Peritectic reaction and solidification in iron-nickel alloys

A high-temperature confocal scanning laser microscope (CSLM) has been used to study austenite (γ) precipitation and growth on δ-ferrite during peritectic reaction and solidification in the Fe-Ni system. The two stages of the peritectic transition involving the liquid phase-the reaction, which sees austenite grow along the ferrite/liquid boundary, and the direct solidification of austenite-were observed. It was found that, for both the hypoperitectic (4.2 pct nickel) and hyperperitectic (4.7 pct nickel) alloys, the reaction rate increases with increased undercooling. It was also observed that the reaction rates were faster in the hypoperitectic alloy. However, for both grades, the solidification rate was found to be a function of local-temperature gradients rather than undercooling. Comparisons to available models are made, and a description of possible rate-controlling phenomena is given.