Microstructure and Microtexture Formation of AZ91D Magnesium Alloys Solidified in a Static Magnetic Field

In the present study, we solidified AZ91D magnesium alloys in a static magnetic field with a magnetic flux density up to 10 Tesla (T). Three different regions can be identified in a solidified alloy, according to their microtexture and microstructure; these regions are: (a) region (A), the central region with equiaxed dendrites, (b) region (B), the transitional region with directional dendrites grown from an unmelted region, and (c) region (C), the edge region with cellular dendrites grown from the substrate of the container. No detectable difference can be discerned with regard to the influence of the magnetic field on the microstructure in region (A). However, the microtexture evolution in the region shows a strong dependence on the magnetic field and the preferential orientation formation is briefly elucidated. Special attention is focused on the orientation development of the directional dendrites in region (B) as a function of the magnetic flux density B ₀, when the electron backscatter diffraction (EBSD) technique is employed. It is shown that the directional dendrites exhibit a random orientation distribution at B ₀ < 5 T, while they have a unique orientation at B ₀ ≥ 5 T. Microstructure observation indicates that the crystallographic orientation selection completes at the interface of the melt/unmelted regions. A theoretical analysis reveals that the competition between the magnetization energy and the anisotropic interfacial energy difference of the crystals controls the crystallographic orientation selection. For the microstructure and microtexture in region (C), heterogeneous nucleation takes place from the polycrystalline Al₂O₃ substrate and thus enables a random crystallographic orientation distribution. The short growth interval may not be sufficient for the preferred growth direction to be selected near the chilling area; this applies to the cellular dendrites grown from the substrate at all levels of the magnetic field.