Melt fluxing to elevate the forming ability of Al-based bulk metallic glasses

Salt-fluxing treatment is an effective technique to improve the glass-forming ability (GFA) of bulk metallic glass (BMG)-forming melts, as demonstrated before in Pd- and Fe-based systems. However, it has been challenging to develop similar fluxing protocol for more reactive melts, such as Al-rich BMG-forming systems. Here we design new fluxing agents, from a thermodynamics perspective that takes into account combined effects of physical absorption and chemical absorption (reaction) between the fluxing agents and oxide inclusions. MgCl2-CaCl2 composite salts were selected, and their fluxing effects were systematically studied on an Al86Ni6.75Co2.25Y3.25La1.75 alloy, the best BMG-forming composition reported thus far for Al-rich alloy systems. The oxygen content was found to continuously decrease in the master alloy with increasing cycles of salt-fluxing treatment, with chlorate products on the surface suggesting concurrent physical absorption and chemical reaction. The fluxing treatment developed has enabled a record critical size (diameter) of 2.5 mm for Al-based BMGs. Our finding is thus an advance in developing highly desirable Al-based BMGs, and also provides guidance for designing processing protocol to produce larger-sized BMGs in other reactive systems.