Atomic-scale study of the {[formula omitted]} twinning and {[formula omitted]}-{[formula omitted]} double twinning mechanisms in pure titanium

Abstract

The {112¯1} twinning and {112¯2}-{112¯1} double twinning mechanisms in pure Ti are systematically investigated using molecular dynamics (MD) simulations. MD simulation results indicate that a {112¯1} twin embryo is composed of a group of pyramidal stacking faults (PSFs) with 6d{112¯1} or 8d{112¯1}, caused by the successive slip of partial dislocations with the Burgers vector of 136〈112¯6¯〉M. The migration of {112¯1} twin boundaries (TBs) is mainly carried out by the slip of twinning dislocations (TDs) b⇀1 = 16(4γ2+1)<1¯1¯26> ±12<11¯00>. Two {112¯2}-{112¯1} double twinning mechanisms are discovered. One is that a {112¯1} twin forms and grows at the {112¯2} twin tip through the dissociation of the TD b⇀1(112¯2) at the {112¯2} twin tip, forming the {112¯2}-{112¯1} double twin structure. The other is that a basal <a>T dislocation interacts with a {112¯2} TB, forming a twin embryo composing of a group of PSF with the 6d{112¯1¯} and leaving a step at the {112¯2} TB, eventually forming the {112¯2}-{112¯1} double twin structure. The interfacial structures of {112¯1} twins have the terraced character with short coherent (112¯2)||(112¯0) facets and coherent twin boundary (CTB) segments. In addition, (0001)||(112¯4) facets with a disclination dipoles feature can form at twin tips. The coherent twin interfaces form at their intersected interfaces during the {112¯2}-{112¯1} double twinning process. Our present findings may provide clues for designing Ti alloys with excellent strength–ductility properties through obtaining a high-volume fraction of coherent interfaces in terms of {112¯2}-{112¯1} double twins.