| Magnesium(Mg)alloy as the lightest known metal structure material is widely used in transportation,military armor,aerospace and biomedical and many other fields.However,the hexagonal close-packed crystal structure of Mg alloy leads to its low plasticity at room temperature,while the low absolute strength and hardness restrict the further development and application of Mg alloy.Fracture,cracking and other failure behaviors of Mg alloy structural parts in actual engineering applications are mainly caused by the surface or sub-surface performance deficiencies that lead to its easy formation of defects such as crack sources and gradually extend inward in service.Therefore,it is of great significance and value to improve the surface properties of Mg alloy parts to enhance its service life and break through the service limitations of Mg alloy in high performance fields and extreme environments.WE43 Mg-RE alloy has good heat resistance.Also,as an age-strengthened magnesium alloy,the rare-earth elements can promote the precipitation of non-basal precipitation phases after subsequent ageing treatment.Therefore,it is important and valuable to improve the strength of WE43 Mg alloy by means of deformation and heat treatment.In this study,WE43 Mg-RE alloy was researched.Firstly,the ultrasonic shot peening(USSP)processes under different temperatures were used to achieve the surface strengthening of Mg alloy.Then,deformation and heat treatment were combined to explore the different age hardening responses of different regions of surface-strengthened Mg alloys under different temperatures through different aging regimes,and to elucidate the aging precipitation behaviors of the nanocrystalline region of surface-strengthened Mg alloys under different temperatures.The main studies are as follows:Firstly,the solid solution treated WE43 Mg alloy was processed by USSP process for surface strengthening.The USSP devices under room temperature and cryogenic temperature were designed to create surface-strengthened magnesium alloys with superficial nanocrystalline region and a gradient grain structure in the depth direction.The effects of mian USSP process parameters on the microstructure and microhardness were studied.And the mechanisms of forming nanocrystals induced by USSP under different temperatures were clarified.Therefore,the process range of USSP treatment was broadened.Secondly,combining the deformation and heat treatment,the surface-strengthened Mg alloys under room and cryogenic temperatures were studied.Combined with the single aging treatment process,the age hardening laws of different regions of surface-strengthened Mg alloys under different temperatures were clarified.The precipitation behaviors of nanocrystalline region of surface-strengthened Mg alloys under different temperatures were revealed.The process parameters of peak aging and over-aging were derived.And the size,morphology,orientation and distribution of the precipitated phases in the nanocrystalline region during single aging treatment were explored.Finally,in order to further improve the strength of surface-strengthened Mg alloy,a double aging "low temperature+high temperature" combination process was used to treat surfacestrengthened Mg alloys under different temperatures.The age hardening laws of different regions of surface-strengthened Mg alloys under different temperatures after double aging treatment were investigated.The precipitation behaviors of one-stage aging and two-stage aging in the nanocrystalline region of surface-strengthened Mg alloys under different temperatures in double aging were elucidated.In combination with the size,morphology,orientation and distribution of the precipitated phases in the nanocrystalline region during the double aging treatment,the relationship between the age hardening response to the precipitation behavior of surface-strengthened Mg alloys under different temperatures was established. |
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