Metastable phases in the Ni75Nb12B13 alloy prepared under nonequilibrium conditions

Abstract

The structure of Ni75Nb12B13 alloys prepared by liquid quenching (LQ) and mechanical alloying (MA) has been studied by x-ray diffraction. The alloy prepared by LQ at a cooling rate of ∼106 K/s is shown to be fully amorphous, while MA yields an amorphous-crystalline material in which the predominant phase is an fcc Ni〈Nb,B〉 solid solution. The thermal stability of the alloys and their structural transformations on heating have been studied by differential scanning calorimetry. The amorphous phase obtained by LQ is shown to crystallize at 490°C. After heating to 720°C, the alloy consists of two equilibrium phases: Ni21Nb2B6 (τ) and Ni5Nb3B2 (z). Heating the MA alloy to 720°C leads to the formation of a stable τ-phase, while the Ni-based fcc solid solution remains supersaturated and, hence, metastable. Increasing the milling time leads to the formation of nanocrystalline τ and Ni3B phases, in addition to the Ni-based fcc solid solution, which corresponds to the equilibrium phase composition of the Ni75Nb12B13 alloy in the Ni-Nb-B phase diagram. The effect of high-energy milling on the phase composition of the alloy is similar to that of heat treatment.