Microstructure of Al3Sc with ternary transition-metal additions

The microstructure of binary Al3Sc and ternary Al3(Sc1−yXy), where X is one of the transition metals from Group IIIA (Y), IVA (Ti, Zr or Hf) or VA (V, Nb or Ta), was investigated as a function of alloying element concentration for 0.1≤y≤0.75. Alloys with Group IIIA and IVA additions exhibited a single L12 solid-solution phase with some Kirkendall porosity. At the highest concentration studied, a second phase precipitated with the D019 (Y), D022 (Ti) or D023 (Zr and Hf) structure. Conversely, alloys with Group VA additions exhibited both the L12 trialuminide phase and a dendritic trialuminide second phase with D022 structure for all concentrations studied. The solubility limit in the ternary L12-type Al3(Sc1−yXy) phase was high for Group IIIA and IVA metals (almost 12.5 at.% or y=0.5), and much lower for Group VA metals (from about 1.8 at.% or y=0.07 for Ta to about 2.7 at.% or y=0.11 for V). Similarly, the solubility limit of Sc in the non-L12 phases decreases from the Group IIIA trialuminide to the Group VA trialuminides. The lattice parameter of the L12 solid-solution decreased linearly with increasing concentration of Group IVA and VA metals, but increased linearly with concentration of Y (Group IIIA). This linear concentration dependence of the lattice parameter is found to correlate with the atomic size mismatch between Sc and the transition metal. The microhardness of the L12 solid-solution increased linearly with increasing concentration of ternary elements. The concentration dependence of hardness is strongest for Group VA metals and weakest for Group IVA metals, for which a correlation is found with the concentration dependence of lattice parameter.