Phase separation in liquid and amorphous Ni–Nb–Y alloys

Abstract

Phase separation in Ni–Nb–Y metallic melts is treated by thermodynamic calculations. The miscibility gap of the monotectic binary Nb–Y system extends into the ternary Ni–Nb–Y system up to 70 at.% Ni as a consequence of the positive mixing enthalpy between Nb and Y. Experimental evidence of phase separation in the melt of Ni 58.5Nb 20.25Y 21.25 (numbers indicate at.%) is obtained by in situ X-ray diffraction at elevated temperatures and differential scanning calorimetry. The phase separated melts of Ni 58.5Nb 20.25Y 21.25 and Ni 58Nb 10.1Y 30.9 can be frozen into a two-phase amorphous metallic alloy by rapid quenching from the liquid. The microstructure consists of two amorphous regions, one Nb-enriched and the other Y-enriched, with a size distribution from several nanometers up to micrometer-dimension. The two amorphous phases crystallize separately. In a first step the cubic Ni 2Y phase is formed by a polymorphous reaction from the Y-rich amorphous phase. At much higher temperature the remaining Nb-rich amorphous phase crystallizes into Ni 7Nb 6.