| Mg-Ca-Zn alloys with significant aging-hardening response and good comprehensive properties have a wide application prospect.It has been reported that the precipitation sequence of Mg-Ca-Zn alloys is:supersaturated solid solution?ordered G.P.zone??(Mg Ca Zn)??(Mg2(Ca,Zn)).However,it is still controversy about the main strengthening phase in the alloys during the aging process.In this paper,the characteristic of precipitates in Mg-1 wt.%Ca-x wt.%Zn(x=0,0.5,1)alloy was systematically investigated using scanning elctron microscopy,HAADF-STEM and EDS-STEM techniques,and the precipitation behaviour was discussed based on the observation.The solution treatment parameters of the alloy were determined at first,then the alloys were isothermally aged at 200?and the age-hardening curved were measured.Finally,the microstrcutre of the alloys at different ageing stages was characterized,and the precipitation behavior was discussed based on the observation.The results show that Zn additions can obviously improve the aging-hardening response of the Mg-1Ca-0.5Mn alloy.The peak hardness value of Mg-1Ca-1Zn-0.5Mn alloy at 200?is 62.2 HV,which is 37.6%higher than that of the Mg-1Ca-0.5Mn alloy,and the time to reach the peak hardness is 1 h.The precipitation sequence of Mg-1Ca-0.5Mn alloy is:SSSS.?Mg2Ca.The precipitation sequence of Mg-1Ca-1Zn-0.5Mn alloy is:SSSS?monolayer GP zone?three-layers GP zone?Mg6Ca2Zn3.The main strengthening phase in the Mg-1Ca-1Zn-0.5Mn alloy is the ordered GP zone,and the equilibrium phase is Mg6Ca2Zn3 phase.In the Mg-1Ca-xZn-0.5Mn(x=0,1)alloy,Mn element exists in the form of Mn particles,which plays an important role in the aging process.At the initial stage of aging process,Ca and Zn segregation at the interface between Mn particles and Mg matrix can provide composition fluctuations for the nucleation of the precipitates.Mn particles can also be as the heterogeneous nucleation sites for Mg2Ca phase and Mg6Ca2Zn3 phase to promote their precipitation. |
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