It is instructive to examine the transitions in creep mechanisms in solid solutions of class-A type such as the results depicted in Fig. 3.10 where we

(a) 10-2

10-3 10-4

^kT 10-5 DEb

note that the stress exponent in the intermediate stress region is 3.5 corre­sponding to viscous glide of dislocations or Weertman microcreep mecha­nism. Since glide and climb occur in sequence, when lower temperatures are approached the climb-controlled creep becomes dominant with five power law thus depicting the fact that the slower climb process controls creep. In

fact this low stress regime is associated with similar characteristics as the climb-controlled creep with distinct subgrain formation and relatively large primary creep region. On the other hand, dislocations may break away from the solute atmospheres at high stresses, thus entering a climb-controlled regime again as noted at higher stresses with higher n value. Following Murty’s work, this breakaway stress can be calculated from the equation74

wm c0

2ekTb3,

where Wm is the binding energy between solute atom and the dislocation, c0 is the solute concentration, and в typically ranges between 2 and 4 depend­ing on the shape of the solute atmosphere. Later, Langdon and co-workers75 showed that this relation is valid for a number of solid solution alloys. Assuming 0.23 eV as a reasonable value for Wm, the critical stress for break­away is estimated to be ~7.5 x 10-4E, which is in agreement with the experi­mental results obtained from various class-A alloys.65 At even higher stresses, another regime may appear involving low temperature climb-controlled creep with a stress exponent value of n + 2 (i. e. 7). This mechanism is asso­ciated with the climb processes involving dominance of dislocation core dif­fusion (Fig. 3.16b). However, this is often masked because the PLB regime starts in the near vicinity.

As lower stresses are approached, one expects to note viscous creep with n = 1 (Fig. 3.16b) either due to N-H or Coble creep mechanisms. Depending on the test temperature, one of the regions such as with n = 3 for viscous glide may completely disappear as noted in Fig. 3.16b. This could get further complicated if an intervening GBS regime with n = 2 appears between vis­cous creep and dislocation creep regimes.