MULTIPHASE INTERMETALLICS

Ordered intermetallic compounds, notably the aluminides of nickel, titanium, iron and niobium, have been under intensive investigation for the past two decades as prospective load-bearing materials for high temperatures, with special emphasis on aerospace applications. The stronger of these phases are all catastrophically brittle at and near ambient temperature and this difficulty has proved obdurate. A promising solution appears to be to alloy such phases in such a way that the result is a two-phase or multiphase material. The extra phases may be disordered and intrinsically ductile: the presence of such phases lowers the high-temperature strength but is apt to enhance the plastic deformability of the material even at ambient temperature. There are also instances of multiphase alloys in which all the phases are ordered intermetallics, some stronger and more brittle, others weaker and more ductile, and the end-result can be a promising combination of strength and ductility, as well as good creep resistance.