A high temperature W2B–W composite for fusion reactor shielding

Abstract

We have developed a new material for neutron shielding applications where space is restricted. W2B is an excellent attenuator of neutrons and gamma-rays, due to the combined gamma attenuation of W and neutron absorption of B. However, its low fracture toughness (3–4 MPa m1/2) and high melting point (2670 °C) prevent the fabrication of large fully-dense parts with adequate mechanical properties. Here we meet these challenges by combining W2B with a minor fraction (43 vol%) of metallic W. The material was produced by reaction sintering W and BN powders. The mechanical properties under flexural and compressive loading were determined up to 1900 °C. The presence of the ductile metallic W phase enabled a peak flexural strength of ∼850 MPa at 1100 °C, which is a factor of 2–3 higher than typical monolithic transition-metal borides. It also enabled a ductile-brittle transition temperature of ∼1000 °C, which is not observed in monolithic borides. Compression tests showed hardening below ∼1500 °C and significant elongation of the phase domains, which suggest that by forging or rolling, further improvements in ductility may be possible. These results have implications for W2B–W shield design; neutronics performance will likely improve with increased boron content, however this study suggests mechanical properties and manufacturability may degrade.