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

•Reactive interdiffusion of a CoCrFeNi high-entropy alloy (HEA) and a sputter deposited Al layer was carried out.•The phase formation during isothermal annealing at 773 K was studied using structural characterization at multiple length scales.•The diffusion and phase evolution are controlled by the Gibbs free energy gradient in front of the interface.•The sluggish diffusion effect in HEAs does not play a dominant role in determining phase constitution in the current system.•Surface modification in HEAs can be realized through a combination of sputter deposition with annealing.