| In this dissertation, Vacuum-Gas Pressure Infiltration process is developed for fabrication of discontinuous reinforced Metal Matrix Composites and the whiskers preform forming technique by wet forming method without any binder is also developed. The micrographs reveal reasonably uniform distribution and random orientation of the Mg2B2O5 whiskers in the as cast Mg2B2O5w/AZ91D composite fabricated by Vacuum-Gas Pressure Infiltration process. The micographs of the composites show that there are no macro and micro pores defect in the composites. Meanwhile the SEM micrographs show that micro structure of the Mg2B2O5w/AZ91D composite is very fine structure. The test results show that the hardness, elastic modulus, tensile strength and proof strengthσ0.2 of the composite are increased significantly compared to that of the magnesium matrix alloys. The thermal expansion coefficient of the composite is also reduced obviously. However, the elongation to failure of the composite is decreased because of the addition of the hard brittle ceramic whiskers into the Mg alloy matrix. It indicates that the addition of Mg2B2O5 whisker causes the AZ91D magnesium matrix to be strengthened and the dimension stability at elevated temperature of the matrix alloy is improved.The micro structure and interface characterization of Mg2B2O5 whisker reinforced magnesium composite are investigated using optical microscopy, transmission electron microscopy and X-ray diffraction. It is found that the Mg2B2O5 whiskers have a twinned structure with the (202) as the twin plane and [010] as the growth direction. The MgB4O7 particles and the globular Mg2Si particles are observed within the Mg2B2O5 whisker, and at the grain boundary between the Mg2B2O5 whiskers and Mg matrix of the magnesium composite, respectively. The MgO and MgB2 phase are formed at the matrix-whisker interface during vacuum-gas pressure infiltration process, which result from the interfacial reaction between Mg2B2O5 whisker and Mg matrix. The crystallographic orientation relationships between the Mg2B2O5 whisker and the interfacial reaction products are found to be [010]Mg2B2O5//[110]MgO, [110]MgO// [2110]MgB2, (202)Mg2B2O5//(002)MgO and (002)MgO//(0001)MgB2, respectively. Based on the surface structure analysis of the Mg2B2O5 whisker, it is induced that interface bonding between Mg alloy matrix and Mg2B2O5 whisker may be directly through Mg-O bonding (Mg from Mg matrix, O from the surface of Mg2B2O5 whisker). Therefore, the bonding ways of the Mg2B2O5w/AZ91D composite interface probably are two and/or more bonding forms.The tensile properties of the composite at elevated temperature are also tested. It is found that the elastic modulus, tensile strength and proof strengthσ0.2 of the composite are decreased when the temperature is elevated. The results show also that the slops of the curves tensile strength and proof strengthσ0.2 of the composite vs. temperature are steeper after 250℃than that of below 250℃. On the other hand, the elongation to failure of the composite is increased when the temperature is elevated. It indicates that the composite is very sensitivity to the performance temperature.The microstructure evolution of the composite is investigated when the composite is heat treated. The relationships between microstructure and micro-hardness of the composites heat-treated are also studied. The results show that the micro-hardness of the solution treated composites is decreased due to resolution of eutectic phase, whereas the micro-hardness of the aged composites is increased gradually and the peak hardness is reached to 201 HV in the composite sample aged for 16 hours. Solution treatment at 415℃for 24h,β-Mg17Al12 phase is dissolved intoα-Mg matrix forming supersaturated solid-solution in the composite and then the dispersedβ-Mg17Al12 phase precipitates after subsequent aging treatment at 200℃for 8 hours, and microhardness of the composite increases 30%. However, as the aging time increases to 24h, hardness of the composite is reduced to 183 HV because the fine platelikeβ-Mg17Al12 precipitates grow to discontinuous coarse phase.The liquid Paraffin-lubricated sliding tribological properties and wear behavior of the composites are investigated using a pin-on-disc wear-testing machine against a GCr15 steel counterface under loads of 5-40 N, and within a sliding velocity range of 0.5-5.0 m/s for a constant sliding distance of 2 km. The transition load is found for a constant sliding velocity. The wear rate increases sharply when the loads are higher than the transition load. The composites exhibit superior wear resistance compared with that of matrix alloy when the loads are below the transition load. The transition loads that are called critical loads in some literatures are 25,18,10,5 N for velocity of 1,2,3,5 m/s, respectively. It is observed that the abrasive and delamination are the dominant wear mechanism for loads above and below critical loads using SEM analysis of the worn surfaces and the subsurface of the composites. It is found that the crossover from abrasive wear mechanism to delamination wear mechanism of the composite is related not only with the loads but with the sliding speeds as well. The dry sliding tribological properties and wear behavior of the composites are also studied. The results show that the Mg matrix alloys exhibit superior wear resistance compared to that of composites at the range of loads.The effect of varied heat treatments on the "thermal ratcheting" phenomenon of the composite combined with the thermal cycling curves is investigated. It is found that composites after varied heat treatments can effectively alleviate the "thermal ratcheting" phenomenon in the composite, which take place during thermal cycling between 30℃and 400℃, although the phenomenon occur at all of the situations. The results show that "thermal ratcheting" phenomenon is most serious in as-cast composites, and followed in the composites after annealing treatment, then in the composites after aging treatment, the quenched composite is the best one for alleviating the "thermal ratcheting" phenomenon when the composites is subjected to thermal cycling.The effect of the thermal cycle times on the mechanical properties of the Mg2B2O5w/AZ91D composite under thermal cycling is also studied. It is found that the mechanical properties of the composites will be degraded after varied thermal cycle times. Meanwhile, with the increase of the thermal cycle times, the mechanical properties of the composites degrade more and more serious. It is observed from SEM tensile fractographs of the composites that the interface between whisker and matrix are damaged, or even debonding after thermal cycling, which makes the mechanical properties of the composites degradation. Moreover, the observations show that when thermal cycle times are increased, whisker breakage are more and more serious in the fracture section, which reveals the reasons for the degradation in mechanical properties of the composites.Thermal fatigue crack propagation morphology of the composites are observed under thermal cycling condition between 30℃and 350℃. The mechanism of initiation and propagation of thermal fatigue crack are investigated. The results show that the thermal fatigue crack is related to thermal cycle times and the micro structure of the composites.Three-dimensional finite element model has been constructed to predict the behavior of the composites under external mechnical loads and thermal cycling. The different possible types of the composite behaviors are discussed in detail. Finally, Composite Behavior Maps, which are diagrams which delineate regions of dimensionally stable and unstable composite behavior, for the composites are constructed.
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