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

During tensile creep deformation, single-crystal nickel-based superalloys d evelop a characteristic internal stress state, which leads to counteracting tetragonal lattice distortions of the two phases gamma and gamma'. The aim of this work was the evaluation of the related elastic lattice distortions in the microstructure of a turbine blade after service in an accelerated m ission test with high lateral resolution by convergent-beam electron diffra ction. For the determination of the local lattice parameters, the higher-or der Laue zone (HOLZ) lines of the central diffraction disc were evaluated u nder the assumption of a tetragonal distorted crystal lattice, Thr zone-axi s patterns were simulated considering dynamical effects or the diffraction process. In order to reduce the computation time, a set of HOLZ line patter ns was calculated in advance. Experimental patterns were compared with dyna mically 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 ev aluation of the experimental patterns. With this new approach a relatively fast evaluation of lattice strains was possible.