| It is well known that magnesium alloys possess many excellent and unique properties, such as low density, high specific strength and stiffness, 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 potential to apply in the conventional and rising industries. However, the further development and application of magnesium alloys have been limited 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 the addition of reinforcement phase into magnesium or its alloy. Base on the shape of reinforcement phase, magnesium matrix composite can be grouped under three heads, i.e. fiber (long fiber and short fiber), whisker and particulate reinforced magnesium matrix composites. The fiber reinforced magnesium matrix composites have received considerable interest due to its evident strengthening effect along the fiber direction, however, its fabrication processing is very complex and the products can not be reutilized. Although, the strength of whisker reinforced magnesium MMCs is very high and the products with excellent fabricating and reprocessing properties can be reutilized, the range of application of the composite has been restricted because of a high cost starting material. At present, the particulate reinforced magnesium matrix composites have become one of the hotspot due to its significant inherent simplicity and potential cost-effectiveness in scale-up manufacturing, as well as its isotropy properties for the science researchers.The fabrication method of magnesium matrix composites is a key scientific problem which influence the development and application of magnesium matrix composites, and the properties, applications and cost of magnesium matrix composite depend on their fabrication methods. Base on the addition manner into the matrix, the fabrication method of magnesium matrix composites can be grouped under two heads, i.e. ex situ manner and in situ manner. At present, the particulate reinforced magnesium matrix composites are mainly fabricated in ex situ manner, namely, the reinforcements are prepared separately prior to the composite fabrication, and then the reinforcements are mixed with the magnesium matrix or directly added into the magnesium melt. In this case, the size of the reinforcement phase is usually the order of microns to tens of microns and rarely below 1.0μm. Furthermore, some other main drawbacks associated with ex situ manner such as the interfacial reactions between the reinforcements and the matrix, and poor wettability between the reinforcements and the matrix have to be overcome. The in situ reinforcing phase is fine in size, uniform in distribution, clean in surface and good in interfacial bonding, and the composites with in situ reinforcing phase possess higher mechanical properties. And hence exploring new in situ synthesis technique will be an emphasis in research field of magnesium matrix composites. However, magnesium and its alloy possesses low melt point and are prone to oxidation or burning, and thus the in situ synthesis technique and the reinforcement, which may be in situ synthesized in the magnesium melt, are rather limited. In addition, the reinforcement is synthesized by exothermic reactions in a magnesium matrix during the in situ synthesis process. In this case, the'inert'matrix acts as a diluent and heat can dissipate in the'inert'metal matrix, resulting in the incompletion of the reaction. Therefore, it is vital for particulate reinforced magnesium matrix composite application in industry to develop the new type reinforcing particulate and to simplify the synthesis technique. The TiB2 particulate reinforced magnesium matrix composites were successfully fabricated using master alloy method combined with mechanical stirring in semi-solid region. It is a breakthrough to solve the key scientific problems, i.e. the introduction of the in situ particulate into matrix and the interface contamination, for the magnesium matrix composites during the fabrication process. The thermodynamics and kinetic of SHS reaction of Al-Ti-B systems is investigated and it provided scientific and theorical foundation for fabricating TiB2-Al master alloy. The remelting process of TiB2-Al master alloy in pure magnesium melt was also studied and it provided foundation for fabricating TiB2/AZ91 composites using master alloy method. Lastly, the solidification behavior of TiB2/AZ91 composite was investigated. The major research efforts of the present study are as follows:(1) The TiB2/AZ91 composites with different TiB2 contents are successfully fabricated using master alloy method. When the molar ratio of Ti to B is 1:2 in TiB2-Al master alloy, the TiB2-Al master alloy and TiB2p/AZ91 composites contained a certain amount of transient TiAl3 phase; however, when the molar ratio of Ti to B is 1:2.5, the TiAl3 phase was almost completely eliminated and the distribution of TiB2 particulates generally appeared to be more homogeneous throughout the magnesium matrix.(2) The non-equilibrium solidification behavior of magnesium matrix composites containing TiB2 particulates reveaves the reason forα-Mg grain refinement in the TiB2p/AZ91 composite, as following,①Compared with the unreinforced AZ91 alloy, the composite possesses lower nucleation temperature of primaryα-Mg, almost the same eutectic reaction temperature and faster cooling rate during the total solidification time, and hence the undercooling and the nucleation rate ofα-Mg increases and the critical nuleus radius ofα-Mg decreases.