Effects of thermal treatment on microstructure and mechanical properties of a Mg-Gd-based alloy plate

The Mg-8Gd-4Y-Nd-Zr alloy plate was prepared by casting, homogenization and extrusion. Microstructure of the as-cast alloy was comprised of α-Mg and eutectics Mg5.05RE. These eutectics were able to dissolve in matrix during subsequent homogenization, but rare earth (RE) solutes precipitated from the matrix again during extrusion in the form of irregular Mg5.05RE particles. In order to dissolve particles and improve mechanical property, companion samples were solution treated at temperatures ranging from 450 °C to 520 °C. With the solution temperature increasing, the volume fraction of Mg5.05RE particles decreased, and finally, almost all of the particles dissolved in the matrix when solution temperature increased to 520 °C. The strength of the solution and ageing treated samples increased first and then decreased as the solution temperature increasing, and the best property was achieved by 475 °C/0.5 h treatment followed by ageing. The ultimate tensile strength, tensile yield strength and elongation of the sample were 419 MPa, 321 MPa and 2.6%, respectively. Fracture analysis indicated that the residual Mg5.05RE particles were responsible for the sample′s failure. It was also the reason for the failure of other samples solution treated at lower temperatures. However, in 520 °C/0.5 h-T6 treated sample, the fracture was induced by promoted twinning behavior, which was caused by the formation of β′ precipitates during ageing. The critical resolved shear stress (CRSS) for basal slip was improved much higher than that for twinning, which suppressed slip but promoted twinning and finally resulted in the deterioration in elongation.