A high-resolution transmission electron microscopy study of interfaces between the γ, B2, and α2 phases in a Ti-Al-Mo alloy

Abstract

Conventional and high-resolution transmission electron microscopy (HRTEM) were used to examine the interfacial structures in a Ti-50Al-5Mo (at. pct) alloy which was processed to produce combinations of γ, B2, and α2 phases in a single sample. A small amount of a fourth phase labeled ζ was also found in the microstructure. It may be the phase Ti2AlN but confirmation requires analysis of the N content in the phase.In this alloy, the orientation relationship between the γ and B2 phases is {111}γ ∥}110}B2 and 〈101]γ ∥ 〈111〉B2 with a coherent habit-plane interface parallel to (474)γ. The orientation relationship between the B2 and α2 (and also the ζ@#@) phases is {110}B2 ∥(0001)α2/ζ and 〈111〉B2 ∥〈11-20〉α2/ζ with a coherent interface parallel to the close-packed planes and along other orientations. The orientation relationship between the α2 (and also the ζ@#@) and γ phases is (0001)α2/ζ ∥{lll}γ and (11•20)α2/ζ ∥<10•1]γ. The α2 phase has a coherent interface parallel to the close-packed planes, while the ζ phase appears to adopt the (474)γ interface plane of the γ phase, similar to the B2 phase. In some cases, the interface configuration between the γ and B2 phases appears to be altered by the presence of α2 phase, resulting in a semicoherent interface. The phase labeled ζ in this study has the same orientation relationship with the γ and B2 phases as α2 but consists of an ABABAC... stacking of close-packed basal planes. The (474)γ habit plane interface between the γ and B2 phases is analyzed by several different theories of interfacial structure, and microstructural evolution in this system is also discussed.