EFFECT OF TRANSVERSE STRESS ON THE CRITICAL CURRENT OF BRONZE-PROCESS AND INTERNAL-TIN NB3SN

The effect of transverse stress on the critical current of two substantially different Nb3Sn superconductors, a bronze-process conductor and an internal-tin conductor, has been measured. Photomicrographs of the two conductors reveal a basic difference in their microstructure. The bronze-process conductor exhibits columnar grains that are radially oriented within the Nb3Sn filaments, while the grains of the internal-tin conductor are more equiaxed and randomly oriented. The radial orientation of the bronze-process grains defines an anisotropy between the axial and transverse directions that might account for the greater sensitivity of the critical current to transverse stress reported previously. The effect of transverse stress measured on the internal-tin conductor, however, is comparable to that of the bronze-process conductor. Thus, these data indicate that the transverse stress effect is not highly dependent on either grain morphology or fabrication process. From an engineering standpoint the similarity of the transverse stress effect for these two types of Nb3Sn superconductors represents an important simplification for setting first-order quantitative limits on the mechanical design of large superconducting magnets.