Studying rapid solid-state phase transformations by combining controlled pulse heating experiments and Calphad consistent modeling considering the non-equilibrium state of the interface – RAPID-SOLID

In rapid solid-state transformations, diffusion kinetics in the bulk are low in both, growing and parent phases, creating a global deviation from equilibrium of the system. However, interface non-equilibrium mechanisms also take place, such as solute drag and attachment kinetics. The energies dissipated by the migration of the interface and the transfer of solute through the interface have to account for these local mechanisms. This leads to a higher amount of undercooling during quenching and a distinct deviation from local equilibrium at the interface. As consequence, a wide variety of solid-state transformations and types of microstructures can be observed as a function of undercooling.

The purpose of the project is understanding the influence of the thermodynamic state of the interface on transformation mechanisms and phase selections. The transitions under focus are (massive-type transformation) from diffusion controlled to interface-diffusion controlled and (martensitic-type transformation) from interface-diffusion controlled to interface controlled. The conditions at the interface are characterized analyzing its thermodynamic state by combining experiment and simulation. Controlled pulse heating experiments that allow for an in-situ, high resolution measurement of T-t curves and their assignment to local microstructures as well as high resolution electron microscopy/spectroscopy are employed. A thermodynamic and kinetic model that considers velocity-dependent phase equilibria and interface thermodynamics is developed. Being fully coupled with Calphad databases, simulations of the experiments are performed to reach direct one-to-one comparisons. The combination of the experimental and numerical investigations is the foundation for this collaborative project and its final goal: revisit interpretations of the transitions between solid-state transformations involving massive type interface diffusion controlled reactions.