On a mechanism of Snoek-like relaxation caused by C, N and H in fcc iron-based alloys

The effect of the grain boundary segregation of carbon, nitrogen and hydrogen in face-centred cubic (fcc) iron-based solid solutions on the concentration dependence of the Snoek-like relaxation strength is analysed based on the results of measurements and available experimental data. It is shown that carbon and hydrogen cause a square-like concentration dependence of the relaxation strength, whereas linear concentration behaviour occurs in nitrogen-containing solid solutions. The role of grain boundaries in the apparent square concentration dependence of the relaxation strength is demonstrated for the case of hydrogen using a comparison of results obtained for polycrystals and single crystals. Taking into account the strong affinity of carbon and hydrogen atoms with the grain boundaries and a weak segregation effect for nitrogen in the fcc iron-based solid solutions, it is concluded that the apparent square concentration dependence of the Snoek-like relaxation strength in the carbon- and hydrogen-doped solid solutions can be due to absorption of interstitial atoms at the grain boundaries, which takes some of the interstitials out of the solid solution. The conclusion is made that the Snoek-like relaxation is mainly caused by single carbon, nitrogen or hydrogen interstitial atoms creating complexes with substitutional solutes. Some additional experiments with single crystals are desirable in order to test the possible contribution of C–C pairs to the Snoek-like relaxation in austenitic steels.