SNOEK RELAXATION IN FE-CR ALLOYS AND INTERSTITIAL-SUBSTITUTIONAL INTERACTION

The internal friction (IF) spectra of alpha-Fe, Fe-Cr ferritic alloys and Cr have been investigated in a frequency range of 0.01 to 10 Hz. A Snoek-type relaxation was found in all the investigated C doped Fe-Cr alloys, starting from pure Fe and finishing with pure Cr. The tempera ture location of the Snoek peak (T-max) in alpha-Fe was found to be 31 5 K (1 Hx). The activation energy deduced from the T-f shift was 0.81 eV. T-max in Cr was 433 K with an activation energy of 1.11 eV. The Sn oek-type peaks in Fe-Cr alloys are much wider than in pure Fe or pure Cr. The ''temperature location of the peak versus chromium content'' c urve exhibits a maximum in the vicinity of 35 wt% Cr (T-max was 573 to 578 K, f approximate to 1.2 Hz and the activation energy was about 1. 45 eV). It is important that Cr atoms in alpha-Fe have a more pronounc ed influence on the temperature location of the peak than Fe atoms hav e in chromium. A new model based on the atomic interactions is propose d to explain the influence of composition on Snoek peak location. The internal friction has been simulated by a Monte Carlo method, using C- C and C-substitutional atom (s) interaction energies. A model of long- range strain-induced (elastic) interaction supplemented by the ''chemi cal'' interaction in the two nearest coordination shells around an imm obile substitutional atom was used for the C-s interaction. The intera tomic interaction was supposed to affect IF by changing both the carbo n atom arrangement (short-range order) and the energy of C atoms in oc tahedral interstices, and therefore the activation energy of IF. The p eak temperatue calculated coincides well with the experimental ones if the value for the chemical interaction in the first coordination shel l (H-chem) for C-Cr in Fe is -0.15 eV and for C-Fe in Cr +0.15 eV. The difference in the influence of Cr in alpha-Fe and Fe in Or is account ed for by a difference in the elastic and chemical interaction both be tween the carbon atoms and the substitutional atoms. The relaxation pr ocess in chromium Fe-based alloys is due to the carbon atom ''diffusio n under stress'' between octahedral interstices of first and second co ordination shells around the Cr atoms, and in Cr-based alloys, between second and third shells around the Fe atoms.