Deformation mechanism maps creep

Frost, H.J. and Ashby, M.F. (1982) Deformation-Mechanism Maps The Plasticity and Creep of Metals and Ceramics (Pergamon Press, Oxford). 

Figure 16.5 Deformation mechanism map for Ag polycrystal a = applied stress, p = shear modulus, grain size = 32 pm, and strain rate = IGF8 s 1. The diffusional creep field is divided into two subfields the Coble creep field and the Nabarro-Herring creep field. From Ashby [20].

For a given material, different creep mechanisms may dominate in different temperature and stress regions. This information can be conveniently given in the form of a deformation mechanism map, as shown schematically in Fig. 7.12. This... 

Depending on the temperature and the stress, different microscopic processes are important in determining creep behaviour. These will be discussed in this section. We will see that different processes are important at different temperature and stress values a fact that can be visualised using so-called deformation mechanism maps. 

H. Frost and M. Ashby, Deformation Mechanism Maps—The Plasticity and Creep of Metals arid Ceramics, Pergamon, 1982... 

Creep data of this nature are represented pictorially for some well-studied systems in the form of stress-temperature diagrams, which are termed deformation mechanism maps. These maps indicate stress-temperature regimes (or areas) over which various mechanisms operate. Constant-strain-rate contours are often also included. Thus, for some creep situation, given the appropriate deformation mechanism map and any two of the three parameters—temperature, stress level, and creep strain rate—the third parameter may be determined. 

Even though the experimental results are still limited, it appears that the predicted occurrence conditions provide the general trend compatible with the experimental results. Thus, it may be possible to examine the fracture process to be expected for hydraulic stimulation in supercritical rock masses on the basis of the fracture mechanism map given in Fig. 4. The ratio of the minimum horizontal tectonic stress to the vertical tectonic stress usually falls in the range of 0.5-1.0 for brittle rock masses (Brace Kohlstedt, 1980.). The actual difference of the tectonic stresses may be significantly smaller in supercritical rock masses due to the high temperature creep deformation (Fournier, 1999 Muraoka et al., 20()0). If we limit... 

Ashby et al. [LI 87] have developed the approach of HIP maps , where, for a material under given conditions, the areas in two-dimensional space (relative density depending on the pressure), in which the predominant phenomenon that controls the densification has been identified, are traced (in particular the grain size and temperature). These maps make the pendant of the creep maps and deformation maps also credited to Ashby et al. (see Figure 7.2 in Chapter 7). The principle of these maps is certainly attractive, but their applicability requires three conditions i) having a sufficient number of experimental data, ii) estabhshing, for each of these data, the nature of the predominant mechanism, and lastly iii) verifying the... 

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