CREEP RESISTANCE OF CMSX-3 NICKEL-BASE SUPERALLOY SINGLE-CRYSTALS

Creep deformation in <001> oriented nickel base superalloy single crystals has been studied in an effort to assess the factors which contribute to the overall creep resistance of superalloys with high volume fractions of gamma'-phase. Detailed observations of three dimensional dislocation arrangements produced by creep have been made with the use of stereo electron microscopy. In the temperature range of 800-900-degrees-C at stresses of 552 MPa or lower, the dislocation-free gamma'-precipitates are resistant to shearing by dislocations. As a result, creep deformation occurs by forced bowing of dislocations through the narrow gamma-matrix channels on {111} planes. At moderate levels of temperature and stress there are incubation periods in virgin crystals prior to the onset of primary creep. The incubations arise because of the difficult process of filling the initially dislocation starved matrix material with creep dislocations from widely spaced sources. When the newly generated dislocations percolate through the cross section, incubation comes to an end and primary creep begins. In primary creep neither work hardening nor any type of recovery plays an important role. The creep rate decelerates because the favorable initial thermal misfit stresses between gamma and gamma'-phases are relieved by creep flow. Continued creep leads to a build-up of a three-dimensional nodal network of dislocations. This three-dimensional network fills the gamma-matrix channels during steady state creep and achieves a quasi-stationary structure in time. In situ annealing experiments show that static recovery is ineffective at causing rearrangements in the three-dimensional network at temperatures of 850-degrees-C or lower. The kinematical dislocation replacement processes which maintain the quasi-stationary dislocation network structures during apparent steady state creep are not understood and require further study. Because of the impenetrability of the gamma'-precipitates, dislocations move through the gamma-matrix by forced Orowan bowing, and this accounts for a major component of the creep resistance. In addition, the frictional constraint of the coherent or semi-coherent precipitates leads to the build-up of pressure gradients in the microstructure, and this provides load carrying capacity. There is also a smaller component of solid solution strengthening. Work hardening is comparatively unimportant. Finite element analysis shows that the non-deforming precipitates are increasingly stressed as creep deformation accumulates in the matrix. In the later stages of steady state creep and during tertiary creep the stresses in the precipitates rise to high enough levels to cause shearing of the gamma'-particles by dislocations entering from the gamma-matrix. The recovery resistance of the material is in part due to a very low effective diffusion constant and in another part due to the fact that the three-dimensional dislocation networks formed in the gamma-matrix serve to neutralize the misfit between the gamma and gamma'-phases.