Morphology of α2 phase in γ-TiAl-base alloy ingots

Abstract

Using an etching technique for the investigation of non-metallic inclusions in iron, the morphology of the α 2 (ordered hexagonal close-packed (h.c.p.)) phase in Ti-(46–54)at.%Al alloy ingots containing 0.04–0.2 mass% of oxygen was examined. On etching, the γ (ordered face-centered tetragonal (f.c.t.)) phase was preferentially attacked leaving α 2 and a third phase. The α 2 phase was composed of plate-like, rod-like and globular shapes. From the morphology of the plate-like α 2 phase, the formation processes of the dendrites of the α (h.c.p. solid solution) phase were estimated. The cracks and peeling observed in the plate-like α 2 phase strongly suggest that a relatively high internal stress existed before removal of the γ phase by etching. With the addition of oxygen, the plate-like α 2 phase tended to become thick and rounded, and in aluminium-rich ingots, the dendrites of the α phase tended to be disjointed. On annealing at 1673 K, the array of the plate-like α 2 phase became more regular in aluminium-poor ingots, whereas in aluminium-rich ingots formed in the previous α dendrite the α 2 phase tended to coagulate and layers were observed along the previous interface between the dendrites and the γ matrix.