Softening of Young's Modulus of Polycrystalline Nb sub 3 Sn

摘要

Ultrasonic measurements show that the shear modulus (C sub 11 -C sub 12 )/2 softens dramatically as single crystals of Nb sub 3 Sn approach the martensitic transition near 50K. It is expected that Young's modulus of polycrystals will olso soften, but previous ultrasonic measurements, which suffer from severe damping, fail to show the expected effect. We have measured Young's modulus of polycrystalline Nb sub 3 Sn between 4.2K and 300K by static beam deflection methods, and observe marked softening. A value of 14.3 +- .5 x 10 exp 11 dyne cm exp -2 was obtained at 300K by the deflection of thin Nb sub 3 Sn-Nb-Nb sub 3 Sn composite strips by external stress. The variation of the modulus with T was obtained from the change in the radius of curvature of internally stressed Nb sub 3 Sn-Nb composite strips. (This method is made possible by the near perfect match between the thermal expansion coefficients of Nb and Nb sub 3 Sn.) The modulus is found to be proportional to 1nT between 50K and 300K and is temperature independent below the superconducting T/sub c/, resulting in a decrease by a factor of approx. 2 between 300K and 18K. The observed softening is somewhat less than that predicted by a polycrystalline average of the experimental single crystal elastic constants, but is much larger than that observed with ultrasonic measurements. (ERA citation 04:053441)