Room-temperature mechanical behavior of FeAl: Effects of stoichiometry, environment, and boron addition

The intrinsic ductility of FeAl (in ultrahigh vacuum) decreases with increa sing Al content, from around 16% in Fe-37A1 to zero in Fe-48A1. The sharpes t decline occurs around the composition where the fracture mode changes fro m transgranular to intergranular. Boron shifts this ductile-brittle transit ion to higher Al levels by segregating to the grain boundaries and suppress ing grain-boundary fracture. However, its effectiveness decreases with incr easing Al concentration, even though the amount of B segregating to the gra in boundaries remains the same, independent of alloy stoichiometry. Consequ ently, even the B-doped alloys become brittle and fracture intergranularly as the stoichiometric composition is approached. Low-pressure hydrogen embr ittles FeAl, although not as severely as atmospheric moisture. Environmenta l embrittlement is most noticeable in Fe-rich FeAl; with increasing Al conc entration, the grain boundaries become intrinsically weak, and brittle frac ture persists even after environmental effects are eliminated. (C) 1998 Act a Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.