Abstract
The strength, ductility, toughness and microstructure of the as-cast and hot isostatic press, HIP, -treated cast TiAl with the compositions of (γ+α2) two-phase structure were investigated. The relationships between fracture mechanism and microstructure of each TiAl were then examined in the present study.
The fracture toughness, Kin, of the as-cast TiAl increases with increasing size of the lamellar grain and decreasing linearity of the lamellar grain boundary when the specimen has little porosity and dendritic structure because the grain boundary fracture is inhibited, and microcracks and secondary cracks are easy to form in these microstructures.
The microstructure of the cast TiAl changes from fully lamellar to (lamellar+γ grain) duplex structure by HIP-treatment. According to this microstructural change, the strength, ductility and fracture toughness decrease. The reductions in strength, ductility and fracture toughness caused by the formation of γ which tends to fracture in the manners of cleavage and decohesion between γ interfaces will be greater than those caused by diminishing microshrinkage and spheroidization of pores.
The cast TiAl used in this study exhibits relatively good fracture toughness and ductility (elongation) compared with those reported in polycrystal TiAl because the direction of lamella is nearly equal to that which exhibits excellent fracture toughness and ductility.
