The Morphology Evolution Mechanism Of The In-situ TiC_x Ceramic Particles Fabricated By Combustion Synthesis In Al-Ti-C-(M) Systems

With the development of industry, the property requirement of metal materials is higher and higher. Hard particle reinforced metal matrix composites have been widely applied due to their high strength, high hardness and wear resistance as well as ease of casting work pieces with complicated structures and forms at low cost. The ceramic particle reinforced metal matrix composites are the most significant hard particle reinforced metal matrix composites. However, different morphologies have a diffident impact on the strengthening effect of the metal matrix composites. For example, the round ceramic particles have less stress concentration in the matrix metal while the non-round ceramic particles result in stress concentration at the tips of the particles. TiC_x possesses many advantages such as high melting point, high strength and elastic modulus, good thermodynamic stability and ease of fabrication by in situ methods. Therefore, the study on the morphologies of TiC_x ceramic particles has great value in theory and practical significance.Therefore, in this thesis, the Al-Ti-C system was studied. The influence rule was studied by the experimental method, which was on the morphology of the in-situ TiC_x particle by the stoichiometry x(0.5, 0.625, 0.75, 0.875 and 1.0) and the addition of different alloy elements to the Al-Ti-C system when x was 1.0. The influence of different stoichiometry x on the interfacial energy of {100} and {111} of TiC_x, as well as the influence of the addition of different alloy elements on the interfacial energy of TiC_x/α-Al,were studied using first-principle calculation. The influences of different stoichiometry x ofTiC_x and the addition of different alloy elements on the interfacial energy of TiC_x/α-Al, the interface structure and the density of states were revealed. The growth mechanism of in-situ TiC_x ceramic particle was revealed combining experiments when the stoichiometry x of TiC_x and the addition of alloy elements were changed.1) It is revealed that in the 30 wt.% Al-Ti-C system, the morphology evolution regularity of the in-situ TiC_x ceramic particle with the increase of the stoichiometry x from 0.5 to1.0 was suggested as from octahedron to truncated-octahedron and near-sphere. The change rule of the exposed crystal surface with the increasing stoichiometry was suggested as from {111} to {100} and {111} existing simultaneously, and finally to{100} existing alone.2) The morphology evolution regularity of the in-situ TiC_x ceramic particle was revealed with the addition of Cu, Mn, Si, Mg and Zn to the 30 wt.% Al-Ti-C system when x was1.0. The addition of Cu could make the morphology of TiC_x change from near-sphere to sphere and truncated-octahedron existing simultaneously, and the proportion of exposed crystal surface {111} increased. The addition of Mn could promote the spheroidization of TiC_x and the proportion of exposed crystal surface {100} increased.The addition of Si could further promote the spheroidization of TiC_x and produce perfect sphere, and the proportion of exposed crystal surface {100} increased further.However, the addition of Mg and Zn had little effect on the morphology of the in-situ TiC_x particles.3) The change rule of interfacial energy of different interface between TiC_x and α-Al with the increasing stoichiometry x from 0.5 to 1.0 in the 30 wt.% Al-Ti-C system was revealed. In the 30 wt.% Al-Ti-C system when x was 1.0, most of Cu, Mn and Si distributed in the interface between TiC_x and α-Al, while Mg and Zn tended to distribute in the melt.i) With the increasing stoichiometry x from 0.5 to 1.0, the hybrid effect of C-Ti in the TiC_x system and the crystal surface stability of {100} and {111} enhanced. The electron cloud interaction of Al-C atoms and Al-Ti atoms as well as the hybrid effect of electrons weakened in the interface between C-site and Ti-site in {111} of TiC_x/Al, leading to the instability of the corresponding interface. In the interface of C-site in {111} of TiC_x/Al, the atom interaction of Al-C and interface stability showed little change. Therefore, with the increasing stoichiometry x from 0.5 to1.0, the interfacial energy of TiC_x/α-Al {100} decreased, while the interfacial energy of TiC_x/α-Al {111} increased.ii) In the 30 wt.% Al-Ti-C system when x was 1.0, the addition of Cu could make the Al2p-Ti3 d hybrid effect in the interface of Ti C/Al {111} Ti-site strengthen and the corresponding interfacial energy decrease. The interface stability could also enhance. The addition of Mn could interact with C atoms in the interface of Ti C/Al {111} C-site intensely and the Mn3d-C2 p hybrid effect would happen. Mn atoms were adsorbed above the C atoms in the interface, which made the interfacial stability strengthen, while hampered the combination between Ti and C during the growth of Ti C. The addition of Si could interact with Ti and C, causing Sils-C2 p and Si2p-Ti3 d hybrid effects, respectively. Si atoms were adsorbed above the corresponding atoms, which made the interfacial stability of Ti C/Al {111}strengthen, while hampered the combination between Ti and C.4) The morphology evolution mechanism of the in-situ TiC_x ceramic particle was revealed with the increasing stoichiometry x from 0.5 to 1.0 in the 30 wt.% Al-Ti-C system and the addition of Cu, Mn, Si, Mg and Zn to the 30 wt.% Al-Ti-C system when x was 1.0.i) With the increasing stoichiometry x from 0.5 to 1.0, the interfacial energy of TiC_x/α-Al {100} C-site decreased and the interface became more and more stable.However, the interfacial energy of TiC_x/α-Al {111} increased and the interface became more and more unstable. As a result, the exposed crystal surface of the in-situ TiC_x ceramic particles changed from {111} to {100} and {111} existing simultaneously, and finally to {100} existing alone. The morphology of TiC_x changed from octahedron to truncated-octahedron, and finally to near-sphere and sphere.ii) The addition of Cu could make the interfacial energy of Ti C/Al {111} decrease, therefore, the {111} of TiC_x grew preferentially, and finally the morphology of the in-situ ceramic particles changed from sphere and near-sphere to truncated-octahedron. The addition of Mn made the interfacial energy decrease and the interface became more stable. The Mn atoms were adsorbed above the C atoms in the interface, which hampered the further growth of {111}. As a result,the {100} grew and exposed preferentially, leading to the more obvious spheroidization of in-situ ceramic particles. The addition of Si had little effect on the interfacial energy of Ti C/Al {111}, however, Si atoms could distribute in the interface and were adsorbed above the Ti atoms and C atoms, which hampered the growth of {111} intensively. As a result, the {100} grew and exposed preferentially, leading to the more obvious spheroidization of in-situ ceramic particles. The Mg and Zn atoms distributed in the melt and had little effect on the morphology of the in-situ TiC_x particles by the growth interface.