| With the entrance to 21st century the resource and environment problemshave become the principle problems in the sustainable development of humanbeings. Therefore in the long term, the trends of the industrial development allover the world can be reduced to the saving of energy and resource, what is more,the pursuit of natural production. Magnesium alloys are kind of the lighteststructural metal alloys, which have many excellent and unique properties, such ashigh specific strength and stiffness, high heat and electrical conductivity, gooddamping and shock absorption, excellent electromagnet shielding capability andeasy processing, as well as a good recovery capability, and therefore, increasingattention has been paid to magnesium and its alloys in automotive, communicationapparatus and aerospace applications. Consequently, it is considered thatmagnesium alloys are the most worthy and exploitative "natural engineeringmaterials"with greatly potential application in this century.With the development of the lightweight automotive and aerospace industries,the application of magnesium alloys depends mainly on their high tensile strengthat the ambient and elevated temperature, erosion resistance, wear resistance and onsuperior machinability. Currently, however, each of the most widely usedmagnesium alloys has its own advantages and limitations, which limit itsapplication to the above areas of advanced technology. For example, the tensilestrength, die-castability and the erosion resistance of the AZ91D magnesium alloyis excellent, but creep resistant properties are poor; the creep resistance of theAS41B magnesium alloy is higher, while both the tensile strength and hardness arelower. Conventional alloying technology do not meets the demands for higherproperties of magnesium alloys. It is interesting to fabrication of magnesiummatrix composites with an optimum set of mechanical properties, such as highstrength, modulus, and thermal stability. According to the morphological feature of reinforcement, magnesium MMCscan be grouped under three heads, i.e. fiber, whisker and particulate reinforcedmagnesium matrix composites. The strengthening effect of fibre reinforcedcomposites is evident along the fiber direction, attracting more extensive interest;however, the fabrication processing is very complex and the production can not bereutilized. Although, the strength of whisker reinforced magnesium MMCs is veryhigh and the production with excellent fabricating and reprocessing properties canbe reutilized, the range of application of the composite has been limited due to ahigh cost starting material. Recently, the particulate reinforced magnesium matrixcomposites have been becoming one of the hotspot in the research field ofmagnesium MMCs due to its significant inherent simplicity and potentialcost-effectiveness in scale-up manufacturing, as well as its isotropy properties. Presently, the particulate reinforced magnesium MMCs are mainly fabricatedby ex situ manner. In this case, the size of the reinforcement is large and irregular,which easily lead to the local stress concentration, and to gas entrapment. Othermain drawbacks that have to be overcome are the interfacial reactions between thereinforcement and the matrix, and poor wettability between the reinforcement andthe matrix due to surface contamination of the reinforcements. Therefore, ceramicparticles are notoriously difficult to add directly to metal melts. In order to improve ofwettability, some processes, such as preheating, degassing in vacuum, surfaceprocessing by high-temperature oxidation or coating are needed, thus, theprocessing technology is complex. TiC can be formed in situ by SHS reaction inthe magnesium melt. In this case, however, the "inert"matrix acts as a diluent,which may make the propagation of the combustion wave unstable owing to thestrong heat dissipation in the "inert"metal matrix. XD technology is employed in this dissertation. In this technology, firstly, aTiC/Al master alloy with as high as 50~70 wt.% TiC particulates was fabricatedby SHS reaction in the Al-C-Ti system. Secondly, the master alloy could be addedeasily and economically to an AZ91 magnesium alloy matrix to achieve acomposite reinforced with the desired volume fraction of TiC particulates. Lastly,with the master alloy remelting in AZ91 magnesium alloy matrix, TiC particulatereinforced magnesium matrix composites was successfully fabricated bymechanical stirring in the semi-solid AZ91 magnesium alloy melt. Compared withthe conventional PRMMCs(particulate reinforced magnesium matrix composites)fabricated by ex situ methods, the in situ PRMMCs exhibit the followingadvantages: ①the in situ formed reinforcement is thermodynamically stable atthe matrix, leading to the less degradation in elevated-temperature service; ②thereinforcement-matrix interface is clean, resulting in a strong interfacial bonding;③the in situ formed reinforcing particulate is finer in size and their distributionin the matrix is more uniform, yielding better mechanical properties. The majorresearch efforts of the present study are as follows:1. Influencing factors of kinetics of SHS reaction to form the TiC/Al master Influencing factors of kinetics of SHS reaction to form the TiC/Al masteralloy were detailedly investigated by experimental research. Some newphenomenons and laws are found, what is more, the experimental parameters offabrication techniques which are appropriate for the SHS reaction to form theTiC/Al master alloy are optimized and suggested respectively.