Simulation Study On Solidification Structure Control And Mechanical Behavior Of Ti-Al Alloy

Ti-Al alloys have been widely used in aerospace and automotive power fields due to their light weight,high strength,creep resistance,and excellent high-temperature mechanical properties.The macroscopic mechanical behavior of Ti-Al alloy depends on its microstructure,and the formation of different microstructures is regulated by its solidification process.Therefore,the in-depth study of the solidification structure regulation and mechanical behavior of Ti-Al alloys is of great scientific significance for the preparation of metal materials with excellent mechanical properties.In this thesis,the molecular dynamics method is used to simulate and study the regulation mechanism of the uniform solidification process cooling on the microstructure of the Ti₃Al alloy solidification process,and the regulation mechanism of the temperature gradient in the directional solidification process on the solidification structure.in-depth analysis.The main conclusions are as follows:（1）An efficient and accurate method for identifying and extracting the grain structure in the solidified structure is established,namely the Density-Based Spatial Clustering of Applications with Noise（DBSCAN）.Compared with the currently commonly used PTM（polyhedral-template-matching）,this method can identify and extract grains with complex crystal structures,and the operation efficiency is higher.（2）During the uniform solidification of Ti₃Al alloy,the effect of cooling rate on the microstructure is only manifested at the liquid-solid transition point,and as the cooling rate decreases,the grain size of the solidified structure increases,and the HCP（Hexagonal Closest Packed）structure（α2-Ti₃Al phase）increased.The specimen tensile tests show that the strength and plasticity of the solidified structure are simultaneously improved with the decrease of the cooling rate;and the mechanism of strengthening and toughening is clarified according to the change of the microstructure during the deformation process.（3）During the uniform solidification of Ti₃Al alloy,the system contains tens of thousands of different types of atomic cluster structures,but only a few types of cluster structures play a key role in the crystallization nucleation process.The critical nucleus of Ti₃Al alloy is a single-phase structure composed of FCC atoms,and the formation of the parallel twinning structure is formed by the layer-by-layer growth of the FCC single-phase nucleus along the close-packed plane.（4）The directional solidification model of Ti₃Al alloy was constructed,and it was found that the solidification structure gradually changed from equiaxed grains to columnar grains during directional solidification,and the average grain size is approximately linear along the direction of directional solidification.During the solidification process,there is a competitive growth of grains,and the grains are more likely to grow parallel to the direction of directional solidification.When the model is uniaxially stretched perpendicular to the direction of directional solidification,it is also found that the strength and plasticity of the generated gradient model are higher than those of the uniform solidification model.The synergistic improvement of strength and toughness in the gradient structure is due to the grain gradient structure inhibiting shear localization occurs.