ANOMALOUS ELASTIC SOFTENING OF POLYCRYSTALLINE Nb₃Sn BELOW THE MARTENSITIC TRANSFORMATION TEMPERATURE

Abstract
Nb₃Sn exhibits an elastic shear-mode softening associated with a structural transformation from cubic to tetragonal symmetry at a temperature of T_m˜ 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 T_m. 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 T_m. 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 T_m is given in terms of stress-induced domain wall motion below T_m.