Study Of Three Dimensional Microstructural Morphologies Of Dendritically Solidified α-Mg

Magnesium has a hexagonal close-packed (h.c.p.) structure, as a result, some newissues appeared. Indeed, it is very unclear to date about how to describe theoretically themorphologies, orientation selection and evolution mechanism of h.c.p. dendrites.The main objective of the present study is to establish a fundamental understandingof the3D morphologies and orientation selection of dendritic primary α-Mg solidsolution phases. In order to elucidate this question, Mg Al, Mg Zn and Mg Snbinary alloys were selected. The present work are principally foused on studying3Dmorphologies characteristics of α-Mg primary dendrite phases formed within the alloymelts with a combination of experimentally3D X-ray tomography technique andcomputationally phase-field simulations of equiaxed dendritic growth and directionalfreezing of magnesium alloys. It includes three main axes:1) as-solidifiedmicrostructures of magnesium alloys were characterized by synchrotron-based X-raytomography, and various dendritic growth patterns of α-Mg were obtained, themorphological feature and orientation selection phenomena were thoroughly analyzed;2) a3D phase-field model of dendritic growth was built, using the ξ-vector formalism totake into account the effect that the anisotropy of solid liquid interface (S/L) freeenergy has on dendritic growth morphologies. The general rule of h.c.p. α-Mg3Ddendritic growth and some valuable conclusions were gained;3) an algorithm of3Dpolycrystalline solidification of dendritic growth was developed and carried out usingthe parallel computing.The results have demonstrated that the morphology of α-Mg solid solution formedfrom the different melts possess the diversity aesthetically. In Mg Al alloy melts, α-Mg3D dendrites are characterized by typical plate-like structures of six-fold symmetry,which suggests that the growth of the third dimension of α-Mg dendrite, that is c-axis, issuppressed. In Mg Zn alloy melts, α-Mg3D dendrites perform the richerstructural feature morphologically. The experimental results also demonstratethat six secondary branched structure behind advancing dendrite tip that is,two secondary arms of six with the60°angle deviated from primary trunks arelocated in {0001} basal plane, and other four lie in the crossing {1011} crystallographic faces, which has not given the obvious hint about the specificangle is one of the main phenomenon.3D phase field simulations confirm theabove experiments and strongly support the hypothetical dendritic orientationgrowth model.2D cross-sections of Mg Sn alloys indicate that3D α-Mg(Sn)morphologies will be more complicated which clearly need to be penetratedfurther. Furthermore, the morphological diversity of α-Mg dendrites is thebyproduct of interaction between nucleation, anisotropy, melts, grain-boundaryand overlapped diffusion etc.It has also been demonstrated both computationally and experimentally thatthe morphologies of α-Mg3D dendrites solidified from the alloy melts exhibitmuch more richer and fascinating than previously anticipated, that is, α-Mg3Ddendrites are remarkably influenced by solid solution elements and thecharacteristics of surrounding melts, in addition, heterogeneous nucleation andassociated wetting angle, distribution etc., and the interaction of multigrainoverlapped diffusion and/or grain boundaries are all responsible for themorphological diversity. Phase field simulations in this contribution forecast tosome extent the formation of topographically and morphologically complicatedα-Mg3D dendritic structures, in addition, this research also gives an uniqueinsight on tailoring of solidified microstructures by controlling orientationsselection.