Tailored dislocation kinematic enhances the ductility of mechanically-stable complex concentrated alloys

The enhanced strength and ductility of face-centered cubic (FCC) alloys at cryogenic temperatures have generally been ascribed to mechanically metastable effects. However, among these metastable effects, the specific contribution of dislocation slip, as the intrinsic origin of crystal plasticity, has not been extensively explored in cryogenic deformation. Here, we present a significant improvement in mechanical properties of a Ni₂CoCrFe complex concentrated alloy (CCA) achieved through tailored dislocation kinematics. At 173 K, the Ni₂CoCrFe CCA shows increased yield strength (∼ 672 MPa), ductility (∼ 67 %), and strain hardening capacity compared to room temperature despite the absence of deformation twinning and phase transformation. With the aid of in-situ synchrotron X-ray diffraction measurements, we elucidate that the enhanced strain hardening ability and thus ductility in this alloy are governed by the coupling effect of temperature-dependent dislocation accumulation rate and variations in dislocation configurational arrangement. Thermal activation modulates dislocation kinematics, impacting both on dislocation accumulation and configuration. At 173 K, the delayed activation of dynamic recovery processes and reduced kinetics contribute significantly to this phenomenon. Additionally, a linear correlation between dislocation activation volume and microstructure-sensitive strengthening factor α indicates that thermal activation controls the evolution of temperature-dependent dislocation configurations. The suppression of thermal barriers at 173 K promotes the delocalization of dislocation slip. Despite a reduction in strain hardening efficacy due to the uniform dislocation distribution, as characterized by strengthening coefficient α (σ=Mαμbρ), excellent strain hardening ability and improved ductility are obtained under cryogenic deformation.