High temperature embrittlement of metals due to helium: is the lifetime dominated by cavity growth or crack growth?

Abstract

Helium produced by (n, α)-reactions can degrade the mechanical stability of fusion reactor materials by reducing their ductility and lifetime, particularly at elevated temperatures. This high-temperature He embrittlement (HTHE) has been attributed to the diffusional growth of cavities on grain boundaries, which is slow in an initial gas-driven (bubble) growth stage and fast in the subsequent stages of stress-driven (void) growth, and crack formation by cavity coalescence. In cases of severe HTHE, the time to rupture t R has been suggested to be dominated by gas-driven bubble growth. Recently, it has been proposed that t R is, instead, controlled by gas-driven crack growth (Borodin et al., 1992). In the present paper, it is shown that, in the temperature range of HTHE, gas-driven crack growth is not possible because of diffusional stress relaxation and would result, at lower temperatures, in unstable growing cracks only at bulk helium concentrations in the percentage range. Experimental evidence for the interpretation of HTHE in terms of gas- and stress-driven diffusional growth of cavities on grain boundaries is presented.