Thermal activation-induced transition of deformation mechanism for the duplex TiAl alloy: From transgranular to lamellar fracture

Abstract

Ti-48Al-2Cr-2Nb alloys fabricated by electron beam melting (EBM) exhibit equiaxed-γ grains and nano-lamellar colonies. However, the nanoscale interfacial interactions under thermal stress in additively manufactured nanostructures remain insufficiently understood. In this work, a combination of tensile experiments at room temperature (RT) and 973 K, multi-scale microstructural characterization, and polycrystalline molecular dynamics (MD) simulations was employed. Experimental evidence revealed a remarkable transition in the fracture mechanism from a transgranular mode at RT to a lamellar mode at 973 K, leading to a simultaneous enhancement in both strength and elongation. Atomic-scale insights from MD simulations clarify that thermal activation is the primary driver for this mechanistic shift by enabling a sequential and synergistic deformation process. Deformation at 973 K initiates with extensive twinning within the equiaxed-γ grains, which accommodates initial plastic strain and enhances overall ductility. As deformation proceeds, the mechanism transitions to one dominated by dislocation reactions at nano-interfaces within the lamellar colonies, leading to significant work hardening and strength. Lomer-Cottrell (L-C) locks formed at these interfaces act not only as stress concentrators but also as dislocation sources. The subsequent emission and cross-slip of Shockley partial dislocations effectively relieve internal stress, while the formation of new L-C locks from these partials shortens the slip distance, contributing to work hardening. This synergistic mechanism of stress relief and dislocation locking fundamentally explains the concurrent improvement in ductility and strength. This study provides a fundamental understanding of the high-temperature deformation and fracture mechanisms in additively manufactured duplex TiAl alloys.