| Summary: | 博士 === 國立交通大學 === 材料科學與工程學研究所 === 85 === The main objective of this dissertation is to study the processing and mechanical behavior of titanium matrix in-situ composites produced by combustion- assisted synthesis. Both systems of TiC/Ti and TiB/Ti were successfully fabricated by combustion-assisted synthesis which consisted of exothermic reactions and vacuum arc remelting. The effects of thermomechanical process (TMP) by hot-swaging on microstructure were explored. In addition to evaluation of tensile and compressive properties, fracture behavior was characterized by in-situ SEM observations and quantitative analyses of fracture surface from 298 K to 1022 K. Creep and fatigue crack propagation tests were carried out according to ASTM specification. The results in this study were summarized as follows：
Both TiC/Ti and TiB/Ti composite systems with 5 to 20 vol. % in-situ reinforcements demonstrated a superior strength and ductility. This was attributed to the clean interface and good bonding strength of reinforcements as revealed by HRTEM observations and analyses of fracture behavior. The particle size of reinforcement increased with increasing volume fraction of reinforcement since the adiabatic temperature, the raised temperature for products due to heat of chemical reaction under adiabatic condition, increased with increasing volume fraction of reinforcement for both TiC/Ti and TiB/Ti systems. A linear relationship between interparticle distance (X) and reduction ratio of hot-swaging (R) for 10% TiC/Ti was established and expressed by
λ(μm)＝18.57-0.21R (R≧4)
Due to the refinement of TiC and the decrease of interparticle distance by TMP, tensile strength (σ) at both ambient and elevated temperatures can be significantly enhanced. The relation of σ in proportional to l/λ indicated that the strengthening mechanism of titanium matrix in-situ reinforced composite at room temperature followed the Orowan mechanism. Based on the in-silu SEM observations, the tensile fracture process at room temperature for TiC/Ti composite consisted few stages, including reinforcement protrusion, cracking in small particles, cracking in larger particles and coalescence of large cracks. There is an obvious transition temperature at 645 K for fracture mechanism being changed from particle cracking to interparticle voiding. The creep mechanism of present titanium matrix in-situ composites, which was proved to be diffusion controlled dislocation climb, was identical to that of Ti matrix. All creep data of pure Ti and composites were merged together, which had the true stress exponent of 4.2 after compensating effects of the modulus, threshold stress and particle size. A constitution equation of creep for present Ti-based in-situ composites was proposed as
The fatigue crack growth rate (FCGR) of titanium matrix in-situ composite at 723 K was approximately one order lower than that at 298 K for same applied stress intensity range under same stress ratio of R=O.1, showing a disparity tendency in comparison with matrix metal. All data of FCGR merged very well after crack closure correction. The difference in fatigue crack growth behavior at room and elevated temperatures was interpreted by crack closure. The mechanism of microcrack-induced crack closure was proposed to account for improvement of fatigue crack growth resistance at elevated temperatures.
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