Pre-stored edge dislocations-enabled pseudo-toughness in chromium

Pre-deformation is an effective mean to tune the ductile-to-brittle transition (DBT) temperature of body-centered cubic metals. However, due to complex microstructural evolution during pre-straining, altering not only dislocation density but also grain size, the mechanism of reduction in DBT temperature remains intriguing. Here, we designed two types of chromium samples, with similar grain size but different initial dislocation density, to uncover the underlying mechanisms that govern the variation in DBT. Small-punch, quasi-in situ crack propagation, and in-situ nanomechanical tests conducted over a wide temperature range revealed that pre-stored edge dislocations only can make pseudo-toughness, i.e., increasing deformability but the sample is still intrinsically brittle-prone to fragile cracking. Activation of the easy glide pre-stored edge dislocations could enhance the crack resistance at the initial stage of mechanical loading, which makes a left-shift of the DBT temperature, while the toughness/brittleness of chromium is still controlled by the relative mobility of screw versus edge dislocations.