Microstructural control and mechanical properties of nickel silicides

The alloying behavior, microstructure and mechanical properties of quaternary Ni3(Si,Ti) polycrystals, which were alloyed with transition elements, V, Nb, Zr, Hf, Cr and Fe beyond their maximum solubility limits, are investigated. The solubility limits of the quaternary elements in the Ll2 Ni3(Si,Ti) phase are ranked in the sequence of Nb>V>Cr≧Mn>Fe>Hf>Zr, and correlated with the size misfit parameter between Si and the quaternary element X, and the difference in the formation enthalpy between Ni3Si and Ni3X. The second-phases (dispersions) formed beyond the solubility limit are identified: fcc-type Ni solid solution for the V-, Cr- and Fe-added Ni3(Si,Ti) alloys and Ni3X-type compounds for the Nb-, Zr- and Hf-added Ni3(Si,Ti) alloys. The second-phase dispersions (and/or the quaternary elements in the Ll2-phase matrix) result in strengthening over a wide range of temperature. Tensile elongation at high temperatures is improved by the introduction of second-phase dispersions. The moisture-induced embrittlement of the V- and Nb-added Ni3(Si,Ti) alloy is reduced while that of other quaternary Ni3(Si,Ti) alloys was accelerated. Among the quaternary Ni3(Si,Ti) alloys observed in this study, the Nb-added Ni3(Si,Ti) alloy with the Nb-containing second-phase dispersion is shown to have the most favorable mechanical properties.