The tensile test at elevated temperatures show that either die cast AZ91 containing 1%Y or 1%MM Mg alloy has the higher strength than AZ91

in which the strength of AZ91 containing 1%Y is the higher than that of AZ91 containing 1%MM.The AZ91 Mg alloy was enhanced due to both the formation of polygonal Al

2

Y in AZ91 Mg alloy containing 1%Y and the formation of rod like Al

4

（Ce

La） in AZ91 containing 1%MM possessing higher thermal stability and microhardness.By friedel model

the critical stress σ

c

for a dislocation moving from the resistant Y atom or Ce atom was calculated in both AZ91 containing 1%Y and AZ91 containing 1%MM alloy.The calculation reveals that the σ

c

of die cast AZ91 containing Y is higher than that of die cast AZ91 containing MM

resulting in higher strength of AZ91 containing Y than that of AZ91 containing MM.