Microstructural instabilities in an industrial gas turbine engine vane

Microstructural changes occurring in a Udimet 500 turbine vane during service have been examined. The gamma prime (γ’) content and carbide contents determined experimentally were in close agreement with numerical estimates obtained by Decker’s⁴ method. The method of Barrows and Newkirk¹⁰ for predicting sigma phase formation was examined and numerical predictions compared with experimental observations. The γ matrix was correctly predicted to be sigma prone when these calculations were applied to experimentally determined γ phase compositions. When the calculations were applied to computationally determined compositions the γ phase was predicted to be stable if residual aluminum in the γ phase was considered to act as a stabilizing element, or unstable if aluminum was considered to promote the formation of sigma. These discrepancies are discussed in terms of the effects of carbide and sigma formation on matrix chemistry and the interdependence of these reactions.