Atomic-scale investigation of interface structures in polysynthetically twinned TiAl single crystals

Abstract

Interface structures of γ-TiAl/α2-Ti3Al and differently oriented γ/γ lamellae in PST-TiAl crystals with 0° lamellar orientation have been systematically investigated on an atomistic scale using high-resolution scanning transmission electron microscopy. The γ/α2 lamellar interfaces are atomically abrupt and coherent except for steps with a height of even number (mainly two) of (111)γ planes. These steps are associated with Shockley partial dislocations that slip on alternate (111)γ or (0002)α2 planes to accomplish the γ ⇋ α2 transformation. Three kinds of γ/γ interface structures are all atomically flat and coherent. The γ/γ twin boundaries contain steps associated with Shockley and Frank partial dislocations, while there are usually no steps and mismatch dislocations on the γ/γ ordered boundaries and pseudo-twin boundaries. But there is a thin α2 lamella with a thickness of two or three lattice cells sandwiched between some γ/γ ordered boundaries. More intriguingly, the γ lamella terminated in the α2 lamella, or vice versa, forms the flat and coherent (1 1‾ 00)α2/(1 2‾ 1)γ end interfaces with the existence of steps associated with dislocations.