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Abstract Nb 3 Al superconductors with high superconducting performance are of interest as potential materials for using in high magnetic fields. The phase formation mechanism plays a vital role in preparing high quality Nb 3 Al superconductor, especially with better superconducting transition temperature (T c). In this work, the phase evolution of Nb Al system during mechanical alloying and subsequent sintering process was studied, and the corresponding kinetics mechanism of Nb 3 Al phase formation was investigated in detail. It was found that the Nb 3 Al phase can be formed through two different routes: one is reaction between Nb 2 Al and Nb, the other is transformation from Nb Al supersaturated solid solution (Nb(Al) ss) directly. Mechanical alloying promotes the transformation of Nb Al pre-alloyed powders from local Nb Al amorphous to Nb(Al) ss , controlling the different formation routes of Nb 3 Al phase. According to the kinetics calculation of the Nb 3 Al phase formation during sintering process, the values of activation energy (E) for the reaction between Nb 2 Al and Nb forming Nb 3 Al is higher than that for the transformation from Nb(Al) ss to Nb 3 Al in the initial stage, but decreases sharply with the reaction going on. Moreover, Nb 3 Al bulk synthesized via reaction between Nb 2 Al and Nb obtained a higher value of T c (about 15.8 K) compared with the Nb 3 Al bulk synthesized via Nb(Al) ss. The results in our work suggest that the phase formation mechanism of Nb 3 Al during mechanical alloying and subsequent sintering process actually influence their final superconductivity performance and can give an effective guide for optimizing the technique of preparing Nb 3 Al superconductors. Graphical abstract Image 1 Highlights • Two formation routes of Nb 3 Al phase are found during mechanical alloying and subsequent sintering. • The formation of Nb 3 Al phase is dominated by interface reaction and random nucleation. • Nb 3 Al bulk synthesized via reaction between Nb 2 Al and Nb obtained a higher value of T c. [ABSTRACT FROM AUTHOR]

The Nb 3 (Al 1-x Sn x) superconductors (x = 0.05, 0.15, 0.25, 0.35) were prepared by mechanical alloying and subsequent low-temperature sintering. It was found that the critical current density (J c) of the obtained Nb 3 (Al 1-x Sn x) sample is enhanced significantly compared to undoped Nb 3 Al prepared using the same method. The maximum value of the J c reaches up to 5.10 × 104 A·cm−2 (4.2 K, 5 T) for Nb 3 (Al 0.85 Sn 0.15) sample. Combined with the phase composition analysis and microstructure observation, it is indicated that Sn enters the A15 crystal structure of Nb 3 Al forming the Nb 3 (Al 1-x Sn x) phase after low-temperature sintering. What's more, Sn doping improves the grain connectivity as well as refines the grain size of Nb 3 Al, which are the main reasons for the enhancement of J c in the Sn doped samples. Based on the exploration of the phase evolution of Nb–Al–Sn system during the mechanical alloying and subsequent low-temperature sintering process, it is also confirmed that Nb 3 Sn forms firstly during the sintering process and then can serve as the perfect nucleus of Nb 3 Al to increase its nucleation rate, leading to the significant decrease of the grain size of Nb 3 Al. • The J c of Nb 3 Al reaches up to 5.10×104 A·cm−2 (4.2 K, 5 T) after Sn doping. • The Sn doping sharply reduced the grain size of Nb 3 Al (from 80-200 nm to 25–40 nm). • During the low-temperature sintering process, the Nb 3 Sn formed firstly and then served as the perfect nucleus of Nb 3 Al to increase its nucleation rate. [ABSTRACT FROM AUTHOR]