Influence of 5 at.%Al-Additions on the FCC to BCC Phase Transformation in CrFeNi Concentrated Alloys

In the context of the recent developments in the field of high entropy alloys (HEAs), CrFeNi based alloys with the addition of Al have gained significant attention due to their interesting combination of mechanical properties and corrosion resistance, enabling an even wider range of applications for HEAs. A key feature of this system is the co-existence of multiple phases such as body centered cubic (BCC) and face centered cubic (FCC) phase, which significantly enhances the mechanical performance of the alloy. However, despite the ongoing research efforts, an in-depth study of the effect of Al on the phase transformation kinetics in this system is not yet available, which undermines the design of effective heat treatments, curbing the optimization of microstructures and properties. In this work, the influence of 5 at.%Al addition on the interdiffusion behavior and phase transformation kinetics from FCC to BCC phase at 700 °C is studied by experiments and state-of-the-art phase field simulations. The kinetics of BCC precipitation are observed experimentally through the sequential characterization of samples heat treated up to 11 days. 2D phase field (PF) simulations of the growth and coarsening of BCC-precipitates in a dual-phase BCC/FCC system are performed on the same alloy system. Experimental observations reveal that the growth of BCC is greatly enhanced by the addition of Al to the ternary CrFeNi system. This result is consistent with findings from the phase field simulation. PF results show that the gradients of the diffusion potentials and interdiffusion mobilities are increased in the case of Al addition, which explains the significant increase of the phase transformation rate from FCC to BCC. Finally, the cross terms in multi-component diffusion equations are found to significantly affect the evolution of the phases. These findings demonstrate the need of multicomponent models to fully understand the complex interdiffusion behavior in high and medium entropy alloys to foster the design of these materials.