Low-Temperature Thermochemical Treatments of Face-Centered Cubic Alloys: New Perspectives for Expanded Austenite From Austenitic Stainless Steels to High-Entropy Alloys

Abstract

Thermochemical treatments performed at low temperatures (< ∼500°C) have received increasing interest for the surface modification of austenitic stainless steels. In fact, when treatment media rich in nitrogen and/or carbon are used at these temperatures, the formation of chromium compounds is inhibited and the interstitial atoms are retained in the face-centered cubic lattice of austenite beyond the solubility limit. The obtained supersaturated solid solution, known as expanded austenite or S-phase, has high hardness and can maintain or even increase the corrosion resistance in many environments. In the international literature, many studies are present that highlight the effects of the formation of this phase on tribological properties, fatigue resistance, corrosion behavior, wettability, biocompatibility, and magnetic properties of austenitic stainless steels. However, using analogous treatment conditions, expanded austenite can be obtained in many other alloys having a matrix with a face-centered cubic lattice, such as austenitic steels, nickel and cobalt alloys, and the more recent medium- and high-entropy alloys, but the studies on this topic are mostly at their very beginning. In this review, the characteristics and properties of expanded austenite and of the modified surface layers in which it is present are analyzed and discussed, considering all the different alloys in which this supersaturated phase can be produced. The role of alloy elements in promoting or hindering the formation of expanded austenite and the competing compound precipitates is taken into account. The opportunities and challenges of the low-temperature treatments are highlighted, and possible future directions for the investigation are suggested.