Study of Mg–Gd–Zn–Zr alloys with long period stacking ordered structures

The microstructure evolution of Mg96−xGd3Zn1Zrx (at%) (x=0, 0.1, 0.2, 0.4) alloys with long period stacking ordered (LPSO) structures prepared by conventional permanent mold casting were investigated by XRD, EDS, SEM and TEM. Also, the deformation behavior and microstructure evolution of Mg95.8Gd3Zn1Zr0.2 (at%) alloy subjected to equal channel angular pressing (ECAP) were examined. The results show that the LPSO structure phases observed in as-cast Mg96Gd3Zn1 alloy did not notably improve the mechanical properties of the alloy owing to their low volume fraction. In contrast, the mechanical properties of the alloy after heat treatment were raised significantly because of the transformation from eutectic structures to LPSO structure phases. Furthermore, moderate Zr addition into Mg–Gd–Zn alloys refined and homogenized α-Mg grains, changed the morphology and distribution of LPSO structure phases and promoted the precipitation of strengthening phases, and hence strengthened the alloys. Besides, tensile test results showed that aging-treated Mg95.8Gd3Zn1Zr0.2 alloy subjected to 2-pass ECAP processing exhibited high ultimate tensile strength (UTS) of 418 MPa, yield strength (YS) of 330 MPa and elongation of 7.5%. Such excellent mechanical properties could be attributed to complex strengthening of kinking LPSO phases and the dispersion strengthening of tiny spherical phases.