Influence of lowering basal stacking fault energy on twinning behaviours

Deformation twinning plays a crucial role in the strong anisotropic strength, poor deformability, and texture and microstructure evolutions of hexagonal metals. Modifying twinning behaviour thus may tailor mechanical behaviour of hexagonal metals. Since propagation and growth of deformation twins inevitably interact with other plastic deformation carriers, such as dislocations, stacking faults and phase transformation bands if they can be activated simultaneously, we hypothesize that lowering basal stacking faults energy (SFE) may promote the activity of partial dislocations, thus generating basal stacking faults (BSFs) and even face centred cubic (fcc) phase. Based on first-principles density functional theory calculations, Al solutes can reduce basal SFE and the cohesive energy difference between hexagonal close packed (hcp) and fcc phases of Ti alloys. Microscopy characterizations of deformed Ti-10at.%Al alloy demonstrated that lowering basal SFE actually promotes the formation of profuse BSFs and fcc nanobands in hcp-phase Ti-10at.%Al alloy during mechanical deformation. Consequently, these defects constrain the propagation and growth of {101¯2} deformation twins. More importantly, BSFs and fcc nanobands are activated from twin boundaries during twinning, pinning deformation twins. As a result, deformation twins in Ti-10at.%Al alloy have an average thickness of ∼130 nm, much thinner than twins in pure Ti and other Ti alloys with the similar grain size and under similar strains. In addition, BSFs and fcc nanobands are activated during twin-twin interactions, releasing local stress/strain concentration. Using topological model and atomistic simulations, we further explored interaction mechanisms between deformation twinning and BSFs or fcc nanobands that formed prior to twinning, and the mechanisms of the emission of BSFs and fcc nanobands from twin boundaries during twinning. This work provides insights into understanding the influence of lowering basal SFE on twinning behaviours in hexagonal metals.