A15 phase formation kinetics study of mono-element internal-Sn Nb ₃Sn superconducting wires

Four sets of mono-element internal-Sn (MEIT) wires which have different Sn-Cu ratio and 1 at% Zr addition in one wire were designed and fabricated for investigating A15 phase formation kinetics of Nb₃Sn superconductors. All samples underwent a 210 °C/50 h + 340 °C/25 h thermal duration for Cu-Sn alloying prior to the A15 phase formation heat treatment (HT). Four reaction temperatures of 650 °C, 675 °C, 700 °C, and 725 °C were chosen to study the temperature and time influence. All the heat-treated samples were examined by scanning electronic microscope for A15 layer thicknesses that were then plotted versus HT time at different temperatures and were nonlinearly fitted. Superconducting quantum interference device magnetization measurements were used to determine the inductive J_C variations for the heattreated wires. The obtained results demonstrate that the A15 phase growth is promoted by four factors: reaction temperature elevation, reaction time extension, Sn-Cu ratio enlargement, and the alloyed Zr addition. The phase formation kinetics of MEIT Nb₃ Sn superconducting wires is in agreement with Yⁿ = K(T)t relation and the A15 growth exponent n is affected by HT temperature and Zr alloying.