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 Tm=50 K, with the c/a of the tetragonal structure increasing as the temperature is decreased until the ratio becomes constant at the superconducting transition temperature Tc. During electron irradiations between 0.4 and 2.0 MeV the electrical resistivity at 20 K initially decreased, contrary to experience with irradiated metals. For electron energies above the energy determined to be threshold for the production of Frenkel defects the resistivity eventually increased with increasing dose as expected. For energies below threshold, however, the resistivity approached a limiting value of about 99.6% of the initial resistivity before irradiation. The resistivity vs. temperature shows a deviation to higher-resistivity values as temperature is lowered commencing at Tm with increasing deviation as the tetragonality increases at lower temperatures. We ascribe the observed decrease in resistivity with electron irradiation to a metastable reversal of the tetragonal phase toward a state of reduced tetragonality (to cubic). This resistivity decrease is completely recovered in annealing to slightly above Tm. High-voltage electron microscopy at 12 K confirms the decrease in tetragonality by a disappearance of the twin boundaries associated with the tetragonal phase. The microstructure responsible for the resistivity changes will be discussed. (orig.)