[Séminaire] - Stability and metastability in phases transformations : examples from solid-state decomposition and oxide/hydroxide formation

The stability and metastability of phases play a central role in controlling phase formation in metallic systems under both heat treatment and aqueous corrosion conditions. This presentation examines phase transformations in a Fe18Mn3Ti steel, followed by an analysis of oxide and hydroxide formation during the aqueous corrosion of stainless steel.
Medium manganese and maraging steels are both typically produced via martensitic transformation, by quenching a face-centred cubic (FCC) austenite high-temperature phase into a supersaturated body-centred cubic (BCC) ferritic phase. In a newly proposed Fe18Mn3Ti (wt.%) alloy, the metastable Mn-rich martensitic phase undergoes compositional redistribution during aging at approximately 450 °C. Subsequent annealing leads to Mn segregation to grain boundaries and dislocations, followed by homogeneous phase decomposition and the formation of α-Mn nanoprecipitates. The strong tendency for segregation in this system is connected to the ferromagnetism of Fe and the antiferromagnetism of Mn. The addition of 3 wt.% Ti enables martensitic transformation during quenching and stabilizes the α-Mn precipitates.
A related interplay between stable and metastable phases is observed during the aqueous corrosion of 420 stainless steel. Thermodynamic calculations show that metal hydroxides are metastable relative to their corresponding oxides, but hydroxide from solution can be kinetically favored due to strong solvation of metal ions. Potentiostatic experiments and ambient-pressure X-ray photoelectron spectroscopy reveal potential-dependent growth of iron oxides, stabilization of chromium hydroxides, and the destabilization of chromium oxides under anodic conditions, accompanied by the formation of Cr⁶⁺ species.
These results illustrate how phase transformations in both solid-state processing and corrosion processes are governed by the relative stability of phases and the kinetic pathways available for phase formation.

Séminaire organisé par le Département SI2M dans le cadre du programme interdisciplinaire MEDICIS.