Study On Microstructure And Properties Of TC4 Titanium Alloy During Heat Treatment In Two-Phase Region

TC4 titanium alloy is widely used in aerospace,medical instruments,chemical industry,ships and other fields due to its high specific strength,strong corrosion resistance and good biocompatibility,etc.How to improve the ductility at room temperature is an important challenge in the processing of TC4 titanium alloy.The effect of heat treatment in two-phase region on the microstructure and mechanical properties of TC4 titanium alloy was studied in this paper,focusing on the change of ?phase stability and its effect on the work hardening and ductility of titanium alloy.This study aims at providing theoretical reference for exploring the treatment process to obtain high ductility of TC4 titanium alloy.The solution treatment of as-annealed TC4 titanium alloy was conducted at low temperature(400??800?)in two-phase region.The results show that the stability of? phase decreases with the increase of solution temperature.When the temperature below 500?,the microstructure is composed of ? phase and ? phase which is relatively stable and the deformation mechanism is mainly dislocation slip of two phases.The microstructure is composed of ? phase,stable ? phase and metastable ? phase during solution treatment at 500??700?.The deformation mechanism of metastable ? phase occurs ???~2 phase transformation.The deformation mechanism includes dislocation slip and deformation induced metastable ???~2 phase transformation and shows an outstanding balance of strength and ductility.The microstructure of solution treatment above 700? is composed of ? phase,?' martensite generated by ? phase transformation and a little ? phase.The deformation mechanism is mainly the dislocation slip of ?phase and ?' martensite which also shows an outstanding balance of strength and ductility.In as-annealed TC4 titanium alloy,the thermal stability of ? phase is affected by composition and grain size while the mechanical stability is affected by composition,grain size and the surrounding ? phase.The solution treatment of cold-drawn TC4 titanium alloy in two-phase region was conducted from 400??850?.The results show that with the increase of solution temperature,the thermal stability of ? phase decreases gradually.Compared with asannealed TC4 titanium alloy at the same solution temperature,? phase in cold-drawn TC4 titanium alloy exhibits higher stability.Metastable ? phase formation is only observed in 750 WQ sample and twinning is formed after deformation.This indicates that in addition to chemical composition,dislocations can also affect ? phase stability.During heat treatments,the changes of cooling rate also affects the composition and stability of ? phase.At the same heat treatment temperature,the content of V and Fe in? phase obtained by air cooling is higher than that obtained by water quenching,which has stronger stability.Compared with 800 WQ sample,metastable ? phase was formed in the 800 AC sample.During the deformation process,???~2 phase transformation occurred and excellent ductility was obtained.By analyzing the factors affecting strength and ductility of cold-drawn TC4 titanium alloys,a heat treatment process of 800AC/30 min was designed.Compared with the simulative annealing process(850AC/60min)commonly used in industry,the metastable ? phase was formed and the deformation-induced ???~2 phase transformation was produced during deformation process.The elongation and strength are about 22.4% and 961 MPa which shows excellent combination of strength and ductility.