Three-Dimensional Characterization And Phase-Field Simulation Of The Dendrite Morphology Of Magnesium Alloys

The dendrite morphology,commonly observed during a wide range of solidification processes,had a profound influence on the mechanical properties of the products.The three-dimensional(3D)dendrite morphology and growth orientation for magnesium alloys with hexagonal close-packed(hcp)structureare still unclear,and accordinglycharacterization of the3D dendrite morphology for magnesium alloy is extremely important.In order to understand the dendrite growth of magnesium alloys,the 3Ddendrite morphology of magnesium alloyswith different alloying elementswere characterized using the synchrotron X-ray tomography,accompanied with the electron backscatter diffraction method.The 3D phase field simulations were then performed to study the dendrite growth of magnesium alloys.Based on the results,the following conclusions had been obtained.The dendrites of magnesium alloys with different alloying elements,including Mg-Sn,Mg-Gd,Mg-Al,Mg-Ca,Mg-Ba andMg-Y alloys,all exhibited an 18-branch morphology with six branches growing along<1120>and the other twelve branches along<1123>directions.The secondary arms of the<1120>-oriented branches are in four or six directions,whereas those secondary arms of the<1123>-oriented branches are in three or five directions.The main influence of the alloying element on the dendrite morphology was shown to be on the anglebetweenthe<1120>and<1123>directions,the growth rate of the branches in the two directions and the detailed feature of dendrite morphology.Three theories were adapted to explain the possibility of the existence of<1120>and<1123>preferred growth directions.For Mg-Zn alloys,the phenomenon termed dendrite orientation transition(DOT)took place.When the Zn concentration is 20 wt.%or less,the a-Mg dendrite grew along both<1120>and<1123>directions,showing an 18-branch morphology,whereas with 45 wt.%or more Zn content,the a-Mg dendrite would only grow along<1123>directions with a 12-branch morphology.Seaweed structures were observed for dendrites of the alloys in intermediate Zn content between 20wt.%and 45wt.%,in which case the preferred growth orientation was shown to be<1121>.For the12-branch microstructure,three secondary arms on the<1123>branchonly grew in<1123>.For the transition state,the secondary armsof the primary<1121>branch could grow in<1121>and<1012>directions.In addition,thea-Zn dendrites of Zn alloys were also characterized and the morphology exhibited a pseudo-twelve-and-two branches structure,which made the DOT behavior possible.Based on the reconstructed 3D dendrite morphology,an anisotropy growth function was firstly established to describe the growth of ?-Mg dendrites,by combining and modifying certain terms of spherical harmonics.Three anisotropy strength parameters were employed in the function,in which the ?1 measures the growth tendency along the c-axis,while the ?2 measures the growth tendency along the 18 preferred directions,and the ?3 affects the growth tendency along<1120>directions.The 3-D phase field simulations were then performed and the results were greatly similar to that obtained from experiments,in particular on the eighteen-branch,twelve-branchdendrite morphology and the prediction of the DOT behaviour.