Mechanical properties and microstructure of helium-implanted beryllium

0.2 mm thick specimens of beryllium have been homogeneously implanted with helium. Implantation temperatures ranged from 100 to 600 °C, and final helium concentrations from 30 to 800 appm. Tensile tests at temperatures between 20 and 600 °C were carried out with testing temperature both equal to and lower than the implantation temperatures. For practicality all conditions of helium-implanted specimens, ductility decreased and yield and ultimate tensile strength increased as compared to the unimplanted specimens. The amount of embrittlement and strengthening, however, depended sensitively on implantation dose, implantation temperature, and tensile test temperature. The formation of helium bubbles, dislocation loops, and dislocation networks and the fracture modes were observed by transmission and scanning electron microscopy, respectively. Two ranges of embrittlement can be distinguished. They are attributed to different mechanisms: matrix strengthening is the dominant mechanism at low temperatures, and loss of grain boundary cohesion at high temperatures. It is concluded that in both temperature regimes the embrittlement is dominated by helium and not by the displacement defects introduced by its implantation.