Stability and growth kinetics of {112} twin embryos in [formula omitted]-Ti alloys

Abstract

{112}〈111̄〉 deformation twin, shortened as {112} twin, is usually the dominant twinning mode in transition metal alloys in a body-centered cubic (BCC) lattice except for many BCC β titanium (Ti) alloys. To understand this twin-mode variation, we investigate stability and early-stage growth kinetics of {112} twin embryos with multiple atomic layers in a series of β-Ti alloys by applying density functional theory (DFT) and classical atomistic simulations. Both simulation methods demonstrate that, as average valence electron concentration (VEC measured in a unit of e/a) of Ti alloys decreases, β→ω phase transformations at {112} twin boundaries, which are confirmed by our transmission electron microscopy characterizations, increase the critical thickness of {112} layers as stable twin embryos, possibly raising {112} twin nucleation energy barriers. In simulations of twin embryo growths under applied shear stress on {112} planes, when VEC (or temperature) values are low (∼4.25e/a at 300 K), the applied shear stress results in β twin →α’/α” phase →β matrix phase transformations through an anti-twinning mechanism inside the existing twin embryos; concurrently, ω phases strongly impede the twin boundary migration; these combined effects result in eliminations of {112} twin embryos. However, when VEC increases slightly (∼4.34e/a at 300 K), ω phases at twin boundaries reduce the required stress for {112} twin embryo growth compared with the cases with high VEC values (e.g., ∼4.50e/a at 300 K). These changes of ω effects on {112} twin embryo growth kinetics could originate from free energy landscape variations of β matrix →ω phase →β twin phase transformations.