Microstructure and Hardness of Scandium Trialuminide with Ternary Rare-Earth Additions

The microstructure of ternary Al3(Sc1-yREy) intermetallic compounds (where RE is one of the rare-earth elements La, Ce, Nd, Sm, Eu, Yb or Lu), was investigated as a function of RE concentration for 0<y≤0.75. Alloys with La, Ce, Nd, Sm or Eu additions consist of a L12 phase containing a dendritic second phase with D019 (La, Ce, Nd, Sm) or C11b (Eu) structure. Alloys with Yb or Lu additions show a single L12 phase. The RE solubility limits at 1373 K in the L12-Al3(Sc1-yREy) phase are very low for La, Nd, Ce and Eu (0.08-0.41 at.% or y=0.0032-0.0164), low for Sm (3.22 at.% or y=0.1288) and complete for Yb and Lu. The lattice parameter of the L12 solid-solution increases linearly with RE concentration and the magnitude of this effect is correlated with the atomic size mismatch between Sc and the RE elements. The Vickers micro-hardness of the L12 solid-solution increases linearly with increasing RE concentration.