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A structural aspect of α(α2) → lamellar α2 + γ transformation in γ-TiAl

A structural mechanism for fine lamellar microstructure formation (α(α2) → α2 + γ fine lamellae) in γ-TiAl-based as-atomized powders is discussed. The powder particles show a high density of basal stacking faults, formed by dissociation (on a basal plane) of a single screw dislocation jogged at multiple locations inside an α–α2 grain. dislocations emitted from the prior α(α2)–α(α2) grain boundaries which repeatedly cross-slip from the basal plane into prismatic plane, give rise to a staircase-like stacking-fault nucleation mechanism. The basal stacking faults eventually escape from the pinning jogs and grow to form γ lamellae. It is inferred that the thickness and distribution of γ lamellae can be modified by controlling the jog heights and the number of dislocations emitted from the grain boundary.