Modeling-based Study On Quantitative Relationships Between Microstructure And Properties Of TA15 Titanium Alloy

The integration of shaping and performancing of high-performance light-weight complex-structured components of titanium alloys is the urgent need for the development of advanced utilities in aviation,aerospace and other fields,of which the key and challenging problem is how to precisely describe the relationship between complex morphology,distribution and property of the microstructural features and the performance of the formed part so as to effectively control the microstructures and properties during thermal processing.To this end,micromechanical finite element simulation method based on the quantitative characterization of the morphology and distribution of microstructure and constitutive modeling of constituent phases was adopted to systematically and thoroughly investigate the quantitative relationship between the microstructural features and the corresponding performance of TA15 titanium alloys.The main contents and results are as follows:A method for separating the complex touching equiaxed and lamellar α phases within the microstructure of titanium alloys was proposed.The method uses concave points to indicate touching locations and then adopts marker circle or direction circle to form a candidate separation line based on the concave points.The correct separation line is determined by the proposed rules.The separation accuracy of the method is higher than 85%.The feature area combined with its aspect ratio is used to identify the equiaxed α or lamellar α phases from the separated results.The identification accuracy is 95%.The method of quantitatively characterizing the morphology and distribution of microstructural features in titanium alloy was established.The volume fraction of microstructural features,the size of equiaxed α and the thickness of lamellar α were measured by area method,mean intercept length method and ellipse fitting method,respectively.The morphology and distribution of microstructural features were quantitatively characterized by the lineal-path and two-point correlation functions using the new proposed row scanning and line segment scanning algorithms,respectively.The computational efficiency is 30 times higher than the traditional algorithm.The relationship between thermomechanical processing parameters and microstructure evolution was quantitatively studied.It was found that the thickness of lamellar α increases after a two-step heat treatment and isothermal compression.The tertiary α appears after both the two-step heat treatment and isothermal compression followed by heat treatments at 10 °C and 25 °C below the compression temperature.The equiaxed and lamellar α phases concentratively distribute,respectively,along 0°(perpendicular to compression direction)and 45° after isothermal compression.The equiaxed α phases are homogeneous along different directions,while the lamellar α phases are inhomogeneous along different directions after the heat treatment.The variation of the properties of microstructural features and their effects on the properties of TA15 titanium alloy were investigated.It was found that the hardness of transformed β matrix increases about 0.1-0.4 GPa from bi-modal microstructure to tri-modal microstructure after isothermal compression,whereas the variation of equiaxed α is slight.The contribution of transformed β matrix to the integrated hardness of TA15 titanium alloy is larger than 55%,is less than 8% by the equiaxed α-β matrix interface,and is between 8% and 40% by equiaxed α and the lamellar α-residual β interface.The thin α plates in bi-modal and tri-modal microstructures deform unevenly and result in the lower elongation of TA15 titanium alloy.The mixed structure of thick and thin α plates in tri-modal microstructure deforms evenly and leads to the higher elongation of TA15 titanium alloy.Geometry model based on the microstructure of TA15 titanium alloy was established.Constitutive models of equiaxed α and transformed β matrix were developed,respectively,by considering strengthening by dislocations and solid solutions and by reverse analysis of nanoindentation curves.Based on the established geometry and constitutive models,an actual microstructure-based finite element model was established.The model is validated by means of comparing the predicted stress-strain responses and mesoscale deformation characteristics of TA15 titanium alloy with experiments.The parameters effect of microstructural features on the properties of TA15 titanium alloy was investigated by the established finite element model.It was found that the ultimate tensile strength of TA15 alloy increases by 18% with the volume fraction of equiaxed α decreasing by 40%.The ultimate tensile strength is higher along the distribution direction of equiaxed α.The yield strength of equiaxed α increases by 12% resulting in minor improvement of ultimate tensile strength but improving failure strain by 25%.The tensile strength and failure strain of TA15 alloy increases by 10% and decreases by 27%,respectively,because of the increase of strain concentration with the yield strength of transformed β matrix increasing by 18%.