High temperature deformation of V-1.6Y-8.5W-(0.08-0.15)C alloys

Full text of publication follows: Vanadium and its alloys are candidate materials for fusion reactor structural applications, however, one of their issues is enhancement of high temperature strength. In order to develop fine-grained and particle-dispersed V alloys with improved strengths at high temperatures, V-1.6%Y-8.5%W-0.08%C and V-1.6%Y-8.5%W-0.15%C (mass %) alloys were fabricated by mechanical alloying with starting powders of V, Y, W and VC and hot isostatic pressing (HIP), followed by annealing at 1273 and 1473 K for 3.6 ks. Tensile tests were performed at temperatures from 973 to 1373 K at initial strain rates from 1.0 x 10-4 to 1.0 x 10-2 s-1. X-ray diffraction analyses show that the 1273 K-annealed specimens exhibit distinct peaks of Y2O3 and W and small peaks of V2C, WC and W2C, whereas the 1473 K-annealed specimens do not show W peaks, indicating that W is dissolved into the V matrix during annealing at 1473 K. Transmission electron microscopy observations and energy dispersive x-ray spectroscopy show that W does not dissolve in the V matrix after annealing at 1273 K, but approximately 5%W dissolves after annealing at 1473 K. The average diameters of the matrix grains and dispersoids after 1473 K-annealing are 420 and 32 nm, respectively, in V-1.6%Y-8.5%W-0.08%C and 410 and 30 nm, respectively in V-1.6%Y-8.5%W-0.15%C. Tensile test results show that V-1.6%Y-8.5%W-0.15%C exhibits considerably higher yield stress over the entire temperature region than V-1.6%Y-8.5%W-0.08%C and that the 1473 K annealed specimens exhibit higher yield stress than the 1273 K-annealed one, due to solution hardening by W. The yield stress of V-1.6%Y-8.5%W-0.15%C is considerably higher than that of V-2.4Y whose yield stress is the highest in our previous studies, over the entire temperature region. Test temperature dependence of yield stress is divided into two regions: high- and low-temperature regions, and the transition temperature is higher in V-1.6%Y-8.5%W-0.15%C than in V-1.6%Y-8.5%W-0.08%C. The stress exponent of plastic strain rate, n, is approximately 3 in V-1.6%Y-8.5%W-0.08%C and approximately 4 in V-1.6%Y-8.5%W-0.15%C in the low temperature region. The activation energy for deformation in the low temperature region is 240 kJ/mol for V-1.6%Y-8.5%W-0.08%C and 250 kJ/mol for V-1.6%Y-8.5%W-0.15%C. These results suggest that the deformation of both alloys is controlled by solute atmosphere dragging. (authors)