiAl-Nb alloy in the ranges of 750?-900? and influence of heat treatment on microstructure and creep properties of TiAl-Nb alloy during creep are investigated.Results show that the microstructure of as-cast TiAl-Nb alloy consists of ?/?₂ phases with the lamellar feature, the boundaries with irregular serrated configuration consist of single y phase, and have different orientations between the lamellar ?/?₂ phases. The crystallography relationship between the lamellar y-TiAl and ?₂-Ti₃Al phases is identified as:?110?_?2//?200? ? and [1 ???0]_?2//[0 ???1]_? The as-cast TiAl-Nb alloy displays a better creep resistance and longer creep life when the temperature is 750?-900?. The creep resistance of the alloy can be further improved by means of solution, solution+ aged treatment, the creep lifetimes of the as-cast alloy at 900?/120 MPa and the aged stated alloy at 800?/220 MPa are measured to be 237 h and 374 h respectively. During creep of as-cast alloy at temperature near 750?, the initiation and propagation of the cracks occur easily in the interfaces of the lamellar ?/?₂ phases, which is perpendicular to the stress axis, up to the occurrence of creep fracture. This is thought to be the fracture mechanism of the as-alloy during creep at 750?. The deformation mechanism of alloy is amount of dislocations shearing into the lamellar ?/?₂ phases in the form of dislocation rows during creep at 800-900?. Wherein, 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 improve the creep resistance of alloy. The strain of the as-cast alloy during creep at high temperature occurs mainly in the grain boundary regions with single y phase. The initiation of the crack for the heat treated alloy occurs firstly in the boundaries parallel to the lamellar ?/?₂ phases along the direction at about 45° relative to the stress axis, and the crack is propagated along the boundaries up to the occurrence of creep fracture. Compared to ?₂-Ti₃Al phase, the y phase possesses a weaker strength, therefore, the crack is easily initiated along the boundaries which consist of single y phase, being at about 45° angles relative to the stress axis to bear the maximum shearing stress of applied load. The crack is propagated along the boundaries parallel to the orientation of the lamellar ?/?₂ phases. Moreover, the fracture parallel to the orientation of the lamellar ?/?₂ phases displays the smooth surface. While in another side fracture of the crack, the inclined fracture surface relative to the orientation of lamellar ?/?₂ phases displays the feature with the tearing edges, which is attributed to the ?₂-Ti₃Al phase with better strength increasing the resistance of the crack propagation during creep.