Segregation-enhanced grain boundary embrittlement of recrystallised tungsten evidenced by site-specific microcantilever fracture

Tungsten is a major candidate as a plasma-facing material in fusion reactors due to its ability to withstand high temperatures and intensive particle fluxes. The heat flux experienced during service can result in recrystallisation of the initial microstructure, leading to an increase in the brittle-to-ductile transition temperature. Such a phenomenon is thought to result from impurity segregation to grain boundaries, however, direct evidence of impurities affecting grain boundary embrittlement has not yet been reported. In this study, microcantilever testing coupled with local chemical analysis using atom probe tomography provides insights into the effects of impurity segregation on the fracture toughness of 99.98 at.% purity tungsten recrystallised at 1600 °C for 1 h. Fracture toughness measurements were performed with the notch placed directly at random high-angle grain boundaries. The in situ measurements showed brittle failure regardless of grain boundary misorientation or grain orientation. For both single- crystalline microcantilevers and the as-received material, significant plasticity was accumulated before failure, while in some cases cracking did not occur during testing. In contrast, the fracture toughness at recrystallised grain boundaries was determined as 4.7 ± 0.4 MPa·√m using a linear elastic approach, lower than that reported for cleavage plane fracture in tungsten microcantilevers. Local atom probe analysis of the high-angle grain boundaries showed clear segregation of phosphorous at the recrystallised interfaces exceeding 2 at.%, imbued during recrystallisation. Atomistic simulations confirmed the role of P on embrittling high-angle grain boundaries in tungsten, while additionally revealing mechanisms of crack-grain boundary interactions and their dependence on P segregation