Phase selection during solidification of Ni-10.95 at. % and B-3.23 at. % Si alloy

Abstract

To gain insight into the solidification of the Ni-10.95 at. % and B-3.23 at. % Si alloy, 29 cast samples extracted from a casting of this alloy were remelted under an Ar atmosphere, using either DSC or an in-house thermal analysis apparatus. Ni(fcc) and Ni3B phases were identified by X-ray diffraction of both the remelted and original cast samples; however, microstructural differences were observed between the two. These microstructural differences are associated with the high undercooling observed during the eutectic reaction in the remelted samples. This high undercooling results from the difficulty of Ni3B nucleation, which allows the formation of a NixBymetastable phase. Metallographic inspection shows that the metastable phase solidifies at least partially through a non-cooperative growth mechanism. This metastable phase may be the elusive Ni23B6 phase. The melting temperature of the metastable phase is 985 °C, and it solidifies below 943 °C under the experimental conditions used in this study. The temperature of solid transformation of the metastable constituent to Ni3B and Ni is strongly dependent on the amount of Ni3B formed during solidification. The minimal temperature of such transformation, 834 °C, corresponds to samples where Ni3B solidification was not detected in the thermal curves.