Effect of rare earth and silicon additions on structure and properties of melt spun Mg-9Al-1Zn alloy

Magnesium based Mg 9Al 1Zn 5RE (RE = Y, La, Nd, Ce, or Pr) alloys with or without an addition of 1%Si were rapidly solidified by chill block melt spinning and splat quenching. The base alloy AZ91 (Mg-9Al-1 Zn) was also rapidly solidified. Isochronal heat treatment for 1 h at 100-400°C showed that the microhardness of the ribbon maintained a similar level to that of the as spun alloy up to 300°C but decreased when heat treated at 400°C. Isothermal heat treatment for up to 24 h at 250-350°C showed that there were aging responses for the sample treated at 250°C while above this temperature, the microhardness decreased as the treatment time increased. The addition of 5% of RE elements to AZ91 displaced the Mg₁₇Al₁₂ phase in AZ91 with fine dispersoids of Al₂RE (RE = Y or Nd) or Al₁₁RE₃ (RE = La, Nd, Ce, or Pr) in Mg-9 Al-1 Zn-5RE alloys. These Al RE intermetallics remained fine and precipitated at the grain boundaries so restraining grain growth during heat treatment at up to 400°C. Although Mg₂Si precipitates were found to be present in the silicon containing alloys after heat treatment at 400°C, their size was greater than those of Al-RE intermetallics, indicating that Mg₂Si has a lower thermal stability than these Al-RE intermetallics. The relationship between microhardness and grain size is discussed.