Young’s modulus of polycrystalline Nb3Sn between 4.2 and 300 K

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Abstract
Ultrasonic measurements show that the shear modulus 1/2 (C11−C12) softens dramatically as single crystals of Nb3Sn approach the martensitic transition near 50 K. It is expected that Young’s modulus of polycrystals will also soften, but previous ultrasonic measurements, which suffer from severe damping, fail to show the expected effect. We have measured Young’s Modulus of polycrystalline Nb3Sn between 4.2 and 300 K by static beam‐deflection methods and observe marked softening. A value of 1.32×1012 dyn cm−2 was obtained at 300 K by the deflection of thin Nb3Sn‐Nb‐Nb3Sn 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 Nb3Sn‐Nb composite strips. (This method is made possible by the near‐perfect match between the thermal expansion coefficients of Nb and Nb3Sn.) The modulus is found to be proportional to lnT between 50 and 300 K and is temperature independent below the superconducting Tc, resulting in a decrease by a factor of ∼2 between 300 and 18 K. 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.