Boriding of tungsten by the powder-pack process: Phase formation, growth kinetics and enhanced neutron shielding

In this study, tungsten boride coating was grown on pure tungsten substrate by pack boriding method for improved neutron shielding ability. The microstructure, growth kinetics and radiation shielding performance were systematically studied. It is found that the boronized layer shows a dual-phase structure under the boriding temperature of 950–1050 °C. The main layer closes to coating surface is identified to be the WB phase, while the inner layer adjacent to the substrate is W2B. Thermodynamics calculation proves the W2B is more favored to nucleate and grow adhering to W than α-WB. Once the W2B layer was formed, further increasing boron concentration enables to introduce the formation of WB at the interface. On the basis of the calculated WB phase diagram, two separated diffusion models were developed to elucidate the growth of WB with composition gradient and W2B with constant composition. The diffusion coefficients of boron in boride phases as well as the activation energy for boron diffusion were estimated. Finally, the large improvement of neutron attenuation ability for tungsten specimen upon boriding treatment was demonstrated. Our findings could provide new directions for developing high performance radiation shielding materials.