The effects of 0.86 percent yttrium on the structure and elevated temperature tensile properties of unirradiated and thermal reactor irradiated Fe-15Cr-4Al ferritic steel have been investigated. Yttrium reduces the grain coarsening tendencies of this alloy; the yttrium is present mainly as YFe ₉ precipitates within the grains and generally aligned in the direction of rolling. Autoradiographic examination shows that the boron is concentrated at the grain boundaries of the yttrium free alloy, while the boron is primarily associated with the YFe ₉ phase in the 0.86 percent yttrium alloy.

The tensile ductilities of the 0 percent yttrium alloy at temperatures ranging from 700 to 950 C are markedly reduced after irradiation at 50 or 650 C; the magnitude of the embrittlement is somewhat greater after irradiation at the higher temperature. This irradiation induced embrittlement is associated with failure along the grain boundaries. There are no significant reductions in the high-temperature ductilities of the 0.86 percent yttrium alloy after irradiation.

Optical and thin film electron microscope examinations suggest that the mechanism of the irradiation induced embrittlement in the 0 percent yttrium alloy is similar to that proposed for austenitic steels, namely, the stress induced growth of grain boundary helium bubbles produced as a result of the B10(n,α)Li7 reaction. The absence of a high-temperature irradiation embrittlement effect in the 0.86 percent yttrium alloy is considered to be primarily because of the association of most of the boron with the YFe ₉ phase and, hence, the retention of the helium within the grains.