| TC18(Ti-5Al-5Mo-5V-1Cr-1Fe)alloy is a kind of(α+β)high-strength and hightoughness two-phase titanium alloy,which is widely used in aviation,aerospace and other fields,especially for manufacturing large-scale load-bearing devices such as airplanes and ships.Previous studies have shown that the alloy is very sensitive to changes in heat treatment regime.Therefore,revealing the microstructural evolution of(α+β)titanium alloy under multi-stage heat treatment regime and its mechanism of action on mechanical properties is a scientific basis for in-depth excavation of high strength and toughness of titanium alloy and guidance of hot processing technology,and is one of the research hotspots in the current field.However,the strengthening and toughening mechanism of TC18 alloy still needs to be further studied,especially the effect of multi-step heat treatment on the microstructure,tensile strength and fracture toughness of TC18 titanium alloy needs to be further improved.It is of great significance to guide the practical production and application of TC18 titanium alloy by deeply studying the microstructure evolution,the action mechanism on mechanical properties and the strengthening and toughening mechanism of this alloy under the multi-stage heat treatment system.The as-forged TC18 alloy was heat-treated by solution,single-step aging and two-step aging processes,and its microstructure evolution law and its action mechanism on tensile properties were studied.The results show that the solution temperature affects the precipitation behavior of aging α-phase,and the number of aging α-phase increases with the increase of solution temperature.When the alloy is aged at 500-600℃ in a single stage,with the increase of aging temperature,the secondary α phase with fine needle shape precipitates from the matrix,and its content decreases,accompanied by the coarsening of precipitated phase.Compared with single-step aging,after two-step aging heat treatment,the third αphase precipitates and the total content of aging α phase increases slightly.The results of microstructure study show that the aging α phase is the main strengthening phase of the alloy.In addition,its quantity and dense or scattered precipitation directly affect the strength,but it is accompanied by the loss of plasticity.Generally speaking,higher solution temperature and lower first-step aging temperature will precipitate a larger number of aging α phases with finer morphology,which will lead to higher strength.At the same time,{11(?) 4} and {1 0(?) 1} twins were found in the tensile specimens of the aged alloy after tensile deformation,which improved the coordinated deformation ability of the alloy during plastic deformation.TC18 alloy can achieve excellent matching between strength and ductility through multi-step heat treatment,and provide sufficient strength range(978-1439MPa)and optional elongation(8.25-18.9%).The effect of multi-step heat treatment on fracture toughness and fracture behavior of TC18 alloy was studied.The results show that a large number of finer nano-sized aging αprecipitates will cause a larger stress concentration,which will promote the growth of micropore polymerization and is not conducive to the deflection of cracks,thus causing the loss of fracture toughness of the alloy.The increase of aging temperature coarsens αprecipitates and reduces their number,which promotes crack deflection and consumes additional energy,thus improving the fracture toughness of the alloy.In addition,the macro fracture morphology analysis shows that the specimen with higher toughness has a larger shear fracture area and a tortuous crack propagation path.Through the analysis of microfracture morphology,it is found that the fracture morphology is mainly high-density dimples,and the higher toughness increases the size of dimples. |
