= Solution
\b[Neutron capture by boron-10 can generate <helium>, leading to <helium embrittlement>, especially at elevated <temperature>.] The relevant <nuclear reaction> is
$$
{}^{10}_{5}\mathrm B+{}^{1}_{0}n\longrightarrow{}^{7}_{3}\mathrm{Li}+{}^{4}_{2}\mathrm{He}.
$$
The emitted alpha particle eventually becomes a <helium> atom in the metal. The product branch can also involve gamma emission, without changing this helium-producing balance. The production depends on boron isotopic content and the neutron energy spectrum; a thermal component is particularly relevant to this reaction.
<Helium> has very low solubility in <steel> and can be trapped at <vacancies>, interfaces and <grain boundaries>. Boron already segregated near <grain boundaries> can therefore create a locally unfavorable distribution of <helium>. Stabilized helium–vacancy clusters develop into bubbles or cavities. Under tensile <stress>, boundary cavities can grow and link, reducing <ductility>, <fracture toughness> and creep-rupture life through <intergranular fracture>. Early thin-foil observations and mechanical tests associated high-temperature ductility loss with helium-containing boundary bubbles and their coalescence.<High-temperature Embrittlement of Stainless Steel irradiated in Fast Fluxes by A. S. Fraser and others>[Abstract and opening discussion, Nature 211, pp.291–292: boron-10 reaction, electron microscopy and boundary-bubble mechanism]{ref}
<Irradiation temperature> controls the transport and clustering of both <helium> and <vacancies>. At lower <temperature>, reduced long-range transport can retain small trapped clusters in the matrix. These can contribute to <irradiation hardening>, but extensive boundary-cavity growth is kinetically less favorable. At higher <temperature>, migration, cavity growth and <material creep> become faster, making <intergranular fracture> and loss of high-temperature <ductility> more likely. Trap distribution and prior <microstructure> can redistribute the gas, so there is no composition-independent critical <temperature> or universal monotonic law.
The mechanical-test <temperature> also matters. Material irradiated cold can become embrittled during subsequent hot testing or service when trapped gas and defects become mobile. Distinguish gas production during <irradiation> from the later conditions under which its mechanical consequences are expressed.
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