Stability Research Of The TiC Reinforcement In Al-Si Melts

Pistons made of Al-Si alloys are one of the most important components in an engine,working in high temperature and loading condition.With the development of high power density engines,the traditional Al-Si alloys are hard to meet the requirements of harsh working conditions.Ceramic particles reinforced aluminum matrix composites are one of the most promising next generation materials for pistons.Titanium carbide(TiC)has excellent properties as reinforcement,but the decomposition of TiC in Al-Si melts restricts its wide application.Therefore,it is primary for its application to reveal the under-lying mechanism of the decomposition of TiC in Al-Si melts.The decomposition process and mechanism based on experiments and the first principle calculation were studied in this thesis.Ex situ method was employed to fabricate the particles reinforced aluminum matrix composites with high percent of TiC,to characterize the whole process conveniently.The scanning electronic microscope,energy dispersive spectrometer and X-ray diffractometer were used to characterize the constitution and microstructures of materials.Based on the density functional theory,the lattice parameter was determined by calculation and measurement.The formation energy of a C vacancy was obtained,and diffusion barrier of Ti in TiCx and the effect of Si on the diffusion barrier were analyzed.The formation of TiAlSi and the effect of Si on the shape of TiAlSi were studied by comparing TiAlSi and Al3Ti.A model was obtained that describes the decomposition of TiCx in Al-Si melts,and a method was found to stabilize the TiCx in Al-Si melts.The main conclusions of the thesis are as followed:TiC could be stable in the pure Al melts,yet Si lowered its stability and it decomposed into TiAlSi and Al4C3.Firstly,TiCx particles were surrounded by TiAlSi,forming the core-shell structure,and then the shape of TiAlSi became to long flake and TiCx particles disappeared with holding time increasing.The lattice parameters of TiC obtained by calculation and experiment are consistent,determined as 4.33A.The C vacancy is easy to form due to the low formation energy.The formation energy of Si is lower than that of Al,implying that Si is easier to take up a C vacancy,influencing the properties of TiCx.The electronic density difference and Bader charge analysis indicates that the bond between Si and Ti is weaker than that between C and Ti,which results in the decrease of Ti diffusion barrier,accelerating the diffusion and immigration of Ti along the tube made of C vacancies,lowering the stability of TiC and causing the decomposition of TiC.TiAlSi formed from some Al sites of Al3Ti being taken by Si,and it indicates,from both XRD patterns and simulation,that the peaks of Ti(Al1-xSix)3 shift to higher angles comparison to the peaks of Al3Ti.Si would lower the energy of(110)in Al3Ti,which makes the TiAlSi growing as flake.B,N and O can take amounts of C vacancies from the calculation of formation energy,which blocks the diffusion of Ti and prevent TiC from decomposition.Al-12Si-3TiC-0.9B was successfully fabricated,confirming the protective effects of B.