The influences of multiscale second-phase particles on strength and ductility of cast Mg alloys

Commercially cast Mg alloys generally contain two kinds of second-phase particles with different length scales, i.e. nanoscale precipitates and microscale particles. Both have substantial effect on the strength and ductility of Mg alloys, which, however, is still insufficiently understood up to now. Here in present work, a strengthening model and micromechanics model were, respectively, developed to quantitatively describe the individual influence and coupling effect of the two second-phase particles on yield strength and ductility. The relationships between the second-phase particles (size, volume fraction, aspect ratio and orientation) and strength or ductility, were ad hoc and experimentally validated in three Mg alloys with different precipitate orientations, i.e. Mg–3Gd alloy with prismatic plate precipitates, Mg–3.5Zn alloy with [0001]_α rod precipitates and Mg–1Gd–0.4Zn–0.2Zr alloy with basal plate precipitates. Comparisons indicate that the calculations are in quite good agreement with the experimental data. It was commonly revealed that the precipitate strengthening (Orowan) makes a major contribution to the strength, which is highly dependent on the precipitate shape and orientation. On the other hand, the ductility is predominantly controlled by the microscale particles, which, serving as the microvoid initiators, have a detrimental effect on ductility much greater than the nanoscale precipitates.