Atomic-level study of {[formula omitted]} deformation twinning in pure Ti and Ti-5at.% Al alloy

Abstract

In this work, pure Ti and Ti-5at.%Al alloy are both cold rolled, and then characterized to observe deformation twins {101¯1}. Many two-layer steps are observed at the {101¯1} twin boundary (TB) in pure Ti by high-resolution transmission electron microscopy (HRTEM). However, in addition to two-layer steps, a large number of four-layer steps are observed at the {101¯1} twin boundary in Ti-5at.%Al alloy. To understand the experimental observations, molecular dynamics (MD) simulations are performed to analyze the {101¯1} deformation twinning at the atomic scale. MD simulations are also showing similar results to the experimental observations in pure Ti and Ti-5at.%Al alloy. The two-layer and four-layer steps are twinning dislocations (TDs) with the Burgers vectors of 12×4γ2−93+4γ2<101¯2¯> ±16<12¯10> (b⇀2) and 4γ2−93+4γ2<101¯2¯> (b⇀4) by topological analysis, respectively. For pure Ti, the twin growth is carried out only by the slip of TDs b⇀2. For Ti-5at.% Al alloy, twinning is carried out by the gliding of TDs b⇀2 and TDs b⇀4. Due to the poor mobility of TDs b⇀4, the nucleation of the twinning dislocations b⇀4 (4-layer steps) requires some special nucleation sites, such as intersections between the stacking fault (SF) and TB (SF-TB intersection) in Ti-5at.% Al alloy. The SF-TB intersection is a sessile imperfect disconnection, and this defect is immobile, which leads to high stress at the SF-TB intersections to facilitate the nucleation of TD b⇀4. The Al element enhances the high density of basal stacking faults (BSFs) and SF-TB intersections at the twin-tip, and indirectly promotes the nucleation of TD b⇀4 in Ti-5at.% Al alloy. In addition, Al element also facilitates the nucleation of matrix dislocations, resulting in twin-dislocation interactions that promote the generation of TDs.