Degradation of single-crystal gas turbine blades

The work presented in this paper aimed to evaluate and classify the microstructural degradation of diffusion coated single-crystal gas turbine blade, so that the characterised features of degradation could be used for assessing the effective in-service material temperatures at the corresponding blade locations. Two potentially useful indicative measures were evaluated for this purpose: a) gamma prime coarsening and degradation in the substrate microstructure to indicate the condition and temperature in the blade interior and b) combined thickness of the intermediate diffusion zone (IDZ) and topologically close-packed (TCP) precipitate layers to indicate the condition and temperature at the blade coating. Both applied measures appear to provide meaningful indications of the service temperature, although not with the same level of uncertainty. The thickness of IDZ+TPC layers can indicate the effective temperatures to a relatively good accuracy as long as the IDZ and TPC layers remain sufficiently intact for the assessment. For verification and demonstration purposes, a case example is shown of an ex-service turbine blade.