Preparation And Mechanical Properties Of Laminated TiAl-based Composites

The density of TiAl alloy is just about 50%of the nickel-based superalloy,which working temperature can reach to 600⁹00?.TiAl alloy has characteristics such as good strength at high temperature and excellent creep resistance,at the same time with high specific strength,wear resistance,magnetization resistance,oxidation resistance,corrosion resistance,and so on.TiAl alloy is expected to replace Ni based superalloy in the near future,which is regarded as the important development direction of the advanced engineering materials in the future.However,as a typical intermetallic compound,the intrinsic brittleness of TiAl alloy leads to the poverty of plasticity and toughness in room temperature,which make it difficult to be molded by conventional means.In addition,the mechanical properties of TiAl alloy will decrease when the working temperature is over 1100?.Above-mentioned disadvantages seriously hinder the further application of TiAl alloy.How to improve the mechanical properties of TiAl alloy has became one of the important research directions.The strengthening mechanism of TiAl alloy are extremely complicated,especially the particle reinforcements caused by in-situ ceramics,the type of reinforcements particles,size,distribution,quantity,shape and other factors will all have significant effects on the mechanical properties of the matrix such as strength and toughness.The ternary layered compound Mn+1AXn,what referred as MAX phase,is a new type of cermet which has attracted wide attentions in recent years?M represents the transition element,A represents the third or fourth group element,X represents C or N element,n=1,2or 3?.Because of the special lamellar structure and the lattice structure coexisting with metal bonds and covalent bonds,MAX has both the advantages of metals and ceramics which make it possible to be widely applied in high-temperature materials,energy storage materials,new functional materials and other fields.Ti_n+1AlC_n?n=1 or 2?is considered as an ideal reinforcements for TiAl based allyos due to its characteristics such as favorable high-temperature performances,self lubrication property,coefficient of thermal expansion similar to TiAl alloys,and others.In this paper,the in-situ MAX ceramic particles are introduced into the composite layers.What is more,laminated structure design is applied to improve the comprehensive mechanical properties of TiAl based composite sheets.In these experiments,the Ti-Al-TiC mixed powder was used as the composite layers,and TC4 titanium alloy?Ti-6Al-4V?sheets as the toughening layers,then different layers were overlapped layer-by-layer in the die.The TC4/Tin+1AlCn-TiAl laminated composite sheets were prepared via spark plasma sintering?SPS?.By using optical microscope?OM?,scanning electron microscopy?SEM?,X-ray diffraction?XRD?and other means to analyze the microstructures,phase composition,and others to study the influences of different contents of MAX phase in composite layers and different numbers of toughening layers on structure and mechanical properties of TC4/Tin+1AlCn-TiAl laminated composite sheets.In addition,the strengthening and toughening mechanisms,and fracture mechanism are studied.The analysis shows that the composite layers are mainly composed of?₂-Ti₃Al/?-TiAl lamellar structure and the densification is fine.Kirkenthal holes are rarely observed by SEM and interfaces between layers are well bonded.There are TiAl-Ti₃Al interface layers generated between the composite layers and toughening layers.The mechanical properties of the TC4/Ti_n+1AlC_n-TiAl laminated composite sheets are tested,the maximum flexural strength reaches 1428.79±71.4 MPa,and the maximum fracture toughness reaches 67.72±3.4 MPa·m^1/2.The test results of mechanical properties about the laminated composites show clearly that flexural strength and fracture toughness of the laminated composite are anisotropic,what means that the mechanical properties of materials in forced direction parallel or vertical to laminated structures are significantly different.The mechanical properties can be effectively improved by doping TiC and enhanced by the in-situ MAX reinforcements.The existence of laminated structure increases the resistance of crack propagation,shortens movement paths of crack acceleration,and greatly improves the mechanical properties of materials.In addition,the energy dissipation mechanism of laminated structure can weaken the sensitivity of composite sheets to drawbacks.