Processing And Properties Of Refined Ti₂AlC/TiAl Composites By In-situ Synthesis

TiAl alloy exhibits remarkable prospect in aerospace, automotive, and power industry due to its high melting point, low density, high specific strength and specific modulus, excellent oxidation resistance and creep resistance. However, low ductility in room temperature and insufficient strength in elevator temperature limit the application of TiAl alloys. The introduction of particle reinforcements can improve the strength, plasticity, and high temperature properties, besides the maintenance of low density and high specific modulus of Ti Al alloy. However, it is difficult to achieve TiAl matrix composites with refine grains and dispersed reinforcements. Therefore, it is important to develop the controllable fabrication method of fine/ultrafine grained TiAl matrix composites with dispersed reinforcements and to make clear its strength mechanism in the exploitation of TiAl matrix composites with high performances.In this dissertation, on the basis of the traditional powder metallurgy methods,the content and uniform distribution of carbon element is enhanced through changing carbon source and ball-milling method during the mechanical alloying of Ti and Al powders. Fine/ultrafine grained TiAl matrix composites with in-situ fabricated and uniform distributed Ti₂ AlC reinforcements is obtained. The preparation method, microstructure, mechanical properties, and strength and toughen mechanism of the composites are systematically investigated. The model for the mechanochemical reaction of Ti and Al in n-hexane is proposed. The formation mechanism of fine/ultrafine grained TiAl matrix composites, as well as the strength mechanism, is illustrated.The fine grained Ti₂AlC/TiAl composite is prepared by surfactant dispersion method with multi-walled carbon nanotubes（MWCNTs） as carbon source for the first time. The pre-alloyed powders of Ti-50 at.% Al are first prepared by mechanical alloying method. Then MWCNTs with zwitterionic surfactant treatment uniformly load on t he surface of Ti-50 at.% Al pre-alloyed powder by ultrasonic dispersion. Finally, Ti₂ Al C/TiAl composite is in-situ fabricated though cold press-vacuum sintering process. The effects of process control agent（PCA） and milling parameters on the morphology and microsturcture of pre-alloyed powders are investigated. The effect of z witterionic surfactant on the dispersion of MWCNTs is discussed. It is found that the addition of stearic acid as PCA can significantly restrain the cold welding and increase the powder yield. The powders are deformed and broken due to ball milling, and solid solution, nanograin, amorphous and intermetallic phases are formed with the milling time prolonged. Zwitterionic surfactant treatment can reduce the clustering of CNTs, improve the dispersion of CNTs on matrix, and result in the uniform dispersion of in-situ fabricated Ti₂ AlC reinforcements during sintering process.The ultrafine grained TiAl matrix composites powders reinforced by Ti₂ AlC are in-situ fabricated through mechanochemical ball-milling process. Ti and Al powders are first mechanochemically ball milled in liquid n-hexane. C atoms from the decomposition of n-hexane prefer interaction with Ti, and then the finely dispersed Ti₂ AlC is formed by a nnealing treatment. The influence of milling time on t he resultant phases and particle sizes is investigated. The fitted curve of carbon addition with milling time is achieved through the mensuration of the relative amount of phases in annealed product. The reaction mechanism of Ti and Al powders in nhexane during ball milling process is proposed.In-situ reinforced ultrafine grained Ti₂AlC/Ti Al composites were fabricated by vacuum hot pressing technique using the composite powders mechanochemically ball-milled. The influence of sintering parameters and reinforcement content on the microstructure and mechanical properties of the composites is investigated. It is revealed that the strength and plasticity of ultrafine grained Ti₂AlC/Ti Al composites increase significantly fabricated by the hot-press preparation. Sintering temperature, hot pressing pressure, and content of reinforcements are the important factors influencing the properties of composites. With the increase of sintering temperature, the density, as well as the grain size, increases, and the yield strength and bending strength are raised first and then decrease. Although the density increases with sintering pressure increasing, the internal stress of composites increases if the pressure is too high resulting in defects with stress concentration and the decrease of mechanical properties. The yield strength increases with reinforcement content increasing. But when the content of reinforcement is too high, reinforcement continuum forms easily, which affects the sintering density and decreases the strength. The strengthening mechanism of composites is mainly cooperative strengthening effect of refine grain strengthening and dislocation strengthening. The enhancement of plasticity and toughness is mainly due to refined grain effect and uniformly distributed in-situ produced Ti₂ Al C particles with layered structure. The break of layered structure of in-situ produced Ti₂ AlC enhances the energy for crack propagation, which benefits the improvement of toughness. The Ti₂AlC/TiAl composites fabricated with sintering temperature 1100 °C, sintering pressure 30 MPa, and 6 w t.% reinforcement content, possess the yield strength 1547 MPa, fracture strain 26.8%, and bending strength 967 MPa, which are much higher than that of TiAl alloys.