Behaviour of implanted helium in boron carbide in the temperature range 750 to 1720°C

Abstract

Alpha implantations in boron carbide samples were carried out different controlled temperatures ranging from 750 to 1720°C and the irradiated samples were observed in an electron microscope.

Below 1500°C one part of the implanted helium produces lenticular bubbles within the grains whereas another part accumulates at the grain boundaries where it produces strain centres. Above 1500°C, tridimensional bubbles are visible in the grains as well as on the grain boundaries.

In boron carbide, the brittle to plastic transition takes place around 1500°C. Below this temperature, the helium accumulation at the grain boundaries and the organization of bubbles into strings within grains are responsible for inter-and intragranular microcracking respectively. Above this temperature, because of plasticity, the cracking disappears. But the swelling increases markedly. At 1720°C, for instance, while less than 1% of the implanted helium is retained by the sample, each helium atom is associated with 100 vacancies within tridimensional bubbles responsible for a swelling of 1% per percent of implanted helium.