Effect Of Cu Doping On The Reaction Pathway And Products Of Combustion Synthesis In Ti-Si System

Recently, high-melting intermetallic compounds have attracted considerable attentionbecause of their potential applications in many fields including aviation, aerospace, military,and so on. Among various intermetallics, titanium silicide （Ti₅Si₃） has been regarded as oneof the most promising high temperature structure materials on account of its high meltingtemperature, low density and excellent thermal stability. However, due to its complexhexagonal structure （D88）, Ti₅Si₃shows very low fracture toughness, which severely limitsits application. Hence, the major challenge in applying Ti₅Si₃is to reduce its brittleness orimprove the fracture toughness.More recently, researchers have conducted a sequence of work including doping thethird element in Ti-Si system to improve the mechanical properties of Ti₅Si₃. First-principlescalculation indicates that some alloying elements, such as Nb, V or Cr can increase bulkmodulus or reduce shear modulus of Ti₅Si₃, which could improve the ductility of Ti₅Si₃effectively. Experimental results suggest that doped metal elements, such as Fe, Ni and Alcan form binary or ternary eutectic liquids with Ti or Si in the Ti-Si system, whichparticipates in the reaction process or even changes the reaction pathway. As a result, metalelements serve as bonding agents distributing in boundaries of Ti₅Si₃grains after the reaction,thereby contributing to improving the performance of Ti₅Si₃materials.Some studies suggest that Cu doping in Ti-Si system can not only decrease the reactiontemperature of combustion synthesis （CS）, promoting the rate of reaction, but also refineTi₅Si₃particulates, improving the density. In short, it is of considerable significance to studythe influence of Cu doping on the improvement of Ti₅Si₃properties. However, less researcheffort has been carried out on the CS reaction pathway and products of Cu-Ti-Si system.Consequently, in the present study, based on the calculation of the adiabatic combustiontemperature, influences of Cu addition as well as Ti and Si particle sizes on the CS reaction pathway of Cu-Ti-Si system are investigated. Besides, particular attention is also paid to theinfluence of dynamic factors, such as Cu content, ratio of reactants and particle sizes, on thephase compositions and microstructures of CS reaction products of Cu-Ti-Si systems. Inaddition, the study also fabricated in situ Ti₅Si₃and TiB2reinforced Cu matrix compositeswith high densification by combustion synthesis plus quick press （CS/QP） process andinvestigated the microstructures and mechanical properties of composites. Results of thepresent studies are as follows:（1） The formation of Cu-Si and Cu-Ti-Si liquids is significantly reducing the reactiononset temperatue; the reaction pathway of Cu-Ti-Si system during CS could be described as:the Cu3Si was formed initially via solid state diffusion reaction between Cu and Si particles;with the temperature increasing, Cu-Si eutectic liquid was formed at about802oC betweenCu3Si and Si; and then Ti diffused into the surrounding Cu-Si liquid and led to the formationof Cu-Ti-Si ternary liquid; meanwhile, once the liquid reached a saturation stage with [Ti]and [Si] in the course of continuous dissolution, plenty of Ti₅Si₃could be developed from theliquids by a solution–reaction–precipitation mechanism. The reaction pathway of Cu-Ti-Sisystem is: Cu_{（s）}+Ti_{（s）}+Si_{（s）}â†'Cu3Si_{（s）}+Ti_{（s）}+Si_{（s）}â†'（Cu–Si）（l）+Ti_{（s）}â†'（Cu–Ti–Si）（l）â†'Cu（l）+Ti₅Si_{3（s）}.（2） The Ti and Si particle sizes have a great influence on the reaction pathway inCu-Ti-Si system: for Ti_[15]Si_[15]Cu_[45]system, fine Ti particle easily dissolves into Cu-Sibinary liquid to form Cu-Ti-Si ternary liquid at⁷95oC, which further participates intothe reaction of β-Ti and Si to yield abundant Ti₅Si₃particulates at⁹17oC; ForTi[150]Si[15]Cu[45]system, however, it is relatively difficult for coarse Ti particles to reactwith the Cu-Si liquid to form the ternary liquid and, thereby delaying the formation ofTi₅Si₃to⁹48oC; for Ti_[15]Si_[150]Cu_[45]system, coarse Si particle results in the formationof insufficient Cu-Si liquid initially, while Cu-Ti liquid forms at⁹60oC instead, whichfurther reacts with coarse Si to form the Cu-Ti-Si liquid, and the Ti₅Si₃are precipitated outof the ternary liquid.（3） When Cu content increases over the range of10-50wt.%, products only consistof Ti₅Si₃and Cu phases. while the average size of Ti₅Si₃decreases significantly from¹5 to²Î¼m and the morphology of Ti₅Si₃transforms from the long strip to thecobblestone–like shape with a relatively smooth surface. Besides, the volume of productsexpanded and followed by shrink with the increasing of Cu content and the expansion ratereached the highest value of14.7%when Cu content was20wt.%. The Cu atoms can takeplace of Ti sites in Ti₅Si₃lattices. The sizes of Cu and Ti powders had little influence onthe type, size and morphology of reaction products, and Ti₅Si₃exhibited cobblestone–likeshape or polygon shape. However, the size of Si powder had a great effect on themorphology of Ti₅Si₃. The polygon-like shape of Ti₅Si₃changed from smooth surface topolyporous surface with the size of Si increasing from15to150μm.（4） In situ high volume fraction Ti₅Si₃, TiB₂and Ti₅Si₃+TiB₂particulates reinforcedCu matrix composites with high densification could be successfully fabricated by CS/QPmethod. The porosities of the three composites were1.1,5.0and1.7%, respecitively.Average values of σUCS（σ0.2） and εffor Ti₅Si₃, TiB₂and Ti₅Si₃+TiB₂reinforced coppercomposites were1040（686）,1262（1009） and1234（876） MPa, as well as13.6,5.9and5.5%, respectively.