Strong size effect on deformation twin-mediated plasticity in body-centered-cubic iron

Deformation twinning serves as an important mode of plastic dissipation processes in nanoscale body-centered cubic (BCC) metals, but its origin and spatio-temporal features are mysterious. Here, applying in situ tensile experiments, we report a strong size effect on mediating the twinning behaviors and twin boundary (TB)-dislocation interaction mechanisms in BCC iron (Fe) nanowires (NWs). There exists a critical diameter (d) of ∼2.5 nm, above which the deformation twinning rather than dislocation slip dominates the plasticity. Unlike the traditional reflection TBs, the intermediate isosceles TBs are consistently observed as mediated by the 1/12<111> partial dislocations. Moreover, we uncover two distinct TB-related deformation mechanisms, including twin variant re-orientation and TB cracking for NWs with d < 17 nm and d > 17 nm, respectively. Further molecular dynamics and statics simulations provide the basic underlying mechanisms for size-dependent plasticity, which have been largely overlooked in previous experimental investigations. Our findings highlight the importance of grain size in mediating the deformation behaviors in Fe, serving as possible guidance for exploring single-crystalline and poly-crystalline Fe-based materials (e.g. steel) with optimized mechanical performance.