The effect of the size of NbTi filaments on interfacial reactions and the properties of InSn-based superconducting solder joints

Highlights: • Interaction of NbTi with InSn produces an intermetallic phase, (Nb,Ti)Sn₂, with randomly crystallographic oriented grains. • Larger filaments form thin uniform (Nb,Ti)Sn₂ phase with negligible Ti-solubility. • Large (Nb,Ti)Sn₂ scallops grow with significant Ti-concentration for finer filaments. • Irreversibility field and flux line pinning force of NbTi changes owing to coarsening of α-Ti precipitates after soldering. • No detectable interaction zone for NbTi–(In,Bi)Sn due to decrease in thermodynamic activity of Sn with addition of Bi. • The property of bulk solder is regarded the limiting factor and not any interfacial reactions. -- Abstract: Interactions of superconducting Nb(47 wt% Ti) with liquid binary In-35 wt% Sn and Bi-containing ternary (In, 15 wt% Bi)-35 wt% Sn solders have been examined for the first time up to 450 °C. Reaction of NbTi with InSn results in an intermetallic phase, (Nb,Ti)Sn₂, the morphology and chemical composition of which depends on the size of NbTi filaments. Larger filaments react to form a thin uniform (Nb,Ti)Sn₂ phase with a Nb:Ti ratio of 6:1, while for finer filaments large (Nb,Ti)Sn₂ grains grow with a Nb:Ti ratio of 0.5:1. Differences for interfacial phase layers have been explained by a diffusion-controlled growth model based on metallurgical state of NbTi filaments. Neither filamentary types exhibit any change in critical temperature (~9.2 K) after soldering treatment at 350 °C, but the irreversibility field and flux line pinning force of NbTi changes by a higher factor for the larger filaments. Interaction of NbTi with ternary (In,Bi)Sn results in a lack of any detectable interaction zone, and characteristic nucleation behaviour of phases in the bulk solder at NbTi–(In,Bi)Sn interface is rationalized on the basis of thermodynamic arguments. Transport measurements suggest that properties of solder are the limiting factor for joint behaviour and not any interfacial reactions.