Ductilisation of tungsten using rolling and rhenium alloying

Abstract

The brittleness of W at low temperatures remains a major challenge for its application in load-bearing components. To address this problem, two strategies were explored in this study: (1) the application of plastic deformation via rolling and (2) alloying with the ductilising element, Rhenium (Re). To study the combined effects of thermo-mechanical processing and alloying, sheets of W and W-25Re (wt.%) were prepared using powder metallurgy and rolling. Average grain sizes of 0.4 μm and 0.6 μm were measured perpendicular to the rolling direction, for the W and W-25Re sheets respectively. Additionally, the W-25Re sheet was annealed at 1650 °C to form large, recrystallised grains that would typically be associated with brittle behaviour in unalloyed W. Characterisation of the as-rolled materials revealed highly textured microstructures with elongated grains with a preferential 〈110〉 orientation aligned parallel to the rolling direction. Tensile yield stresses of 1.82 GPa and 1.33 GPa were obtained for the rolled W and W-25Re respectively, while the recrystallised W-25Re had a yield stress of 0.99 GPa. The elongation to failure was significantly increased by Re alloying from 4 % up to 22 % at room temperature in the rolled material. Examination of the fracture surfaces revealed that the rolled materials failed via transgranular cleavage at room temperature, whereas the recrystallised W-25Re failed by intergranular fracture. There was also evidence of microcracking at room temperature for all three materials which suggested the initial stages of a transition to a delamination failure mode at elevated temperatures. An increase in necking was seen for W-25Re compared with W and an increase in plastic deformation because of alloying. Microcracking in the rolled W may accommodate some of the 4 % elongation measured despite the fact the material had a fracture surface typical of brittle failure. We demonstrate that Re alloying imparts an increase in ductility that can be utilised in combination with rolling to produce strong materials with high temperature compatibility and exceptional room temperature ductility.