Occurrence of an Ordered Phase in an As-Cast Titanium-Aluminium Alloy

Abstract

PHASE equilibria in the titanium–rich region of the titanium–aluminium constitution diagram have not been elucidated despite many experimental investigations. The results of earlier work¹ indicated an extensive region of hexagonal close-packed α solid solutions. At a temperature of 800° C. it was reported that 36.5 atomic per cent aluminium was soluble in the low-temperature modification of titanium. It was also concluded that the first intermediate phase on this side of the diagram was γ, which exists over a region of homogeneity and resulted from a pertectic reaction involving a β solid-solution and the melt. A study of the variation in lattice parameters as a function of composition in the proposed α phase region has been made². Several papers have presented X-ray powder diffraction evidence for an intermediate phase in this same region of the diagram. This crystallographic species, the δ phase, has been observed in heat-treated specimens containing 25 and 33$\frac{1}{3}$ atomic per cent aluminium^3,4. Experiments performed with an electron microscope⁵ have demonstrated the epitaxial relationship of α and δ. A constitution diagram has been proposed⁶ in which two intermediate phases exist between the terminal α phase and γ.