Helium gas is produced through transmutation of the component elements in aus­tenitic (FCC) stainless steels and other materials. This can lead to embrittlement behavior that cannot be eliminated by high-temperature annealing. Helium is practi­cally insoluble in metals and hence after generation, it tends to precipitate into bub­bles particularly when the temperature is high enough (>0.5Tm) for helium atoms to migrate. Helium may produce severe embrittlement (intergranular cracking) to such an extent that at elevated temperatures even if the yield strength recovers in the irradiated alloy, the ductility is never regained. The extent of helium embrittlement depends on fast neutron fluence, alloy composition, and temperature.

The source of helium in steels is the component elements present in them. Helium is generated in threshold reactions due to the interaction of neutron with the specific isotopes comprising (n, a) reaction. Boron (B10) and Ni58 are such ele­ments important in generating alpha particle (i. e., helium nucleus or He4) through the following reactions:

B10 + n1 ! Li7 + He4. (6.10)

Ni58 + n1 ! Ni59; Ni59 + n1 ! Fe56 + He4. (6.11)

These reactions occur in both thermal and fast neutron spectra. Similarly, helium (n, a) reactions between fast neutrons and Ni, Fe, Cr, and N atoms occur, although with different reaction cross sections. In fast reactors, helium embrittlement is more pronounced simply because the fast neutron flux in the fast reactor is about three to four orders of magnitude higher than the thermal flux, whereas the ratio is 1:1 in the thermal reactors. Helium embrittlement remains a widely studied topic. Olander [11] summarized various theories of helium embrittlement:

i) Woodford, Smith, and Moteff postulated that the increased strength of the matrix material due to the presence of helium bubbles in the grain interiors would lead to stress concentration at the grain boundary triple points during deformation. But as the stress concentration at the grain boundaries would not be able to relax itself, this would induce grain boundary triple point cracks and eventual propagation of cracks along the grain boundaries. So this theory espouses an indirect origin on helium embrittlement.

ii) Kramer and coworkers showed that helium bubbles can form on the grain boundary carbide particles (M23C6), thereby allowing cracks to form. Now

the question is why helium bubbles tend to nucleate on the carbide parti­cles. One aspect of it could be that these carbide particle surfaces reduce the critical nucleation barriers of the helium bubbles. But it is not clear why it happens to be on these M23C6 particles. Boron was found to be asso­ciated with these structures as [M23(CB)6]. So, when boron transmutes through (n, a)-type reaction, helium is produced close to the carbide parti­cles and forms bubbles on the particle itself. Thus, bubbles are formed on or very near to the grain boundary, thus promoting the possibility of helium embrittlement.

iii) Reiff showed that the presence of helium in triple-point cracks permits unstable growth of these cracks at stresses much lower than that required for a gas-free crack to propagate. The presence of helium deteriorates the grain boundary cohesion, thereby leading to weaker grain boundaries that cannot sustain larger loads.

iv) A majority of researchers believe that this phenomenon occurs due to the stress-induced growth of helium bubbles on the grain boundaries that eventu­ally link up and cause intergranular failure. However, as you can see from above, all these theories are related and would explain the behavior depending on the situation. Perhaps all the above factors play a role in helium embrittle­ment. Figure 6.36 shows the various schematic locations of helium bubble for­mation. On the other hand, Figure 6.37 shows the formation of helium bubbles in different 82 series alloys with a base composition of Fe-25Ni-15Cr.

It demonstrates the formation of helium bubbles at the secondary MX-type precipitates, primary MX precipitates, on the grain boundary M23C6 precipi­tates, and on the grain boundaries themselves.

temperature of 650°C): (a) on secondary MX particles, (b) on primary MX particles, (c) on M23C6 particles, and (d) on the grain boundaries [20].

The effect of irradiation temperature on the ductility (percentage elongation to fracture) of the irradiated 304-type stainless steel is shown in Figure 6.38. The fast neutron fluence was kept at >1022 ncm~2 s-1 during irradiation. Tensile tests were conducted at 50 °C. Note the dip in ductility due to helium embrittlement near ~580 °C.

Interestingly, BCC metals/alloys are less vulnerable to helium embrittlement (i. e., no drastic loss in ductility). It is believed that the large diffusion coefficients in BCC materials due to their more open structure help in relaxing stress concentra­tions at the grain boundaries effectively and thus minimize the stress-enhanced helium bubble growth.

6.2.4