Ca And Bi Composite Alloying Of The The Am50 Alloy Microstructure And Mechanical Properties Of Impact

In this thesis, alkaline earth element Ca and Bi are added to the AM50matrix magnesium to get new alloys, X-ray diffraction (XRD),opticalmicroscopy, scanning electron microscopy and comes with energy dispersivespectroscopy,electronic universal tensile test machine and other test means areused to analysis and research the microstructure and mechanical properties ofthe alloys. The results show that:The microstructure of cast AM50alloy is composed of-Mg phasematrix and-Mg17Al₁2phase, and the-Mg17Al₁2phase distribute along thegrain boundaries as continuous network,bone-shape or massive. When thecontent of Bi is1%, the addition of Ca can refine the cast microstructure ofAM50, but with the increase of Ca,the amount of-Mg17Al₁2phase decrease,the herringbone network is disconnected and tend to be dispersed, then, a newphase Al2Ca which has a higher melting point and good thermal stability hasbeen formed in the alloy, that is helpful to improve the heat resistance of thealloy; When Bi content is2%,the grain of alloys become coarse,and the phasecomposition does not change.The result of XRD suggest that Mg2Ca phase andMg₃Bi2phase were not found in the alloys.The addition of Ca and Bi makes the yield strength of alloys have beenimproved, but it also reduces tensile strength and elongation, especially whenCa and Bi content are higher, the elongation of alloy sharply decreased to onlyabout2%.The observation of the tensile fracture of SEM shows that the fracturetype of AM50alloy is a hybrid of quasi-cleavage and dimple, and the fracture ofalloys shows trend of conversion from quasi-cleavage to cleavage,with theaddition of alloying elements Ca and Bi. The torn edges,large but shallowdimples are found,which are typical character of quasi-cleavage fracture. Thehard particle Mg₃Bi2is not found in the fracture, and the high content of Ca isincline to make the alloy perform tear fracture.By410℃,8h solution treatment, most of second phase in the alloy haveintegrated into the matrix,the hardness of alloys have a slight decreasecompared to cast alloys, grains grow coarse in some alloys. After aging treatment of200℃at different time between12h and22h, the hardness firstlyincrease and then decrease when Bi content is lower, and have a trend ofdownward when Bi content is higher.