The deformation of Nb–N single crystals at low temperatures

Abstract

Previous work by Bowen and Taylor in 1977 on the mechanical properties of Nb–N single crystals tested in compression in the temperature range 50–300 K has be supplemented by new experiments on similar alloys in tension in the range 4.2–77 K. The characteristics studied include the temperature dependence of the yield and flow stresses, the strain rate dependence of the flow stress, and the slipline morphology. Measured yield stresses at 77 K were generally compatible with those of Bowen and Taylor in compression, but the spread in values with nitrogen content was appreciably smaller. Alloys with up to 70at.p.p.m. N behaved like pure niobium and exhibited a loss of temperature dependence of the yield stress in the range 77–50 K where anomalous slip often occurs, followed by a steep rise with decreasing temperature at lower temperatures. Alloys with 200 and 320 at.p.p.m. N did not show the low-temperature plateau, anomalous slip was suppressed and the yield stress at low temperatures was significantly higher. The flow stress results did not show a plateau even for ‘pure’ niobium, and the total temperature dependence in the range below 77 K was approximately the same for all alloys. Single-temperature-change experiments between room temperature and 77 K showed a larger temperature dependence in this range for the alloys of high nitrogen content, but the spread of values more nearly approached those of the yield stress than in Bowen and Taylor's compression tests, so that the differential effect of nitrogen on the temperature dependence of the yield and flow stresses, although still detected, was not as pronounced as that in the earlier work. At very low temperatures, twinning was the dominant mode of deformation, even after pre-strain at room temperature or 77 K.