C. M. Sellars

Bio: C. M. Sellars is an academic researcher. The author has contributed to research in topics: Creep & Deformation (engineering). The author has an hindex of 3, co-authored 3 publications receiving 580 citations.

Strength and structure under hot-working conditions

Abstract: The main feature of hot working is that extremely large strains are applied to materials at high rates of strain at temperatures above ∼ 0.6Tm, where Tm is the melting temperature in degrees Kelvin. Strength and ductility under these conditions are markedly dependent on both temperature and rate of straining. Although this review is confined to strength and structure during hot working, ductility is intimately related to the deformation processes that govern plastic flow. This aspect has been recently reviewed by one of the authors. These large strains can be achieved with little or no strain-hardening, indicating that dynamic softening processes can operate sufficiently rapidly to balance the strain-hardening processes. In this situation, the structural changes involved can be used to obtain information on the mechanism of deformation. However, as emphasised later, care must he taken in the interpretation of such hot-worked structures, since significant structural changes can occur on holding at ...

Creep resistance of CMSX-3 nickel base superalloy single crystals

Abstract: 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 γ′ 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°C at stresses of 552 MPa or lower, the dislocation-free γ′ precipitates are resistant to shearing by dislocations. As a result, creep deformation occurs by forced bowing of dislocations through the narrow γ 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 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 γ and γ′ 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 γ 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°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 γ′ precipitates, dislocations move through the γ 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 γ′ particles by dislocations entering from the γ 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 γ matrix serve to neutralize the misfit between the γ and γ′ phases.