Phase separation in Ni₇₀Nb_30-xY_x glasses

Phase-separated metallic glasses in the ternary Ni-Nb-Y system were prepared by rapid quenching of the melt. For Ni-rich alloys, an early stage of spinodal decomposition or an almost homogeneous glassy state is obtained due to a lowering of the critical temperature of the liquid-liquid phase separation near to the glass transition temperature. In-situ small-angle X-ray scattering at elevated temperature gives evidence for ongoing phase separation of the glass prior to crystallization. The structural changes during isothermal heat treatment point to a spinodal mechanism of the decomposition. For glass with low Y-content (5 at%), no indication for phase separation is found in accordance with the composition dependence of the metastable miscibility gap of the supercooled liquid. Upon heating, the phase separated glass acts as a precursor and is the reason for the nanostructure of the Ni₂Y-phase formed during the first stage of crystallization.