| Particle reinforced metal matrix composites possess high strength, high specificmodulus, high wear resistance and other excellent properties, so they have been widelyapplied in many fields, such as military, aerospace, automotive, machinery, electronic, andso on. However, the disadvantages of the micro-sized particle reinforced metal matrixcomposites are that the ductility decreases when the strength increases, and theimprovement of the high temperature mechanical property is not obvious. These are theunsolved bottleneck problems of the particle reinforced metal matrix composites for manyyears. Some researchers reported that these problems could be solved by making metalmatrix composites reinforced by nano-sized particles. Nevertheless, because of the highspecific surface energy of the nano-sized particle, they are easily agglomerate in the metalmatrix. Meanwhile, the interfacial bond is weak because of the low wettability between thenano-sized particle and the metal matrix, resulting in the easily contaminated surface.Therefore, though the nano-sized particle reinforced metal matrix composites possessexcellent mechanical properties, the composites with homogenously dispersed nano-sizedparticles and good interfacial bonding between the particles and matrix are difficult tofabricate. Simultaneously, the studies on the microstructures and properties of thenano-sized particle reinforced metal matrix composites are rare. Thus, the fabrication of thenano-sized particle reinforced metal matrix composites and influence mechanism of thenano-sized particles on the microstructures and properties need to be further explored andrevealed.Therefore, in this thesis, the Al-Cu alloy and nano-sized TiC particle were chose asmatrix alloy and reinforcement, respectively. The morphology evolution mechanism of thein-situ TiC was revealed by first-principle method. The nano-sized TiC particle reinforcedAl-Cu matrix composites were fabricated by stir casting method, reaction in melt method and master alloy method, respectively. The influence mechanism of the nano-sized particleson the microstructures and mechanical properties of the Al-Cu matrix alloy were revealed.The best manufacturing method to fabricate the nano-sized TiC particle reinforced Al-Cumatrix composite was present.The main results are as follows:1) The relationship between the morphology evolution of the in-situ fcc TiCxparticle andthe stoichiometry x was built, and the critical value0.625of the TiCxmorphologychanging from octahedron to sphere was revealed by using first-principle calculationand experimental proof for the first time. The morphology evolution mechanism of theTiCxwas put forward. When x is0.5, the surface energy of {111} is lower than that of{100}, and then the TiCxmorphology is octahedron. With the increase of x, the {100}surface energy decreases faster than that of the {111} surface. When x>0.625, thesurface energy of {100} is lower than that of {111}, and the morphology of TiCxparticle changes to sphere.2) It is revealed that α-Al dendrites of the nano-sized TiC particle reinforced Al-Cu matrixcomposite were refined, and the strength and ductility were improved:i) The α-Al dendrites of the Al-Cu matrix alloy are coarse with mean size about180μm. The ultimate tensile strength and elongation of the Al-Cu matrix alloy are485MPa and6.6%, respectively.ii) The mean sizes of the α-Al dendrites of the1.0wt.%nano-sized TiC particlereinforced Al-Cu matrix composites fabricated by stir casting method, reaction inmelt method and master alloy method are about150,80and70μm, respectively.The ultimate tensile strength and elongation of the composites are527MPa and14.3%,526MPa and15.1%,572MPa and11.4%, respectively.iii) The mean sizes of the α-Al dendrites of the3.0wt.%nano-sized TiC particlereinforced Al-Cu matrix composites fabricated by stir casting method, reaction inmelt method and master alloy method are about120,50and58μm, respectively.The ultimate tensile strength and elongation of the composites are452MPa and10.1%,521MPa and12.2%,630MPa and10.8%, respectively. iv) The mean sizes of the α-Al dendrites in the composites fabricated by reaction inmelt method and master alloy method are fine, and the tensile strength of thecomposite fabricated by master alloy is high.3) It is put forward that the θ’ precipitations in the nano-sized TiC particle reinforcedAl-Cu matrix composites were refined obviously. The diameter and thickness of the θ’precipitations in the Al-Cu matrix alloy and composites fabricated by stir castingmethod, reation in melt method and master alloy method are194nm,80nm,45nm,70nm and14.24nm,2.25nm,2.55nm,2.12nm, respectively.4) The influence mechanism of the nano-sized TiCxparticles on the α-Al dendrites andtensile properties at room temperature and200oC is revealed. When the size of TiCxparticle is small and the morphology is sphere, the size of α-Al dendrite is fine and thetensile strength at room temperature and220oC are high.i) The α-Al dendrite in the Al-Cu matrix alloy is coarse with mean size of about180μm. The ultimate tensile strength and elongation of the Al-Cu matrix alloy atroom temperature and220oC are485MPa,6.6%and220MPa,10.4%,respectively.ii) The mean sizes of the α-Al dendrite in3.0wt.%120nm and60nm TiCxparticlereinforced Al-Cu matrix composites are70and58μm, respectively. The ultimatetensile strength and elongation at room temperatrue are562MPa,10.9%and630MPa,10.8%, respectively. The ultimate tensile strength and elongation at220oCare300MPa,9.6%and344MPa,6.5%, respectively.iii) The mean sizes of the α-Al dendrite in3.0wt.%spheral and octahedral TiCxparticle reinforced Al-Cu matrix composites are58and65μm, respectively. Theultimate tensile strength and elongation at room temperatrue are630MPa,10.8%and606MPa,6.8%, respectively. The ultimate tensile strength and elongation at220oC are344MPa,6.5%and330MPa,6.7%, respectively.5) It is revealed that the improvement in strength and ductility is suggested to be inducedby the refinement of the α-Al dendrites, more homogeneously distribution of the θ’precipitates with higher diameter/thichness ratio and the exitence of the nano-sized TiC particles inside the α-Al grains. At evalated temperature, the strengthening mechanismis mainly TiC particle strengthening, while the strengthening effect of the θ’ precipitatesdecreases.6) It is put forward that the master alloy method is the best fabrication method bycomparing the microstructures, mechanical properties and technology. The3.0wt.%spheral TiCxparticle with size of60nm reinforced Al-Cu matrix composites possessthe best tensile properties at both room and elevated temperatures. The yield strength,ultimate tensile strength and elongation of the composite at room temperature are374MPa,630MPa and10.8%, increased by23.4%,29.9%and63.6%than those of theAl-Cu matrix alloy, respectively. The ultimate tensile strength at200oC and315oC are344MPa and144MPa, increased by56.4%and51.6%than those of the Al-Cu matrixalloy, respectively.The achievement obtained in this thesis laid the necessarily theoretical andtechnological basis for development of the nano-sized particle reinforced Al matrixcomposites with high strength and ductility at both room and elevated temperatures andtheir fabrication technology. |
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