Playing with Limp <c + a> Dislocations

Hexagonal close-packed (HCP) metals are widely used in various applications due to their unique mechanical and functional properties. The ductility and toughness, however, remain intrinsically limited because of the restricted slip systems. The slip of <c + a> dislocations, a critical <c>-axis deformation mode, can provide sufficient <c>-axis strain but needs high critical resolved shear stress to operate. This perspective highlights the nature of <c + a> dislocation—a typical limp dislocation—that leads to the difficulty in self-multiplication and relies on the existing dislocation source to proliferation. Owing to the marked difference in mobility between edge and screw components, the limp <c + a> dislocations have poor self-multiplication ability, which can be resolved by directly incorporating a high density of interfacial dislocation sources. For example, twin boundaries or phase interfaces are unique interface structures that can readily nucleate <c + a> dislocations to mediate <c>-axis plasticity. This strategy can substantially enhance the uniform deformation and strain-hardening ability of HCP metals, offering an effective approach to overcome their intrinsic limitations in plasticity and toughness.