Thermally-Activated Glide via Cross-Slip

One of the known equations for steady-state creep, indicating stress dependence [10] is  

Qc is the activation energy for creep and n is the stress exponent. A similar expression may be given for climb-controlled creep  

But in this case, Qc is independent of applied stress [10]. At lower temperatures, cross-slips made by screw dislocations are the process by which obstacles in the slip plane may be bypassed. 

The activation energy for cross-slip is rendered by Schoeck and Seeger [76] as  

AHq is the energy for cross-slip, cr is the critical resolved shear stress, a is the applied stress and C and c are constants. A model of creep controlled by cross-slip from the 111 to the 100 plane in the temperature range of 530-680 °C over the stress range of 360-600 MN was found to be in good agreement with the experimental results. The energy to form a restriction between the partials, namely to recombine the Shockley partials, was evaluated on the basis of Dorn s expression [8]. (See also Hemker et al. [51] for the creep mechanism at intermediate temperatures in Ni3Al). 

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Bulk-diflRision-assisted creep occurs in the processes listed above, namely in (b) climb (c) climb-assisted glide and (d) thermally-activated glide via cross-slip. All these are obviously associated with dislocation motion. High stress, below yield stress, causes creep by conservative dislocation motion, namely by dislocation glide within its slip plane. This readily occurs at high temperatures above 0.3 Tm for pure metals and at about 0.4 Tm for alloys, where the dependence on strain rate becomes quite strong. For ceramics, T > 0.4—0.5 T (K). A formulation used for such creep is ... 

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