Transformation-mediated twinning governs plasticity in body-centered cubic nanocrystals under extreme loading

TL;DR

This paper uncovers unconventional transformation-mediated twinning pathways in BCC nanocrystals under extreme loading, revealing new mechanisms involving phase transitions that challenge traditional shear-driven models.

Contribution

It introduces a unified framework for understanding transformation-mediated twinning in BCC nanocrystals, highlighting pathways involving HCP and FCC phases under high pressure.

Findings

Transformation-mediated twinning pathways involve HCP or FCC phases.

Plasticity in BCC nanocrystals can be triggered by elastic instabilities.

Different twinning mechanisms depend on the material's elastic stiffness.

Abstract

Plasticity in body-centered cubic (BCC) nanocrystals is often associated with twin nucleation phenomena under extreme loading conditions. Here, we reveal unconventional twinning pathways that operate at the intersection of crystal plasticity and structural phase transitions. We show that the classical shear-driven twinning mode becomes progressively suppressed with increasing pressure, giving rise to transformation-mediated twinning pathways involving transient HCP or FCC phases. In BCC Fe, Ta, and Nb nanocrystals of moderate elastic stiffness, plasticity is consistently initiated by an elastic instability that triggers a dual-shuffle process mediated by stable or metastable hexagonal closed-packed (HCP) phases. This pathway operates independently of the characteristic {112} twin boundary planes and is driven by compression, challenging the conceptual paradigm for metal plasticity in which plastic deformation arises from shear stresses resolved on specific planes. By contrast, in the archetypal elastically stiffer BCC Mo and W nanocrystals, plastic deformation proceeds via two alternative twinning pathways associated with shear-driven elastic instabilities mediated by highly-distorted face-centered cubic (FCC) phases. Comprehensive analyses of the energy landscapes to the competing nanoscale twinning modes provide mechanistic insight into their activation, establishing a unified framework for transformation-mediated twinning in BCC nanocrystals across a broad range of loading conditions.