Microstructure and hardness of scandium trialuminide with ternary rare-earth additions

The microstructure of ternary Al-3(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 O <= y <= 0.75. Alloys with La, Ce, Nd, Sm or Eu additions consist of a L1(2) phase containing a dendritic second phase with D0(19) (La, Ce, Nd, Sm) or C11(b) (Eu) structure. Alloys with Yb or Lu additions show a single L12 phase. The RE solubility limits at 1373 K in the L1(2)-Al-3(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 L1(2) 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 L1(2) solid-solution increases linearly with increasing RE concentration.