Tensile Strength of Large Grain Niobium at Liquid Helium Temperature

Tensile tests at liquid helium temperature were performed using specimen taken from high purity large grain niobium ingot produced by CBMM. The measured RRR of large grain niobium is 242. The ingot, a diameter of 260 mm, and was sliced by a multi wire saw to 2.8 mm thickness. 5 specimens were cut off from one sliced disk. 3 disks were set in the same phase to obtain the same grain distribution. 3 specimens, each of 5 grain patterns, 15 in total, were used for the tensile testing. We manufactured the tensile testing stand using a cryostat and liquid helium. The measured tensile strength varied from 379 to 808 MPa. The average tensile strength value is 611 MPa, which is 70% of that of fine grain. The tensile strength at room temperature is 84 MPa. The strength becomes high at low temperature like in fine grain niobium. The specimen includes some grains whose strength depends on the crystal orientation, which causes the variation of strength.