| It is well known that magnesium alloys have many excellent and unique properties, such as low density, high specific strength and stiffness, perfect damping capacity and shock energy absorption capacity, easy processing, as well as a good recycling capacity and compatibility with environment, and the resource of the magnesium is very abundant in the nature. Therefore, it is believed that magnesium alloy can be one of the most hopeful metallic materials adopted in the high-tech industry facing 21st century. However, the further development and commercial application of magnesium alloys have been restricted due to their some disadvantages, such as low elastic modulus, poor tensile strength and wear resistance, especially the poor heat resistance. It is one of the effective ways to improve the performance of magnesium alloys by adopting the magnesium matrix composite reinforced by some phase with high melting temperature, hardness, elastic modulus in different shapes, such as fibre, whisker or particulate. Aiming at the above mentioned disadvantages of the current used magnesium alloys, the present study investigated the application of in-situ synthesis process to produce magnesium matrix composites reinforced by particulate such as TiC, MgO or Mg2Si to improve their high temperature tensile strength and creep resistance, and to promote their under high-temperature field of about 200℃.The magnesium matrix composites were produced in a vacuum induction furnace under the protection of Ar atmosphere by adding some element/compounds such as Ti-C-Al powder, SiO2 or Si into melting magnesium alloy. With the aid of analysis and test equipments/technologies, such as OM, SEM, TEM, XRD, EDAX, electronic materials testing machine and high temperature creep testing machine, the production process, the microstructure and its mechanical behavior at different temperatures of those in-situ particulate reinforced magnesium matrix composite were studied. The main results of the present study are as follows:1. The production process, the microstructure and their mechanical behavior of in-situ synthesized TiC, MgO or Mg2Si paniculate reinforced magnesium matrix composites fabricated by three techniques, i.e. exothermic dispersion, direct reaction synthesis and foundry process, respectively, were studied. It was found that the in-situ synthesized Mg^Si reinforced magnesium matrix composite fabricated by casting process has the best industry application prospect among these techniques due to its simple process flow, excellent mechanical properties, especially the high-temperature performance.TiC particles can be obtained by adding directly Ti-C-Al, Ti-C or Ti-C-Mg powder into the melting magnesium alloy. It was found that the chemical reaction between the adding powder and the matrix alloy was too drastic and the in-situ synthetical TiC particles were sintered badly, and therefore, it is almost impossible to produce a magnesium matrix composite with uniformly distributed TiC strengthening particles. Then, the two-step process was used, i.e., the TiCp/Al pre-fabricating alloy was firstly produced, and then this pre-fabricating alloy was put into the melting magnesium alloy directly. In this way, the TiCp/Mg composite with uniformly distributed TiC particles was obtained successfully. The melting mechanism of TiCp/Al pre-fabricating alloy into melting magnesium alloy can be explained as follows: the melted Al diffuses into melting Mg directly; with the help of shear force of melting alloy flow aroused by mechanical stirring, the cohesive force among TiC particles will be destroyed, and therefore, the particles can easily distribute uniformly into the melting magnesium. The interface between TiC particle and ZM5 matrix is clean, and no obvious interface was found. Comparing to the ZM5 alloy, the tensile strength, yield strength and hardness of the composite are improved due to the addition of TiC particles, but its elongation decreases.The in-situ synthesized MgO particulate reinforced magnesium matrix composite fabricated by direct reaction synthesis were studied.The thermodynamic calculation shows that the additives such as SiO2, B2O3 and AI2O3, can all have the in-situ reaction with the molten magnesiumand create the reinforcement of MgO particles to reinforce the composite. But the dynamic study shows that within the melting temperature region of magnesium only the SiO2 can easily react with magnesium and create MgO. It's almost impossible to get MgO particles by adopting A12O3 or B2O3 as the reactant additives. Most of the MgO particles resulted from the reaction between SiO2 and magnesium were aggraded in the bottom of the crucible, and therefore, the mechanical stirring should be used to obtain the uniformly distributed MgO particles in the matrix. And in this way, the mechanical behavior, especially the high-temperature strength of magnesium alloy can be improved evidently.Foundry process