TRUQ (TRansformation-Heat based up-quenching) for high-performance Nb3Al superconductors

Heat treatment conditions and procedures of the transformation-heat based up-quenching (TRUQ) process particular to Nb₃Al coils are studied. It is found that pre-heating at 600°C before TRUQ process makes the bcc phase ordered but does not cause transformation. Noticeable is that TRUQ process combined with such a pre-heating does not depress the J_c performance, compared with TRUQ process without pre-heating. Thus, pre-heating before TRUQ process can be used to help fast-heating of coils. In this work, a vacuum furnace having two heaters or a set of fluidized-bed furnaces are used for TRUQ process combined with pre-heating. Samples used are small coils where the conductor of several tens of centimeters in length is wound on a stainless-steel bobbin. It is found that both ways improve J_c characteristics, compared with the conventional transformation process. Additionally, untransformed short samples are ohmically heated as TRUQ process, controlling current or voltage of the power supply, to control the application of the ohmic-heating to provide a practical coil heat treatment.