Shrinked bifilms in Mg-Gd-Y-Zr alloy

Mg-Gd-Y-Zr alloy has excellent mechanical properties at room and high temperatures, but its high oxidation properties can also result in a significant number of bifilm defects. However, its formation mechanism was not revealed so far, which was explored in the present work. The results showed that the surface oxide film folds into a grid-like double layer structure defect during solidification, due to the difference of thermal expansion coefficient between surface oxide film and matrix. In addition, the oxide film produces microcracks. As the magnesium alloy melt fills the crack, Zr precipitates into crystal nucleus with peritectic reaction and Y₂O₃ accumulates. These results would shed new lights on the understanding of shrinkage formation of bifilm defects and improving the defects control of magnesium alloy castings.