Competing mechanisms toughen bcc binary and medium-entropy alloys at 20 K.

Plastic deformation in body-centred cubic (bcc) metals and alloys is conventionally mediated by screw dislocations. Their temperature-sensitive motion is attributed to a steep rise in strength with decreasing temperature that constricts plastic zones at crack tips, leading to a ductile-to-brittle transition1. Here we evaluated the mechanical properties of group IV-V bcc refractory alloys with similar electronic structures at 20 K and found that these materials instead exhibit a plateau in tensile yield strength and fracture toughness as a function of temperature, with no apparent ductile-to-brittle transition. This behaviour arises from the global activation of twinning and an ω-phase transformation, which nucleate dislocations at interfaces before global dislocation motion. The initial mechanism is temperature insensitive and controls yielding, thereby preventing a crack-tip plastic zone from constricting with decreasing temperature.