Creep regimes with a stress exponent greater than 7 are sometimes described as the PLB regime. There are certain dispersion strengthened alloys that exhibit high stress exponents, but such a behavior is rationalized by invoking a threshold stress for the operation of the mechanism of five power-law creep. The PLB, also known as the exponential creep regime, is described by

[3.37]

where A and B are material constants. The constant B is related to the acti­vation volume given by the area swept by dislocation and its Burgers vector (b). It has been observed that the PLB regime occurs at a > 10-3£. At such high stresses, the dislocation density increases more than that predicted by the Taylor equation (a2) and thus PLB could be controlled by the motion of dislocations. However the mechanism of steady-state creep deformation in the PLB regime is not clearly resolved.

The activation energy, Qc, for PLB has been found to be smaller than the activation energy for self diffusion, Qsd. In some cases, the activation energy was found to be equal to the activation energy for pipe diffusion suggesting that the mechanism of deformation could be dislocation climb but facilitated by short circuit diffusion of vacancies through the large

number of dislocation cores generated at high applied stresses. Sherby and Burke16 suggest that vacancy diffusion due to vacancy supersaturation at high applied stresses can be associated with the rate controlling mechanism. Others have considered breaking down of subgrain walls70 and cross slip or cutting of forest dislocations71 instead of dislocation climb as the rate con­trolling mechanism. In any case PLB remains poorly understood and this is due to the relatively small number of studies that have been carried out in this regime.