AZ series magnesium alloys sheet are widely used in automotive and aerospace industry

in which the morphology and quantity of main strengthening phase Mg

17

Al

12

have important influence on the formability of the sheets. The rolled AZ31

AZ61 and AZ91 magnesium alloy sheets suffer annealing at 300 ℃ for 5 h

liquid nitrogen cryogenic treatment for 6 h at 160 ℃

aging at 160 ℃ for 18 h and different combination heat treatments. The results indicate that the main strengthening mechanisms of rolled AZ series magnesium alloys are solid solution and dislocation strengthening

which increases the strength with decrease of the plasticity. After annealing and cryogenic treatments

the second phases are less precipitated

and some large and slender grains are broken under the thermal stress of cryogenic treatment. The yield and strength ratio is decreased

and the strain hardening index is increased with the stable plasticity. After annealing and cryogenic as well as aging compound heat treatments

the strain hardening index is increased

and a large number of fine second phases are precipitated and dispersed at the grain boundaries and within grains

which is conducive to activate different slip systems during deformation and improves the uniform deformation ability and strength. The yield and strength ratio is further reduced accordingly

which is beneficial to the subsequent forming process of magnesium alloy sheets.