Stability and Growth Kinetics of {112} Twin Embryos in Β Ti Alloys

Abstract

{112}<111> 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 thin layers as twin embryos 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 [[EQUATION]]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} plane, when VEC (or temperature) values are low (~4.25 e/a at 300 K), the applied shear stress results in β twin[[EQUATION]]α'/α'' phase[[EQUATION]]β  matrix transformations inside the existing twin embryos, meanwhile, ω phases at twin boundaries strongly impede the twin boundary migration, leading to eliminations of {112} twin embryos. However, when VEC increases slightly (~4.34 e/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., ~4.50 e/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[[EQUATION]]ω  phase [[EQUATION]]β  twin phase transformations.

Keywords: β Ti alloys, {112} twin embryos, stability and growth, phase transformation, First-principles calculations, atomistic simulations