Microstructure and elemental analysis of iron-based powder composite materials

This paper studies the kinetics of structure formation of an iron-bronze composite containing solid lubricants. Depending on the compacting pressure and sintering temperature, binary and complex phases are detected in the iron-bronze structure. The presence of solid lubricants in the composition of the composite material significantly reduces interaction of the liquid (bronze) and solid (iron) phases during sintering. Talc and graphite, which are heat–resistant at a sintering temperature of 850 – 1150 °C, were used as solid lubricants. The presence of talc, located on the surface of compressed particles of iron, copper, tin and graphite, significantly reduces the effect of their interaction. At the same time, the micro-talc particles envelop them, and its thermal stability retains this state up to high temperatures (approximately 900 °C). It was established that there is no perlite in the microstructure of iron-bronze sintered at a temperature of 850 °C. This can be explained by the talc adsorbing ability on the surface of iron particles which prevents diffusion of carbon into the iron crystal lattice. An increase in the sintering temperature up to 1000 °C leads to the formation of perlite in the iron-bronze structure, while the amount of perlite predominates over ferrite. This indicates the partial burnout of talc from the surface of iron particles and the opening of diffusion paths to carbon. At a sintering temperature of 1150 °C, perlite and a grid of light inclusions are formed in the microstructure of the iron-bronze samples. According to the results of electron microprobe analysis, the light inclusions are solid solutions of variable compositions such as Fe – Cu – Sn, Cu – Fe – Sn, Cu – Sn – Fe. In order to confirm these assumptions, a phase X-ray diffraction analysis was performed. Diffraction patterns of these samples are represented by reflections of iron and copper crystals. The absence of diffraction effects (characteristic of tin crystals) is conditioned by tin solubility in the copper lattice. This is due to the low melting point of tin (232 °C) and its ionic radius, which allows isomorphically replacing of copper and iron ions with tin ions (their difference is less than 15 %).