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

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 → γ_T processes from an atomistic perspective. The results show that high temperature annealing facilitates the formation of lamellar microstructure including γ + α₂ and γ_T twin lamellae in the vicinity of core/shell interface, due to rapid diffusion of principal atoms followed by atomic shuffling. Specifically, plenty of lenticular γ_T can nucleate in B2 regime by accidental misalignment of atoms on a 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 plane. The formation of γ_T mediated by 9R involves γ → 9R and γ ← 9R → γ_T 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 γ_T 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 γ_T 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.