New insights into lamellar and twinning transformations in TiAl/Nb composites with core-shell structure during high temperature annealing

Abstract

Lamellar and twinning transformations play a crucial role in stabilizing the ordered phases in TiAl based composites for property improvement. In this work, we have systematically investigated the lamellar and twinning transformations in TiAl/Nb composites with core-shell structure and proposed novel insights on γ → α2 and γ → γT processes from an atomistic perspective. The results show that high temperature annealing facilitates the formation of lamellar microstructure including γ + α2 and γT twin lamellae in the vicinity of core/shell interface, due to rapid diffusion of principal atoms followed by atomic shuffling. Specifically, plenty of lenticular γT can nucleate in B2 regime by accidental misalignment of atoms on a 110B2 plane adhering to Kurdjumov-Sachs relationship, and then grow even into α2 lath driven by the release of internal stress via Shockley partial slips on the 111γ plane. The formation of γT mediated by 9R involves γ → 9R and γ ← 9R → γT transformations. Among them, a full period of atomic stacking …ABC|BCA|CAB… (LPSO) can be achieved through three Shockley partials being sequentially activated on the C, A and B atomic layers in the stacking of …ABC|ABC|ABC… (L10) with the total shift of 3δA. Then 9R will change synchronously to γ and γT once the migrating Shockley partials react with Frank partials to yield a full dislocation, i.e., δA + Dδ→DA and δA + δC + δD→BD, which can also be considered as detwinning and twinning behaviors, respectively. Additionally, the transition of bulk γ to α2 or γT lamellae is closely associated with the stacking faults, i.e. whether the activated atomic layer is located at the boundary or periphery of the updated stacking fault region.