Time-Dependent Deformation Creep

Most simply and generally defined, creep is time-dependent deformation under constant stress . Even though creep may occur at relatively moderate temperatures, most ceramics are intended for use at high temperatures, where they are ductile. For ceramics with low-temperature ductility, creep may occur at 0.5 T or even at lower temperatures. The term defined below, indicating homologous 

Pelleg, Mechanical Properties of Ceramics, Solid Mechanics and Its Applications 213, DOI 10.1007/978-3-319-04492-7 6, Springer International Publishing Switz land 2014 

Low-temperature creep, at or below 0.5Tj , is believed to be governed by non-dififusion- controlled mechanisms, whereas high-temperature creep, above 0.5T , is diffusion controlled. Stress, time and temperature, mentioned above, act simultaneously during creep. These three parameters determine the creep rate and may be expressed in terms of strain rate as  

As indicated throughout the sections of this book, deformation, including creep deformation, occurs by some form of dislocation motion. The objective of selecting materials or developing new materials is to slow down dislocation motion, as far as possible, in order to ensure their long lifetime of service. 

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Further below, time-dependent deformation (creep) iiutiated by climb will be extensively discussed. In this section, an example of dislocation climb is illustrated. Figure 3.70 shows dislocation climb in an AI2O3-YAG specimen. Here, climb was assisted by thermal activation. Such a dislocation network, resulting from the reaction of dislocations from the basal and pyramidal slip systems, involves dislocation climb. It is a diffusion-controlled deformation mode characterizing creep deformation and, in this particular case, the activation energy determined is Q = 670 kJ/mol. 

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