Secondary hardening in Co-Ni-Cr super-alloy investigated by Mechanical Spectroscopy

The effects of thermal treatments on a Co-Ni-Cr super-alloy in the temperature range from 250 °C to 900 °C, resulting in secondary hardening, have been studied by Mechanical Spectroscopy. The simultaneous evolution of the internal friction spectra and of the dynamic modulus suggests that precipitates are involved in complex mechanisms, which control the dislocation mobility increasing the strength of the material. Precipitation induces pinning of dislocations and depinning is induced by a threshold stress. Such a precipitation process occurs in the temperature range from 400 °C to 600 °C and it is confirmed by reversible changes on thermoelectric power (TEP) due to the depletion of alloying elements from solid solution and by hardness measurements showing a hardness increase at the same annealing temperature as the maximum in TEP curve. The amplitude-dependent internal friction spectra as well as the interaction between dislocations and precipitates are interpreted by proposing a phenomenological model based on the Granato-Lücke theory.