On constitutive equations for various diffusion-controlled creep mechanisms
Résumé
Plastic flow of polycrystalline solids at elevated temperatures occurs by one of three independent deformation mechanisms: slip by dislocation movement, sliding of adjacent grains along grain boundaries, and directional diffusional flow. All three mechenisms are considered to be thermally activated and controlled by the diffusion of atoms. Constitutive equations have been developed which accurately describe each of the three independent mechanisms. These equations center on a power law dependence of the creep rate. Thus the stress exponent, n, in εωασn, is shown to have discrete values depending on the plastic flow mechanism. For deformation by slip, n can take on values ranging from n= 1 to n=8 depending on the specific dislocation mechanism. For grain boundary sliding, n can be either 2 or 4, and for diffusional creep, n is unity. The microstructure is an important factor In establishing the magnitude of the creep rate for each of the three independent deformation mechanisms. Grain size is the principal microstructural feeture in determining the creep rate for deformation by grain boundary sliding and by diffusional flow. On the other hand, the subgrain size and the dislocation density plays an important role in determining the creep rate for deformation by dislocation motion; examples are shown for ODS alloys. Competition between these various mechanisms can be described quantitatively through the use of constitutive equations and deformation mechanism maps. It is shown that diffusional creep is not as dominating a process as has been considered in the literature, and that grain boundary sliding or Harper-Dorn creep are the more likely deformation mechanisms occurring at low stresses and high temperatures for fine grain size materials.
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