(1) In the influencing factors of kinetics of SHS reaction to form a TiC/Almaster alloy, Al contents have a decisive effect on the size of TiC particle. As theamount of aluminum incorporated was increased over the range 20 to 50 wt.%, theTiC size decreased from approximately 5~7μm to 0.7~0.8μm.(2) In the influencing factors of kinetics of SHS reaction to form a TiC/Almaster alloy, the size of graphite powders and C/Ti ratio have an important effecton the morphologies of TiC particles and construction of phases in the finalproduct. A number of octahedral TiC particles besides spherical TiC particles existin the TiC/Al master alloy when graphite powders with the size of 75 μm and amolar ratio of C/Ti=1 were used. In addition, there are quantities of plate-like andneedle-like Al3Ti metastable phases besides Al and TiC in the final SHS reactionproduct.(3) An addition of TiC dilution agent leads to forming TiC particles ofdifferent size during SHS reaction. On the one hand, the TiC particle additiveplays an role of nucleation, which makes itself spherical and enlarged; On theother hand, TiC additions as a dilution make the size of TiC particles formed insitu during SHS reaction be finer. In addition, TiC additions postulate thetransformation from Al3Ti to Al and TiC.(4) TiC/Al master alloys with the size of 5~7μm and 0.7~0.8μm, arefabricated through SHS reaction in the Al-Ti-C system, respectively, such as theAl-Ti-C system (30 wt.%Al, Al, Ti, C powders with the size of 29, 75, 50μm,respectively) and the Al-Ti-C system (50 wt.%Al, Al, Ti, C powders with the sizeof 29, 15, 0.5μm, respectively).(5) The reaction between Al3Ti and C during the SHS reaction in the Ti-C-Alsystem, i. e. Al3Ti + C= TiC + Al, was confirmed experimentally when a TiC/Almaster alloy was remelted in pure magnesium melt, which provide directconclusive evidence for understanding deeply the reaction mechanism of the SHSreaction in the Al-Ti-C system.2. The growth mechanism of TiC crystal was proposed through study on thegrowth striations of TiC crystal The growth striations of TiC crystal formed during the SHS reacion in the Al-Ti-Csystem were found for the first time. From the growth striations of TiC crystal, thegrowth mechanism of TiC crystal is postulated to be lateral growth from thetwo-dimentional-nucleus steps. In addition, TiC grows mainly by Multilayer stacks orwrapping. TiC is regarded as a typical faceted crystal due to its high Jackson factor, which wasestimated to be between 5 and 7, thus, the growth mechanism transition from lateral tocontinuous growth modes, which is hypothesized by the classical crystal growththeory, is not observed for the TiC crystal during the SHS reacion in the Al-Ti-Csystem. The morphology of octahedral TiC crystal enclosed by {111} planes isexplained based on the PBC theory.3.Influencing factors of kinetics of remelting in pure magnesium for theTiC/Al master alloy The remelting of the TiC/Al master alloy in pure magnesium was conducted,the results show that diffusion reciprocally between Al in the TiC/Al master alloyand Mg in magnesium melt occurs. Al in the TiC/Al master alloy diffuse intomagnesium melt thoroughly, which results in the alloying of the magnesium melt.At the same time, Mg in magnesium melt diffuse into the TiC/Al master alloy,which made the TiC/Al master alloy change into the TiC/Mg master alloy, thus,TiC can exist stably in the magnesium melt. No product of the oxidation reactionwas found at the interface between the TiC particulate and the matrix. TiCparticles are suitable reinforcement for fabrication of the magnesium matrixcomposites. Preheating temperature in vacuum, remelting temperature and timehave an obvious effect on the remelting of the TiC/Al master alloy in puremagnesium.4. Influencing factors of kinetics of TiC uniformly distribution inAZ91magnesium alloy matrix Influencing factors of kinetics of TiC particulates uniformly distribution inAZ91magnesium alloy matrix were detailedly investigated by experimentalresearch. The experimental parameters of fabrication techniques which are...
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