Insights into microstructural evolution of tungsten-tungsten carbide plasma facing composite materials prepared by field assisted sintering technique

Abstract

Combining a stable, resilient W-W2C or WC as plasma-facing material, with thermally more conductive and ductile W as a structural material, may provide a solution for the demanding environment in the fusion reactor chamber. In order to ensure the structural stability of such layered composite, their phase composition and thermal stability at elevated temperatures and cyclic heat loads needs to be evaluated.In situ densification and joining of W and W-xWC materials have been performed by field-assisted sintering technique. Microstructural analysis revealed that the interface layer formed between W and pure WC consists primarily of large W2C grains. Initially, equiaxed W2C grains are formed following the decarburisation of WC due to the large C concentration gradient. Further C diffusion results in the formation of larger anisotropic grains growing preferentially in the 〈0001〉 direction. A similar microstructure was observed when joining WC-reinforced W (W-xWC, x = 7, 17, 28, 33 at. % C) with WC. On the other hand, the joining of W with WC-reinforced W resulted in a sound joint with no observable grain growth at the interface, even at large C concentrations (e.g. 28 at. %). As formed, microstructures were stable after thermal ageing at 1600 °C for 24 h with no observable change in major microstructural features.The combination of WC-reinforced W as high strength plasma facing armour material and W-structural material offers a promising composite solution for improved thermal stability under high cyclic heat loads expected in future fusion power plants.