Anomalous twinning in body-centered cubic metals

Anomalous shear is an unusual mode of plastic deformation in body-centered cubic (BCC) metals, traditionally associated only with dislocation slip. Here, we show it can also arise from deformation twinning. Using in situ transmission electron microscopy straining and atomistic simulations, we find twins on {112} planes at low resolved shear stresses nearly half those for the primary systems. These anomalous twins originate from screw dislocation transformations and are dominant when sheared in the anti-twinning direction. They promote dislocation emission, enable relaxation-induced detwinning, and substantially enhance deformability. Our findings establish anomalous twinning as a previously unrecognized plasticity mechanism in BCC metals.