Reactive isothermal solidification in the Ni–Sn system (original) (raw)

AbstractÐThe kinetics, morphologies, and mechanisms involved in Reactive Isothermal Solidi®cation 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 dierent durations at temperatures in the range 235±6008C. The following results were obtained. During solidi®cation, Ni 3 Sn 4 was the only phase detected by SEM, optical microscopy, and XRD. However, in electron diraction during TEM examination, the Ni 3 Sn phase was also found. When solidi®cation was complete, the other two equilibrium intermediate phases were detected by SEM and XRD. The Ni 3 Sn 4 grew mainly as a layer at the solid/liquid interface. The Ni 3 Sn 4 /liquid interface was not¯at, containing deep grain boundary grooves, and part of the layer had crumbled into the liquid. As the Ni 3 Sn 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 3 Sn 4 layer, too, was found to be parabolic, and with an activation energy of 27.621.7 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 3 Sn 4 layer grains crumble into the liquid, a process related to the grooving of the Ni 3 Sn 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 aect the growth kinetics. #