In the present study,AZ31 Mg-alloy sheets with bimodal non-basal texture were subjected to heating treatment(520℃/5 h),and then immediately water-quenched and quenched into liquid nitro-gen for 12 h.Then,their ambient temperature mechanical performance and microstructure evolution were studied by means of uniaxial tension testing,electron backscatter diffraction(EBSD)and transmis-sion electron microscopy(TEM).The results show that nano-precipitates Mg17Al12 and Al8Mn5 all exist in AZ31 Mg-alloy sheets.However,compared to the Mg-alloy sheet subjected to water-cooling treatment,the volume fraction and size of precipitates increase about 65.5％and 78.7％respectively for the sheet subjected to cryogenic treatment.Meanwhile,the volume fraction of {10(1)2} extension twin(ET)increases by 38.0％and 36.7％for the sheet being subjected to 6％and 12％deformation,respectively.The yield strength(YS)and ultimate tensile strength(UTS)of the cryogenic treated sheets are increased by 43.8％and 5.2％,respectively,compared with the water-cooling treated ones,however,the fracture elongation(FE)decreases by 20.4％.The increase in YS and UTS may mainly be due to the generation of high-den-sity dislocations and precipitation strengthening by Mg17Al12 and Al8Mn5 precipitates during cryogenic treat-ment.The decrease in FE is mainly due to the accumulation of high-density dislocations near {1012} ET boundaries during tensile deformation at room temperature,which would hinder the movement of basal slip and benefit in propagation of microcracks to expand to this region.
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