| In this dissertation, the solid-state phase transformation and reaction mechanism induced bymechanical alloying and subsequent discharging of Ti-C-N systems have been systematicallyinvestigated by X-ray diffraction （XRD）, scanning and transmission electron microscopy （SEM andTEM）, energy dispersive X-ray spectroscopy （EDS）, X-ray photoelectron spectroscopy （XPS）,positron annihilation lifetime spectroscopy （PALS）.In the first part of the dissertation, the development and present research situation of mechanicalalloying over past20years have been critically overviewed, which includes the mechanism, modeland theory, the influence of technological parameters of ball milling on the phase composition,morphology and grain size of mechanical alloying products, recent development of mechanicalactivation and after treatment, and the progress of producing technology of Ti（C,N） powder. Based onabove work, the purpose and significance of the dissertation have been pointed out.Solid-gas reaction of Ti with N2driven by mechanical alloying has been investigated.Nanocrystalline TiN compound with average grain size about50nm was obtained. The influence ofthe ball-to-powder weight ratio on the phase transformation, morphology and grain size of TiN grainwas analyzed. The thermodynamic and kinetics related to the transformation from chemisorption tometal nitrdes also addressed.The formation mechanism of Ti-C-N system and the reaction difference between mechanical andthermal activation were exhaustively investigated. It is found that the formation of Ti（C,N） driven bymechanical alloying follows the mechanically induced self propagating reactive synthesis（MSR）.However, the reaction proceeded in a form of gradual diffusion under thermal processes, namely,nitrogen atoms diffused into the titanium, formed TiN0.26and Ti2N in sequence. When the annealingtemperature equal to or higher than800℃, C and N atoms interdiffused to form Ti（C,N） solidsolution.The effect of milling energy on the solid state reaction of Ti-C-N system driven by mechanicalalloying has been examined in detail. The formation mechanism is MSR when the effective extensivefactor is89.64～316.85KJ/（g·s）. The improvement of milling rotate speed could remarkably increasethe linear speed of single milling ball, the collision frequency of balls and energy transfer to thepowders in each collision. Thus ignition temperature of the MSR declined rapidly, incubation timeshortened and the total energy for fabricating Ti（C,N） required for MSR reduced.The phase transformation of Ti-N system driven by mechanical activation and glow discharging or arc discharging has been investigated. It is found that TiN formation chose a different pathway ineither glow discharging or arc discharging processes. A formation sequence of TiN0.26â†'Ti2Nâ†'TiNwas observed in the glow discharging, while TiN was directly formed during arc dischargingprocesses. Both discharge treatment can promote the synthesis of TiN, however effect of the arcdischarge was more obvious.The phase transformation of Ti-C-N ternary system driven by mechanical activation and glowdischarging/arc discharging has also been investigated. It is found that Ti（C,N） formation follows thesame mechanism （self-propagation high temperature synthesis, SHS） during both dischargingprocesses. With the glow discharge power increases, the collision of energetic particles on the powdersurface significantly increased the temperature of the powders, accelerated carbon and nitrogen atomsinterdiffusion in the Ti lattice, induced the reaction ignite and synthesis of Ti（C,N）. In the arcdischarging process, when unit mass of powder stored energy Et>0.81×109J/g, the reaction can becompletely; when Et>1.29×109J/g, the as-milled powder can be ignited in an instant.Mechanical alloying and arc discharging of Ti(C1-x,Nx) solid solution with different Ti/C molarratio of mixed powder have been studied. When the C content in the mixed powders was30at%, theinterphase Ti2N appeared; the reaction product contains Ti（C,N）, middle phase Ti2N and microcontentunreacted Ti powder when it was15at%during mechanical alloying.The content of the interphaseTi2N and unreacted Ti powder reduced, the lattice parameter of Ti(C1-x,Nx) solid solution becamelarger and the nitrogen content in solid solution reduced with the mechanical activation time extendedin the arc discharge treatment. X value of Ti(C1-x,Nx) solid solution ranged from0.19to0.96drivenby mechanical alloying, while X value of the range of0.012to0.92generated in the arc dischargingprocess. |
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