A HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPY STUDY OF INTERFACES BETWEEN THE GAMMA, B2, AND ALPHA(2) PHASES IN A TI-AL-MO ALLOY

Conventional and high-resolution transmission electron microscopy (HRT EM) were used to examine the interfacial structures in a Ti-50Al-5Mo ( at. pct) alloy which was processed to produce combinations of gamma, B 2, and alpha(2) phases in a single sample. A small amount of a fourth phase labeled zeta was also found in the microstructure. It may be the phase Ti2AlN but confirmation requires analysis of the N content in t he phase. In this alloy, the orientation relationship between the gamm a and B2 phases is {111}(gamma) parallel to {110}(B2) and [10 (1) over bar](gamma) parallel to [111](B2) with a coherent habit-plane interfa ce parallel to {474)(gamma). The orientation relationship between the B2 and alpha(2) (and also the zeta) phases is {110}(B2) parallel to (0 001)(alpha 2/zeta) and [111](B2) parallel to [11 (2) over bar 0](alpha 2/zeta)) with a coherent interface parallel to the close-packed plane s and along other orientations. The orientation relationship between t he alpha(2) (and also the zeta) and gamma phases is (0001)(alpha 2/zet a) parallel to {111}(gamma) and [11 (2) over bar 0](alpha 2/zeta) para llel to [10 (1) over bar](gamma). The alpha(2) phase has a coherent in terface parallel to the close-packed planes, while the zeta phase appe ars to adopt the {474), interface plane of the gamma phase, similar to the B2 phase. In some cases, the interface configuration between the gamma and B2 phases appears to be altered by the presence of alpha(2) phase, resulting in a semicoherent interface. The phase labeled zeta i n this study has the same orientation relationship with the gamma and B2 phases as alpha(2) but consists of an ABABAC.... stacking of close- packed basal planes. The {474), habit plane interface between the gamm a and B2 phases is analyzed by several different theories of interfaci al structure, and microstructural evolution in this system is also dis cussed.