Oxygen impurity and its alloying effect in sputter-coatings of Zr-based metallic glass alloy for a brazing filler metal

The sensitivity of intrinsic impurities in a normal sputtering process to the coating of Zr-based metallic glass alloy as a brazing filler metal (BFM) is demonstrated. In a sputtering atmosphere, the oxygen is the most critical impurity, mostly resulting from the outgassed water vapors. Its preferential adsorption onto the highly reactive species during sputtering causes a depletion of easily oxidizable Zr and Ti in the resultant coating, as well as a substantial decrease in the coating's reactivity with the Zircaloy-4 base metal upon melting. In a brazed joint, the coating with high levels of oxygen (about 12 at.%) induces the segregation, comprising of the predominant α-Zr grains alloyed by oxygen, which has a deleterious effect on the mechanical strength owing to an oxygen-induced embrittlement of α-Zr. Not more than about 5 at.% is proposed as the degree to which oxygen impurity needs to be controlled in BFM coatings to achieve the sound joints with properties comparable to those of the Zircaloy-4 base metal.