Microstructure of Al 3Sc with ternary transition-metal additions

Abstract

The microstructure of binary Al 3Sc and ternary Al 3(Sc 1− y X y ), 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 L1 2 solid-solution phase with some Kirkendall porosity. At the highest concentration studied, a second phase precipitated with the D0 19 (Y), D0 22 (Ti) or D0 23 (Zr and Hf) structure. Conversely, alloys with Group VA additions exhibited both the L1 2 trialuminide phase and a dendritic trialuminide second phase with D0 22 structure for all concentrations studied. The solubility limit in the ternary L1 2-type Al 3(Sc 1− y X y ) 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-L1 2 phases decreases from the Group IIIA trialuminide to the Group VA trialuminides. The lattice parameter of the L1 2 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 L1 2 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.