Characterization For Properties Of Bulk Ti₂ALC Synthesized By Combusion Synthesis And First-principle Study On MAX Phases

A class of layered ternary compounds, with the general formula Mn+1AXn（MAX phases for short） where n=1,2or3, M is early transition metal, A is anA-group element （mostly group13and14）, and X is C or N, exhibits a uniquecombination of both metal-and ceramic-like properties: high fracture toughness,high Young’s moduli, good thermal and electrical conductivities, easymachinability, high damage tolerance, excellent thermal shock and oxidationresistance, and so on. These render it potential candidates for high-temperaturestructural and functional applications. This thesis develops a new process tosynthesis Ti₂AlC bulk firstly, and investigates its structures and properties in detail.And First-principle method is used to investigate the properties of MAX phasessystematically, aiming to give the theoretical evidence for understanding origin ofexperimental phenomenon and designing new materials, and guide the followedexperimental work, after realizing structure-property relationship.A patented method, self-propagating high temperature combustion synthesiswith pseudo hot isostatic pressing process （SHS/PHIP）, was employed for densepolycrystalline Ti₂AlC bulk using elemental reactants, in which the effect ofprocess parameters was also examined. The applied pressure in SHS/PHIP processplays a critical role in the densification of Ti₂AlC. The resultant sample mainlycontains typical plate-like nonstoichiometric Ti₂AlCx（x=0.69） with finemicrostructures （2.5-3.0μm）. The as-synthesized Ti₂AlC bulk is rich in the latticedefects. The equal inclination fringes in the local area indicate that Ti₂AlCsynthesized by SHS/PHIP process is deformed under high residual stress.The properties of Ti₂AlC bulk synthesized by SHS/PHIP process were studiedin detail. It exhibits a typical metal-like conduction. An abnormal thermalexpansion behavior was observed that the curve of thermal expansion againsttemperature bends up dramatically at around1050oC. There is the higher thermalexpansion along c-axis than a-axis. The electronic component of the thermalconductivity is always the dominant mechanism in the researched temperaturerange. The high-density lattice defects result in the fact that the phononcontribution to thermal conductivity almost can be neglected. The fine microstructures are related to the high flexural strength （606±20MPa）.Interestingly, a metal-like non-catastrophic failure is present in the SENB test,with the high work of fracture. The brittle-ductile transition temperature （BDTT）under flexure （900-950oC） is higher than compression （700-800oC）. Theflexural and compressive strengths both keep almost unchanged in the zone ofbrittle failure below BDTT, but decrease sharply as the plastic deformationoccurs above BDTT. A behavior of dynamic extension of initial short crack wasobserved in the thermal shock of Ti₂AlC. The critical thermal shock temperature ofTi₂AlC is estimated to be300-500oC. And the fracture mode changes fromtranscrystalline to intercrystalline. With increasing quenching times, the retainedstrength decreases more slowly.The crystal structure, electronic structure, elastic properties, lattice dynamics,and compressibility of Mn+1AlCn（n=1-3） phases were systematically investigatedusing First-principle calculations, especially considering effects of VEC （valenceelectronic concentration） and d electronic shell for transition metal M. The latticeconstants of Mn+1AlCnphases are both a linear function of M diameter. Thetheoretical density increases with increasing VEC of M and M-C slabs. Also, theformation energy increases with the increase of VEC. In addition, the bond lengthand bond angle were studied.The lattice dynamics of Mn+1AlCnphases was studied in detail, as well asRaman and infrared active modes. The imaginary frequency is present in somecompounds, which are all member of312and413phases, and mostly contain Mwith VEC=6. Moreover, such the imaginary frequency of Nb3AlC2explains why acomposition gap is present in Nb-Al-C system （Nb4AlC3and Nb2AlC exist, butNb3AlC2does not）. The highest Raman and infrared frequencies all correspond tolongitudinal or shear vibration of C atoms along c-direction.A simple model is established to investigate bond stiffness quantificationally.M-Al bond has the lowest bond stiffness, approximately1/3¹/2of M-C bondstiffness. Interestingly, the same bonds in different systems （M2AlC, M3AlC2andM4AlC3） have the similar stiffness. VEC affects the bond stiffness as a function ofpressure. Of most importance, B is0.256that of average bond stiffness. Withincreasing pressure, M-Al bond shifts towards to basal plane, but M-C bond shiftsalong c-axis. As a result, the compression becomes more difficult along c-axis. Using First-principle calculation, the electronic structure, elastic and opticalproperties, and compressibility of some typical MAX compounds （M2InC, Ti₂CdC,Ti₃SnC2and Ti4GaC3） were predicted. A-group element has an obvious effect onbulk moduli （B） of M2AC, in which Zr2InC has the lowest B （113GPa） in211phases. Correspondingly, the lowest shear moduli （70GPa, G） appears in Ti₂CdC.The low G and G/B ratio of Ti₂CdC indicate the low friction coefficient, whichrends Ti₂CdC as a potential electrical-friction material. In the low frequency rangefrom radio waves to visible light, Ti₃SnC2behaves similarly with TiC. In otherword, Ti-Sn bond only affect the optical properties in the high frequency range. Thelow G/B ratio partially explains why Ti₃SnC2is relatively soft and damage tolerant.Both bond stiffness and bond angles play roles in the compressibility.A new Ti4GaC3polymorph （γ-type） was proposed according to the reportedexperimental results, with the Ti and Ga （underlined） atomic arrangements ofABCBACBABC. Since the α-to γ-phase transition only involves shuffling of theA-atoms, it occurs much more easily than those to β-Ti4GaC3despite the factthat the latter is thermodynamically less stable than γ-Ti4GaC3. Threepolymorphs have similar electronic structures, elastic properties, andcompressibility. The electrons occupy all the bonding states for α-Ti4GaC3, but thebonding states are partially occupied for both β-and γ-Ti4GaC3. In general, withincreasing pressure, all the bonds become stronger, and the rate of increase inbond stiffness also increases.