From an Anomalous Peak Effect to a Second Magnetization Peak in Nb-rich Nb-Ti Alloys
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We investigated the evolution of the DC magnetic hysteresis curves of Nb 0.89Ti 0.11 alloys thermo-mechanically processed by intermediate heat treatments at 900 ∘C in vacuum and cold rolling (∼ 50 % thickness reduction). Starting with a rectangular piece (∼ 0.4 mm thick) cut from the as-grown alloy, after the first thermo-mechanical treatment, the specimen exhibits over a wide temperature T interval a peak effect not far from the DC irreversibility line. With a supplemental thermo-mechanical treatment, the peak effect disappears and is substituted by a second magnetization peak (where the characteristic fields are significantly lower) induced by pinning enhancement. The second magnetization peak was clearly seen at high temperatures only, due to the occurrence of thermo-magnetic instabilities in the low-T domain. In both cases, analysis of magnetic relaxation evidences a crossover towards plastic vortex creep accompanying the maximum in the effective critical current density. These results suggest a common nature of the observed effects, related to the disordering of the vortex system induced by pinning.
KeywordsNb-rich Nb-Ti alloys DC magnetic measurements Peak effect Second magnetization peak Pinning-induced vortex system disordering
Work supported by UEFISCDI, Romania, under Core Programs PN16-480101 and PN16-470104.