Regulation Of ?/? Phase Transformation In Ti Matrix And Its Effect On Ti-B₄C System Diffusion Reaction

By studying the accelerated diffusion phenomenon of the titanium matrix around the ?-transition temperature(?t),it is expected to realize the low-temperature reaction sintering of the Ti-B4C system to achieve fine structure and performance control.However,in order to control microstructure characters,such as the morphology,size and amount of reinforcements,and thus adjust mechanical properties of(TiCp+TiBw)/Ti composites,the accelerated diffusion mechanisms and in-situ reaction behavior of Ti-B4C powder system by solid-state sintering near[3t of Ti matrix must be further clarified.In this thesis,the O element is added to pure titanium to adjust the ?-transition temperature,and Ti-B4C diffusion reaction couples are prepared near the corresponding ?-transition temperature.The product characteristics and the thickness of the diffusion layer are determined by structure characterization.The element diffusion behavior of the reaction process is studied by establishing a kinetic model.Finally,the reliability of theoretical results is verified by preparing composite materials and examining their organization and mechanical properties,which provides a theoretical basis for the research and application of high-performance(TiCp+TiBw)/Ti composite materials.The main conclusions are as follows:1.Ti-2wt%O solid solutions were prepared by powder metallurgy method using pure Ti powder and TiO2 powder as raw materials.The optimal sintering process parameters for the decomposition of TiO2 and the solid solution of oxygen into the Ti matrix are determined by conducting density tests,X-ray diffraction(XRD)analysis,and microhardness tests on the samples prepared at different sintering temperatures.2.The metallographic method was used to test the ?/? transition temperature of the Ti-O solid solutions with 0,0.25 and 0.5wt%O content.The relationship between the phase transition temperature of the solid solutions and the content of O element is determined.Compared with the phase transition temperature obtained by theoretical calculation based on elemental analysis,the phase transition temperature obtained by the metallographic test method is more accurate than the theoretical calculation,and the phase transition temperatures are determined to be 990?,1070?,and 1140?,respectively.3.Ti-B4C diffusion couple samples were prepared using Ti,Ti-0.25wt%O and Ti-0.5wt%O solid solutions as the matrix,respectively,and spark plasma sintering(SPS)at four different temperatures near the phase transition temperature,and the samples were isothermal heat-treated.The thickness of the TiB2 layer in the diffusion reaction layer is studied by microstructure characterization,and the influence of temperature on the diffusion ability of B and C atoms in Ti is clarified.The results show that the thickness of the TiB2 layer at the ?/? transition temperature is higher than other temperatures,including the higher sintering temperature in the ? phase region.It is confirmed that the ?/? phase transition of the titanium matrix itself can accelerate the atomic diffusion behavior.4.The exponential empirical formula was used to calculate the relationship between the thickness of the diffusion layer and the diffusion reaction time.The n index of the reaction diffusion mechanism is less than 0.5.It shows that the Ti-B4C reaction is diffusion-controlled at all temperatures,which confirms that this paper can use the abnormal self-diffusion behavior of the titanium matrix to accelerate the in-situ reaction of the Ti-B4C system.5.The mechanical properties of(TiCp+TiBw)/Ti composite material with Ti-0.25wt%O solid solution as matrix are studied.The results show that the size of the reinforced phase,especially TiB whiskers,generated by the reaction of the sintered samples at the phase transition temperature is significantly larger than other temperatures.It is even larger than the whiskers of composites prepared above the phase transition temperature.Compared with Ti-0.25wt%O solid solution the hardness of(TiCp+TiBw)/Ti composites increases significantly,which is mainly due to the addition of the reinforcing phase.The bending strength of the composite material prepared by sintering at the phase transition temperature of the titanium matrix is lower than that of the same composition prepared by sintering at other temperatures.The cross-sectional analysis results show that the decrease in bending strength is caused by the preferential cracking of the interface between the course TiB and the titanium matrix during the bending process.