Enhanced strength by precipitate modification in wrought Mg–Al–Ca alloy with trace Mn addition

Effects of trace manganese (Mn) contents (0.1 wt% and 0.3 wt%) on mechanical properties and microstructure of the Mg-5.5Al–3Ca wt.% alloy were systematically investigated. By addition of 0.3 wt% Mn, the yield strength of the as-extruded alloy is significantly increased from 366 MPa to 402 MPa. The as-cast alloys show a dendritic structure consists of α-Mg and (Mg, Al)₂Ca phases. Mn addition leads to the formation of Al₈Mn₅ phase, but has no refining effect on the microstructure of the as-cast alloys. The as-extruded Mg-5.5Al–3Ca-xMn alloys exhibit a bimodal grain structure composed of dynamic recrystallized (DRXed) fine grains and coarse unDRXed grains. Compared with Mn free alloy, nano-scale Mg₂Ca particles with a smaller diameter and higher number density are precipitated in the DRXed regions with 0.1Mn and 0.3Mn addition. These Mg₂Ca particles are more effective in dispersion strengthening and grain growth inhibiting, which is considered to be the main reason for the increment on yield strength by Mn addition. Based on the thermaldynamic calculation and experimental results, the thermal stability and formation mechanism of the Mg₂Ca precipitates were discussed.