Reactive interdiffusion of an Al film and a CoCrFeNi high-entropy alloy at elevated temperatures

The phase evolution of alloys is closely related to atomic diffusion. The influence of reactive diffusion on phase formation in high-entropy alloys (HEAs) is however still unclear. The present work systematically investigates the phase evolution of a multicomponent CoCrFeNi/Al diffusion couple through isochronous-reactive interdiffusion experiments. This provides a direct way to study the influence of enthalpy and entropy on the phase formation and element diffusion behavior. At temperatures below 1173 K, the enthalpy contribution dominates the total energy, leading to the formation of intermetallic compounds. When the temperature is in the range of 1173–1573 K, the entropy of mixing starts to play a more important role. This causes diffusion of Al towards the HEA without phase transformation, forming a more disordered state on the microscale. Even after the system reaches a disordered state, the enthalpy contribution cannot be totally ignored, which is reflected by the uphill diffusion of Ni towards Al. This demonstrates the combined effects of entropy and enthalpy on the phase formation in HEAs at elevated temperatures. Considering the homologous temperature for different equimolar alloys reveals that a multicomponent configuration does not stabilize the disordered state, while the mixing enthalpies between atomic pairs have a large impact on the transition temperature from ordered to disordered state. Finally, it is shown that surface modification of the HEA can be realized through a combination of film deposition and annealing processes. Compared to the HEA matrix, the formation of intermetallic compounds results in a hard surface layer. After the system becomes disordered, the higher hardness of the film side compared to the matrix can be attributed to the lattice distortion induced by Al.