Liquid-State Interfacial Reactions of Sn-Zn/Co Couples at 250°C

This study examined the interfacial reactions between liquid Sn-Zn alloys and Co substrates at 250°C with different Zn compositions from 1 wt.% to 9 wt.%. The reaction phases formed at the interface were significantly affected by the Zn content. One irregular reaction phase, γ2-CoZn13, was observed in the Sn-9 wt.%Zn/Co reaction. Decreasing to 7 wt.% Zn, the interfacial product changed to the γ1 phase. After a longer reaction time, a new ternary compound with composition Sn-20.2 at.%Zn-31.4 at.%Co, the T phase, was formed between the γ1 phase and Co substrate. Only one very thin and continuous T phase layer was found, and it was stable at the Sn-4 wt.%Zn/Co interface, even after 120 h reaction. However, the Sn-3 wt.%Zn/Co couples revealed significant microstructural evolution in both the T and CoSn3 layers. Moreover, the dominant phase became CoSn3 as the Zn content decreased below 2 wt.%. Based on these interfacial results, a predicted Sn-Zn-Co phase diagram (less than 30 at.% Co) at 250°C is proposed. The corresponding reaction paths of these couples are illustrated on the phase diagram.