Snoek Relaxation in Fe-Cr Alloys (O to 100%Cr)

Abstract

Internal friction (IF) spectra of α-Fe-FexCry-Cr ferritic alloys have been investigated with the help of forced and free decayed pendula in the frequency range 0.01 to 10 Hz. he Snoek-type relaxation has been found in all the investigated ternary alloys, iron-chromiumcarbon (Fe-Cr-C), starting with pure iron and finishing with pure chromium. The temperature of the Snoek peak (Tmax) in α-Fe is found to be 315 K (1 Hz), the activation energy deduced from the “T-f' shift is 78 kJ/mol, and the Tmax in Cr is 433 K with an activation energy 107 kj/mol. The Snoek-type peaks in Fe-Cr alloys are much wider than those in pure Fe and Cr. This is connected with the redistribution of atoms between octavoids with different numbers of Fe and Cr atoms in neighboring sites around octavoids and therefore different relaxation times. The “temperature location versus chromium content” curve passes through a maximum in the vicinity of 35 wt.%Cr, where Tmax is 573 ÷ 578 K,f = 1.2 Hz, and the activation energy is about 140 kJ/mol. The new approach, based on interaction of carbon-carbon (C-C) and carbon-substitutional C-s) atoms, is suggested for the explanation of the influence of composition on Snoek-peak location. The long-range strain-induced interactions supplemented by the “chemical” interaction in the two nearest atomic shells around a fixed substitutional atom have been used for the modeling of C-s interaction. It is suggested that the interatomic interaction affects IF by changing the carbon atom arrangement (the short-range order was simulated by the Monte Carlo method) and the energy of C atoms in octahedral interstices, that is, the activation energy of IF. The carbon atoms contribution to IF has been calculated by the Debye equation with the use of energies of C-C and C-s interaction. The Tmax shift due to the iron alloying by Cr is much stronger than that due to chromium alloying by Fe and is explained from the point of view of the above-mentioned interactions.