was used to obtain magnesium matrix composite reinforced by in-situ synthesized Mg2Si reinforcement by adding Si directly into the melting magnesium. The mechanical behavior, especially the high-temperature strength of magnesium alloy can be improved evidently. This process, compared to the exothermic dispersion process to produce TiC/Mg composite and the direct reaction synthesis process to produce MgO/Mg composite, has the most simple technologic flow and need not additional mechanical stirring technique.2. The in-situ synthesized Mg2Si reinforcements existed in two modalities: eutectic Mg2Si shaped like Chinese-script, and the primary Mg2Si shaped in bulk or dendrite. The morphology of the Mg2Si in Mg2Si/ZM5 composite was affected by the content of Si. It appears mainly eutectic Chinese-script shape when Si content is less than 1.0%; and becomes the mixture of eutectic Chinese-script and primary bulk when the Si content is around 1.5%; when the Si content reaches 2.0% or higher, it forms dendritic primary shape and eutectic Chinese-script shape, and the higher the Si content, the more integrated of the dendritic structure. It is found using SEM that the liquid-solid interface structure of the Mg2Si phase is a facet interface, and its growth mechanism is step-mode.Under the solid-solution treatment condition, the Mg2Si phase shows a favorable thermal stability and hard to be soluted into the matrix. During the aging process, similar to the case in ZM5 alloy, the incoherentequilibrium phase of y directly precipitates from the supersaturated solid solution in Mg2Si/ZM5 composite without any intermediate process. The y phase precipitates in two forms, i.e. continuous precipitation and discontinuous precipitation. The discontinuous precipitated phase forms in the grain boundary, and grows into the inside of the grain. Mg2Si decreases the amount of the discontinuous precipitated phase, and conduces the increase of the amount of continuous precipitated phase. It is widely accepted that the properties of the continuous precipitated phase is much better than that of the discontinuous precipitated phase, and therefore, the appearance of Mg2Si phase in the composite is in favor of improving its properties. The TEM analysis shows that the precipitated y phase does not contain element Si.Under as-cast and T6 treatment states, the strengths of the Mg2Si/ZM5 composite both at room temperature and elevated temperature (200°C) are higher than those of ZM5 alloy. The strengths of the Mg2Si/ZM5 composite with 1.5%Si content has the highest value: under the as-cast state, the elevated-temperature (200 °C) yield strength is increased by 25.7% than that of ZM5 alloy; and under the T6 treatment state, it increases by 13.4%. But the elongation in both states all decreases. The creep properties of Mg2Si/ZM5 composite are much better than those of ZM5 alloy. Under the loading conditions of 175°C, 50Mpa, the creep properties of the composite reach the highest values, for example, the creep velocity of the material decreases evidently and barely corresponds 13.6% of the value of the ZM5, and its creep life improved obviously and reaches 171.2 hours, which is 5.84 times higher than that of ZM5.The control mechanism of the creep in ZM5 alloy is dislocation climb; and in Mg2Si/ZM5 composite is the second-phase particle strengthening mechanism.3. Modification of Mg2Si/ZM5 composite was studied.lt was found that the addition of Sr/Ca has an evident modification function on the Mg2Si phase, and with certain Sr or Ca contents in the composite, the size of the Mg2Si particles was considerably reduced and its shape was changed from a rough dendridic, bulk or Chinese-script shape to a fine,multi-angle granular shape. There is an appropriate range of content of Sr or Ca. As a surface activator, Sr or Ca can easily absorb on the surface of the Mg2Si phase. Sr probably can enter the Mg2Si phase and ameliorate its shape. A small amount of Ca can dissolve in Mgi7Ali2 phase, and slows down the dissolution rate of Mgi7Ali2 phase into the matrix during the solid-solution process and extends the dissolution time. During the aging process, the addition of Ca can also restrain the precipitation of the discontinuous aging phase. The addition of Sr or Ca can improve evidently the mechanical properties of the composite at ambient and elevate temperatures, especially at elevated temperature, the elongation can also be improved as well as the tensile strength. The size of the Mg2Si particle modified by 0.1 %Ca is smaller than that by 0.35%Sr, and the size of Mg2Si particles modified by 0.1%Ca+0.35%Sr is appreciably bigger than that modified by separate addition of Sr or Ca. The tensile elongation both at room temperature and elevated temperature of the composite modified by Sr is higher than that modified by Ca lonely or by Ca+Sr together, but the yield strength is lower.
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