Effect Of Heat Treatment On Tensile Properties And Toughness Of Ti-55531 Alloy With Multilevel Lamellar Microstructure

It is found that the lamellar structure of high strength and toughness titanium alloy is multilevel microstructure?including prior?grain,?colony and?plate?.However,there are few researches on the microstructure and properties of multilevel lamellar microstructure.The influence of heat treatment process on the multilevel lamellar microstructure parameters and the relationship between the characteristic parameters and the properties of each level are still unknown.Therefore,the wide application of lamellar structure is limited of high strength and toughness titanium alloy.In this paper,the experimental material object is Ti-55531 alloy,adjusting the lamellar microstructure by adjusting solution or annealing temperature,using metallurgical microscope?OM?,X-ray diffraction?XRD?,scanning electron microscopy?SEM?,transmission electron microscopy?TEM?combined with image analysis software Image-pro-plus 6.0,to analyze parameters of the multilevel lamellar microstructurehe after different heat treatments.Then the mechanical properties such as room temperature tensile,impact and fracture toughness of multilevel lamellar microstructure were tested,and the relationship between each level and strength,plasticity and toughness was discussed.Then through the notch tensile test,the notch sensitivity of the multilevel lamellar microstructure of the alloy was analyzed.The main conclusions are as follows:The effect of solution temperature on the microstructure and properties of multilevel lamellar microstructure was studied by adjusting the solution temperature between 870? and 930? with the annealing process was the same.With the increase of solution temperature,the?grain size and?colony effective size increase,while the?plate effective size decreases.Although the size of?grain and?colony increases,the decrease of?plate thickness leads to the increase of total number of interfaces,and the increase of interface strengthening effect leads to the strength increased of the alloy.?grain size was increased make the effective volume fraction of?_GB increase,resulting in the decrease of plasticity.The contribution of strength increase to toughness is greater than that of plasticity decrease to toughness damage,and finally the fracture toughness and impact toughness increase thus the increase of solution temperature.At different solution temperatures,the effective control unit for the strength of the multilevel lamellar is?colony,and the key units affecting its plasticity are?grain size and?_GB effective volume fraction.As the plasticity of the lamellae after solution above 870? is low and the industrial application value is not high,the lower solution temperature?860??is selected to further study the effect of annealing temperature?610?,640? and670??on the microstructure and properties of the lamellae.When other parameters are the same,with the decrease of annealing temperature,the?grain size remains unchanged,the volume fraction of?phase increases,the thickness of?colony and?plate decreases gradually,the percentage of the?phase low angle interface is also reduced;the tensile strength and yield strength of the multilevel lamellar increase,but the plasticity and impact toughness decrease.Although the reciprocal square root of effective width?d_c,d_??and strength of?colony and?plate are in line with Hall-Petch relationship,the fitting coefficient of?colony with UTS and YS is higher than that of?plate,so the contribution of?colony to the the multilevel lamellar strength is greater than that of?plate.Although the effective width of?colony and?plate is linearly related to the plasticity,the fitting coefficient of?plate to El and RA is higher than that of?colony,so the contribution of?plate to the plasticity of the multilevel lamellar is greater than that of?colony.During the impact process,crack initiation work accounts for most of the total work?more than 76%?.The TEM analysis results of the deformation characteristics of the microstructure near the impact crack initiation show that the larger the alpha plate,the higher the probability of twinning in the impact deformation process,the better the plastic deformation ability,and the more energy is consumed in the deformation process;after the twins are formed,the twin interface becomes The obstacle of dislocation movement eventually becomes the preferential nucleation position of the microcrack.After the microcrack is formed,it spreads along the grain boundary or through the grain,and many crack steps are formed in the broken?piece,which causes the crack initiation process to consume more More work done by the outside world,thus improving the crack initiation work.The impact toughness has a negative correlation with the strength and a positive correlation with the plasticity,so the impact of plasticity on the impact toughness is greater than the strength,that is,the contribution of?plate to the impact toughness of the multilevel lamellar is greater than that of?colony.Although the notch has a significant effect on the tensile properties of the alloy,it does not show the"notch strengthening"effect.Compared with the smooth specimen,the notch makes the plasticity and strength of the alloy decrease obviously.In addition,with the increase of annealing temperature,the notch sensitivity coefficient of the alloy increases;with the increase of notch radius,the tensile strength of the alloy increases,but the plasticity is almost the same,and both are very low.The tensile fracture mode of the alloy is a mixed fracture mode composed of intergranular fracture,dimple and dissociation fracture.The existence of notch increases the brittle fracture tendency of the alloy.