Dendritic Growth in Mg-Based Alloys: Phase-Field Simulations and Experimental Verification by X-ray Synchrotron Tomography MINGYUE WANG, YANJIN XU, QIWEI ZHENG, SUJUN WU, TAO JING, and NIKHILESH CHAWLA Changes in polycrystalline dendritic growth patterns during solidification result in a variety of solidified dendritic structures and morphologies. These microstructural changes are induced by a variety of effects such as the random distribution of nucleation sites and orientations, the interaction of growing individual dendritic grains, and effects of solid-liquid interfacial energy anisotropy. Here, we have studied the formation of the complicated and diverse dendrite morphologies both experimentally, by electron backscatter diffraction and by X-ray tomogra- phy; and numerically by three-dimensional phase-field simulations. Three binary magnesium alloys were considered in this study: Mg-Al, Mg-Zn, and Mg-Sn alloys. We show that the solidification microstructure can be attributed to the following factors: The interaction of the growing dendrites, the anisotropy of the growth, and the distribution and initial random ori- entations of nucleation sites. DOI: 10.1007/s11661-014-2200-x Ó The Minerals, Metals & Materials Society and ASM International 2014 I. INTRODUCTION DENDRITIC microstructures, which result from the dendritic growth of the solid-liquid interface (S/L), are ubiquitous in a wide range of solidification processes, such as casting, welding, etc. The dendritic microstruc- ture, along with the inter-dendritic distribution and segregation of elements and/or impurities, plays a crucial role in determining the uniformity and quality of the final casting. [1–5] In addition to its technological importance, dendritic solidification represents a rather classic example of self-organized formation in systems far from equilibrium. It is a phenomenon that occurs in nature and has been studied extensively. [6–8] Neverthe- less, the quantitative understanding of dendritic evolu- tion is still a major theoretical and experimental challenge within the materials community. Over the past few years, most of the research on dendritically solidified microstructures has been focused on single crystals with well-defined crystallographic axes. Studies on single crystals have provided an understanding of the orientation selection and evolution of dendritic growth. [9–12] However, microstructures of most materials are polycrystalline three-dimensional (3D) structures. Solidification patterns and morpholo- gies in polycrystalline materials in three dimensions are quite complex and many fundamental questions remain unanswered. [13] The interactions between multiple grains, which perturb the bulk diffusion behavior of the advancing arms of dendrites due to, for example, geometry and competing growth anisotropies, have a profound impact on polycrystalline morphological evo- lution, yielding a wide spectrum of topologically com- plex dendritic patterns, especially in three dimensions. [14] It is challenging to predict the complex microstructures of alloys in 3D, especially when we take into account the distribution, preferred directions of heterogeneous nucleation, and the wetting conditions for a grain on the surface of preexisting nuclei or impurity particles. Actually, considerable observations on microstructures of the industrial/commercial alloys indicate that most solidify into wide range of morphologies and topologies. It has long been suggested that inherent crystalline anisotropy plays a critical role in the evolution of solidified microstructures, especially in the context of dendrites. [5,9,15,16] Historically, the orientation-related anisotropy in face-centered cubic (FCC) and/or hexag- onal close-packed (HCP) metals has been described with just a single anisotropy term. More recently, however, atomistic-scale calculations, such as molecular dynam- ics, have demonstrated that accurate parameterizations of the S/L free energy for FCC and HCP metals generally require two or more anisotropy parameters MINGYUE WANG, formerly Ph.D. Candidate with the School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P.R. China, and Visiting Scientist with the Materials Science and Engineering, Arizona State University, Tempe, AZ 85287, is now Postdoctoral Fellow with the International Research Institute for Multidisciplinary Science, Beihang University, Beijing 100191, P.R. China. YANJIN XU, Postdoctoral Fellow, is with Beijing Aeronautical Manufacturing Technology Research Institute, Beijing 100024, P.R. China. QIWEI ZHENG, Ph.D. Candidate, and TAO JING, Professor, are with the School of Materials Science and Engineering, Tsinghua University. SUJUN WU, Professor, is with the International Research Institute for Multidisciplinary Science, Bei- hang University. NIKHILESH CHAWLA, Fulton Professor of Materials Science and Engineering, is with the Materials Science and Engineering, Arizona State University. Contact e-mail: nchawla@asu.edu Manuscript submitted October 1, 2013. Article published online February 13, 2014 2562—VOLUME 45A, May 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A