Laws And Mechanisms For The Effects Of Transition Metal Elements And In Situ Ceramic Particles On The Compression Propetries Of TiAl

The TiAl alloy possesses the potentiality for substituting Ni–base superalloy to becomehigh temperature light weight alloy, due to its low density, high melting point, high specificstrength and specific modulus, low creep rate and excellent high temperature oxidationresistance. Moreover, the specific strength, creep resistance and oxidation resistance of TiAlalloy are better than those of the Ti alloys and the Ni–base superalloy. However, the lowroom temperature ductility and high temperature strength limit its extensive application.Thus, the investigation for improving the strength and ductility of TiAl alloy has importanttheory and application value for TiAl to substitute Ni–base superalloy and become importantaerospace material. At present, the investigation about improving the strength and ductilityof TiAl alloy is still a problem to be solved quickly in the world. The influence mechanismsof the kind and content of metal elements and the kind, size, shape, quantity and distributionof in situ ceramic particles on the strength and ductility of the TiAl alloy are still unclear,which limits the extensive application of TiAl in practice. Thus, the influence mechanisms ofdifferent metal elements and in situ ceramic particles on the strength and ductility of TiAlalloy need to be further explored and revealed.Therefore, in this thesis, the alloying TiAl alloys and the in situ ceramic particlesreinforced TiAl matrix composites fabricated by the method of combustion synthesis and hotpress consolidation were studied. The influence law of different kinds and contents oftransition metal elements （Zr/Hf/V/Nb/Ta/Cr/Mo/W/Mn/Fe/Co/Ni/Cu/Zn） and differentkinds, sizes, shapes, quantities and distribution of in situ ceramic particles（Ti₂AlC/TiB₂/Ti₅Si₃/TiB₂–Ti₂AlC） on the compression properties of TiAl alloy wereexplored by the combination methods of first principle calculation and experiments. Therelationship between microstructures and properties was built and the mechanisms forimproving the strength and ductility of the TiAl alloy were discussed.The main results are as follows:1) The in situ ceramic particles （Ti₂AlC/TiB₂/Ti₅Si₃/TiB₂–Ti₂AlC） reinforced TiAl matrix composites were successfully fabricated by the method of combustion synthesis and hotpress consolidation for the first time. It is found that the in situ ceramic particles are allin submicron–size and nano–size. The sizes of Ti₂AlC, Ti₅Si₃and TiB₂are⁷00nm,60–80nm and30–50nm, respectively.2) It is found that the metal elements （Fe, Co）, in situ ceramic particles （Ti₂AlC, TiB₂,Ti₅Si₃, TiB₂–Ti₂AlC） and metal elements+in situ ceramic particles （Mn+Ti₂AlC,Mn/Fe/Co+TiB₂） all could refine the grain size of TiAl alloy.ⅰ) With the addition of3at.%Fe and Co, the grain size of TiAl alloy decreases from⁶6μm to¹5μm and25μm, respectively.ⅱ) With the generation of6vol.%Ti₂AlC, TiB₂, Ti₅Si₃and TiB₂–Ti₂AlC, the grain sizeof TiAl alloy decreases to³0μm,¹5μm,²2μm and¹0μm, respectively.ⅲ) The2at.%Mn+4vol.%Ti₂AlC and the2at.%Mn/Fe/Co+4vol.%TiB₂decreasethe grain size of the TiAl alloy to²6μm and¹0μm, respectively.3) It is revealed that the4vol.%Ti₂AlC/TiAl–2Mn composite possesses the bestcompression properties, of which the yield strength （637MPa）, ultimate compressionstrength （1694MPa） and fracuture strain （22.2%） are172MPa,279MPa and4.9%higher than those of the TiAl alloy; The4vol.%TiB₂/TiAl–2Co composite possessesthe highest compression strength, of which the yield strength （820MPa）, ultimatecompression strength （1906MPa） are355MPa and491MPa higher than those of theTiAl alloy, the fracture strain is similar with that of TiAl alloy.4) The influence mechanisms of metal elements （Mn, Fe, Co）, in situ ceramic particles（Ti₂AlC, TiB₂, Ti₅Si₃, TiB₂–Ti₂AlC） and metal elements+in situ ceramic particles（Mn+Ti₂AlC, Mn/Fe/Co+TiB₂） on the strength and ductility of TiAl alloy were putforward.ⅰ) Mechanisms for improving strength:a) Metal elements: the solid solution strengthening and the grain refinementstrengthening.b) In situ ceramic particles: the second phase strengthening and the grainrefinement strengthening.c) Metal elements+in situ ceramic particles: the solid solution strengthening, thesecond phase strengthening and the grain refinement strengthening.ⅱ) Mechanisms for improving ductility:a) Metal elements: the grain refinement and the improvement of crystal structures,electronic structures and elastic properties.b) In situ ceramic particles: the grain refinement and the uniform distribution of fine in situ ceramic particles.c) Metal elements+in situ ceramic particles: the improvement of crystalstructures, electronic structures and elastic properties, the grain refinement andthe uniform distribution of nano–size in situ ceramic particles.In summary, the influence law of different kinds and contents of transition metalelements and different kinds, sizes, shapes, quantities and distribution of in situ ceramicparticles on the compression properties of TiAl alloy were explored by the combinationmethods of first principle calculation and experiments; the mechanisms of transition metalelements and in situ ceramic particles for improving the strength and ductility of TiAl alloywere put forward, which laid the necessary theoretical basis for the development of the insitu ceramic particles reinforced TiAl matrix composites with high strength and ductility andits new fabrication technology.