Wettability and Reactivity of Liquid Magnesium with a Pure Silver Substrate

Abstract

For the first time, experimental data on the high-temperature interaction of liquid Mg with pure Ag are presented. The study was performed by the sessile drop method and capillary purification procedure. The test was carried out under isothermal conditions at 720 °C in a protective atmosphere of Ar + 5 wt.% H2. The solidified couple was subjected to detailed microstructural observations by scanning electron microscopy (SEM) coupled with energy-dispersive x-ray spectroscopy (EDS). Under the used conditions, immediately after contact with the Ag substrate, liquid Mg drop showed a good wetting (θ0 ~ 65°) followed by fast spreading over the substrate in subsecond time to form the final contact angle of θf ~ 31°.SEM/EDS analysis showed that θf is apparent because of a deep crater (200 μm) formed in the substrate under the drop by the dissolution of Ag in liquid Mg. SEM/EDS observations of complex structural transformations in the Mg/Ag couple due to high-temperature contact and subsequent cooling are in good agreement with the Ag-Mg phase diagram. Besides substrate dissolution, the interaction between liquid Mg and solid Ag at 720 °C is accompanied with the alloying of the Mg drop with Ag and the formation of a continuous layer of the β-AgMg phase at the Mg/Ag interface. During cooling, the chemical composition of the Mg(Ag) drop continuously changes, and this process is followed by the formation of the β-AgMg phase by secondary precipitation from Ag-saturated liquid, a partial transformation of the β-AgMg to ε′-Ag17Mg54 phase by peritectic reaction, followed by the solid-state transformation of the ε′-phase to the ε-AgMg3 phase, and finally, the solidification of residual liquid in the form of the two-phase eutectic mixture of AgMg4 + (Mg). The results obtained suggest that a very good wetting and fast spreading observed experimentally for the Mg/Ag couple is caused by high reactivity between liquid Mg and Ag substrate leading to the combined effect of two reactive wetting mechanisms, i.e. through dissolutive wetting and wetting through the formation of the interfacial reaction product (β-phase).