Microstructural control and mechanical properties of nickel silicides

The alloying behavior, microstructure and mechanical properties of quaternary Ni-3(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 Lit Ni(Si,Ti) phase are ranked in the sequence of Nb > V > Cr greater than or equal to 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 Ni-3(Si,Ti) alloys and Ni3X-type compounds for the Nb-, Zr- and Hf-added Ni-3(Si,Ti) alloys. The second-phase dispersions (and/or the quaternary elements in the Ll(2)-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 Ni-3(Si,Ti) alloy is reduced while that of other quaternary Ni-3(Si,Ti) alloys was accelerated. Among the quaternary Ni-3(Si,Ti) alloys observed in this study, the Nb-added Ni(Si,Ti) alloy with the Nb-containing second-phase dispersion is shown to have the most favorable mechanical properties. (C) 2000 Elsevier Science Ltd. All rights reserved.