Phase/grain boundary assisted-3D static globularization mechanism of TC17 alloy based on the microstructure reconstruction and in-situ TEM observation

Abstract

Lamellar globularization in the dual-phase titanium alloy is the key to improving plasticity and strength. However, the mechanism has not been fully elucidated so far. In this work, the role of phase/grain boundary in the static globularization of TC17 alloy was systematically studied by setting different α phase content before annealing through low- and high-temperature deformation. Isothermal compression causes the parallel distribution and fragmentation of 3D α plates and few globular α particles are formed at a strain rate of 1 s–1. Post-deformation annealing promotes the static globularization of α phase while it is affected by initial α phase content. After 730 °C deformation, the development of α/α interface by absorbing dislocations promotes the formation of globular α grains based on the nucleation of separated α particles and pre-recovery α subgrain during subsequent annealing. The α/α/β and α/β/β triple junctions formed due to high α content with about 36% volume fraction are favorable for the further nucleation and growth of globular α grains by reducing interface energy, forming a 3D irregular α plate. Then nucleation and growth of the β phase dominate the microstructure evolution during subsequent annealing, resulting in the local dissolution of the plate and formation of α rods. After 850 °C deformation, the α phase tends to nucleate at the β/β/β triple junctions and grow into a lamellar shape along the high energy β/β grain boundary due to low α content with about 7% volume fraction. The α nucleation that maintains the Burgers orientation relationship (BOR) with the surrounding β phase grows along the habit plane and thickens slowly, resulting in the formation of a precipitated α plate with a flat surface and the suppression of static globularization. The comprehensive investigation of lamellar globularization provides guidance for optimizing the 3D microstructure and properties of dual-phase titanium alloy.