Evolution of microstructure in neutron irradiated cold rolled tungsten and its correlation with hardness

Abstract

The understanding of the neutron irradiation effects in tungsten is of significant importance for its use as a plasma facing material in future fusion devices. In this study, cold rolled tungsten is neutron irradiated at the Belgian BR2 fission reactor at the temperatures 600, 800, 900 and 1200 °C to a dose of 0.18 displacements per atom (dpa). The neutron induced changes in the microstructure are investigated as a function of irradiation temperature using transmission electron microscopy, positron annihilation spectroscopy and electrical resistivity measurements. The influence of the irradiation on the elastic properties and the hardness is examined using the impulse excitation techniques and depth-sensing indentation. Voids and dislocation loops are observed at all the irradiation temperatures. As the irradiation temperature increases the number density of both voids and loops decreases whereas their size increases. The total dislocation density increases after irradiation at 600 °C whereas it decreases for higher temperature irradiations. Furthermore, the formation of very small vacancy clusters in the temperature range of 800 - 900 °C is revealed by PAS measurements. A systematic decrease in the values of Young's and shear moduli is observed as the irradiation temperature increases and this decrease is of about 3.5% after irradiation at 1200 °C. A considerable hardening effect is observed at all irradiation temperatures. The hardness increases with irradiation temperature reaching a maximum at 800 °C and remains almost constant at higher irradiation temperatures.