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 (gamma 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 tem-peratures. Substantial Cr segregation can occur in gamma N-304/316 under CrN formation at elevated treatment temperatures (typically at above 450 degrees C), resulting in degradation in corrosion performance. In contrast to 316 SS (mainly based on the Fe-Cr-Ni system), a FeCrNiSi0.5 medium entropy alloy (MEA) was investigated after active screen plasma nitriding (ASPN) at 430-480 degrees 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 gamma N-MEA at treatment temperature as low as 430 degrees C. However, STEM-EDX analysis of those nanoprecipitates (-5-30 nm) at 480 degrees 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.

Automated summary

Nitrogen-expanded austenite (γN), also known as S phase, is a metastable phase that has been extensively studied on AISI 304/316 stainless steel following low temperature N diffusion treatments. Nitrogen absorption and Si segregation were observed on the surface of FeCrNiSi0.5 medium entropy alloy (MEA) after active screen plasma nitriding (ASPN) treatment. The sluggish Cr segregation in the N-modified MEA surface could be attributed to Si-induced nanoprecipitation and the high equimolar Ni/Cr content in the host matrix.