Refinement of LPSO structure in Mg-Ni-Y alloys by ultrasonic treatment

Effect of ultrasonic treatment (UT) on the microstructures and mechanical properties of Mg_99.0-xNi_xY_1.0 (x=0.5, 1.0, 1.5, at.%) alloys was investigated. With the increase of Ni content, the amount of eutectic structure, consisting of Mg, Mg₂Ni phase and LPSO structure, increased while the mechanical properties decreased in as-cast alloys. After introducing UT to the melt, the secondary phases in these alloys were refined significantly and distributed uniformly, especially for long period stacking ordered (LPSO) structure. In Mg_98.5Ni_0.5Y_1.0 alloy, the formation of MgNi₄Y phase, which was distributed adjacent to the LPSO structure at the grain boundaries, was stimulated by UT. In Mg_97.5Ni_1.5Y_1.0 alloy with UT, not only the width of grain boundaries were reduced, but also both the width of LPSO structure and that of eutectic structure were reduced. The optimal mechanical properties were obtained in Mg_98.5Ni_0.5Y_1.0 alloy, which exhibited 206MPa in ultimate tensile stress and 7.96% in elongation, respectively. After UT, these properties were enhanced to 231MPa and 14.56%, respectively. The other alloys' mechanical properties were also enhanced significantly by UT.