Nitriding of a high entropy FeNiMnAlCr alloy

The as-cast, two-phase, f.c.c. + B2, lamellar-structured, high-entropy alloy Fe28.2Ni18.8Mn32.9Al14.1Cr6 was annealed in pure nitrogen at either 800 °C or 1000 °C for 100 h in order to harden the surface by nitridation. Similar nitriding treatments were performed on a comparably-structured, four-component alloy Fe30Ni20Mn35Al15 for comparison. Scanning electron microscopy and energy dispersive X-ray spectroscopy were used to analyze the resulting microstructures and phase compositions. AlN formation near the surface completely depleted the matrix of Al resulting in a f.c.c. matrix containing AlN particles, while a two-phase lamellae structure remained in the center of the specimens. Nevertheless, the AlN-containing region exhibited a higher hardness than the AlN-free area. Interestingly, the presence of Cr led to much more rapid diffusion of nitrogen into the material.