Study On Technology Of TiAl Matrix Composites Fabricated By Combution Synthesis And Hot Pressing

TiAl alloy possesses the potentiality as aerospace engine with high temperature lightweight alloy because of high melting point, low density, high specific strength and modulus,low creep rate and high temperature oxidation resistance, but the intrinsic brittleness at roomtemperature limits its application and development. So the study about TiAl alloy is mainlyconcentrated in improving strength and plasticity. At present, introduction of ceramicparticles to the TiAl alloy is an important method to improve the strength and plasticity ofTiAl alloy, but the comparative study of the ex situ method and in situ way of these twokinds of ceramic particles into strength of TiAl based composites materials and the plasticityis less, the influences of the strength and the plasticity of TiAl based composites, and thedifference in the action mechanism are unclear.Therefore, this paper adopts the combustion synthesis heating pressure integratedmolding preparation technology for external and in-situ particle reinforced TiAl basedcomposite material as research object, to explore the ex situ α-Al2O3ceramic particlescontent, in-situ Ti5Si3ceramic particle content, bipolar in-situ TiB2and Ti5Si3ceramicparticles proportion on the microstructure and compression properties of TiAl alloy, toestablish the relationship between the microstructure and compression properties, to proposemechanism to improve the strength and plasticity of TiAl alloy.The main results of this thesis are as follows:1)The ex situ α-Al2O3ceramics nanoparticles and in situ ceramic particles (Ti5Si3, TiB2and Ti5Si3-TiB2) reinforced TiAl matrix composites were successfully fabricated by themethod of combustion synthesis and hot pressing. It is found that the ex situ α-Al2O3ceramics nanoparticles are in micron-size, and the in situ ceramic particles are all innano-size. The sizes of α-Al2O3, Ti5Si3and TiB2are1-2μm,60-90nm and30-50nm,respectively.2)It is found that in situ TiB2and Ti5Si3ceramics nanoparticles can refine the grain sizeof TiAl alloy,4vol.%TiB2and4vol.%Ti5Si3ceramic particles can make TiAl matrix grainsize decrease from~65μm to~20μm and~25μm, respectively. There is little difference inthe influence of different Ti5Si3content on TiAl grain size. Bipolar (TiB2-Ti5Si3) nanoceramic particles make TiAl matrix grain refinement effect is better, especially whenTiB2:Ti5Si3=2:1(∑M(TiB2+Ti5Si3)=4vol.%), the average grain size of TiAl matrix of about15μm.3) It is found that adding a small amount of nano-sized α-Al2O3ceramic particles canimprove the strength of TiAl alloy, but will reduce the plasticity of TiAl alloy. The yieldstrength (497MPa) and the ultimate compression strength (1454MPa) of the2vol.% Al2O3/TiAl composites are32MPa and39MPa higher than those of TiAl alloy, while thecompression fracture strain (14.5%) is2.8%lower than that of TiAl alloy. Increasing thecontent of α-Al2O3will deteriorate the performance of the TiAl composites. The TiAlcomposites enhanced by a duplex nanometer ceramic particles have good comprehensivemechanical properties, the comprehensive mechanics performance is best whenTiB2:Ti5Si3=2:1(∑M(TiB2+Ti5Si3)=4vol%), of which the yield strength (761MPa),ultimatecompression strength (1647MPa), and compression fracture strain (26.1%) are296MPa,232MPa and8.8%higher than those of TiAl alloy.4) To explore the mechanism for improving strength and ductility of TiAl alloy by exsitu α-Al2O3ceramics nanoparticles and∑4vol.%different proportion in situ TiB2and Ti5Si3ceramics nanoparticles.i)Mechanism for improving strength and ductility of TiAl alloy by ex situ α-Al2O3ceramics nanoparticles: the second phase strengthening.ii)Mechanism for improving strength and ductility of TiAl alloy by in situ Ti5Si3andTiB2ceramics nanoparticles: effect on strength of TiAl alloy by dispersion and distributionof Ti5Si3and TiB2ceramics nanoparticles matrix and the grain refinement of TiAl alloymatrix.