镁合金凝固过程三维枝晶形貌和生长取向研究进展:三维实验表征和相场模拟

RESEARCH PROGRESS ON 3D DENDRITE MORPHOLOGY AND ORIENTATION SELECTION DURING THE SOLIDIFICATION OF Mg ALLOYS: 3D EXPERIMENTAL CHARACTERIZATION AND PHASE FIELD MODELING

Mg作为一种典型的hcp结构金属,其枝晶形貌和生长取向受到多种因素的影响,目前针对镁合金中a-Mg枝晶生长模型的描述以及多样性的起源等问题的研究都比较缺乏,基于此,本文综述了本课题组在镁合金凝固微观组织结构三维表征方面取得的研究成果.借助同步辐射X射线微观断层扫描技术以及相场数值模型,研究了镁合金凝固过程中a-Mg(X)枝晶生长选择多样性的形成机理以及固溶合金元素(Al,Ca,Zn和Sn等)、固溶元素含量(溶质浓度)等因素对a-Mg枝晶生长选择和演化的影响.研究结果表明,固溶元素、固溶元素含量等因素都会对镁合金中a-Mg三维枝晶形貌和择优取向产生重要影响.在Mg-Ca和Mg-Al(hcp-fcc)合金中,枝晶倾向于以<1120>或<2245>为择优方向生长.在Mg-Sn(hcp-bct)合金中,等轴生长的枝晶沿着基面上<1120>和偏离基面的<112X>(X≈2)方向形成一种18次分支的结构,在Mg-Zn(hcp-hcp)合金中,a-Mg枝晶的择优取向会随着Zn含量的增加,从<1120>方向朝偏离基面的<1121>方向发生连续转变,并在转变的过渡区,发现了超支化的藻状枝晶结构,其原因可能是高各向异性Zn元素的引入带来的固/液界面自由能各向异性的变化.研究结果从一定程度上揭示了镁合金凝固过程中a-Mg枝晶生长形貌和分支结构选择多样性的规律.同时基于快速X射线成像技术率先开展了镁合金凝固过程三维微观结构演化原位表征研究,获得了镁合金凝固过程三维枝晶的生长演化过程.

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 patterns and their origination, therefore, this paper reviewed recent research progress of this group on 3D characterization of microstructure in solidified magnesium alloys. Using synchrotron X-ray tomography and phase-field modeling, the formation mechanism of the diverse aMg（X） dendrites and the affections of alloying element（such as Al, Ca, Zn, and Sn）, solute concentration on the growth selection and evolution of a-Mg dendrites during solidification were studied. The results indicate that the alloying elements and solute concentration would impose a significant influence on the morphology and orientation selection of the primary a-Mg dendrites. In Mg-Ca and Mg-Al（hcp-fcc） alloys, dendrites tend to grow with preferred orientation of 〈1120〉 or 2245 which is in good agreement with the traditional expected direction. 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 〈1120〉 and the remaining 12 along 〈112X〉（X≈2） off the basal plane. For the case in Mg-Zn alloys, an orientation transition from 〈1120〉 on the basal plane to 〈1121〉 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 of the growth interface which results in a dendrite orientation transition（DOT）.These findings partially reveal the underlying formation mechanism and origination of the diversity dendritic morphologies and branching structures of a-Mg dendrites in Mg alloys. Furthermore, with the fast X-ray imaging facility, in situ observations of the 3D microstructure evolution in Mg alloys during solidification are also carried out and the evolution of a-Mg dendrites are obtained for further analysis.