Microstructure Evolution in Mg-Zn-(Y, Gd) Casts

Commercial magnesium alloys have a great potential for structural applications in automotive due to their significant weight saving. However, they have poor creep resistance at temperature over 125 degrees C, thus making them inadequate for power train applications such as engine pistons, which are operated at temperature up to 300 degrees C. Recently, creep resistant magnesium alloys with rare-earth elements and zinc have been developed and it is suggested that Mg-2Zn-10Y-5Gd-0.5Zr alloy is promising as a candidate material for the engine piston application. However, the detail strengthening mechanism is not clear, hence the detail microstructure of Mg-Zn-(Y, Gd) alloys was investigated in this paper. Gravity casting was performed with Mg-Zn-(Y, Gd)-Zr alloy, followed by T6 treatment. In the as cast condition, both Mg-12(Y, Gd)Zn and Mg-24(Y, Gd)(5) were observed at the alpha-Mg grain boundaries, while metastable Mg-24(Y, Gd)(5) was dissolved into alpha-Mg matrix and surplus (Y, Gd) and Zn were precipitated on the Mg-12(Y, Gd)Zn via solution heat treatment at 535 degrees C. After the aging treatment for 24 hours at 225 degrees C, fine beta' precipitates were formed in alpha-Mg matrix.