Spheroidization and subsequent homogenization of lamellar α phase in metastable-β Ti alloys during severe plastic deformation

The spheroidization and homogenization of lamellar α in metastable-β Ti alloys during subtransus severe plastic deformation were investigated. The study was based on the interactions between the α phase and the β matrix, e.g., heterostructured deformation, α-induced extra work hardening, and continuous dynamic recrystallization (cDRX) of β phase. Two kinds of initial microstructures containing lamellar α with different sizes were employed. The α_p dispersion was dominated by boundary migration and lattice rotation during the cDRX of the β matrix, and the growth of cDRXed β grains and the α globularization were competitive in time sequence. That is, localized dislocation storage near phase interfaces induced by heterogeneous deformation improved the formation of fine β subgrains through cDRX. The decomposed α particles could achieve a uniform distribution by boundary migration and lattice rotation of the fine cDRX β grains. However, as the fine cDRXed β grains fully grew, the dispersion of α particles became more difficult due to the lack of assistant from the β boundary migration and lattice rotation. A deformation mechanism map for α_p phase dispersion during subtransus processing of the Ti-55531 alloy was established. Based on this, a uniform α_p dispersion was achieved. An investigation on the solution-aging heat treatment of the fabricated material with uniform α_p was subsequently performed. After a solution treatment under 800 °C for 1 h followed by aging under 560 °C for 3 h, the tensile strength, elongation, and fracture toughness were tested to be 1281 MPa, 6.31 %, and 77.6 MPa·m^1/2, respectively.