Effects of stress coarsening on coherent particle strengthening

Abstract

We have determined the effects of stress coarsening of coherent, γ’ [Ni3 (Al,X)] precipitates on the yield behavior of nickel-base super alloy single crystals. In so doing, we have also determined the influence of coherent precipitate shape on yield strength, and the strengthening role of dislocation substructures innickel-base superalloys. The γ’ morphologies considered are large plates, long rods, and the familiar cuboids. These morphologies were obtained by annealing the crystals under applied tensile stress, applied compressive stress, and zero stress conditions, respectively. The tensile test axes were perpendicular to the γ’ plates and parallel to the γ’ rods. Between room temperature and 1400°F, the γ’ rod and γ’ plate morphologies are found to increase the yield strength of the crystals over that due to the γ’ cuboids by about 10 ksi (10 pct) and 30 ksi (35 pct), respectively. Above 1400°F, the yield strengths of the crystals are found to be essentially independent of precipitate morphology. The increase in yield strength in going from crystals with γ’ cuboids to those with γ’ rods is shown to be due to dislocation networks, which are produced as a natural consequence of annealing under stress. A simple analysis of the interaction between dislocations and the precipitates show that the increase in yield strength in going from γ’ rods to γ’ plates is due to dislocation line tension and consistent with the difference in the shape of these coherent and ordered precipitates. The results of this investigation are comforting in that they show creep applications should not degrade the strength of nickel-base superalloy single crystals. The results are also discussed with respect to polycrystalline nickel-base superalloys and composite structures of γ and γ’ phases.