Grain boundary ridge formation during initial high temperature oxidation of Mn/Al TRIP steel

Abstract

Confocal scanning laser microscopy (CSLM) was used in real-time observation of alloy element oxidation of a Mn/Al TRIP steel in an Ar–O₂ atmosphere. CSLM images reveal a marked role of grain boundaries in the overall initial oxidation kinetics of the alloy, and consequently in the morphology of the initial surface oxide. The oxidation on the alloy surface is dominated by the formation of Mn-rich oxide ridges along grain boundary traces on the surface. Oxide ridge formation kinetics was quantified by measurements on images extracted from real-time recordings of surface oxide evolution. Oxide ridge growth was found to take place at a constant rate. Scanning electron microscopy (SEM) images of the oxidized surfaces showed homogenous oxide ridges along straight grain boundary traces and heterogeneous oxide ridges along non-straight grain boundary traces. A transport mechanism of Mn to the surface is proposed, which relies on grain boundary segregation of Mn and on a relationship between grain boundary diffusivity and grain boundary character. It is suggested that when regarding alloys with significant grain boundary segregation of a solute, separate Wagner balances for internal vs. external oxidation is required for the grain lattices and the grain boundaries, respectively.

Acknowledgements

Wayne Jennings of the Dept. of Materials Science at Case Western Reserve University, Cleveland, Ohio, is gratefully acknowledged for his careful work on the Auger Electron Maps. The thoughtful comments of Dr. Lidong Teng are acknowledged and appreciated. The research results presented in this paper were supported by funds from the National Science Foundation under grant DMR-0307188.