Influence of Zirconium Additions on the Epsilon Phase of the Titanium-Aluminium System

Abstract

THE TiAl phase (ɛ) of the aluminium-titanium system exists as a single-phase field and can be achieved as a high-temperature phase without a polymorphic phase transformation. Results of an early investigation by the Armour Research Foundation indicated that an alloy of 36 per cent aluminium with 64 per cent titanium had superior oxidation resistance of 1,500°F, good hot hardness, and low density; however, it lacked any degree of room-temperature or elevated-temperature ductility¹. The cause of this brittleness appeared to be the ordered face-centred tetragonal lattice structure, in which [002] planes of aluminium atoms were alternated with titanium atoms. It was assumed that if the c/a ratio could be reduced to unity, and the atomic arrangement could be disordered, the ductility of the ɛ, phase could be improved. Kessler and McAndrews¹ tried to reduce the c/a ratio to unity by the additions of 13 different elements. Davies² in 1959 suggested the addition of Zr as a means of reducing the c/a ratio. Sandlin and Klug³ and Troup⁴ continued to study the effect of Zr additions to the TiAl binary. The purpose of our investigation was to refine the 2,325° F ɛ-phase boundary of the Al-Ti-Zr system outlined by the earlier investigators and to determine the influence of zirconium on the c/a ratio within the ɛ phase at this temperature.