Some specific features of fine Fe and Fe-Ni particles

Abstract

By comparing results of Mössbauer spectroscopy and x-ray diffraction investigations of ultrafine Fe and Fe-Ni particles, a formerly stated supposition on the existence of two spin states of the fcc phase in pure iron and Fe-Ni alloys has been verified. Some structural peculiarities of particles under study have been observed also. For the pure bcc-Fe particles with an average diameter of 30 and 50 nm the existence of two hyperfine fields (H1=330 kOe, H2=360 kOe) at room temperature has been found. It is supposed that H2 in the bcc phase could be considered as a remnant of a high spin state of the fcc-Fe phase at high temperature. Particles of Fe-Ni(30.3 wt %), Fe-Ni(35 wt %), and Fe-Ni(52 wt %) alloys with an average diameter ranging from 5 to 15 nm were studied also. Particles of Fe-Ni (30.3 wt %) and Fe-Ni(35 wt %) alloys with diameter of 5–8 nm had the bcc structure. A mixture of the bcc and fcc phases appeared with an increase of the particle size. At the same time only the fcc structure remained for the largest particles. The observed size structural dependences and the existence of the stable bcc phase in small particles can be explained by the martensite fcc-bcc transition.