Microstructure evolution of spray-formed Mg-9Al-xZn alloys

The billets of AZ91,AZ92 and AZ93 magnesium alloys were synthesized by the spray-forming(SF) processing.The microstructure evolution of the alloys was observed.The microstructure analysis shows that the microstructure of the conventional as-cast AZ91 alloy consists of coarse grains,and the brittle β-Mg17Al12 phases distribute along the grain boundaries continuously to form network.The grains of the as-sprayed AZ91,AZ92 and AZ93 alloy are homogeneous and refined,and the networks of brittle β-Mg17Al12 phases are broken.The rapid solidification process under a higher supercooling condition can remarkably increase the forming of supersaturated microstructure of α-Mg,which results in the absence of macrosegregation and the improvement of precipitated phase shape.The solution limitation of Al in Mg decreases with increasing addition of Zn.The high content of Zn accelerates the precipitation of β-Mg17Al12 phases along the grain boundary and formation of α-Mg+β-Mg17Al12 divorce eutectic structure.The grain growth is limited because the segregation tendentiousness of Zn offers drive force for activation of nucleation sites in the component undercooling area.