Mechanism Research On Effect Of Al And/or Zn Additions On The Microstructure Evolution Of Mg-Ca Based Alloys During Hot Extrusion

Magnesium alloy,as the lightest metal structure material,has been widely concerned due to the advantages of low density and significant weigh saving.Ca element shows similar characteristics with the rare earth elements and is also cheap.Therefore,Mg-Ca based alloy is superior to other Mg alloys in the aspects of cost and density,exhibiting great potential in application.Based on the optimization of the composition and process design of Mg-Ca alloy,this study clarified the mechanism effect of the Al and/or Zn additions on the microstructure evolution of Mg-Ca based alloy during hot extrusion.The main results have been obtained as follows:(1)The dilute Mg-0.8 wt.%Ca shows the yield strength of?389 MPa and the elongation to the failure of?1.2%while extruded at a low speed(0.41 mm/s).With the Ca content increases to 1.2 wt.%,the yield strength is?360 MPa and the elongation increases to?3.9%.The microstructure analysis shows that the addition of Ca is conductive to the aggregation of high-density dislocation defects and the nucleation of recrystallization,and the fine and dispersed nano-Mg2Ca has an effective pining effect on the recrystallized grain boundaries,resulting in the formation of refined grains(0.56?0.85 ?m)and high strength.(2)The microstructure analysis of Mg-Ca-Al alloy with low Al content(0.6 wt.%)exhibits similar high recrystallization temperature characteristics to the Mg-Ca binary alloy.The degree of recrystallization is low(?32.1%)and the residual dislocation density is high after indirect extruded at 250?.As a result,the alloy shows the high yield strength(?398 MPa)and the poor plasticity(?1.3%).High density of the needle Al2Ca phase precipitated with high Al content(0.9 wt.%)added,which results in the decrease of the equivalent Ca content in the matrix and the increase of recrystallization degree(?60.5%).Hence the yield strength keeps to?367 MPa and the ductility increases to?6.1%.(3)The analysis of the microstructure of the alloy with the Zn added alone shows that the addition of Zn(0.6 wt.%)also increases the degree of recrystallization in the alloy and the density of Mg2Ca and other nano-phases has been decreased.In addition,though there is co-segregation of Ca and Zn atoms in the early stage during hot extrusion,the hindering effect on the recrystallized grain boundaries migration is lower than that of Mg-Ca and Mg-Ca-Al alloys due to the decrease of nano-phase density.Thus,the recrystallized grains grow rapidly to the average grain size of?1.26 ?m in the final stage and the yield strength decreases to?331 MPa.(4)By the combined addition of Al and Zn,Mg-1.6Ca-1.6Al-0.5Zn(wt.%)alloy with good comprehensive mechanical properties was prepared,with the yield strength of?383 MPa and elongation of?12.8%.The analysis of the microstructure evolution shows that the combination of Al and Zn increases the density of needle Al2a and other nano-phase,which can effectively pin the dislocation and promote the entanglement and promote the nucleation of recrystallization;the high-density nano particles and the co-segregation of Zn/Ca at the grain boundaries effectively hinder the grain boundaries migration and improve thermal stability of the recrystallization grains,leading to the formation of high-density sub-grain lamellar(spacing 100?200 nm),which plays a key role in the enhancement of strength and plasticity of the alloy.