A dislocation source efficiency-based model for grain-size-dependent ductile-to-brittle transition in tungsten

The ductile-to-brittle transition (DBT) in body-centered cubic (BCC) metals is a long-standing and critical scientific challenge. For over half a century, the relationship between the DBT temperature (DBTT) and grain size has been described by an empirical Hall-Petch-type relationship originally established for yield strength, yet the validity of its physical mechanism has remained questionable. Here, a new paradigm based on the physics of dislocation dynamics is established to re-examine this fundamental relationship. It is proposed that the DBT is governed by a critical transition in the efficiency of dislocation multiplication, which is dictated by the relative mobility of screw versus edge dislocations. The theoretical model established from this mechanism quantitatively predicts the DBTT of tungsten by employing a novel three-dimensional geometric descriptor, the grain boundary area-to-volume ratio (Σ_r), together with the density of pre-existing mobile dislocations (ρ_i). This approach successfully captures the dependence of DBTT on both grain size and shape, revealing a distinct nonlinear relationship that demonstrates superior predictive accuracy compared to the conventional linear description. This work transcends traditional empiricism, establishing a foundational, mechanism-informed framework for understanding and predicting brittleness in BCC metals. © 2026 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.