Measurement of service-induced internal elastic strains in a single-crystal nickel-based turbine blade with convergent-beam electron diffraction

Abstract

Abstract During tensile creep deformation, single-crystal nickel-based superalloys develop a characteristic internal stress state, which leads to counteracting tetragonal lattice distortions of the two phases γ and γ'. The aim of this work was the evaluation of the related elastic lattice distortions in the microstruture of a turbine blade after service in an accelerated mission test with high lateral resolution by convergent-beam electron diffraction. For the determination of the local lattice parameters, the higher-order Laue zone (HOLZ) lines of the central diffraction disc were evaluated under the assumption of a tetragonal distorted crystal lattice. The zone-axis patterns were simulated considering dynamical effects of the diffraction process. In order to reduce the computation time, a set of HOLZ line patterns was calculated in advance. Experimental patterns were compared with dynamically simulated patterns by the analysis of characteristic areas between HOLZ lines. The dependence of two area fractions on the lattice parameters were fitted by an interpolation function. This function was used for the evaluation of the experimental patterns. With this new approach a relatively fast evaluation of lattice strains was possible.