METASTABLE REVERSAL OF THE MARTENSITIC PHASE IN NB3SN INDUCED BY ENERGETIC-ELECTRON IRRADIATION

Nb3Sn upon cooling undergoes a structural martensitic phase transition from the cubic A-15 structure to a tetragonal one commencing at about T-m = 50 K, with the c/a of the tetragonal structure increasing as th e temperature is decreased until the ratio becomes constant at the sup erconducting transition temperature T-c. During electron irradiations between 0.4 and 2.0 MeV the electrical resistivity at 20 K initially d ecreased, contrary to experience with irradiated metals. For electron energies above the energy determined to be threshold for the productio n of Frenkel defects the resistivity eventually increased with increas ing dose as expected. For energies below threshold, however, the resis tivity approached a limiting value of about 99.6% of the initial resis tivity before irradiation. The resistivity vs. temperature shows a dev iation to higher-resistivity values as temperature is lowered commenci ng at T-m with increasing deviation as the tetragonality increases at lower temperatures. We ascribe the observed decrease in resistivity wi th electron irradiation to a metastable reversal of the tetragonal pha se toward a state of reduced tetragonality (to cubic). This resistivit y decrease is completely recovered in annealing to slightly above T-m. High-voltage electron microscopy at 12 K confirms the decrease in tet ragonality by a disappearance of the twin boundaries associated with t he tetragonal phase. The microstructure responsible for the resistivit y changes will be discussed.