Effect Of Ce/La Mischmetal On Microstructure, Mechanical Properities And Corrosion Resistance Of Mg-Zn-Ca Alloys

The unsalable Ce/La mischmetal as a byproduct of Nd/Pr separation process hasaroused wide interest due to its low price. The effect of Ce/La microalloying onmicrostructure, mechanical properties and corrosion resistance of ternary Mg-Zn-Caalloys was explored in current study. Firstly, Mg-5.3wt.%Zn-0.5wt.%Ca-xCe/La（x=0,0.5wt.%,1.0wt.%）ingots were produced through gravity casting. The kinds andmorphologies of secondary phases and grain size of Ce/La-containing alloys wereresearched during solution treatment process. Subsequently, the thermal deformedMg-Zn-Ca-xCe/La alloys were successfully prepared by hot extrusion and hot rolling.Several test methods were used to represent the effect of Ce/La mischmetal onmicrostructure and mechanical properties of thermal deformed alloys. Furthermore,the effect of grain refinement and texture on mechanical properties was analyzed.Lastly, the influence of Ce/La mischmetal on corrosion resistance of as-extrudedMg-Zn-Ca alloy was investigated.As-cast Mg-Zn-Ca alloy, with an average grain size of114μm, was composedof a large number of Ca2Mg6Zn3/-Mg eutectic structures along grain boundaries, andsmall amounts of irregular shaped Mg-Zn particles and spherical Ca2Mg6Zn3phaserandomly dispersed within grains. Ce/La addition resulted in the grain refinement ofas-cast Mg-Zn-Ca alloy (~80μm), decreased the amount of Ca2Mg6Zn3/-Mgeutectic structures and intragranular Mg-Zn particles, and contributed to theformation of MgxZnyREz-Ca phase. In addition, a small amount of CeMg12andMg17La12phases were also detected. Ce/La mischmetal can effectively restrict thegrain growth during solution heat treatment process, but this kind of effect mighthave a critical value.As-extruded Mg-Zn-Ca-xCe/La alloys were composed of fine equiaxial grain structures (<5μm) and coarse non-recrystallization grains distributed along extrusiondirection. The strength and plasticity of as-extruded alloys were related to the areafraction of recrystallized grains (fDRX) and the average size of recrystallized grains(dDRX), which meant that the optimization of addition amount of mischmetal couldimprove strength obviously. There were a large number of severe deformed grainsand twins in as-rolled Mg-Zn-Ca-0.5Ce/La alloy. After rolling, the ultimate tensilestrength (UTS) and the tensile yield strength (TYS) had increased30%and80%,respectively, which might attribute to the basal texture strengthening and twinstrengthening. The strength of as-rolled alloy was not sensitive to deformation whenit increased from38.3%to57%, but plasticity had a peak value in45.4%(UTS=337.47MPa, TYS=305.14MPa,=9.46%).As-extruded Ce/La-containing alloys experienced a poor corrosion resistance in3.5wt.%NaCl solution. To sum up, the main reason had the following three aspects.a) The formation of MgxZnyREz-Ca phase which had a more positive relativepotential than Ca2Mg6Zn3could increase the number of micro galvaniccorrosion pair.b) The amount and morphology of MgxZnyREz-Ca phase had a dual function inthe corrosion of-Mg.c) Ce/La mischmetal might influence the compactness and protective ability ofthe corrosion product films.