Annealing Effects in Mg-Y-Zn Alloys Prepared by Powder Metallurgy and by Squeeze Casting

Abstract

Response to isochronal annealing up to 440°C was investigated in squeeze cast Mg2Y1Zn alloy and in the same alloy prepared by powder metallurgy and extrusion at 280°C (PM). Electrical resistivity measurements at 77 K and at room temperature after each annealing step and differential scanning calorimetry performed at various heating rates characterized phase changes proceeding during the heat treatments. Transmission and scanning electron microscopy and optical microscopy revealed ribbons of a long-period ordered structure and a relatively high density of stacking faults in grain interiors of the cast alloy having the grain size of ~50 μm. Well pronounced subgrains were observed in the PM prepared alloy. Secondary phase particles decorate grain boundaries in this alloy. Electrical resistivity response of the cast alloy to isochronal annealing up to 440°C shows three precipitation processes, whereas one significant process was revealed in the PM alloy. Activation energies of precipitation processes were determined. Microhardness exhibits good thermal stability in the whole temperature range in the cast alloy and up to 360°C in the PM alloy.