Measurements of mechanical properties in Ni-base superalloys using nanoindentation and atomic force microscopy

The morphology and the nanomechanical properties of two Ni-base superalloys at length scales of <1 μm were measured, using an atomic force microscope in combination with an add-on nanoindentation device, varying both the γ′-precipitate volume fraction and the annealing time. The γ′-precipitate phase was found to be harder up to 45% than the matrix phase. In both phases the hardness decreased with increasing indentation depth, well known as indentation size effect. For the precipitates this decrease appeared uniformly and independent of alloy composition and annealing time, whereas for the matrix the hardness decrease was less pronounced, moreover it did not appear in a uniform manner. The latter observation is explained by an influence of neighbouring precipitates as well as by a high supersaturation of γ′ and γ (matrix) in the early stages of phase separation. The elastic moduli showed no significant difference between the two phases, no dependence on the annealing time and no depth dependence as long as the indentation size was clearly below the size of the particular phase domain but increased by up to 20%, if the size of the indented area approached the size of the measured phase domain.