Microstructure And Mechanical Properties Of Micro-laminated TiB₂-TiAl Composite Sheets Prepared By Roll Bonding And Reaction Annealing

In order o satisfy the requirements of supersonic aircrafts, future gas turbineengines and thermal protection systems for high-strength light-density alloy sheetsapplied in the range of800¹000oC, it is of crucial importance for development andmanufacture of TiAl-based alloy sheets. However, owing to intrinsic brittleness, it israther difficult to to fabricate TiAl-based alloy sheets by means of direct rolling ofTiAl-based alloy ingots. Therefore, in this paper micro-laminated TiB₂-TiAlcomposite sheets have been successfully produced by the processing route of hotrolling multi-layered Ti-（TiB₂/Al） composite sheets which have excellent plasticdeformation capacity and subsequent multistep heat treatment. The relative densityof micro-laminated TiB₂-TiAl composite sheets has been significantly improved byhot-pressing sintering densification process. Preparation processes of TiAl matrixcomposite sheets were investigated and optimized. Phase compositions of thereaction layer and reaction kinetics in the process of reaction annealing weresystemically studied by scan electron microscopy （SEM）, X-ray diffraction （XRD）and transmission electron microscopy （TEM） and the reaction mechanism wasrevealed. In addition, the effect of volume fractions of TiB₂on microstructures andmechanical properties of TiAl matrix composite sheets were investigated. Moreover,microstructures and mechanical properties of micro-laminated TiB₂-TiAl compositesheets were reasonably characterized and evaluated.Multi-layered Ti-（TiB₂/Al） composite sheets with good interface bonding canbe fabricated by hot pressing and hot rolling. However, rolling deformationreduction in thickness should be not more than65%to ensure the thicknessuniformity of Ti and TiB₂/Al composite layers. Deformation compatibility ofmulti-layered Ti-Al composite sheets have been pronouncedly improved by thereplacement of pure Al sheets by TiB₂/Al composite sheets, which is benefit fordesigning and controlling compositions and microstructures of resulting TiAl matrixcomposite sheets.For multi-layered Ti-（TiB₂/Al） composite sheets, after the initial reactionannealing between520oC and700oC, TiAl₃was primary phase and the introductionof TiB₂has little influence on reaction and diffusion between Ti and Al. Formationand growth of TiAl₃layer is a reaction-diffusion process accompanied withorder-disorder transition and Growth velocity of TiAl₃towards TiB₂/Al layer ismuch faster than that towards Ti layer. When annealing temperature is below themelting point of pure Al （660oC）, growth of TiAl₃layer is in consistent with theparabolic growth law, while annealing temperature is the melting point of pure Al or above, its growth accords with linear growth law. Based on reaction kinetics,controlling leakage of liquid Al and reducing formation of pores within TiAl₃layer,optimum initial reaction annealing is660oC.Because of Kirkendall effect during initial raction annealing, when Al has beenexhausted, porous TiB₂layer forms in the middle of TiAl₃layer, which severelyreduces the laminate’s relative density and mechanical properties. Relative densityof micro-laminated TiB₂-TiAl composite sheets can be significantly improved byhot-press sintering in vacuum under60MPa at1225oC for2h, which avoidsformation of secondary cracks. When the laminate is subsequently annealed at950¹200oC, the reaction diffusion proceeds and TiB₂still keep stable. Finally,multi-layered TiB₂-TiAl-Ti3Al composite sheets are obtained. Finally，after lamellartreatment at1400oC for22min, micro-laminated TiB₂-TiAl composite sheetsconsisting of TiB₂-rich layer containing numerous TiB₂particles connected by asmall quantity of γ-TiAl and fully lamellar TiAl layer were obtained. Moreover, theinterface between TiB₂and TiAl phases exhibits sound bonding and follows theorientation relationship:[1100]TiB₂//[112]TiAland （0001）TiB₂//（111）TiAlwthin the localTiB₂-rich layer. TiB₂-rich layer significantly hinders growth of α2γ lamellar colony.Results indicate that2.6vol.%TiB₂-TiAl have optimum composition, microstructure,the lowest density and the highest elastic modulus value of172.15GPa.Fracture toughness testing shows that micro-laminated TiB₂-TiAl compositesheets exhibit excellent fracture toughness and with increasing thickness ofTiB₂-rich layer, fracture toughness increases. Furthermore, fracture toughnessdepends on loading direction, fracture toughness of2.6vol.%TiB₂-TiAl compositesheets parallel to normal direction （ND） is15.12MPa·m1/2, which is27.6%higherthan the value parallel to transverse direction （TD）. TiB₂-rich layer hinderspropagation of cracks and induces crack deflection and crack branching, andmicrocrack toughening occurs. All above factors make crack propagation consumeamount of energy, which contributes to higher fracture toughness of themicro-laminated composite sheets in parallel to normal direction.With raising tensile testing temperature, strength of micro-laminated TiB₂-TiAlcomposite sheets increases firstly and then decreases, elongation increases andelastic modulus reduces. Yield strength at750oC reaches the highest value of339.92MPa. Therefore, miro-laminated TiB₂-TiAl composite sheets have potentialfor high temperature structural applications. Improvement of properties at hightemperature is because of strenghening of TiB₂and refinement of α2γ lamellarcolony due to existing of TiB₂-rich layer.Fracture research results show that initial cracks nucleate within TiB₂-richlayers. With increasing loading, cracks propagate into TiAl layer and interlamellar crack and translamellar cracks are produced. Interlamellar cracks propagate bynucleation, growth and linkage with interfacial microcracks, while translamellarcracks propagate by nucleation, growth and linkage with two different types ofmicrocracks, e. g. interface delamination and translamellar microcracks.