STRUCTURE, TENSILE DEFORMATION, AND FRACTURE OF A TI3AL-NB ALLOY

The development of Ti3Al-Nb alloys is an excellent example of the recent resurgence of interest in the use of intermetallics for high-temperature applications. We examine, in this contribution, the structure of a typical alloy Ti-24A1-11Nb and show it to consist primarily of the ordered α2 phase (based on Ti3Al, DO19) and βo, (based on Ti2NbAl, B2) phases, with small amounts of a third phase, which is distorted slightly to an orthorhombic symmetry from the D019 (hexagonal) structure. Tensile properties have been examined on samples heat-treated to vary the size, shape, and volume fraction of α2 phase and the deformation and fracture behavior of the ordered, two-phase mixture established. The tensile ductility is seen to maximize at intermediate volume fractions of the α2 and βo phases (∼30 pct) at values of 6 to 10 pct elongation to fracture, depending on the grain size of the βo phase. A rationale incorporating the failure modes of the two phases-cleavage of α2 and slipband decohesion of βo-has been evolved to explain the trends in ductility with heat treatment. © 1990 The Metallurgical of Society of AIME.