A comparative study on the oxidation behavior and failure mechanisms of conventional NiCoCrAl alloy and in-situ composite AlCoCrFeNi_2.1 eutectic high-entropy alloy at 1300 C

We present a comparative study on oxidation behavior and failure mechanisms of conventional NiCoCrAl alloys doped with Y and Hf (CNA) and in-situ composite AlCoCrFeNi2.1 eutectic high-entropy alloy doped with Y and Hf (ISC-EHEA) at 1300 degrees C. We demonstrate that the ISC-EHEA has much stronger resistance to surface rumpling and oxide spallation than CNA. Hybrid molecular dynamics (MD) and Monte Carlo (MC) simulations show that the diffusion coefficients of the metal elements in the Al-depletion layer of the ISC-EHEA are 50 % lower than those in the CNA. The low diffusion coefficients lead to low growth stress in the thermally grown Al2O3 scale on the ISC-EHEA and improve the creep resistance of the metal in contact with the scale, thus preventing the occurrence of rumpling. The oxidation rate constant of the ISC-EHEA is similar to 32 % lower in comparison to those of the CNA, which is attributed to the coarser columnar Al2O3 grains that effectively mitigate grain boundary diffusion. The rumpling-free metal/oxide interface, the low residual stress and the low oxide growth rate for the ISC-EHEA result in strong resistance to scale spallation. Owing to the strong scale/alloy bonding at the interface and the build-up of strain energy during prolonged oxidation, damage in the Al2O3 scale on the ISC-EHEA is initiated as surface cracks rather than interface decohesion. When re-oxidized at 1300 degrees C, the ingress of oxygen along the surface cracks results in fast growth of new oxides at the metal/oxide interface, which causes local stress concentration, interface crack propagation and scale spallation.