HIGH-TEMPERATURE EMBRITTLEMENT OF METALS DUE TO HELIUM - IS THE LIFETIME DOMINATED BY CAVITY GROWTH OR CRACK-GROWTH

Helium produced by (n, alpha)-reactions can degrade the mechanical sta bility of fusion reactor materials by reducing their ductility and lif etime, 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-driv en (void) growth, and crack formation by cavity coalescence. In cases of severe HTHE, the time to rupture t(R) has been suggested to be domi nated by gas-driven bubble growth. Recently, it has been proposed that t(R) is, instead, controlled by gas-driven crack growth (Borodin et a l., 1992). In the present paper, it is shown that, in the temperature range of HTHE, gas-driven crack growth is not possible because of diff usional stress relaxation and would result, at lower temperatures, in unstable growing cracks only at bulk helium concentrations in the perc entage range. Experimental evidence for the interpretation of HTHE in terms of gas- and stress-driven diffusional growth of cavities on grai n boundaries is presented.