Evolution of deformation twinning mechanisms in magnesium from low to high strain rates

Abstract

We present a systematic investigation of {101¯2} extension twinning mechanism in single crystal magnesium micropillars deformed over seven orders of magnitude of strain rate, from 10–4 to 500 s−1, revealing how the accommodation of newly formed twins evolves with and depends on the kinetic compatibility of interfacial processes when high deformation rates are imposed. By combination of post-mortem 3D Electron Backscattered Diffraction, Transmission Kikuchi Diffraction and Transmission Electron Microscopy techniques, this work unveils the progressive evolution of the accommodating twin mechanisms from low to high strain rate, correlating differences in mechanical behavior with differences in twin crystallography. Away from quasi–static conditions, simple considerations of twinning shear do not suffice to describe unconventional twin morphologies, requiring the competition between newly activated dislocations and lattice distortions for allowing the evolution of the twin boundary along non–invariant twin planes. Under shock compressions, the basal/prismatic transformation establishing a lattice misorientation of 90° entirely governs the parent → twin conversion. The results illustrated here confirm that some of the recent interpretations deduced by particular twin morphologies are not universally valid and that deformation twinning is not only stress- but also strongly time–controlled.