The temperature-dependence A(T) arises because dislocation jumps are thermally activated (Weertman, 1973). As temperature increases, molecules are increasingly agitated and it becomes more probable that a strained bond adjacent to a dislocation will be broken. Thus the speed of dislocation motion increases with temperature according to statistics of the Boltzmann distribution, giving the Arrhenius relation (Equation 4).
The value for activation energy at T < — 10°C is similar to that for self-diffusion of ice (Weertman, 1983). The increase of apparent activation energy above this temperature indicates that the rate-limiting mechanisms are changing qualitatively as the melting temperature is approached (Paterson, 1994). This suggests an increasing accommodation of incompatible deformations by grain-boundary mechanisms, because the grain boundaries are probably significantly broadened and altered by liquid films at these temperatures (see Dash et al., 1995).
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