Strength and ductility at cryogenic temperature in Cu-Ti alloy

A quenched material (No. 1), an aged material (No. 2), a cold-rolled and aged material (No. 3) and an aged, cold-rolled and anneal-hardened material (No. 4) of a Cu-4 wt%Ti alloy were tensile-tested at temperature range from 4 to 70 K and at room temperature. Microstructures were also observed before and after the tests. Results obtained were summarized as follows: (1) The strength at cryogenic temperature in each material was high in the order of the material No. 4, 3, 2, 1. On the other hand, the ductility was large in the reverse order. These orders were as the same as those at room temperature. (2) The yield stress (the elastic limit) and U.T.S. in all the materials increased essentially in straight lines with decreasing temperature from room temperature to about 4 K. The increases in the strength were not large, and the temperature dependence of the yield stress was ascertained to be due to that of the shear modulus in the materials No. 1 and 2, but the temperature dependence of the yield stress in the materials No. 3 and 4 could not be ascribed to the temperature dependence of the shear modulus only. (3) The elongation to fracture was divided into the uniform elongation and the locallied elongation. The uniform elongation increased exponentially with decreasing test temperature in all the materials. The increase in the uniform elongation was due mainly to the difficulty of cross slip of dislocations at lower temperature, and more twins generated during deformation at lower temperature might play a role in retarding cross slip of dislocations. When the uniform elongation, epsilon sub(U) was larger than about 0.15, the value of epsilon sub(U) was in good agreement with an average value of the two work-hardening exponents, (n1 + n2)/2. Differences in the uniform elongation among the materials could be explausible in terms of the stacking fault energy and inhomogenity of the deformation structure induced during thermomechanical processing. (author)