Precipitation-induced thermal-athermal shift in dislocation plasticity of a Mg alloy

Precipitation hardening is key to strengthening magnesium (Mg) alloys, yet its impact on dislocation-mediated plasticity requires further exploration. To clarify how precipitates alter dislocation mechanisms, we conducted tensile testing across a wide strain-rate range (10^-4 s^-1 to 800 s^-1) on solid-solution and aged samples. Our study reveals, possibly for the first time, that precipitates trigger a fundamental mechanistic shift in dislocation behavior throughout tensile plastic deformation, evidenced by distinct strain-rate dependencies in both yielding and work hardening. At yielding, aging-induced formation of basal nano-precipitates lead to an unusually large activation volume and rate-insensitive yield stress. This signifies a mechanistic transition in 〈a〉 dislocation glide—from thermally activated cutting of Ca clusters in the solution-treated state to an athermal Orowan bypass of Al₂Ca nano-precipitates in the aged state. During work hardening, precipitation alters the hardening response from rate-insensitive (solution-treated) to rate-sensitive (aged), primarily attributed to a change from cluster-controlled, fixed activation volume to forest-controlled, strain-dependent activation volume. These results establish direct mechanistic links between obstacle characteristics and strain-rate-dependent plasticity in Mg alloys.