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The intrinsic ductility of FeAl (in ultrahigh vacuum) decreases with increasing Al content, from around 16% in Fe–37Al to zero in Fe–48Al. The sharpest decline occurs around the composition where the fracture mode changes from transgranular to intergranular. Boron shifts this ductile–brittle transition to higher Al levels by segregating to the grain boundaries and suppressing grain-boundary fracture. However, its effectiveness decreases with increasing Al concentration, even though the amount of B segregating to the grain boundaries remains the same, independent of alloy stoichiometry. Consequently, even the B-doped alloys become brittle and fracture intergranularly as the stoichiometric composition is approached. Low-pressure hydrogen embrittles FeAl, although not as severely as atmospheric moisture. Environmental embrittlement is most noticeable in Fe-rich FeAl; with increasing Al concentration, the grain boundaries become intrinsically weak, and brittle fracture persists even after environmental effects are eliminated. |
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