Growth Mechanism Of Ti5Si3 Phase And Its Effect On Properties In Titanium-silicon Brass

Complex brass is commonly used in the manufacture of wear-resistant mechanical devices due to its high mechanical strength,excellent wear resistance and low cost.Especially in the field of the ring synchronizer of the automobile,complex brass occupies an irreplaceable position.At present,the performance of domestic complex brass still falls much behind foreign advanced level.The main reason is that the poor strength,coarse,defective structures of second-phase and second-phase agglomeration are prone to appear in the solidification,and the reasons for these phenomena are not clear enough.As a new complex brass,titanium-silicon brass is a potential application material for high-performance synchronizer ring through the strengthening of the Ti₅Si₃ second phase.Ti₅Si₃ intermetallic compound is an ideal strengthening phase in titanium-silicon brass due to its high melting point,high hardness,outstanding wear resistance,and excellent thermal stability.However,it is short of understanding of the effect mechanism of the Ti₅Si₃ phase on the titanium-silicon brass strengthening mechanism and wear mechanism.It is a serious obstacle to the precise control of the morphology,orientation distribution,dispersion of second phase and the development of high-performance complex brass.In this work,the growth mechanism and morphology evolution of the Ti₅Si₃ were investigated by the titanium-silicon brass with different Ti and Si solute content.Using a combination of experiment and finite element analysis,the effect mechanism of Ti₅Si₃ volume fraction,morphology and orientation distribution on the mechanical properties,fracture behavior and wear resistance of titanium-silicon brass were systematically analysed.The main results can be described as follows:（1）3D morphologies of the Ti₅Si₃ phases in titanium-silicon brass with different Ti and Si solute contents were analyzed,and the growth model of hexagonal prism Ti₅Si₃ was established.The preferential growth directions of the Ti₅Si₃ crystal are<0001>and<1120>,the close-packed planes such as{1120},{1122}and{10 11}degrade to comers and edges during the growth process,respectively.Thus,the morphology of Ti₅Si₃ phase will forms a long hexagonal prism surrounded by six{10 10}side planes and two{0001}top planes.In addition,the agglomeration and prism facet of Ti₅Si₃ can be ascribed to the restrained solute diffusion within the prism and the difference in the growth rate of the prism facet.（2）With the content of Ti and Si elements in titanium-silicon brass increase,the size and volume fraction of synthesized Ti₅Si₃ increase,the aspect ratio decreases,and the morphology changes from slender hexagonal prism fibers to short hexagonal prisms.Ti₅Si₃ nanowires with a diameter of 110–180nm and a length of 30–50μm were generated in titanium-silicon brass with a smaller solute content of 0.3Ti and 0.1Si.According to this evolution law,a method for preparing Ti₅Si₃ nanowires has been developed by the casting-extraction method.The method has the advantages of simple operation,low cost and catalyst-free,and the Ti₅Si₃ nanowire exhibits excellent field emission performance.（3）The effects of Ti₅Si₃ volume fraction,morphology and distribution on the mechanical properties of titanium-silicon brass were investigated,and it is found that Ti₅Si₃ with large aspect ratio and high volume fraction contributed more to load transfer strengthening and dislocation strengthening.However,the defect morphology of Ti₅Si₃ is prone to fracture under load,resulting in a significant decrease in the mechanical properties and wear resistance of the alloy.The specific performance is that as the volume fraction of Ti₅Si₃ in the titanium-silicon brass increases from 2.14vol.%to 11.83vol.%,the tensile strength of the alloy increases from498MPa to 583MPa,the elongation decreases from 7.7%to 3.1%,and the fracture diverts from ductile fracture to cleavage fracture.The dry friction test results show that the titanium-silicon brass with a larger volume fraction of Ti₅Si₃ exhibits better wear resistance,titanium-silicon brass 3#shows a smaller average friction coefficient（?）=0.44 and wear rate Q=0.88×10^-11m³/m.However,the defect-type morphology reduces the ability of the subsurface layer to resist plastic deformation,resulting in a sudden increase to（?）=0.48.It proves that the titanium-silicon brass 3#with high volume fraction and defect-free Ti₅Si₃ has higher comprehensive properties.（4）Hot extrusion deformation greatly improves the longitudinal tensile properties of the titanium-silicon brass by refining the grains,obtaining the uniform orientation of Ti₅Si₃,and welding casting defects.The tensile strength increasing from 546MPa to 759MPa,increases by39%;the yield strength increasing from 270MPa to 371MPa,increases by 37%;the elongation after fracture increasing from 6.8%to 13.7%,increases by 101%.The wear resistance of the titanium-silicon brass is significantly improved after hot extrusion,and the Ti₅Si₃ orientation has a greater influence on the wear resistance of the titanium-silicon brass.Under the friction conditions with an applied load of 216N,a sliding speed of 0.4m/s,and a test time of 20min.The Ti₅Si₃ orientation parallel to the friction surface and parallel to the sliding direction is most conducive to the wear resistance of titanium-silicon brass,the wear rate is only Q=0.48×10^-11m³/m.（5）Finite element analysis was used to investigate the tensile deformation behavior of complex brass with different second phase elastic modulus and morphology,and the stress and strain changes of Ti₅Si₃ phase and matrix were observed during the loading process.It is found that the morphology and orientation of Ti₅Si₃ phase will affect the strength of the titanium-silicon brass by changing the corresponding the phase stress partition parameter and stress gradient.As the aspect ratio of the Ti₅Si₃phase increases,the bearing capacity of the Ti₅Si₃phase in the titanium-silicon brass increases,which reduces the matrix stress without increasing its maximum stress,thereby increasing the yield strength of the titanium-silicon brass.When the orientation of Ti₅Si₃ phase is consistent with the tensile direction,it can increase the effective area for constraining the plastic deformation of the matrix and increase the stress gradient of titanium-silicon brass.And it is observed that with the increase of the applied load,the degree of uncoordinated deformation between Ti₅Si₃ and the matrix increases,the dislocation density near the interface increases.Thereby expanding the influence of Ti₅Si₃morphology and orientation on the deformation behavior of titanium-silicon brass.This is of great significance to deeply understand the mechanism of Ti₅Si₃ morphology and orientation on titanium-silicon brass.