Effects of recovery and recrystallization on microstructure and texture during annealing of a cold deformed superconducting Nb-50(wt.)%Ti alloy

Abstract

A Nb-50(wt.)%Ti alloy was melted and remelted in an electron beam furnace and the cast ingot was subsequently deformed into a bar by cold swaging, undergoing a maximum reduction in the area of 90%. Samples of the cold deformed bar were subjected to isochronous annealing cycles for 1 h in the temperature range between 250 and 1000 °C. The microstructural changes of the samples were monitored by optical microscopy, scanning electron microscopy with backscattered electron diffraction (EBSD), X-ray diffraction for pattern and texture determinations, and Vickers microhardness measurements. Recovery is the main softening mechanism in the samples annealed at temperatures up to 600 °C, but recrystallization is significant at temperatures equal to or above 750 °C and might also contribute to softening. Grain growth is also noted after annealing at the temperatures of 900 °C and 1000 °C. The microstructures of the cold deformed sample and of the samples annealed at temperatures up to 600 °C display a curly structure caused by deformation bands and their dislocation substructures. At the annealing temperatures of 900 °C and 1000 °C, complete recrystallization occurs and eliminates the deformation bands, but the curly pattern still exists and is probably due to a residue of the microsegregation of elements that occurs during solidification of the ingot. An intense fiber texture that is typical of cold swaging is observed in the cold deformed sample and in those annealed at temperatures up to 750 °C, but annealing at temperatures of 900 °C and 1000 °C weakens this texture owing to complete recrystallization. The results of the present work show the importance of the strong interactions among the phenomena of recovery, recrystallization, and microsegregation in determining the microstructure and texture of cold deformed and annealed Nb-Ti alloys.