Effect Of Ca Additions On Microstructure And Mechanical Properties Of Mg-Gd-Zr Alloy

Magnesium alloys with high specific strength and good thermal stability have received an increasing interest in the past decade for potential applications in the automotive,aircraft,aerospace,and electronic industries.However,the strength of magnesium is still relatively low which limits their extensive application.Precipitation hardening is one of the most effective method to improve the strength of Mg alloys.Therefore,the research on precipitates is important to design new Mg alloy and improve the strength of Mg alloy.In this thesis,by means of scanning electron microscopy(SEM),transmission electron microscopy(TEM),scanning transmission electron microscopy(STEM)and X-ray energy dispersive spectroscopy(EDS),the additions of Ca on the microstructure and mechanical properties of Mg-15Gd-0.5Zr alloy(wt.%)are investigated,and the relationships between them are discussed.The effects of Ca additions on the ageing behavior and ageing structure are studied.It is found that Ca addition can improve the as-quenched hardness value of Mg-15Gd-0.5Zr alloy,and reduce the ageing time to reach peak hardness value.The increased as-quenched hardness value is mainly attributed to the formation of Mg2 Ca particles and the solid solution strengthening caused by Ca additions.The precipitates in peak-aged Mg-15Gd-0.5Zr-0.6Ca alloy are β’,Mg2 Ca and novel plate phase on the basal plane of the magnesium matrix.With continued ageing to 32 h,β phase occurs which is not observed in the Ca-free alloy at the same ageing time.The orientation relationship between β’,β phases and the matrix are identical to those reported in binary Mg-Gd alloy,but Ca segregation in them are detected using EDS.A novel basal plate phase containing Mg,Gd and Ca elements is observed and its structure is determined.The novel basal phase has a hexagonal structure with lattice parameters a = 0.59 nm,c = 1.01 nm,and the ORs with Mg matrix is described as:(0001)_P//(0001)_α (?)Ca additions on the microstructure and mechanical properties of extruded Mg-15Gd-0.5Zr alloy are also investigated.The mechanical properties of alloy with 0.6 Ca wt% are better than those of Ca-free alloy.The tensile yield strength increases from 238 MPa to 278 MPa,and the ultimate tensile strength increases from 288 MPa to 348 MPa.The addition of Ca leads to the formation of small β particles with high number density,which can inhibit the growth of grains during hot extrusion,and refine grain sizes.