Simultaneous improvement of strength and ductility in near β Ti alloy with (α + β) hybrid structure via intermediate phases transformation

The influences of simultaneous precipitation of isothermal ω and O′ on deformation mechanisms and mechanical properties of near β Ti alloy Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe with (α + β) hybrid structure are revealed. During continuous heating, the phase transformation sequence in this alloy is β + α_p + ω-embryos + O′ → β + α_p + ω_iso + O′ → β + α_p + ω_iso + O′ + α_s → β + α_p + α_s. Low-temperature heat treatments from 200 °C to 400 °C are carried out to develop such intermediate phases. Upon aging at 200 °C and 300 °C, the isothermal ω and O′ domains are continuously generated. In comparison with solution state, the deformation behaviors of aged samples are altered, and the yield stress and ductility are simultaneously increased. When aged at 200 °C, the yield strength is increased by 13.8 %, and fracture elongation is increased by 63.2 %. When aged at 300 °C, the yield strength is increased by 18.2 %, and preserves large ductility same to solution state. This is due to the development of isothermal ω and O′ domains suppress long-range stress induced martensitic transformation during loading, and the deformation mechanisms evolve into dislocations slipping gradually. When the aging temperature increases to 400 °C, α precipitation takes place leading to an obvious decrement in ductility. Upon loading the primary α can also accommodate partly the strains and relax local stress concentration. The results in this work clearly proves that better mechanical properties can be achieved through tailoring intermediate phases generation.