Dislocation-related creep

In this section, the basic dislocation processes involved in the progressive deformation of a crystalline solid are discussed briefly to provide background for the detailed discussion of the deformation microstructures observed by TEM in specific minerals to follow. Particular attention is given to relating the nucleation, glide, climb, multiplication, and interaction of dislocations to the various stages of the creep and stress-strain curves. More discussion can be found in the texts referred to in Section 9.1. 

The history of the development of the theory of low-temperature plasticity of solids resembles very much the development of tunneling notions in cryochemistry. This resemblance is not casual it is related to the similarity of the elementary act pictures this was noted by Eyring, who successfully applied the theory of absolute rates to a description of fracture kinetics [202]. Plastic deformation at constant stress (creep) is stipulated by dislocation slip... 

The creep of alumina based fibers can be reduced by adding a second phase. For example, the steady state creep rate of a PRD-166 a-alumina/t-ZrOa (Y) fiber is about one order of magnitude lower than that of the single phase a-alumina fiber (Fiber FP) [78]. This effect is thought to be related, at least at low temperatures (T<1100°C), to the fact that the dispersed zirconia particles at grain boundaries limit the mobility of intergranular dislocations [67],... 

It has been suggested that this high creep resistance might be related to the low density and the low mobility of dislocations present in YAG single crystals [105]. Further, YAG is stable In contact with alumina up to about 1700°C. Both materials display similar CTEs they do not react with each other and their mutual solubility in the solid state is negligible. Hence, YAG... 

Andrade postulated that jS creep is related to dislocation glide within the grain, while K flow is related to slip along grain boundaries. Ascribing k flow to grainboundary sliding is known to be in error. Equation (6.18) may also be expressed as ... 

Low temperature (in relation to the melting point of the material) creep of metals is usually controlled by dislocation movements, because their structures contain sufficient active slip systems and have small Peierls stresses (the force needed to bring about dislocation movement) [4-8]. Deformation can also be controlled by dislocation climb, a process requiring vacancy diffusion. At high temperatures, deformation in metals is usually controlled by diffusion creep mechanisms that do not involve dislocation movement. In ceramics, however, diffusion creep may be the dominant mechanism under most processing conditions due to the small number of slip planes, the high Peierls stresses, and to the need to move stoichiometric amounts of the different atomic species present in the material (both anions and cations for an ionic compound). 

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