Regulating the microstructures and creep behaviors of an LPSO-containing Mg alloy via heat treatment

The microstructures and creep behaviors of a hot-extruded Mg-Gd-Y-Zn-Zr alloy containing long period stacking order (LPSO) phases prepared by different heat treatment procedures were investigated in this work. Three samples were made, including Sample #1, which was in the as-extruded state; Sample #2, which was preserved at 500 °C for 12 h; and Sample #3, which was preserved at 500 °C for 12 h and 460 °C for 10 h. The average grain sizes of the heat treated samples were much larger than those of the as-extruded samples. The second phases included mainly blocky 18R LPSO phases, lamellar 14H LPSO phases and needlelike ZnZr compounds in the three samples, but Sample #2 had much fewer 14H LPSO phases than the other two samples. The grain boundary migration resulting from the small grain size induced the lowest creep resistance in Sample #1. In contrast, the hardening effects generated from the pyramidal <c+a> dislocations and the dynamic precipitation of dense β’ phases dramatically retarded the cross-slip, resulting in the highest creep resistance in Sample #2. Although the aging treatment introduced numerous 14H LPSO phases in Sample #3, they delayed the dislocation movements to a limited degree, and the creep resistance was inferior to that of Sample #2. Therefore, the creep resistance ultimately decreased in the order of Sample #2 > Sample #3 > Sample #1. It is suggested that the microstructures and creep behaviors of LPSO-containing Mg alloys can be well regulated through appropriate heat treatment, and this work can hopefully offer important guidance for the design of highly creep resistant Mg-Gd-Y-Zn-Zr alloys.