Deformation behaviour of dilute niobium--nitrogen alloys

A detailed study has been made of thermally-activated slip in niobium nitrogen single crystals at four doping levels from 0.3 to 300 at. ppM at temperatures from 300 down to 590K, in compression for two orientations, respectively ideal for (211) and (101) slip. It is demonstrated that there is no significant change in the mechanical properties below 3 at. ppM and thus that a considerable temperature dependence of the yield stress is intrinsic to the pure metal. The temperature dependence increased substantially with nitrogen content, however, and at 300 ppM was approximately double that of the pure metal. In contrast the temperature dependence of the thermal stresses, measured by both differential strain-rate and differential temperature tests, was almost insensitive to nitrogen content, indicating that the mobility of screw dislocations is little affected by these doping levels but that the yield stress does not depend solely upon this mobility. The strain-rate sensitivity at around 63K was complicated by the presence of transients on changing the strain-rate. The activation volume vs. thermal stress curves showed clear maxima at around 25 kg mm-2, indicating a change of mechansim of dislocation movement near this stress. ''Anomalous'' (011) slip was observed in the lower-level alloys at 90K and below but appeared to be suppressed by the addition of around 50 ppM nitrogen. Nitrogen doping also appeared to increase the probability of twinning and the twinning stress