The process of precursor formation for the high-entropy Fe–Ni–Co–Cu–Sn system is investigated by galvanic replacement in aqueous chloride solutions using dispersed aluminum as a reducing agent. Based on synchronized measurements of electrode potential and temperature, as well as analysis of literature data, a multi-stage process mechanism is proposed. It has been established that the galvanic replacement process can be conditionally divided into four stages. At the initial stage 1 (0–30 s) after the introduction of aluminum powder into the solution, the system is characterized by a stable potential (+0.38 V) and a constant temperature. This induction period is associated with a certain delay in the activation of the aluminum surface coated with a passive Al2O3 oxide film. The absence of heat generation directly indicates that exothermic replacement reactions have not yet begun. At the end of the induction period, the second stage begins (30–75 s), that is a sharp, two-stage drop in potential, accompanied by an explosive increase in temperature. The first stage is characterized by a rapid decrease in potential to a quasi-stationary value of about 0.0 V, due to the reduction of Fe3+ ions to Fe2+, which act as the primary oxidizer of aluminum. As soon as the local concentration of a strong oxidizing agent (Fe3+) decreases at the surface of the particle, the potential reaches a minimum of –0.38 V. The second stage corresponds to the beginning of the massive co-reduction of Fe2+, Cu2+, Ni2+, Co2+, and Sn2+ ions to form a solid precursor phase of a high-entropy alloy. The third stage characterizes the quasi-stationary state and diffusion control of the process (75–170 s). After reaching the peak of reactivity (minimum potential and maximum rate of temperature increase), the potential begins to slowly shift towards positive values up to –0.26 V, and the rate of temperature increase slows down until it reaches a maximum of 90°C. The steady-state potential is a compromise and is determined by the coupled reactions of delayed aluminum dissolution, diffusion-controlled metal deposition, and hydrogen release. At the final stage, a gradual cooling of the reaction mixture is observed at a stable potential, which indicates an almost complete depletion of oxidizing ions in the solution. Exothermic reactions cease, and the system begins to strive for thermodynamic equilibrium with the environment. X-ray fluorescence analysis confirmed the joint deposition of all five metals, the ions of which were initially present in the solution. The obtained elemental composition of the sample is close to the theoretical one for the equimolar Fe–Ni–Co–Cu–Sn system, which indicates the high efficiency of the galvanic replacement method in obtaining multicomponent precursors. Using EDS, a fairly uniform distribution of deposited metals over the surface of the particles has been established. It has been found that the accumulation and rapid release of hydrogen in the pores of particles has a mechanochemical effect on them, leading to the dispersion and formation of particles of characteristic morphology with signs of split and hollow structures.
Kolpakov et al. (Mon,) studied this question.