Elastic shielding mediated by deformation twin facets in hexagonal close-packed metals

• Local strain field of a deformation twin tip was mapped by 4D-STEM while under load. • The strain map agrees with phase field simulations of deformation twinning. • An elastic shielding mechanism mediated by twin facet junctions was revealed. Diffusionless transformations (DTs), such as twinning, are key plastic deformation modes in metals with a limited number of slip systems. DTs can simultaneously confer excellent strength and ductility to these materials under thermomechanical loads. The efficiency of DTs is fundamentally governed by the balance between plastic dissipation (as the domains grow) and the buildup of internal elastic strain energy in the matrix. The present study examines this duality by quantitatively mapping the mechanical fields at the nanometer scale near the propagation front of a deformation twin under load in hexagonal close-packed Mg. To this end, a novel combination of in situ straining and patterned-probe four-dimensional scanning transmission electron microscopy (4D-STEM) strain mapping is utilized. Cross-examination of the experimental results using an atomistically-informed phase field model suggests that the junctions between interfaces/facets bounding twin domains can shield the matrix from a large fraction of the transformation-induced rotations, thereby significantly lowering the buildup of elastic strain energy that is induced by twinning. This sheds new light on the effects of twinning on the evolution of the internal energy landscape during plasticity. Comparison of the normalized Twin Resolved Shear Stress (TRSS*) maps. A) Normalized TRSS map obtained from the 4D-STEM experiment, and B) Normalized TRSS map obtained via phase field (PF) simulations. The regions of agreement between the experimental and the PF TRSS maps are also indicated.