Temperature creep mechanisms

Creep is a thermally activated process under which materials will slowly stretch under stress when the temperature exceeds 50% of the melting temperature. Creep mechanisms include dislocation climb, vacancy migration, and grain boimdary slip. Creep is especially important in gas turbine engines and a class of Ni-based superaUoys has been developed to minimize its effects. 

At high temperatures the glassy phase may become less viscous and even liquid and as a consequence may account for the plastic deformation. However, viscous flow creep is not regarded as a viable creep mechanism for superplasticity due to its limited deformation, which corresponds to the redistribution of the glassy phase and therefore to the squeeze of these secondary phases from grain boundaries subjected to compression.8... 

As a matter of fact, an E(T) curve provides much more information than the singlepoint tests on softening temperatures however, due to the time dependency of the mechanical properties, not even enough to provide a basis for construction purposes. For the use of plastics under load at elevated temperatures creep data are required ... 

R. Morrell and K. H. G. Ashbee, High Temperature Creep of Lithium Zinc Silicate Glass-Ceramics, Part 1, General Behavior and Creep Mechanisms, /. Mater. Sci., 8, 1253-1270 (1973). 

Fine-grained materials, when subjected to high temperatures and low applied stresses, deform by mutual accommodation of grains assisted by grain boundary sliding and transport of matter (diffusion). Under conditions where lattice diffusion dominates, the diffusional creep rate is reasonably well characterized by the Nabarro-Herring creep process. (For a review of this and other classical creep mechanisms, see Refs. 5 and 6.) Here the strain rate is expressed as... 

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... 

Low-temperature creep was first studied by Meissner et al in 1930 [ ]. They found that above the yield stress there is appreciable creep even at liquid-helium temperatures. This result gave impetus to further studies at low temperatures. Characterizing creep in cadmium as athermic at 1.4 to 4.2 K, Glen [ assumed that creep proceeds bv dislocation tunneling through crystalline lattice barriers. Arko and Weertman [ revealed the sensitivity of creep to temperature at 4 K and inferred that it was the common thermally activated creep. Gindin etai assumed combined thermal activation and tunneling mechanisms. At the present time, there is not unanimous opinion on the nature of low-temperature creep. 

---