| By promoting the unique synergistic effect between the light,soft and tough aluminum matrix as well as the high melting point and high hardness ceramic reinforcement phases,the aluminum alloys under the manipulation of ceramic particles could have an optimized solidification microstructure and a good balance of strength and ductility.Herein,ceramic particles with excellent size and morphology can not only reduce the lattice mismatch in the matrix but also facilitate the heterogeneous nucleation of the matrix aluminum alloy.At the same time,better interfacial bonding between the ceramic particles and the matrix can be obtained.By controlling the spatial distribution of ceramic particles in the matrix,the microstructure can be further optimized,so that the matrix alloy can obtain better mechanical properties.Therefore,based on the inherent characteristics of ceramic particles,designing and controlling the synthesis of ceramic particles with specific size and exposed crystal surface shows unique innovation and practical value.In this thesis,inspired by the synthesis of in-situ ceramic particles,we explored and revealed the synthesis and morphology control mechanism of ceramic particles in ternary Al-Ti-C,Al-Ti-B systems and quaternary Al-Ti-B4C-BN system.By reasonable optimization and manipulation of the combustion synthesis reaction systems,TiC,TiB2 and TiCx Ny-TiB2 ceramic particles with controllable size and morphology were successfully prepared.Then we selected the optimized ceramic particles and dispersed into pure aluminum,Al-Si-Mg and Al-Si-Cu-(Mg)alloys.The solidification behavior and solidification microstructure under the combined effects of various factors such as particle type,particle addition amount,cooling rate and alloy composition were systematically investigated and revealed.Finally,the strength-ductility synergy optimization of aluminum alloy was realized.The main innovations of this thesis are as follows:(1)A control strategy was proposed to predict and manipulate the growth of insitu TiC particles.The synthesis mechanism of TiC nanoparticles was revealed: TiC inspired from the instantaneous endothermic reaction between the reaction intermediates Al3 Ti and Al4C3;TiC nanoparticle morphology transformation mechanism was revealed by reasonably controlling the interface properties between Al and TiC particles: with the increase of C/Ti ratio in the reaction system,the TiC(111)surface became weaken,and the particles morphology changed from octahedron to tangent-octahedron and near-spherical.The doping of Mg and Zn in the system did not directly affect the Al/TiC interface,and the synthesized TiC particles were octahedral.Cu atom enhanced the stability of Al/TiC(111)interface and TiC transformed into octahedron.Doped with Mn and Si atoms,the Al/TiC(111)interface stability decreased,and TiC particles evolved into near-spherical.The characteristics and common rules of the morphology transformation of TiCx under stoichiometric ratio x and alloying element doped were revealed: it was found that TiC growth was mainly controlled by C deficiency,stoichiometric ratio x directly affected C deficiency,while alloying elements doped affected the dissolution and diffusion of C atoms by changing the melt environment,and then affected the stability of specific crystal planes.(2)An in-situ reaction synthesis strategy of nano/submicron TiB2 particles with controllable size and morphology was proposed.The reaction mechanism of the formation of TiB2 particles from the Al-Ti-B reaction system was revealed: Al3 Ti firstly formed and released heat,then B began to react with Al3 Ti until Al3 Ti was depleted.TiB2 was the final stable end product.The evolution mechanism of size and morphology of TiB2 particle was revealed.With the increase of synthesized TiB2 amount,the residual amount of Al3 Ti increased and the TiB2 particle size became larger and the morphology was typical hexagonal prism;It was found that when the ratio of B/Ti atoms exceeded 2.4,there was no residual Al-Ti compound in the product.The size of TiB2 nanoparticles decreased slightly while the morphology changed into polyhedral and near-spherical;B prevented the growth of TiB2 particles by adopting on the specific crystal surfaces,which further resulted in the exposure of high index crystal surfaces such as(1101),(1120),(1212).And the morphology of TiB2 particles changed from hexagonal prism to polyhedron or near-spherical.(3)A method for the design and preparation of dual phases nano/submicron hybrid TiCx Ny-TiB2 particles with controlled size and morphology was proposed.The evolution mechanism of size and morphology were revealed: when the ratio of B4 C : BN : Ti increased to 2.8 : 2.8 : 9.0,the products were nearly spherical TiCx Ny nanoparticles and TiB2 submicron particles with hexagonal prismatic morphology,without Al3 Ti interphase residual.With the decrease of Al content in the system,Al3 Ti decreased and disappeared,while the average particle size increased significantly.Most TiB2 particles were hexagonal prisms,a few of them were cubic or polygonal prisms with clear edges,and TiCx Ny particles were more rounded and spherical;The synthesis mechanism of TiCx Ny-TiB2 particles in Al-Ti-B4C-BN system was revealed: the reaction mechanism of the synthesis of TiCx Ny-TiB2 particles was revealed: Al3 Ti was formed firstly and released a large amount of heat,which promoted the gradual dissolution of all reactants and formed Al-Ti-B-C-N melt.When the intermediate product was completely consumed,the final products were only TiCx Ny,TiB2 and a small amount of Al N.(4)The manipulation mechanism the solidification microstructure of pure aluminum and aluminum alloy by micron/nano-sized ceramic particles was revealed.It was found that the effect of refining pure aluminum ranked from strong to weak: nano/submicron-TiCx Ny-TiB2 > nano/submicron-TiB2 >(111)crystal plane exposed octahedron TiC0.5 and Cu-doped TiC >(100)crystal plane exposed TiC1.0 and Mn-doped TiC nanoparticles;Under the same adding level,nano/submicron-sized TiCx Ny-TiB2 particles had the best ability than submicron/micron-TiB2 or submicron/nano-TiB2 particles to refine the solidification microstructure of pure aluminum and Al-Si alloy: The pure aluminum changed from columnar crystal to fine equiaxial grain.Also,the α-Al dendrites,secondary dendrite arm spacing,eutectic silicon length and length-to-diameter ratio,and precipitate-phase size in Al-Si alloy were significantly reduced;It was found that the addition of ceramic particles could effectively manipulate the solidification behavior of pure aluminum and AlSi alloy: micron/submicron particles would significantly increase the nucleation temperature,reduce the degree of recalescence supercooling,shorten the nucleation and growth time of α-Al and eutectic Si;The effect of the spatial distribution of ceramic particles on the solidification microstructure of pure Al and Al-Si alloys was revealed.It was found that most of the nanoparticles would be pushed by the interface and eventually distributed around the dendrites,while the micron-sized particles at the front of the solid/liquid interface were more easily engulfed by the interface and finally distributed inside the dendrites.It was found that the nanoparticles at the front of the solid-liquid interface can hinder the migration rate of solute atoms,thus restrict the growth of dendrites.At the same time,the nucleationfree zone in front of the growing dendrites can be reduced before the occurrence of recalescence,and the heterogeneous nucleation rate then can be increased.It was found that the sensitivity of the solidification microstructure of the matrix alloy to the nanoparticles decreased with the increase of cooling rate.The increase of cooling rate changed the spatial distribution of ceramic particles: at a slower cooling rate,most of the nanoparticles were pushed by the solid/liquid interface during solidification and finally distributed at the grain boundary,while the increase of cooling rate leaded to part of particles being engulfed by the solid/liquid interface. |
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