Microstructure Correlation And Prediction Model For Fracture Toughness Of TC4-DT Titanium Alloy

TC4-DT titanium alloy is one of the most widely used damage tolerance titanium alloys in aerospace field.However,in practice,the development and application of the alloy still faces many problems,such as the lack of systematic research on the relationship between process,microstructure and mechanical properties;the crack propagation behavior needs to be discussed in detail;a efficient fracture toughness prediction model is missing.Based on the above research status,this paper takes TC4-DT titanium alloy as the research object,and carries out various heat treatments by controlling the solution temperature and cooling rate.Firstly,the microstructure evolution of the alloy under different heat treatment processes was studied based on quantitative metallography.Secondly,the influence law and mechanism of different microstructure characteristics on the tensile properties,impact toughness and fracture toughness of materials were explored.Thirdly,the crack propagation behavior and mechanism under different organizational characteristics were revealed by means of advanced characterization methods such as in-situ tensile and DIC.Finally,the fracture toughness prediction model of TC4-DT titanium alloy was constructed.This paper will provide engineering reference for the performance improvement and further development and application of TC4-DT titanium alloy.The main research content and results of the paper are as follows:（1）A variety of TC4-DT titanium alloys were prepared based on different heat treatment processes,and the effects of solution temperature and cooling rate on microstructure evolution were investigated.The results show that during the heat treatment in the duplex phase region,with the increase of solution temperature,the volume fraction of equiaxedα_E decreases gradually,the grain size of equiaxedα_E increases gradually,and the thickness of lamellarα_Lincreases gradually.However,with the increase of solid solution temperature,the change trend of lamellarα_L aspect ratio under different cooling rates is not the same.Under water cooling conditions,the lamellarα_L aspect ratio decreases significantly.Under air cooling condition,the aspect ratio of lamellarα_L initially increased and then remained unchanged;under furnace cooling condition,the aspect ratio of lamellarα_L remains unchanged.When heat treated in single phase region,the grain size ofβincreases linearly with the increase of solution temperature.With the increase of cooling rate,the thickness of lamellarαL decreased significantly and then remained unchanged.The colony size decreased gradually at 990°C and1010°C,while the colony grew abnormally at 1040°C under air cooling.This may be due to the rapid growth of favorable orientationα_L and the lack of sufficient time for unfavorable orientationα_L to segment the colony.（2）After comparison,the yield strength and tensile strength of TC4-DT titanium alloy are significantly affected by the cooling rate,but the change is not obvious with the solution temperature.For the bimodal microstructure,the elongation of the alloy is significantly affected by the content of equiaxedα_E.For lamellar microstructure,the relative size ofα_GBandα_L,colony size andβgrain size all have important effects on the plasticity of the alloy.（3）The fracture toughness of TC4-DT titanium alloy with different microstructure is tested.It is difficult to extract FQ and measure the average length of prefabricated cracks in samples under water cooling condition.There is missing straight line segment of force-displacement curve and a large difference of prefabricated crack length at different positions of specimen.The analysis shows that the internal cause is that the temperature difference between the surface and the core of the sample is too large due to the low thermal conductivity of the titanium alloy itself,and the martensitic transformation after water cooling leads to the lattice expansion on the surface of the material,resulting in the residual compressive stress on the surface of the material.The corresponding solution is proposed in accordance with the existing specifications.Compared with the bimodal microstructure,the lamellar microstructure has higher fracture toughness due to the existence of a large number of lamellarα_L and the absence of equiaxedα_E under water cooling and furnace cooling conditions.However,under air cooling conditions,the fracture toughness of the lamellar microstructure is decreased due to the relatively flatα_GB with thickness very close toα_L.（4）Through the characterization method,αE andαL in equiaxed microstructure and bimodal microstructure have weak deflection effect on the crack due to their small size.Therefore,the crack can easily bypass or cut through theαphase for propagation,resulting in relatively smooth crack propagation path.For lamellar microstructure,there are large number of long and coarseα_L and clusters with large orientation differences inβgrains.It can not only consume the stress concentration at the crack tip through plastic deformation and phase boundary cracking,but also hinder the crack propagation through the deflection effect,so the crack propagation path is Z-shaped.（5）Through comparison,the seven common fractal dimensions have no significant correlation with the crack propagation path curvature and fracture roughness.Therefore,the fractal dimension cannot be simply used as an alternative parameter for the roughness or curvature of the crack propagation path.A variety of fracture toughness prediction models were tried to build.By comparison,the evaluation of material fracture toughness by impact toughness was more effective.