Creep behavior and deformation feature of TiAl–Nb alloy with various states at high temperature

Abstract

By means of creep properties measurement and microstructure observation, an investigation has been made into the creep behaviors and deformation features of TiAl–Nb alloy with various states at high temperature. Results show that the microstructure of as-cast alloys consists mainly of lamellar γ/α2 phases with various orientations, the boundaries located in between the ones consist of single γ phase. The bigger plastic deformation of alloy during forging may refine the grains and increase the volume fraction of boundaries which consists of the fine block-like γ/α2 phases with weaker strength. During creep, the bigger volume fraction of boundaries increase the probability of crack initiated and propagated along boundaries, which is thought to be the main reason of the forged alloy displaying a lower creep resistance. Compared to the forged alloy, the as-cast alloy displays a better creep resistance and longer creep life at high temperature. The deformation mechanism of as cast alloy during creep is significant amount of dislocations shearing into the lamellar γ/α2 phases in the form of dislocations rows and rings. Compared with α2-Ti3Al phase, the γ phase possesses a weaker strength, therefore, the crack is easily initiated and propagated along the inclined boundaries. Wherein, the propagation of crack along the direction parallel to the lamellar γ/α2 phases displays the smooth surface of fracture, while the tearing edges appear on the surface of fracture in one side of the inclined boundaries relative to lamellar γ/α2 phases, which is attributed to the α2-Ti3Al phase with better strength hindering the propagation of crack during creep.