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Effect of temperature and interstitial composition on the yielding of niobium single crystals Harding, Hugh James

Abstract

An investigation of the effect of temperature and interstitial composition on the yielding characteristics of high purity niobium single crystals has been carried out. Single crystals of [110] axial orientation were purified and grown using an electron beam, floating-zone, zone-refiner. Tensile specimens prepared from the single crystals were tested in tension at temperatures from +25° to -197°C. at a strain rate of 0.057 per minute. The variation of yield stress with testing temperature was found to be discontinuous and consisted of two curves which intersected at approximately -100°C. The tensile results can be correlated to both Fisher's interpretation of the Cottrell-Bilby theory of yielding and to the theory of Zener and Holloman, which involves a thermally activated process. The effect of additions of hydrogen, oxygen and nitrogen to niobium single crystals over the temperature range +25° to -197°C was to increase the yield stress. The unusual temperature dependence of yield stress has been explained in two possible ways: 1. Ordering or superlattice formation at the temperature at which the anomalous behaviour occurs. 2. The preferential release of screw dislocations at stress levels lower than that required for edge dislocations. The result is a decrease in the temperature dependence of yield stress at the temperature when edge dislocations begin to take part in yielding.

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