High-temperature interactions between molten Mg-10Ca alloy with Ti substrate under different atmospheric conditions

Abstract

This work investigates the combined effects of atmosphere and alloying with calcium on the high-temperature interaction between a molten Mg-10Ca alloy and a polished titanium substrate. The sessile drop method was employed with non-contact hea­ting and capillary purification, enabling the generation of an oxide-free Mg-10Ca drop. Experiments were conducted isothermally at 700°C under two flowing atmospheres: (i) pure Ar and (ii) Ar + 5 wt.% H₂.

In pure Ar, the evolution of the average contact angle results from both evaporation of the oxide-free Mg-10Ca alloy and chemical interaction between Mg-Ca vapor and the oxidized Ti surface. Initially, non-wetting behavior (θ > 90°) was observed. A non-wetting-to-wetting transition occurred at ~200 s (θ = 90°), followed by progressive improvement in wettability, reaching an average final contact angle of ~70° at 300 s.

SEM/BSE/EDS analysis of the Mg-10Ca/Ti couple formed in pure Ar revealed interfacial discontinuities, absence of permanent bonding, and pronounced triple-line pinning, which led to an asymmetric drop geometry arising from the inhomogeneous structure and chemistry of the Ti surface. In contrast, excellent wettability and bonding were obtained under the Ar + 5 wt.% H₂ atmosphere. Under identical testing conditions, the contact angle decreased rapidly from 19° to 11° within ~50 s and remained nearly constant during holding at 700 °C up to 300 s, with a final contact angle of ~9° at the end of the test. The results of structural characterization suggest that the remarkable improvement in wettability arises from the synergistic effect of several factors: (1) alloying magnesium with calcium, (2) presence of hydrogen in the surrounding atmosphere, (3) the evaporation of both Mg and Ca from the drop and (4) the enhanced chemical interaction between the Mg-Ca alloy and the Ti substrate in the presence of hydrogen.