SOLID SOLUBILITIES OF TERNARY ELEMENTS AND THEIR EFFECTS ON MICROSTRUCTURE OF MOSI2

Alloying effects on the microstructures and solid solubilities have been investigated experimentally using alloyed MoSi2 specimens slowly cooled from 1473 K. It was found that alloying elements in MoSi2 were classified into three groups: (a) W and Re, both of them were mainly substituted for Mo atoms, and made a complete solid solution of MoSi2. (b) Cr, Zr, Nb, Ta and Al were soluble in MoSi2 to some extent, but the excess addition of them resulted in the formation of the second phase with a lamellar-like or a massive domain structure. The solubility limits of Cr, Zr, Nb and Ta were about 1.0 mol%, 1.1 mol%, 1.3 mol% and 5.0 mol%, respectively. Al was also soluble in MoSi2 up to about 3.0 mol%, but it was mainly substituted for Si atoms in MoSi2, in contrast to the substitutional behavior of the other alloying elements. (c) Ru and Hf were not soluble in MoSi2, but made the second phase with a lamellar-like structure. For example, the silicides, Ru2Si3 or HfSi2, were formed by the Ru or Hf addition. In addition, the eutectic phases appeared in the Al, Nb or Ta containing system. Furthermore, it was shown that the Vickers hardness increased largely by the addition of most soluble elements, due to the solid-solution hardening. However, the solid-solution softening was also observed in the case of Ta and Al additions.