Tensile strain measurement using mechanical and in situ optical techniques in bi-metallic Ni-base superalloys

The mechanical performance in terms of tensile properties of a nickel-base superalloy bi-material was investigated, to study the tensile strength of the joint. The results of strain measurement and control by means of usual clip-on extensometry, which gives the overall strain, cannot reveal the local deformation. To have more detail about the local strain fields, particularly in the vicinity of the interface which remains the most brittle part of the bi-metal, an in situ image capture and processing technique adapted from particle image velocimetry was used. The two techniques showed a perfect correlation with the peculiarity of a precise assessment of the local strain in the case of the optical method. Tensile curves obtained at constant strain rate showed serrated yielding associated with inhomogeneous plastic flow at high temperatures. This instability of strain rate softening type (strain localization) was well detected by means of the optical technique.