Snoek-Type and Zener Relaxation in Fe-Si-Al Alloys

Abstract

Carbon-containing Fe - Si and Fe - Si - Al alloys were studied with respect to the carbonrelated Snoek-type and Zener relaxation using different mechanical spectroscopy techniques. In all alloys the temperature-dependent profile of the Snoek peak, relative to that in pure iron, is modified on its high-temperature side by the substitutional atoms. At least two components, an Fe - C - Fe (which correspond to C atom jumps (diffusion) in areas where it is surrounded by Fe atoms only) and Fe - C - Me peaks, where Me = Si, Al, can be distinguished in the Snoek-peak profile. In both binary Fe - Al and Fe - Si and ternary Fe - Si - Al alloys, a higher annealing temperature prior to quenching leads to an increase in the Fe - C - Fe and a decrease in the Fe - C - Me component of the Snoek peak. Heating to 1173K and above often lowers the peak height due to thermal vacancies. Low-temperature (<670K) ageing of quenched Fe - Si - Al and Fe – Si specimens reduces both the Fe – C - Fe and Fe – C – Al / Si peaks. Ageing at T > 670 K changes the temperature- as well as the amplitude-dependent parts of internal friction due to a redistribution of carbon between solid solution and dislocations. Both the Snoek-type peak height and the dislocation mobility – as can be concluded from the slope of the amplitude-dependent internal friction – increase, and a new peak appears at temperatures higher than that of the Snoek peak, which probably is a Snoek-Köster peak resulting from the motion of weakly pinned dislocations. A Zener peak appears if the concentration of substitutional atoms is > 6 at. %. The Zener peak relaxation strength is much lower in ternary alloys than in the binary ones probably due to mutual compensation of elastic distortions in presence of Al and Si atoms which are bigger and smaller, respectively, than Fe atoms.