Effect of an aluminide coating on precipitate rafting in superalloys

The microstructure of single crystal superalloy SRR99 is shown to be significantly affected by the combined effect of the application of an aluminide diffusion coating and the mechanical deformation caused by thermomechanical fatigue. In the case of an R = - infinity cycle (compressive strains), plates parallel to the coating surface develop in a layer underneath the subcoating zone, markedly differing from the precipitate coalescence observed in the bulk. This effect is absent in the R = 0 cycles (tensile strains). This phenomenon is successfully explained in the light of a criterion for the prediction of gamma' rafting, by considering the superposition of the strains produced both by the TMF cycling and by a layer of gamma' which is observed to develop during the tests with R = - infinity.