Reactive isothermal solidification in the Ni–Sn system

Abstract

The kinetics, morphologies, and mechanisms involved in Reactive Isothermal Solidification were investigated. Experiments involved the Ni–Sn binary system. Three types of samples were prepared: powder samples, samples for dipping experiments, and model multi-layer samples. The samples were heat treated for different durations at temperatures in the range 235–600°C. The following results were obtained. During solidification, Ni 3Sn 4 was the only phase detected by SEM, optical microscopy, and XRD. However, in electron diffraction during TEM examination, the Ni 3Sn phase was also found. When solidification was complete, the other two equilibrium intermediate phases were detected by SEM and XRD. The Ni 3Sn 4 grew mainly as a layer at the solid/liquid interface. The Ni 3Sn 4/liquid interface was not flat, containing deep grain boundary grooves, and part of the layer had crumbled into the liquid. As the Ni 3Sn 4 layer grows, lateral grain growth occurs. The time-dependence of the mean grain size obeys a parabolic law. In short-duration experiments, the growth kinetics of the Ni 3Sn 4 layer, too, was found to be parabolic, and with an activation energy of 27.6±1.7 kJ/mol. This value corresponds to the activation energy for the diffusion of Ni in liquid Sn. In long-time experiments deceleration of the growth kinetics was observed. Simultaneously with the phase growth and the lateral grain growth the coarse Ni 3Sn 4 layer grains crumble into the liquid, a process related to the grooving of the Ni 3Sn 4 layer which takes place at the microstructural transition zone (MTZ). The crumbling as well as the lateral grain growth are considered to be the factors that affect the growth kinetics.