Thermal stability and microstructure development of cast and powder metallurgy produced Mg-Y-Zn alloy during heat treatment
Abstract
Response to isochronal annealing up to 440 °C of squeeze cast Mg-Y-Zn alloy and of the same alloy prepared by powder metallurgy (PM) and extruded at 280 °C was studied by resistivity and microhardness measurement, differential scanning calorimetry (DSC) and microstructure investigation. Electrical resistivity was measured at 77 K and microhardness was measured at room temperature after each annealing step.
DSC measurement was performed at various heating rates. Transmission and scanning electron microscopy and optical microscopy revealed ribbons of long-period ordered structure (LPSO) 18R and planar defects within grain boundaries.
Relatively high density of planar defects was found in grain interiors of the cast alloy with the grain size approximately 50 µm. Well pronounced subgrains were observed in the PM prepared alloy.
Secondary phase particles decorate grain boundaries in this alloy. Three precipitation processes were detected in the cast alloy during repeated isochronal annealing up to 440 °C, whereas only one significant process was revealed in the PM alloy.
These processes were identified as embedding of stacking faults by solutes, development and rearrangement (18R RIGHTWARDS ARROW 14H) of LPSO phase and development of grain boundary particles. A coarsening of grain boundary particles rich in Y and Zn only proceeds in the PM alloy.
Activation energies of the precipitation processes were determined. Microhardness exhibits good thermal stability against annealing up to 360 °C in the PM alloy.