Microstructure And Properties Of TiAl Matrix Composite Sheet Fabricated By In-situ Technique

TiAl compoistes are characterized by their low density, high melting point, as wellas excellent specific strength and specific modulus. However, because of theirpronounced brittleness, there are many difficulties in processing titanium alumindecomposites. In this paper, titanium elemental foils and aluminum matrix composite foilswere adopted to prepare titanium aluminde composite by the processing route ofreactive annealing. The phase transformation mechanism and the microstructure of thematerial was studied by analyzing techniques such as scanning electron microscope（SEM）, transmission electron microscope （TEM）, X-ray diffraction （XRD） and highresolution transmission electron microscopy （HRTEM）. Preparation processes of TiAlmatrix composite sheets were investigated and optimized. Besides, mechanical propertytests were carried out at elevated temperatures.During the low temperature annealing, a layer of TiAl₃was formed at the Ti/Alinterface with Si element solved into TiAl₃, which changed TiAl₃lattice constants andelastic modulus. Growth of TiAl₃layer was in consistent with the parabolic growth law.SiC decomposed to generate Al4C3and reactive Si atoms at low temperature annealingprocess. After hot pressing sintering at1200℃, the bonding strength between themulti-layered sheets improved. Ti（Al,Si）₃phase transformed into TiAl and Ti₃Al, whichcaused the Si solid solution decreased, resulting in the precipitation of Si compounds.The precipitates Ti₅Si₃phase dispersed in the TiAl matrix. TiAl₃reacted with Al4C3togenerate [Ti6C] octahedron and active Al atoms, subsequently to generate Ti₂AlC andTi₃AlC₂. During the homogenization heat treatment, TiAl₃、TiAl、Ti₃Al transformed intoTi₃Al and TiAl. The presence of Si and C has not changed. In both sides of the bondedinterface, the interdiffusion of components was evident.（α₂+γ） lamination wasproduced during the laminarization annealing. The presence of Si has not changed.Ti₅Si₃dispersed phase limited the size of the lamellar clusters. Ti₃Al contacted with theceramic layer to generate a small amount of Ti₃AlC phase. Si solid solution makedTi₃Al and Ti₃AlC ordering. Moreover, the interface between Ti₃Al and TiAl exhibitedsound bonding and followed the orientation relationship:（111）γ//（0001）α₂[110]γ//[1120]α₂The microstructure study of the composite in the final state indicated that thematrix of the material consisted of （α₂+γ） lamination, during which dispersed Ti₅Si₃phase. Between the lamellar layer, ceramic layer consisted of Ti₂AlC and Ti₃AlC.The mechanical properties tests showed that the hardness of the sheets was8.6GPa,and the elastic modulus was about270GPa, while that of ceramic layer was10.9GPa and290GPa respectively.Tensile tests at elevated temperature showed that the failure ofthe materials was caused by brittle fracture. Initial cracks nucleated within the ceramiclayers. Subsequently, cracks propagated into TiAl layer and the unique lamellarstructure makes the crack to deflect until the fracture.