ANOMALOUS ELASTIC SOFTENING OF POLYCRYSTALLINE Nb3Sn BELOW THE MARTENSITIC TRANSFORMATION TEMPERATURE

Nb3Sn exhibits an elastic shear-mode softening associated with a structural transformation from cubic to tetragonal symmetry at a temperature of Tm∼ 45K. Ultrasonic propagation measurements in single crystals indicate that the shear modulus ½(C11-C12) softens dramatically as the temperature is decreased towards 45K, but increases rapidly below Tm. We have measured Young's modulus and internal friction of polycrystalline Nb3Sn between 300 and 4.2K. Our results indicate that transforming polycrystals soften monotonically down to 18K with no discontinuity in the slope at Tm. These results are found to be incompatible with polycrystalline averages of single crystal elastic constants, even when the possibility of the presence of a fraction of non-transforming material is taken into account. A tentative explanation of this anomalous softening below Tm is given in terms of stress-induced domain wall motion below Tm