On the formation of a new vacancy structure in the zirconium-aluminium system

The structures of intermetallic compounds Zr5Al4 and Zr5Al3 are well known. The latter differs from the former by the replacement of Al atoms in the first and third layers by two vacancies. During the observations of these two compounds a new superstructure has been discovered which can be described as the superstructure of Zr5Al4 by the doubling of the unit cell in the basal plane. From the observation of antiphase boundaries and twin boundaries from the superstructure, it appeared that the new phase could be described in the orthorhomic system with parameters asub(s) = asub(h), bsub(s) = asub(h) and csub(s) = csub(h). From the relationship between Zr5Al4 and Zr5Al3 the model of the superstructure could be proposed where the part of the sites occupied either by Al atoms in Zr5Al4 or by vacancies in Zr5Al3 will be occupied in Zr5Al3 by Al atoms. The most probable superstructure is Zr20Al15 where 3/4 of the sites into consideration are occupied by Al atoms. An experimental determination of composition by the energy-dispersive analysis of X-rays emitted by the specimen confirmed this composition. The determination has been done with reference to grains of superstructure Zr3Al2 and Zr5Al3. The new phase appears to be formed near the surface and stabilized by an atom having a small size e.g. H, O, N, C. The formula would then be Zr20Al15. (author)