Influence of Undercooling on the Kinetics of the Peritectic Transition in an Fe-4.2wt%Ni Alloy

In-situ Laser Scanning Confocal Microscopy observations are presented that assess the influence of undercooling before the initiation of the peritectic transition in a Fe-4.2wt%Ni alloy on the resulting kinetics of the peritectic reaction and transformation. In a series of experiments varying the cooling rate, increasing the cooling rate led to a lower temperature at the L/delta interface. The resulting peritectic reaction changed from slow 840 mu m/s - 1500 mu m/s, with limited growth into the delta to rapid similar to 11mm/s with significant growth into delta. In continuous cooling experiments when the nucleation temperature was low, growth into delta was high and the reacting species was observed to propagate along the liquid/delta-ferrite interface at a rate of similar to 11mm/s. The peritectic reaction rate did not appear to be a function of temperature over a measured nucleation temperature range of 5 K. Conversely, the growth rate of austenite into the delta-ferrite in the first 0.03 seconds was observed to increase from 1.5mm/s to 8mm/s as the measured temperature at nucleation decreased.