Effect of atmosphere on the high-temperature interaction of molten Mg-10Ca alloy with steel substrates

Abstract

Experimental studies of the high-temperature interaction of a molten Mg-10Ca alloy (10 wt% Ca) with steel substrates were performed, for the first time, using the sessile drop method combined with non-contact heating and capillary purification procedure. The investigations were carried out under isothermal conditions at a temperature of 700 °C in two different flowing gas atmospheres: (i) a pure Ar and (ii) a mixture of Ar + 5 wt% H2. The high-resolution side-view images of Mg-10Ca/steel couples collected during the experiments were used to determine the contact angle values (θ) formed between the alloy drop and steel substrates. In both atmospheres, under the experimental conditions used, liquid Mg-10Ca alloy exhibited non-wetting behavior on steel substrates: after 180 s of dropping alloy from a graphite capillary, the average contact angle value in the Ar atmosphere was θav = 120°, which was slightly higher than that for the test performed in the Ar + 5 wt% H2 atmosphere, where θav = 114°. The microstructure of the solidified couples was analyzed by scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS). The SEM/EDS analysis revealed interfacial discontinuities throughout the cross-sectioned couples and a lack of permanent bonding between the alloy drop and substrates. Moreover, neither new phases nor mass transfer through the drop/substrate interface were noted. The non-wetting and weak bonding between the alloy drops and the steel substrates is consistent with the Mg–Fe and Ca–Fe phase diagrams, i.e., the non-reactive nature of the Mg-10Ca/steel couples, since at the testing temperature, Fe does not form any compounds with both Mg and Ca, and it does not dissolve in liquid Mg.