The high-temperature decrease of the yield strength of the γ′-strengthened superalloys NIMONIC PE16 and NIMONIC 105

Above Tmax≈1050 K, the yield strength σt of the commercial γ′-strengthened nickel-base superalloys NIMONIC PE16, NIMONIC 105, and of many similar superalloys decreases drastically. In order to elucidate the physical reasons for this softening effect, σt has been measured as a function of the deformation temperature and of the plastic strain rate. Moreover, the configurations of dislocations in deformed specimens have been studied by transmission electron microscopy (TEM). The results are as follows: in NIMONIC 105, the relevant dislocation processes change at Tmax: below Tmax pairs of dislocations with Burgers vectors of the type 12〈1 1 0〉 shear the strengthening γ′-precipitates athermally whereas above Tmax single 12〈1 1 0〉-dislocations overcome them thermally activated. The effects of changes of the γ′-precipitate dispersion with temperature are negligible. In NIMONIC PE16, however, both mechanisms—changes of the γ′-dispersion and thermally activated dislocation processes—strongly affect σt above Tmax.