Research On The Synthesis Of Nanocrystalline Ti(C,N) Powders By Mechanical Alloying And Application In Cermets

In this thesis, the solid state reaction and phase transformation mechanism induced by mechanical alloying (MA) of various systems, such as Ti-C, Ti-N, Ti-C-TiN, etc., have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).In the first part of the thesis, a historic progress of mechanical alloying has been critically overviewed, which includes the basic process and the theoretic model, the influence of technological parameters of ball milling on the appearance, grain size, phase constituent of the products prepared by mechanical alloying, and synthesis of various alloys. Based on the above work, the purpose and significance of the thesis have been pointed out.Mechanical alloying of Ti-C under various milling atmosphere with composition of TiC has been studied. TiC was synthesized by mechanically induced self-propagating reaction under vacuum and Ar. However, TiC was obtained by diffusion-solid solution under H₂. The influence of H₂ on the solid state reaction during milling has been investigated. It is found that H₂ could decrease the reaction rate.The formation of TiN by MA and the effects of subsequent heat treatment on the phase transformation have also been studied. Nanocrystalline TiN was prepared successfully by milling Ti powder in nitrogen. The MA process was controlled by the diffusion of N atom. In addition, the influence of heat treatment on the microstructure, appearance and size of MA products was also investigated. It was found that the degree of order of TiN synthesized by milling for 48h increased with increasing annealing temperature.Based on the above research, the solid state reaction of Ti-C-TiN during MA and the effects of Ni and Mo on the phase transformation have also been studied. Ti(C,N) was prepared successfully by high energy milling. It was found that the formation of Ti(C,N) was controlled by interface diffusion and the alloy elements Ni and Mo could accelerate the reaction.Ti(C,N)-based cermets were produced by vacuum sintering with mechanical alloyed Ti(C,N) with nano-structure being raw materials. The influence of addition of mechanical alloyed Ti(C,N) on the microstructure and properties of Ti(C,N)-based cermets has been studied. It was found that proper addition of mechanical alloyed powder could reduce the sintering temperature and increase the diffusion coefficient, which was advantageous to inhibiting the grains from growing and improving the mechanical properties of Ti(C,N)-based cermets.