THE MICROSCOPIC MECHANISM OF ALLOYING ELEMENTS AFFECTING THE DUCTILITY OR BRITTLENESS OF FeAl INTERMETALLICS

The valence electron densities of bulk and microdefects in binary FeAl, and FeAl doping with B,Zr or Si have been calculated by using the positron lifetime parameters of the alloys. The density of valence electron is low in the bulk of FeAl alloy. It indicates that, in Fe atom the 3d electrons have well-localized properties and tend to form covalent bonds with Al atoms, thus the bonding nature in FeAl is a mixture of metallic and covalent. The large-open-volume defects occur in grain boundaries in FeAl and the cohesion of the grain boundaries is weak due to the low density of valence electron there. When a small amount of B atoms is added into FeAl alloy, some of B atoms segregate to the grain boundaries and increase the valence electron density; this will strengthen the bonding cohesion in the grain boundaries. The other B atoms are dissolved interstitially into the bulk, they interact with Fe and Al, increase the density of valence electrons and thus the bonding cohesion in the bulk. The addition of Zr into FeAl results in the increase of the density of valence electrons in the bulk and grain boundary. Whereas, if FeAl is alloyed with Si, the density of valence electrons in the grain boundary will decrease. B and Zr are found to be beneficial elements for improving the brittleness of FeAl alloy, while Si exhibits an opposite effect.