Tailoring of microstructure and critical currents in directionally solidified

Abstract

Directionally solidified composites display a complex microstructure with several defects coexisting in the matrix and hence the identification of the flux-pinning mechanisms requires a detailed analysis of the critical currents. In this work we review the formation mechanism of twins, stacking faults, microcracks, subgrain boundaries and dislocations in connection with the refining strategies of (211 phase) inclusions and modifications of the microstructure generated through prolonged oxygen annealings and high-temperature plastic deformation of the ceramic. The influence of these defects and post-processing treatments on the critical currents is then analysed focusing on their relative importance as flux-pinning centres at different temperatures and magnetic fields up to 20 T. A complete magnetic phase diagram is finally proposed as a basis for a systematic classification of the observed irreversible superconducting behaviour in single-grain melt-textured composites.