Relation between oxygen activity gradient in the internal oxidation zone of Mn alloyed steel and the composition of oxide precipitates

An internal oxidation zone with (Mn1-x,Fe-x)O mixed oxide precipitates occurs after annealing a Fe -1.7 at% Mn steel at 950 degrees C in N-2 plus 5 vol% H-2 gas mixture with dew point of 10 degrees C. Local thermodynamic equilibrium in the internal oxidation zone is established during annealing of the Mn alloyed steel. As a result, the composition of (Mn1-x,Fe-x)O precipitates depends on the local oxygen activity. The oxygen activity decreases as a function of depth below steel surface, and consequently the concentration of Fe decreases in the (Mn1-x,Fe-x)O precipitates. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.