He cavities (T >> Stage V)

Irradiation at temperatures near or above 0.5 TM typ­ically results in only minor microstructural changes due to the strong influence of thermodynamic equi­librium processes, unless significant amounts of impu­rity atoms such as helium are introduced by nuclear transmutation reactions or by accelerator implantation. When helium is present, cavities are nucleated in the grain interior and along grain boundaries. The cavity size increases and the density decreases rapidly with increasing temperature. Figure 15 compares the helium cavity density for various implantation and neutron irradiation conditions in austenitic stainless

steels as a function of temperature.118,119 The tem­perature dependence of the cavity density is dis­tinguished by two different regimes. At very high temperatures, the cavity density is controlled by gas dissociation mechanisms with a corresponding high activation energy, and at lower temperatures by gas or bubble diffusion kinetics.118 The cavity density decreases by nearly two orders of magnitude for every 100 K increase in irradiation temperature in this very high temperature regime. The helium cavity densities in materials irradiated at low temperatures (near room temperature) and then annealed at high temperature are typically much higher than in materials irradiated at high temperature, due to excessive cavity nucleation that occurs at low temperature. In the absence of applied stress, the helium-filled cavities tend to nucle­ate rather homogeneously in the grain interiors and along grain boundaries. If the helium generation and displacement damage occurs in the presence of an applied tensile stress, the helium cavities are preferen­tially nucleated along grain boundaries and may cause grain boundary embrittlement.12