SNOEK-KOSTER RELAXATION - NEW INSIGHTS - NEW PARADIGMS

Common understanding of the Snoek-Koster (SK) relaxation in bcc metals is based on Schoeck's and Seeger's models. The former considers dragg ing 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 k ink pair formation on dislocations. A new insight into the SK relaxati on, however, can be obtained if the coupling model, developed by K. L. Ngai, is used to interpret mechanical loss spectra induced by the mot ion of dislocations dragging along the Cottrell cloud of FIAs. This ph enomenological model opens a new line of thinking in mechanical spectr oscopy of metals to describe complex cooperative phenomena. It provide s a very general scenario for two distinct regions of the relaxation p rocess: exponential and stretched exponential. This model implies that the relaxation function may continuously cross over from a Debye func tion to a stretched exponential function. The one-sided Fourier transf orm 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 alpha-Iron. This paper is a resume of th e experimental observations of SK relaxation such as the effect of the amount of plastic deformation, of deformation temperature, of conditi ons for thermal stability of the SK peak, and its broadening. It is co ncluded that the concept of the cooperative migration of FIAs in the C ottrell cloud elucidates the primary and secondary features of SK peak . Very low value of the activation enthalpy of the stable SK peak dete cted in deformed ultra-high purity alpha-Iron doped with 25 at. ppm of carbon (i.e. H-SK = 0.95 eV) is given. This small value is interprete d in terms of the coupling model. It is concluded that the basic featu res 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 revea l secondary features of the SK peak.