Thermal Processes in the Heating of Powder Compacts of Metals and Their Compositions IV. Thermokinetics of Recrystallization and Interaction Processes in the Heating of Compacts Produced from a Mixture of Aluminum and Iron Powders

Abstract

The thermokinetics of recrystallization and interaction processes in the heating of porous compacts produced from a mixture of ultrapure aluminum and iron in a 20 : 80 ratio after cold pressing in a steel die was studied using direct thermal analysis. Recrystallization of the aluminum component and relaxation of the iron component were observed in the temperature range 170–265°C. The iron relaxation and recrystallization exhibited a wavelike behavior. The iron component recrystallized completely in the temperature range 500–700°C. The interaction between aluminum and iron initiated with a reduction reaction through a small amount of surface iron oxides. The reduction of surface iron oxides, involving insignificant heat release, occurred in two stages, reflecting the existence of several iron oxides. Active interaction commenced at the melting point of aluminum. A cascade of exothermic effects, attributed to the interaction of intermetallic compounds with lower stoichiometries, was revealed. In this case, temperature exceeded the existence of the intermetallics, leading to their decomposition and subsequent cooling through the endothermic effect. The cooling rate during the decomposition of intermetallics closely resembled the rate of reaction synthesis. When the nonequilibrium solid solution cooled within the temperature range 500–600°C, the ironbased solid solution decomposed and the intermetallic compound synthesized. Thermokinetic oscillations emerge and gradually subside. At all stages of transitions from stationary states to temperature surges or drops, thermokinetic oscillations with varying frequencies and amplitudes were observed.