Abnormally fast crack propagation induced by short-range ordering in iron-cobalt alloys: A combined experiments and molecular dynamics simulations

Abstract

We experimentally found that FeCo21.5Cr0.2Ni0.8 alloy quenched from 500 °C, slightly above the order-disorder transition temperature, surprisingly exhibited an extreme impact brittleness. Impact fracture morphology observed by scanning electron microscope (SEM) showed that the impact brittleness was caused by the fast crack growth and poor local plasticity. Short-range ordered (SRO) structures were found in the sample quenched from 500 °C by high-resolution transmission electron microscope (HRTEM) characterizations, and the average SRO size is ~1.58 nm. We contribute the impact brittleness to the formation of SROs. Molecular dynamics (MD) simulations were carried out to reveal the SRO-induced embrittlement mechanism caused by fast crack propagation under high-speed loading. MD simulations showed SROs significantly inhibited the deformation plasticity near crack tips by suppressing the dislocation proliferation, dislocation slip, and twinning. Generalized stacking fault energy (GSFE) curves were calculated to evaluate the effect of SROs on antiphase boundary (APB) energies, unstable stacking energy γusf, and intrinsic stacking energy γisf. SROs were found to prominently increase the APB energies and γisfγusf, fully supporting the SRO-induced poor dislocation and twinning plasticity near crack tips in MD simulations. These findings revealed the potential SRO-induced impact brittleness and its embrittlement mechanism, and provide a design consideration for the engineering application of materials under impact loading.