In-situ study and simulation of incipient melting of single crystal superalloy

Directionally solidified single crystal superalloys commonly exhibit significant levels of microsegregation, which makes solution heat treatments necessary. In order to avoid incipient melting during the heat treatment, temperature is commonly ramped up in a slow fashion allowing diffusion to partially homogenise the structure. This however is adding a lot of time to the heat treatment. This work aims at developing a better understanding of the kinetics of incipient melting and its link to the microstructure inherited from solidification processing.
Samples CMSX-4 that were produced via directional solidification at different rates were rapidly heated under protective Ar-atmosphere within the hot-chamber of a confocal-laser-scanning microscope (CSLM). The onset of melting and the development of the melt pool was directly observed, while the temperature of the sample was measured using a thermocouple placed underneath the crucible. Thermodynamic calculations using the Scheil model within ThermoCalc were used and additionally microstructure evolution was simulated using the phase-field software MICRESS.
The experiments show that melt pools form in the interdentritic region and spread along dendrite boundaries before extending into dendrites. The fraction-liquid vs. temperature profile obtained from CSLM and MICRESS simulations both show deviation from the Scheil curve upon melting.