Estudo por espectroscopia Mössbauer e difração de raios-x da cinética de reação da solução sólida FeMo obtida por moagem mecânica

Abstract

The reaction kinetics of Fe 50 at.% Mo solid solutions produced by high-energy mechanical milling was studied by X-ray diffraction and Mössbauer spectroscopy. The material was milled with velocity of rotation of 400 rpm and 500 rpm for several milling times in order to obtain samples of various reaction stages. X-ray diffraction patterns shown the presence of two BCC phases, corresponding to metallic Fe and Mo, in all the samples. Analysis of these diffractograms pointed to diffusion of Mo within the Fe matrix and diffusion of Fe within the Mo matrix as well as the expected grain size reduction of both phases, as a result of the milling process. It was also observed that the Williamson-Hall method is not appropriate to analyze these samples due to the high degree of disorder of the milled material. While the X-ray diffraction technique did not allow for the identification of a crystallographic phase being clearly transformed, which makes it difficult to study reaction kinetics, Mössbauer spectroscopy allowed for the identification of 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 posses magnetic moment). Both magnetic phases may be present in the BCC Fe matrix or the BCC Mo matrix. Transformation kinetics of the sextet was studied using a method based on phase volume transformation, depicted by the tribochemical affinity parameter. Results were compared to previous work on the application of these method to Fe-Cu and Fe-N systems. It was observed a behavior similar to that o Fe-N, which as the Fe-Mo system and as opposed to the Fe-Cu system, posses positive enthalpy of mixing.