Development of Microstructure and Properties of Mg-Y-(Nd)-Zn Alloys during Heat and Mechanical Treatment
Abstrakt
This work is focused on development of microstructure and properties of Mg-Y-Zn and Mg-Y-Nd-Zn alloys during heat and mechanical treatment. In the as-cast state both alloys exhibit almost equiaxed grains with little larger size in Mg-Y-Zn alloy and grain boundaries decorated by different structures - long period ordered structure (LPSO) was detected in Mg-Y-Zn alloy and eutectics of Mg3Nd type structure in alloy with Nd addition.
A high density of stacking faults is evident in both alloys. Both alloys were repeatedly isochronally heat treated from room temperature up to 440 oC.
Resistivity and microhardness measurement was performed after each heating step. Stacking faults persist both annealings in both alloys and microhardness development shows no remarkable differences.
LPSO in Mg-Y-Zn alloy disappears after the first annealing and was again detected after repeated annealing up to 340 oC. After the whole treatment no grain growth appeared.
Differential scanning calorimetry measurement was performed at both repeatedly heated alloys up to 540 oC. There are three exothermic peaks in DSC curves of Mg-Y-Zn alloys that can be ascribed to embedding solute atoms in stacking faults, LPSO development and transformation and coarsening of grain boundary particles.
DSC curves of Mg-Y-Nd-Zn alloy exhibit two exothermic peaks that probably correspond to precipitation of basal plates of γ'and γ phase. Measurement of microhardness was performed after sequential deformation of both alloys in the as-cast state.
The alloys were cold rolled in steps of 0,9 % thickness reduction up to cracks formation. Strengthening of both alloys is very similar but formation of cracks in the alloy with Nd addition begins after a lower reduction (about 11 %) compared to Mg-Y-Zn alloy (about 15 %).