Subsurface microstructural changes in a cast heat resisting alloy caused by high temperature corrosion

A cast HP ModNb alloy (Fe-25Cr-35Ni-1Nb, wt.%) was oxidised and carburised in CO-CO2 corresponding to a(c)=0.1 and p(O2) = 3 x 10(-16) atm at 1080 degrees C. Formation of an external, chromium-rich oxide scale led to depletion of this metal in a deep alloy subsurface zone. Within that zone, secondary chromium-rich carbides dissolved, primary carbides oxidised. solute silicon and aluminium internally oxidised, and extensive porosity developed. Pore volumes correspond to the difference between metal loss by scaling and metal displacement by internal oxidation. assuming the scale-metal interface to be fixed. The pores are concluded to be Kirkendall voids. (C) 2009 Elsevier Ltd. All rights reserved.