Improvement of small to large grain A15 ratio in Nb₃Sn PIT wires by inverted multistage heat treatments

The next generation of superconducting accelerator magnets for the Large Hadron Collider at CERN will require large amounts of Nb₃Sn superconducting wires and the Powder-In-Tube (PIT) process, which utilizes a NbSn₂-rich powder core within tubes of Nb(7.5wt%Ta) contained in a stabilizing Cu matrix, is a potential candidate. However, the critical current density, J_c, is limited by the formation of a large grain (LG) A15 layer which does not contribute to transport current, but occupies 25-30% of the total A15 area. Thus it is important to understand how this layer forms, and if it can be minimized in favor of the beneficial small grain (SG) A15 morphology which carries the supercurrent. The ratio of SG/LG A15 is our metric here, where an increase signals improvement in the wires A15 morphology distribution. We have made a critical new observation that the initiation of the LG A15 formation can be controlled at a wide range of temperatures relative to the formation of the small grain (SG) A15. The LG A15 can be uniquely identified as a decomposition product of the Nb₆Sn₅(Cu_x), surrounded by a layer of rejected Cu, thus the LG A15 is not only of low pin density, but is not continuous grain to grain. We have found that in single stage reactions limited to 630 °C - 690 °C, the maximum SG A15 layer thickness prior to LG A15 formation is very sensitive to temperature, with a maximum around 670 °C. This result led to the design of four novel heat treatments which all included a short, high temperature stage early in the reaction, followed by a slow cooling to a more typical reaction temperature of 630 °C. We found that this heat treatment (HT) modification increased the SG A15 layer thickness while simultaneously suppressing LG A15 morphology, with no additional consumption of the diffusion barrier. In the best heat treatment the SG/LG A15 ratio improved by 30%. Unfortunately, J_c values suffered slightly, however further exploration of this high temperature reaction region is required to understand the limits to A15 formation in Nb₃Sn PIT conductors.