Dislocation Induced FCC Twins at the HCP/FCC Interfaces in a Deformed Ti-5Al Alloy: Experiments and Simulations

This paper proposes a new formation route for the face-centered cubic (FCC) twins at the hexagonal close-packed (HCP)/FCC interface in a deformed Ti-5at.%Al alloy. From the experimental results, an FCC band that orients in a prismatic-type (P-type) relationship with the HCP matrix is surrounded by a group of FCC lamellae that align in parallel and orient in a basal-type (B-type) orientation relationship within the HCP matrix. The high-resolution transmission electron microscopy (HRTEM) reveals a local { } twinning relation between the FCC band and the FCC lamellae at their contacting boundaries. Molecular dynamics (MD) simulations indicate that basal stacking faults (BSFs), which later evolve into the FCC lamella in a B-type OR with the HCP matrix, initiate from the misfit regions at the relaxed P-type HCP/FCC interfaces, which then provide the nucleation source for a full dislocation that continues to slip in the initial FCC grain. Repeating these two processes produce the { } twinning relation between the new-forming FCC lamella and the initial FCC grain. The lattice correspondence analysis further interprets the formation route for the { } twins proposed in the present study from the aspect of crystal geometry.