The double exponential rate equation for thermally activated dislocation motion

The double exponential rate equation for dislocation motion developed by Nichols is examined. It is shown that for the usual case where the back activation area is greater than the forward activation area, a plot of dislocation velocity versus effective stress, τ, exhibits an inflexion point at intermediate levels of effective stress. In this case the stress sensitivity of dislocation velocity, n, is shown to assume values of less than unity near the inflexion point. At lower τ levels n approaches unity and decreases with increasing τ while at higher values of τ, n increases in a manner characteristic of a single exponential function. Since this behaviour is at variance with experimental evidence it is concluded that the basic assumptions required in the application of the equation to experimental results are possibly invalid