The valence electron densities of bulk and microdefects in binary Fe3Al

and Fe3Al doped with Cr or Mo have been calculated by using the positron lifetime parameters of the alloys. The result indicates that

the 3d electrons in Fe atoms have well--localized properties and tend to form covalent bonds with the valence electrons in Al atoms

the bonding nature in Fe3Al is a mixture of metallic and covalent. The large--open--volume defects occur on grain boundary in Fe3Al and the bonding cohesion of the grain boundary is weak due to the low density of valence electron there. The addition of Mo into Fe3Al results in the decrease of the ordering energy of the alloy and the open--volume defects on grain boundary. Since the radius of Mo atom is larger than that of Fe atom

when Mo atoms substitute for Fe atoms

they will distort the lattice

enlarge the volume of the lattice and therefore decrease the density of valence electron in the bulk of the alloy. The addition of Cr into Fe3Al increases the densities of valence electrons in the bulk and the grain boundary

thusincreasing the metallic bonding cohesion. Cr is found to be a beneficial element in improving the brittleness of Fe3Al alloy.