The phase evolution and microstructural stability of an orthorhombic Ti-23Al-27Nb alloy

Abstract

The phase evolution and microstructural stability were studied for an orthorhombic Ti-23Al-27Nb alloy. Monolithic sheet materials were produced through conventional thermomechanical processing techniques comprising nonisothermal forging and pack rolling. Phase evolution studies showed that, depending on the heat treatment schedule, this alloy may contain several constituent phases including: α2 (ordered close-packed hexagonal D019 structure), B2 (ordered body-centered cubic, bcc), β (disordered bcc), and O (ordered orthorhombic based on Ti2AlNb). Differential thermal analysis studies indicated that the B2 transus temperature was 1070 °C. Heat treatment and transmission electron microscopy studies showed that the α2+B2 phase field extended between 1010 and 1070 °C. From 875 to 975 °C, a two-phase O+B2 field existed. Sandwiched between these two-phase regimes was a narrow three-phase α2+B2+O field. Below 875°C, an O+β field existed. All heat treatments at or above 875°C, followed by quenching, resulted in equiaxed microstructures. However, below 875 °C, the B2 phase transformed into a mixture of O and bcc phases with lenticular morphologies. Cellular precipitation of O+β platelets at O/B2 and α2/B2 grain boundaries occurred depending on solutionizing and aging temperatures, which is explained by the compositional gradient between the bcc phases.