Microstructural Evolution and Phase Formation in Rapidly Solidified Ni-25.3 At. Pct Si Alloy

Abstract

The drop-tube technique was used to solidify droplets of the Ni-25.3 at. pct Si alloy at high cooling rates. XRD, SEM, and TEM analysis revealed that the metastable phase, Ni₂₅Si₉, formed as the dominant phase in all ranges of the droplets, with γ-Ni₃₁Si₁₂ and β ₁-Ni₃Si also being present. Three different microstructures were observed: the regular and anomalous eutectic structures and near single-phase structure containing small inclusions of a second phase, termed here as heteroclite structure. Both eutectic structures comprise alternating lamellae of Ni₂₅Si₉ and β ₁-Ni₃Si, which, we conjecture, is a consequence of an unobserved eutectic reaction between the Ni₂₅Si₉ and β ₁-Ni₃Si phases. The matrix of the heteroclite structure is also identified as the metastable phase Ni₂₅Si₉, in which twined growth is observed in the TEM. As the cooling rate is increased (particle size decreased), the proportion of droplets displaying the entire heteroclite structure tends to increase, with its fraction increasing from 13.91 pct (300 to 500 µm) to 40.10 pct (75 to 106 µm). The thermodynamic properties of the Ni₂₅Si₉ phase were also studied by in-situ heating during XRD analysis and by DTA. This showed the decomposition of Ni₂₅Si₉ to β ₁ and γ-Ni₃₁Si₁₂ for temperatures in excess of 790 K (517 °C).