The thermally activated deformation of niobium-molybdenum and niobium-rhenium alloy single crystals

Abstract Measurements of the yield-stress and of the strain-rate sensitivity in compression over the temperature range 14—373°C are reported for single crystals of niobium of two purities and for niobium alloy single crystals containing up to 25 a/o molybdenum or 9 a/o rhenium. As the solute content increases, the maximum in the curve of strain-rate sensitivity versus temperature first decreases in magnitude and then moves to higher temperatures, and in the more concentrated alloys a second peak appears at lower temperatures. A corresponding inflection appears in the curves of yield stress versus temperature, and a similar temperature dependence of the yield stress is also observed in ultra high vacuum annealed niobium. Analysis of the activation parameters indicates that there are two rate-controlling mechanisms in different ranges of temperature. These processes occur in a parallel manner in such a way that, at constant strain rate, the mechanism requiring lowest stress is selected. It is therefore suggested that the processes are similar in character but involve two separate saddle-point configurations which are affected by the additions of substitutional solute atoms.