Active screen plasma nitriding of a Si-alloyed FeCrNi medium entropy alloy: High interstitial absorption and an anomalous Si-induced decomposition mechanism in N-expanded austenite

Nitrogen-expanded austenite (γ_N), also known as S phase, is a metastable, interstitial supersaturated phase, that has been extensively studied on AISI 304/316 stainless steel (SS) following N diffusion treatments at low temperatures. Substantial Cr segregation can occur in γ_N-304/316 under CrN formation at elevated treatment temperatures (typically at above 450 °C), resulting in degradation in corrosion performance. In contrast to 316 SS (mainly based on the Fe-Cr-Ni system), a FeCrNiSi_0.5 medium entropy alloy (MEA) was investigated after active screen plasma nitriding (ASPN) at 430–480 °C in this study. Attributable to the equimolar Cr content in the substrate, the ASPN-treated MEA surface showed “excessive” N absorption, which was accompanied by severe surface micro-cracking and a topmost nanocrystalline layer. Nanoprecipitates (∼3-8 nm) can be seen in γN-MEA at treatment temperature as low as 430 °C. However, STEM-EDX analysis of those nanoprecipitates (∼5-30 nm) at 480 °C showed significant Si segregation without observable Cr redistribution, resulting in a uniform “greyish” etched treatment layer. The sluggish Cr segregation in the N-modified MEA surface could originate from both the minor Si addition (via Si-induced nanoprecipitation) and the high equimolar Ni/Cr content in the host matrix.