Reaction kinetics of equiatomic Fe-Mo solid solutions obtained by mechanical alloying: an X-ray diffraction and Mössbauer spectroscopy study

Abstract

The reaction kinetics of equiatomic Fe–Mo solid solutions produced by mechanical alloying were studied by X-ray diffraction and Mössbauer spectroscopy. The materials were milled with velocities of rotation of 400 and 500 rpm for several milling times to obtain samples of various reaction stages. The X-ray diffraction patterns showed the presence of two bcc phases, corresponding to metallic Fe and Mo, in all samples. Analysis of these diffractograms indicated diffusion of Mo within the Fe matrix and diffusion of Fe within the Mo matrix, as a result of the milling process. While the X-ray diffraction technique did not allow for the identification of a crystallographic phase being clearly transformed, making it difficult to study the reaction kinetics, using Mössbauer spectroscopy, it was possible to identify two magnetic phases that suffer transformation during the course of milling: a ferromagnetic sextet corresponding to metallic Fe and a paramagnetic doublet corresponding to Fe atoms with a majority of Mo neighbors (metallic Mo does not possess a magnetic moment). Both magnetic phases may be present in the bcc Fe matrix or the bcc Mo matrix. The transformation kinetics of the sextet were studied using a method based on a local description of phase volume transformation, depicted by the tribochemical activity parameter. The results were compared to published work on the application of this method to Fe–Cu and Fe–N systems. A behavior similar to that of Fe–N was observed, which, similar to the Fe–Mo system and as opposed to the Fe–Cu system, possesses a positive enthalpy of mixing.