Additional Approaches to Experimental Validation of the Phenomenon of Electrochemical Phase Formation in Metals Via a Supercooled Liquid State Stage

Abstract

A previously unknown phenomenon of electrochemical phase formation in metals via a supercooled liquid state stage was recently established by synthesis of experimental and theoretical findings. Essentially, this phenomenon is to be understood as follows: during metal electrodeposition onto a solid cathode in an aqueous environment, a deeply supercooled metal liquid evolves in the form of multiple liquid atomic clusters avalanching at various sites near the cathode or the growing deposit and solidifies ultra rapidly at the deposition temperature to appear in a crystalline, amorphous or quasi-crystalline phase. The unconventional nature of this phenomenon requires additional approaches to experimental validation of its existence. In this context, the intention of the investigation at hand was to perform experimental verification of the existence of the discovered phenomenon by analysis of electrodeposited metals' structural features as resulting from ultra-rapid solidification of highly supercooled liquid metal phase. The hypothesis of this study was that the structure in metals that were ultra-rapidly solidified under high supercooling has certain typical traits that distinguish it from the metals' structure evolved under ordinary conditions. Besides, the degree of supercooling in metal electrodeposition must be a major factor that may affect characteristics of the structure. Idea One and Its Realization. It is well-known that metal solidification at a minor degree of melt supercooling follows a dendritic pattern due to morphological instability at solid-melt interface. At high supercooling, however, dendritic solidification gives way to spherulitic one. In this case the spherulites generally occur at the metal-crucible interface. Additionally, quasicrystals occur in ultra-rapid quenching of highly supercooled melts. Therefore, if metals indeed pass a supercooled liquid state stage during electrochemical phase formation and are rapidly solidified at the deposition temperature, then their layers adjacent to the cathode should contain spherulites and quasicrystals. To verify this idea, cathode-adjacent layers in electrodeposited copper, lead and cobalt were investigated. It has been established that spherulites and pentagonal quasicrystals occur in cathode-adjacent metal layers, which is typical of metals produced by ultra-rapid solidification of a highly supercooled liquid metal phase (Fig. 1). Idea Two and Its Realization. It is well-known that as the deposited layer grows in thickness, the spherulites disappear and the deposit then develops in druse form. For a more convincing argument in favor of the existence of an intermediate liquid phase in metal being electrodeposited, one should completely prevent transition from the spherulitic growth process to the drusy one with increasing thickness of deposit and thus produce a purely spherulitic deposit. The above idea was realized in complex alloying of iron with nickel and chromium during electrodeposition. It has been shown that electrodeposited metals appear solely in spherulitic form if transition from spherulitic to drusy growth form with increasing deposit thickness is prevented completely. Idea Three and Its Realization. Based on the concept of electrochemical phase formation in metals via a supercooled liquid state stage, it may be expected that the enrichment of crystalline defects will increase with higher supercooling in electrodeposition. This is because the high nucleation rate at greater supercooling values dominates over the linear crystal growth rate. Accordingly, increasing supercooling must lead to higher vacancy concentrations, greater dislocation densities and smaller grain sizes in metals being electrodeposited. The above idea was verified in experiments on a number of metals (cobalt, chromium, copper, iron and nickel) which were deposited under ordinary conditions from ordinary electrolytes with no organic additions. Consistent change has been found in characteristics of point, linear and planar crystalline defects as degree of supercooling is increased in electrodeposition. The obtained experimental results prove the existence of the phenomenon of electrochemical phase formation in metals via a supercooled liquid state stage. Fig 1. SEM image of pentagonal quasicrystal in the cathode-adjacent layers of the electrodeposited copper Figure 1