Discrete twinning dynamics and size-dependent dislocation-to twin transition in body-centred cubic tungsten

Abstract

• We present a direct visualization of twinning dynamics in BCC W nanowires, with a discrete behaviour at atomic level. • Quantitative experimental studies directly demonstrate that deformation twins in W nanowires have a minimum size of six-layers and grow in increments of approximately three-layers, in contrast to the layer-by-layer growth of deformation twins in FCC. • These discrete twinning dynamics are generally applicable to different BCC metals across different length scales, which can help interpret the major twinning characteristics observed in previous and current studies of bulk BCC metals. • These findings not only advance the understanding of size-dependent dislocation-twinning competition in BCC nanocrystals, but also stimulate the investigation of plasticity in a broad class of small-volume BCC metals and alloys. Body-centred cubic (BCC) metals are known to have unstable intrinsic stacking faults and high resistance to deformation twinning, which can strongly influence their twinning behaviour. Though twinning mechanisms of BCC metals have been investigated for more than 60 years, the atomistic level dynamics of twinning remains under debate, especially regarding its impact on competition between twinning and slip. Here, we investigate the atomistic level dynamics of twinning in BCC tungsten (W) nanowires using in situ nanomechanical testing. Quantitative experimental studies directly visualize that deformation twins in W nanowires have a minimum size of six-layers and grow in increments of approximately three-layers at a time, in contrast to the layer-by-layer growth of deformation twins in face-centred cubic metals. These unique twinning dynamics induces a strong competition with ordinary dislocation slip, as exhibited by a size-dependent dislocation-to-twin transition in W nanowires, with a transition size of ∼40 nm. Our work provides physical insight into the dynamics of twinning at the atomic level, as well as a size-dependent dislocation-twinning competition, which have important implications for the plastic deformation in a broad class of BCC metals and alloys.