Activation energy dislocation diffusion

Several theoretical mechanisms have been proposed to explain the creep behavior for various materials these mechanisms involve stress-induced vacancy diffusion, grain boundary diffusion, dislocation motion, and grain boimdary sliding. Each leads to a different value of the stress exponent n in Equations 8.24 and 8.25. It has been possible to elucidate the creep mechanism for a particular material by comparing its experimental n value with values predicted for the various mechanisms. In addition, correlations have been made between the activation energy for creep (Qc) and the activation energy for diffusion (g Equation 5.8). 

Figure 9.1 presents self-diffusivity data for DD(dissoc), DD(undissoc), DB, DS, DXL, and DL, for f.c.c. metals on a single Arrhenius plot. With the exception of the surface diffusion data, the data are represented by ideal straight-line Arrhenius plots, which would be realistic if the various activation energies were constants (independent of temperature). However, the data are not sufficiently accurate or extensive to rule out some possible curvature, at least for the grain boundary and dislocation curves, as discussed in Section 9.2.3. 

The acceleration of diffusion by the point defects produced during irradiation has been shown experimentally (49), and an extrapolation to the temperature of bombardment indicated that the enhancement was adequate (48). A surface smoothing would be expected as well as a removal of point defects, and the authors showed that the surface area was indeed lowered by 20 to 50% by the irradiation. Other examples of this effect on surface area are given in Section IV,A. Electrolytic polishing, which should also remove point defects and surface irregularities but not dislocations, produced a surface with similar low activity and low activation energy (48). 

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. 

It was suggested by the dislocation-climb model that the activation energy for creep in many materials at high temperatures is equal to their activation energy for self-diffusion. According to both models, the activation energy for high-temperature... 

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