Critical Experimental Data on the Snoek-Köster Relaxation and Their Explanation by the Coupling Model

Abstract

This paper is devoted to the Snoek-Köster (SK) relaxation in deformed α-iron containing carbon as foreign interstital atoms (FIAs). In this study crucial experimental data on SK relaxation in deformed ultra-high purity α-iron doped with carbon at different concentration levels will be presented. The experimental results include the relaxation enthalpy, HSK, the relaxation time, τSK, the shape of the SK peak and their dependences on carbon concentration, deformation temperature as well as the amount of plastic deformation will be used to critically test theoretical models. In fact, the interpretation of activation enthalpy is still the main unresolved issue in the studies of SK relaxation. Schoeck has recently called into attention that the essential question every theory of SK relaxation must answer is why the HSK is larger than the diffusion enthalpy of the corresponding interstitial in the lattice. Therefore we will focus our discussion on the problem of relaxation enthalpy and relaxation time of SK peak. From an analysis of the experimental data we find that the activation enthalpy of the carbon SK peak in α-iron actually varies over a wide range, decreasing towards the activation enthalpy, HS, of the Snoek relaxation with decreasing C concentration. This, together with an observed influence of the deformation temperature on the relaxation strength of the SK peak raise serious question on the validity of the Seeger model based on kink pair formation (KPF) mechanism. Schoeck earlier had also ruled out the KPF mechanism by calling into attention the fact that some measurements found the strength of the SK relaxation far exceeds that predicted by the KPF mechanism. Therefore the hypothesis that only screw dislocations are responsible for SK relaxation is not confirmed in this study. In our theoretical approach presented here, the Shoeck model with the dragging mechanism is refined so as to take into account mutual interactions between FIAs possessing a long-range mobility in the Cottrell cloud surrounding dislocations. The cooperative migration of the FIAs caused by the mutual interactions between them with the FIAs locked in time-dependent strain field of dislocations subjected to an external, dynamical mechanical field is considered to be the basic “physical picture” of SK relaxation in our theoretical approach. We shall work out the consequence of this physical picture by using the coupling model to describe the cooperative migration of the FIAs and incorporating this into the dragging mechanism of Schoeck. The results can explain quantitatively the critical experimental facts that HSK increases over a wide range from 0.95 < HSK < 2.12 eV with carbon concentration and the corresponding preexponential factors. The relaxation strength is the same as the string model of Schoeck which is in agreement with experiment.