Study Of Three Dimensional Dendritic Morphology Evolution Of Mg-based Alloys During Solidification

As a typical hexagonal close-packed structure metal,the dendritic morphology and preferential orientation of Mg would be influenced by many factors.Current investigations still fall short on the thorough description of the diversity and complexity of dendrites growth pattern and their origination.In this study,we focus on exploring the formation mechanism of the diverse ?-Mg(X)dendrites and the affections of alloying element(such as Al,Ca,Zn,and Sn),solute concentration and thermal condition on the growth selection and evolution of ?-Mg dendrites during solidification.Based on these objectives,binary Mg-Al,Mg-Zn,MgSn and Mg-Ca hypoeutectic alloys are prepared,using Synchrotron X-ray tomography,the microstructure of ?-Mg dendrites in Mg alloys under different solidification conditions are characterized,a bunch of dendrite morphologies are obtained and several dendrite growth patterns are proposed based on the morphologies.The factors which would influence the dendrite growth pattern are exhaustively analyzed.In addition,with the fast X-ray imaging technique,the microstructure evolution of ?-Mg dendrites during solidification of Mg alloys and how the alloying elements,solute concentrations and cooling rate would affect it are studied.The experimental results indicate that the alloying elements,solute concentration as well as the thermal conditions would impose a significant influence on the morphology and orientation selection of the primary ?-Mg dendrites.In Mg-Ca(hcpfcc)alloys,dendrites tend to grow with preferred orientation of <112 0> and <224 5> which is in good agreement with the traditional expected directions.The equiaxed growth dendrites in Mg-Sn(hcp-bct)alloys evolve as a structure with 18 branches,six of which grow on the basal plane along <112 0> and the remaining 12 along <112 X>(X ? 2)off the basal plane,while the columnar growth dendrites are more cellular like.For the case in Mg-Zn alloys,an orientation transition from <112 0> on the basal plane to <112 X> off the basal plane are observed with the increasing addition of Zn alloying element,a hyperbranched seaweed structure is also revealed with an interim composition.A probable explanation is that the addition of high anisotropy Zn would slightly alter the anisotropy of interfacial free energy in front the growth interface which results in a dendrite orientation transition(DOT).The results of the in situ investigation of 3D microstructure evolution of Mg alloys during solidification demonstrate that the alloying element affects the dendrite evolution through undercooling,diffusion and the redistribution of solute.For Mg-Zn alloys with different content of Zn,besides the expected DOT,the morphology of dendrites are also observed to be finer with increasing addition of Zn.Additionally,dendrite are refined for the beneficial of the final properties with the increase of cooling rate,however,an unexpected increase in crack and porosity is also observed under a high cooling rate.The results from present study partially reveal the origination of the formation of diverse ?-Mg dendrite growth patterns in Mg alloys.