Austenitic-to-austenitic-ferritic stainless steel transformation via PVD powder surface functionalization and spark plasma sintering

The present work investigates a new alloy design approach to elaborate stainless steel grades with an austenitic-ferritic microstructure. The originality of the study is the use, as starting material, of a 316 L austenitic powder coated by a thin chromium layer deposited by physical vapor deposition (PVD) technique. The coated powder was then consolidated by spark plasma sintering (SPS), a powder metallurgy process allowing the fast elaboration of dense materials with a fine-grained microstructure. The chromium coating, characterized by scanning and transmission electron microscopy, presents a columnar microstructure, formed by nanometric crystallites, well reproduced by the simulation of the film growth. The characterizations performed after sintering show that the initial austenitic powder particles are still visible in the bulk microstructure. On the other hand, a tetragonal σ phase enriched in chromium and molybdenum forms in the interparticular regions. After annealing treatment followed by quenching, the tetragonal phase transforms into the expected ferrite. The results prove that using a coated powder is a promising and innovative way to elaborate new steel grades with a two-phase austenitic-ferritic microstructure. This original approach can have the advantage of obtaining steels with a controlled microstructure and the desired amount of phases in the final bulk. [Display omitted]