②The presence of TiB2 particulates around the growing Mg crystals would create diffusion barriers for the growth of primaryα-Mg and restrict the growth of primaryα-Mg.(3) The thermal analysis experiment reveales that the TiB2 particulates can not serve as the nuleus substrate of the primaryα-Mg and a model about the microstructure evolution of the TiB2/AZ91 composite during solidification was proposed.(4) Based on one-dimensionally simplified Fourier transfer heat model, the effect of roller rate on cooling rate of melt-spun 7.5wt.%TiB2/AZ91 composite ribbon is investigated. And the cooling rate equation of melt-spun 7.5wt.%TiB2/AZ91 composite ribbon is obtained: According to the above result, the cooling rate of free surface of melt-spun 7.5wt.%TiB2/AZ91 composite ribbon at 500, 1000, 2000 and 3000 r/min is 4.16×104, 1.77×105, 7.62×105 and 3.05×106 K/s, respectively. When the cooling rate is equal to 20.7K/s Mg17Al12 is presence in the TiB2/AZ91 composite, and while the cooling rate are varied from 4.16×104 and 3.05×106 K/s Mg17Al12 dispeared. In addition, when the cooling rate of the composites are varied from 20.7 to 4.16×104 K/s the serious agglomeration of the TiB2 particulates can be observed, and when the cooling rate of the composites are varied from 1.77×105 to 3.05×106 K/s the distribution of TiB2 particulate is relatively homogeneous. (5) The DTA and CFQT (Combustion Front Quenching Technique) experiments have shown that the mechanism of SHS reaction in 50wt.%Al-Ti-B systems is reaction-dissolution-precipitation. At first, the reaction between molten Al surrounding Ti powder and Ti occurs to form TiAl3 and releases a lot of heat which, in return, accelerates the formation reaction of TiAl3. When the temperature of the perform is above 1613K, and then B powder dissolves into the Al-Ti melt to form Al-Ti-B melt due to the size of B powder being small. In this case, some of B and Ti in Al-Ti-B melt react to precipitate TiB2 and the formation reaction of TiB2 release a lot of heat which, in return, accelerates the formation reaction of TiB2. At this time, the maximum temperature of the preform achieved. In addition, the calculated results by thermodynamics have shown that the formation reaction of TiB2 has the greatest tendency to take place in the Al-Ti-B system under the standard state and the SHS reaction in Al-Ti-B systems, whose Al content is less than 58.8wt.%, can occur.(6) The effect of Al content, green density and TiB2 diluent content on the velocity of the propagating wave, combustion temperature, relative fraction of phases in the products and size of the TiB2 particulates are investigated.①As the amount of aluminum incorporated was increased over the range 40 to 60 wt.%, the combustion temperature decreases from 2120 to 1603℃and the velocity of the propagating wave decreases from 38 to 9 mm/s, and the size of TiB2 decreases from approximately 1μm to the order of submicrometer. Furthermore, the relative content of TiAl3 in the product decrease.②As the green density increase from 55 to 78%, the combustion temperature increase and the velocity of the propagating wave achieve the highest value at 65% green density.③The velocity of the propagating wave and the combustion temperature in 40wt.%Al-Ti-B-TiB2 systems decrease with the increasing of TiB2 diluent contents. Furthermore, the ex situ TiB2 powder can serve as the nucleus of in situ TiB2 synthesized in SHS reaction of Al-Ti-B systems.(7) The ex situ TiB2 particulates reinforced AZ91 composite with different TiB2 contents are successfully fabricated using two-step processing method which successfully solves the difficult problem, i.e. the higher porosity in conventional ex situ particulate reinforced magnisium matrix composites. Furthermore, the distribution of fine TiB2 particulates is relatively homogeneous throughout the Mg matrix and the porosity of the composites is relatively lower.(8) The wear resistance of the TiB2/AZ91 composite is higher than that of AZ91 alloy under the same condition and increases with the increasing of TiB2 content. For example, the wear resistance of AZ91 composite with 2wt.%, 5wt.% and 7.5wt.% TiB2 increases by 10.0%, 37.9% and 73.4%, respectively, compared with the AZ91 alloy. In addition, the abrasive wear behavior of the TiB2 particulate reinforced AZ91 composites fabricated by master alloy method is a"plough"abrasion.In a word, the present study not only provides theorical foundation for investiging the mechnesium and kinetics of SHS reaction of Al-Ti-B systems, but also revealed the fabrication technique and solidification behavior of new type particulate reinforced magnesium matrix composite. At the same time, the present study enriched the research field of particulate reinforced magnesium matrix composite and provided the theoretical basis and a new way for the novel magnesium MMCs with the independent knowledge authority.
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