Effects of Heat Treatments on Tungsten for Armours in NFR

Abstract

Tungsten is a promising armour material for plasma facing components of nuclear fusion reactors (NFR) because of its low sputtering rate and favourable thermo-mechanical properties (high melting point and good thermal conductivity). This paper reports some results of an experimental campaign carried out for investigating the microstructural characteristics and the mechanical properties of tungsten (99.97% purity; 5% porosity) for fusion applications. Tungsten has been heat treated at 500 °C and 800 °C with increasing soaking time. The samples in as-supplied condition and after each step of the heat treatments have been examined by optical microscopy and TEM observations, X-ray diffraction (XRD) and micro-hardness tests. The original material has a dislocation density of 1.5 x 1010 cm-2 and a mean grain size of 65 μm. Grain size is not affected by the heat treatment at 500 °C which induces only a weak decrease of dislocation density leading to a little smaller hardness. The microstructure can be considered substantially stable even if a weak recovery of dislocations takes place. On the contrary, grain growth is observed after heating at 800 °C: 10 hours of treatment nearly doubles the average grain size.