Study On Microstructure And Mechanical Properties Of Directionally Solidified TiAl Based Alloy With Varied Cross-Section

TiAl-based alloys have low density,high specific strength and good creep resistance,making them promising materials for aviation applications.However,they have poor plasticity at room temperature and anisotropy in mechanical properties.Directional solidification technology is proposed to control the orientation of the lamellar structure to improve its strength and toughness.TiAl base alloy is a peritectic alloy,and the alloying degree increases during development,making the solidification path very complicated.The microstructure of directional solidified TiAl-based alloys is very sensitive to temperature field fluctuation,making the process difficult.As a result,the specimens with constant cross-section are used in the current research,so as to reduce the influence of the change of section on the temperature field and the microstructure.To advance the development and application of directional solidified TiAl-based alloys,the microstructure evolution in specimens with varied cross-sections must be studied.In this paper,directional solidified TiAl alloys with constant and varied crosssections were prepared.The evolution processes of microstructure continuity,columnar grain diameter,lamellar spacing and lamellar orientation under changing cross-section and alloy composition were analyzed.The effect of microstructure evolution on mechanical properties was elucidated.By analyzing the variation of magnetic field and temperature field in the growing process,combining with theoretical calculation and machine learning,a method with real-time adjusted processing parameter was developed to optimize the microstructure and mechanical properties of TiAl alloy specimen with varied cross-section.Primary phase,solidification path,phase transformation process and microstructure formation mechanism of β-solidified TiAl,γ-TiAl and high-Nbcontaining TiAl alloys were illustrated via quenching experiment and thermal analysis,the influences of the alloying element were also analyzed.The evolution of the columnar grain and lamellar structure in the directional solidified specimen with constant cross-section was analyzed.It is found that the columnar grain can grow continuously in β-solidified TiAl,γ-TiAl and high-Nb-containing TiAl alloy,and the lamellar spacing and lamellar angle at different positions do not change significantly.Moreover,during the directional solidification process,the contact time between the melt and the mould is longer and the contamination is heavier at the upper part of the specimen,which makes the mechanical properties of TiAl-based alloys decrease with increasing distance from the bottom of the specimen.The absorbed heating power in the specimen with varied cross-section is fluctuated in the process of directional solidification,which leads to the fluctuation of temperature field and decrease of the temperature gradient in the solid-liquid interface.The growth of columnar grain was interrupted and the equiaxed grain region was formed in the directionally solidified TiAl alloy.In turn it causes the deflection of columnar grain and the decrease of the directional degree as well as increase of lamellar angle and inter-lamellar spacing.Formation mechanism of equiaxed grain and the criterion of continuous growth were proposed by establishing growth model of columnar grain and equiaxed grain.It is found that increase of heat power and withdrawing rate is beneficial to the improvement of columnar grain deflection and equiaxed grain precipitation,respectively.The processing parameters in directional solidification is adjusted in real-time,based on the curve of absorbed heating power and the heat flow balance equation in the mushy zone.The temperature field fluctuation in the specimen with varied crosssection is significantly decreased,and the melt temperature as well as temperature gradient are increased after using the real-time adjusted processing parameter.Directionally solidified TiAl alloys were prepared by the new method.The processing parameters were taken as input values,and the angle between the preferred growth direction and the actual growth direction was taken as output,the relationship model between the input and the output was established by machine learning method,and the real-time adjusted method was optimized.The microstructure continuity,lamellar orientation and inter-lamellar spacing in the directionally solidified TiAl alloy with varying cross-sections can be improved under the ML-optimized method.Under constant processing parameters,the inter-lamellar spacing and lamellar angle increase along the growth direction,while the tensile properties decrease along the growth direction.When the withdrawing rate is 1.0 mm/min,the strength variations at different positions of three directionally solidified TiAl alloys are 11.2%,9.3% and 9.3%,respectively.In addition,the intergranular fracture is caused by the growth interruption of columnar grains,which significantly reduces the tensile strength of the specimens.Under the real-time adjusted method,the increasing of lamellar spacing and lamellar angle is reduced,and the strength variations is reduced to 6.4%,5.8% and 5.2%,which is similar to that of specimens with constant crosssection.Moreover,under the real-time adjusted method,columnar grains can grow continuously,the equiaxed grain region disappears,and the tensile properties are also basically restored to the level of the specimen with constan cross-section.During the tensile process of TiAl alloys,the deformation mainly occurs in theγ phase,deformation behaviors in γ phase has significant effect on the mechanical property of TiAl alloy.Twin mesh is formed in the γ lamellae of γ-TiAl and high-Nbcontaining TiAl alloys by annealing twin layers in different directions,the mesh not only limit the movement of dislocation,but also allow dislocation to pass through when the dislocation is piled up,which reduces the internal stress and strengthens the TiAl alloy.Deformation nanotwins are also formed in the deformed γ-TiAl and highNb-containing TiAl alloy.In high-Nb-containing TiAl alloy,the stacking fault energy of γ phase is low,forming SISF and LPSO phase,which can be served as the nucleus of deformation twins.Therefore,the density of deformation twins in high-Nbcontaining TiAl is larger,and the strengthening effect is also stronger.The lamellar colony in β-solidified TiAl alloy is surrounded by the coupled microstructure,which limits deformation of γ lamellae,so deformation γ twin is not formed.The mechanical property is mainly controlled by the shape and size of coupled microstructure,the effect of dislocation-twin interaction and twining deformation is not presented.