The taming of the screw: Dislocation cores in BCC metals and alloys

Body-centred cubic (BCC) transition metals tend to be brittle at low temperatures, which poses significant challenges in their processing and major concerns for damage tolerance. The brittleness is largely dictated by cleavage fracture at crack-tips and high lattice frictions of screw dislocation cores; the nature and control of which remain a puzzle after nearly a century. Here, we introduce a crystal geometry-based, semi-empirical material index χ, the energy difference between the BCC and face-centred-cubic structures, that guides engineering of crack-tip and screw dislocation core properties. The unstable stacking fault energy on $\left(110\right)$ planes and screw dislocation Peierls barrier have near-linear scaling with χ and the screw core transforms from non-degenerate to degenerate when χ drops below some thresholds in homogenized BCC alloys, as demonstrated in binary transition metal alloys. The index χ has its origin in crystal geometry and can be extended to finite temperatures; its value is related to entropy and valence electron concentrations, which can be quantitatively predicted by first-principles calculations and effectively tuned in solid solution alloys. The χ-model and computational approach provide a practical path to screening of favourable solutes and compositions for enhanced ductility and toughness in BCC alloys.