Strength-ductility Of β-type Ti-Mo-O Titanium Alloys In Synergy Between Mechanical Twin And Precipitated Phase

β-type titanium alloys have a wide range of application prospects in aerospace,biomedical,offshore and other fields owing to their good corrosion resistance,low elastic modulus,TRIP/TWIP effects and other structural and functional features.However,the strength of β-type titanium alloys can be enhanced by means of traditional dislocation strengthening methods but reduce its ductility,which limit the application range of the β-type titanium alloys.Thus,improving and optimizing the mechanical properties through the combination of the new strengthening approaches and traditional strengthening methods become an important research field in the β-type titanium alloys.In this paper,the plastic deformation modes and precipitation behavior of second phases in Ti-15 Mo alloy with different oxygen content from 0.1% to 0.5%(mass%)were investigated.The effect and mechanism of pre-deformation induced {332}<113> twins on mechanical properties combined with isothermal ω-phase in Ti-15 Mo alloys with different oxygen content were studied.The results shows that the deformation modes of Ti-15 Mo alloy gradually translated from {332}<113> twinning to dislocation slip with increasing of oxygen content.The increase of oxygen content led to an enhancement of β-phase stability and a significant suppression of isothermal ω-phase formation during aging at 300℃-500℃,while the precipitation of α-phase was promoted by the increase of oxygen content during aging at600 ℃.The combination of pre-tensile strain induced {332}<113> twins and isothermalω-phase significantly improved the mechanical properties of Ti-15 Mo alloys with 0.1% and0.2% oxygen content,these two alloys not only had high yield strength(861MPa and 997 MPa,respectively),but possessed good uniform elongation(19% and 9%,respectively).However,the brittleness tendency of Ti-15 Mo alloy with 0.4% oxygen content was enhanced by the combination of pre-strain and precipitated phase,which did not have good balance in strength and ductility.Furthermore,A combination of pre-cold rolling induced {332}<113> twins and subsequent precipitated isothermal ω-phase was confirmed to be effective for enhancing uniform elongation from 0% to 9% in Ti-15 Mo alloy at high yield strength level of 890 MPa.The enhancement of uniform elongation was attributable to the static microstructural refinement caused by pre-deformation induced twins,and the dynamic microstructural refinement due to twinning activation during further tensile deformation in Ti-15 Mo alloy with low oxygen content.The increase of oxygen content hindered the precipitation of isothermal ω-phase and formation of twins induced by pre-deformation,and restrained thetwinning activation during further tensile deformation.Hence,the high oxygen content Ti-15 Mo alloy could not obtain a good balance of strength and ductility after pre-strain and subsequent aging.Thus,the appropriate deformation and phase transition microstructure could be obtained through adjusting the oxygen content,which provided the possibility to widely control and optimize the mechanical properties of β-type titanium alloys.