Effect of heat treatment process and extrusion on microstructure and properties of WE91 alloy were investigated by means of optical microscopy

scanning electron microscopy

X-ray diffraction

transmission electron microscopy and mechanical property tests.The results reveal that the as-cast WE91 alloy is composed mainly ofα-Mg matrix and eutectic structures of Mg24Y5 and Mg12（MM）phases.After homogenization treatment

most of eutectic structures in the alloy are dissolved fully besides the Mg12（MM）phase.After extrusion treatment

the alloy undergoes dynamic recrystallization

so the grain size of the alloy is decreased significantly.The residual Mg12（MM）phase is distributed along extrusion direction.With treating at 225 ℃ for 28 h

numerous long lens-shapedβ′phases are precipitated.After aging treatment

the alloy exhibits good resistance at high temperatures.The strength of the alloy is reduced slightly below 250℃

however

strength of the alloy is reduced significantly at 300℃.As the testing temperature increase

the crack propagation is changed from transgranular to intergranular.