Cyclic Oxidation of High Temperature Materials (EFC 27) PDF

ABSTRACT

Finite-element analyses have been performed of the oxide spallation process in which decohesion occurs by the growth of a wedge crack along the oxide/metal interface. For conditions of direct cooling, the growth of the interfacial crack is inhibited by stress relaxation at its tip associated with alloy creep but the final stage of crack growth occurs rapidly at intermediate temperatures where creep is limited. Results are provided for the alumina-forming Haynes 214 alloy cooled from 1100°C from which it is shown that high cooling rates impair spallation resistance. These general cracking kinetics where an extended quiescent period is followed by rapid propagation gives confidence in the validity of the Critical Strain Energy Criterion for predicting spallation. Initial results are also provided on a chromia-forming austenitic steel of the effect of thermal cycling on stress development within the oxide layer and across the oxide / metal interface. The importance of creep relaxation is again emphasised since this allows in-plane tensile stresses to develop in the oxide layer on the return to temperature. The significance of these on oxide cracking and oxidation resistance is discussed.

Edited by: M. Schütze, W.J. Quadakkers