Regulating microstructure and improving precipitation hardening response of fine-grained Mg-RE-Ag hot-extruded alloy by extreme short-time heat treatment

The difficulty in exerting beneficial effects of fine grains, high number density nano-precipitates as well as weakened texture simultaneously is currently the bottleneck limiting the development of high performance (high strength, high plasticity, and isotropic) age-hardenable Mg-RE alloys. The key difficulty is that the generation of excessive dynamic precipitates is always accompanied by the formation of fully dynamic recrystallized ultrafine grains during thermal deformation, causing a significant depletion of solutes within the entire matrix and finally seriously weakening the subsequent aging nano-precipitation. In this work, the process of extreme short-time (2 min) heat treatment (ESHT) followed by peak aging treatment is applied to an ultrafine-grained weak-textured Mg-RE-Ag as-extruded alloy with numerous dynamic precipitates. The precipitation hardening response is dramatically increased (ΔHV = 37.2 HV) and the yield strength is increased by nearly 50 MPa. This is attributed to the fact that concentration of main solutes in matrix is greatly increased within only 2 min of heat treatment promoting the precipitation of nano-precipitates (prismatic β′ and basal γ′′) in the absence of excessive grain growth. The large difference between the initial and equilibrium solute concentrations, the extremely short distance required for solute diffusion within sub-microscale grains, and the effective pinning of grain boundaries by excess dynamic precipitates are responsible for the unusual microevolution during the ESHT. In addition, the formation of precipitation-free zones (PFZs) is attributed to 'uphill' diffusion during aging, and novel γ′′-PFZs and β′-PRZs are found. The ESHT followed by peak aging treatment provides a new way to simultaneously exert the beneficial effects of fine grains and high number density nano-precipitates.