This thesis explores solidification orientation relationships (ORs) in intermetallic compounds (IMCs) and Al and Mg alloys.
In the Al3Ti-TiB2 system, it is found that the nucleation of Al3Ti on TiB2 and the pushing and engulfment of TiB2 particles by growing Al3Ti crystals both form reproducible ORs during solidification. The nucleation OR is identified by solidifying multiple small Al3Ti crystals on one large (0001) facet of TiB2. Pushing and engulfment ORs are investigated by statistical analysis of EBSD measurements, DFT calculations of interface energies, and imaging of TiB2 particles being pushed and engulfed by Al3Ti facets. It is shown that the lowest energy OR is formed by nucleation as well as by pushing/engulfment. The higher energy ORs, formed by pushing and engulfment, correspond to local interfacial energy minima and can be explained by rotation of TiB2 particles on Al3Ti facets during pushing.
ORs formed by cyclic twinning of low symmetry IMCs is studied in Al3Ti, Ag3Sn, Al45Cr7 and Al13Fe4. It is argued that deeper undercooling induced by higher cooling rate favours the nucleation of metastable phases and/or the formation of short-range order with high symmetry in the melt, which then nucleated/transformed into stable phases with all orientation variants to the higher-symmetry parent phases.
This thesis then applies the new understanding developed in the previous chapters to explore the formation mechanism for the above-random proportion of special grain boundaries in FCC-Al and HCP-Mg after equiaxed solidification. Two main mechanisms are examined and, by combining statistical EBSD analysis and DFT calculations, it is found that the measured preferred grain boundaries with twin ORs correspond to local interfacial energy minima and, for the alloy systems studied here, it is likely due to the rotation and movement between neighbouring grains during solidification instead of nucleation from icosahedral quasicrystals and/or icosahedral short-range order.Open Acces
