Snoek-Köster Relaxation. New Insights - New Paradigms

Abstract
Common understanding of the Snoek-Köster (SK) relaxation in bcc metals is based on Schoeck's and Seeger's models. The former considers dragging of the cloud of foreign interstitial atoms (FIAs) by a dislocation in terms of the string model. The latter is based on the concept of kink pair formation on dislocations. A new insight into the SK relaxation, however, can be obtained if the coupling model, developed by K. L. Ngai, is used to interpret mechanical loss spectra induced by the motion of dislocations dragging along the Cottrell cloud of FIAs. This phenomenological model opens a new line of thinking in mechanical spectroscopy of metals to describe complex cooperative phenomena. It provides a very general scenario for two distinct regions of the relaxation process : exponential and stretched exponential. This model implies that the relaxation function may continuously cross over from a Debye function to a stretched exponential function. The one-sided Fourier transform of the derivative of the normalized relaxation function gives the same asymmetry of a loss peak as that observed for the carbon SK peak in deformed ultra-high purity α-Iron.
This paper is a résumé of the experimental observations of SK relaxation such as the effect of the amount of plastic deformation, of deformation temperature, of conditions for thermal stability of the SK peak, and its broadening. It is concluded that the concept of the cooperative migration of FIAs in the Cottrell cloud elucidates the primary and secondary features of SK peak. Very low value of the activation enthalpy of the stable SK peak detected in deformed ultra-high purity α-Iron doped with 25 at. ppm of carbon (i.e. H^SK = 0.95 eV) is given. This small value is interpreted in terms of the coupling model. It is concluded that the basic features of the S-K peaks in Fe-C, Nb-O and Ta-O are essentially the same. To test, however, the coupling model in other bcc alloys, special care must be taken to "design" favourable experimental conditions to reveal secondary features of the SK peak.