Fabrication Of TiB₂-TiC Particulates Reinforced Magnesium Matrix Composites

In recent years, saving the energy sources and environment pollutionproblems have received more and more attention, and effective method to solvethem is to use light-weight and high-effective materials. Magnesium and its alloyspossess many excellent properties, such as low density, good damping capacityand castability, and their deposits is very abundant. Consequently, it is consideredthat magnesium and its alloys are one of the metal materials with greatly potentialapplication in the high and new-technology industries of 21st century. However, itis more and more difficult that single magnesium alloys materials meet therequirement of integrative properties of the high-tech development today;presently, the appearance of magnesium metal matrix composites (MMCs)provides a better approach to solve the problem. Compared with conventionalMMCs and Al MMCs, Magnesium MMCs provide more potential for jumped-uptechnique field due to their low density, high specific strength, high specificstiffness as well as excellent mechanical and physical properties. And therefore,since the 1980s later, magnesium matrix composites have been becoming one ofthe hotspot in the research field of the MMCs.According to the morphological feature of reinforcement, magnesium MMCscan be grouped under two heads, i.e. fiber (long and short fiber, whisker) andparticulate reinforced magnesium matrix composites. Although the properties offibre reinforced magnesium MMCs is higher than those of particulate reinforcedcomposites, their great application is limited, owing to the limitation in thecomplexity and high cost of fabrication processing, and directional properties. Onthe contrary, particulate reinforced magnesium matrix composites have beenreceiving considerable attention because of low cost of the reinforcement, uniformmicrostructure, isotropic properties, and processing using the traditional metalprocessing technics. During the twenty years of the study and exploitation, particulate reinforcedmagnesium MMCs have been produced by several processing routes such aspowder metallurgy, melt infiltrating, stir casting and spray deposition etc..However, in all these methods, the reinforcing second-phase particles areincorporated into molten magnesium by ex situ methods, and the reinforcingparticle size is coarse, rarely below ~1μm; the surface of the reinforcements isprone to contaminative, leading to the interfacial reactions and poor wettabilitybetween the reinforcements and the matrix, which has been one of the mainsubject of the intensive investigation for the science researchers. Aiming at the drawbacks of the ex situ methods, recently, a new processingmethods i.e. in situ synthesis has been developed to fabrication particulatereinforced MMCs. Compared with ex situ methods, the reinforcement issynthesized during the processing by in situ methods. And hence, the in situformed reinforcing particulate is not only finer in size and clean in surface but alsothermodynamically stable, resulting in the absence of interfacial reaction. However,the reinforcement which may be in situ synthesized in the magnesium melt israther limitedly, since the melting point of the magnesium and its alloys with thelightest weight and very high specific strength is very low, and magnesium alloysare prone to oxidation or burning and reaction. Furthermore, the reinforcement issynthesized by exothermic reactions in a metallic matrix during the in situparticulate reinforced composite fabrication. In this case, the 'inert'matrix acts asa diluent, which may make the propagation of the combustion wave unstableowing to the strong heat dissipation in the 'inert'metal matrix, resulting in theincompletion of the reaction. Therefore, according to the feature of magnesiumalloys, combining with the existing process that have been used for the fabricationof particulate reinforced MMCs, the exploiture and development of new processmethod for the fabrication of magnesium matrix composites is critical for itslarge-scale application in industry. Based on the above standpoint and statement, the master alloy route isselected for the fabrication of magnesium matrix composite, i.e. the in situparticulates reinforced magnesium MMCs were successfully fabricated by addingthe master alloys containing in situ particulates processed via self-propagatinghigh-temperature synthesis (SHS) reaction under the vacuum atmosphere intomolten magnesium and using the stirring casting technique. In consideration of thepoint, a low cost starting material system of Al-Ti-B4C is used to fabricate in situceramic reinforcements TiB2-TiC, and reaction mechanisms of the system andeffect of Al content, the size of B4C particulate as well as the addition of C and Bon the products of SHS reaction have been investigated detailedly. On the basis ofthese work, the (TiB2-TiC)p/AZ91 composites were successfully fabricated bymaster alloy route, and its microstructure and wear resistance have also beenstudied deeply and systemically. The major research efforts of the present studyare as follows:(1) The mechanism of the SHS reaction formations of TiB2 and TiC in Al-Ti-C system was as follows: Al melted initially, and then, the reactions between liquid aluminum and solid titanium to form TiAl3 occurred. Subsequently the reaction between Ti and B4C is initiated by the heat liberation of Al3Ti formation reaction to form the more thermodynamically stable TiB2 and TiC, followed by a reaction between Al3Ti and B4C, and this further accelerated the SHS reaction in the Al-Ti-B4C system.(2) It was found by the experiment that the Al content has important influence in the phase composition and morphology of the SHS products. When the Al content is 30~40wt.% in the Al-Ti-B4C system, apart from the three predominant phases of TiB2, TiC and Al, there is also a small quantity of transient Al3Ti in the final SHS reaction products; as the Al content increased to 50~60wt.%, no Al3Ti phase was found in SHS products. Furthermore, TiB2 is present typically as hexagonal prisms, while TiC is irregular butnear-spherical, and the sizes of in situ formed TiB2 and TiC particulates become finer with the increasing of Al content.(3) When C was added according to C:Ti=1.0 (mol) into the Al-Ti-B4C system and Al content was 30~40wt%, the SHS products consist of Al phase, TiB2 and TiC particulates, and no Al3Ti interphases were found; however, as the Al content increased to 50~60wt.%, the Al3Ti phase was detected by the XRD results. When C was added according to C:Ti=1.2 (mol), Al3Ti disappeared in the Al-Ti-B4C system with 50wt.% Al. The sizes of formed TiB2 and TiC particulates decrease as the Al content increases, which is similar to the Al-Ti-B4C system. At the same time, it is found that the addition of C forms more round TiC.(4) It was found by the experiment that whether B was added according to B:Ti=2.0 (mol) or 2.2 into the Al-Ti-B4C system, no Al3Ti phase was found in the XRD results of the system with 50wt.% Al, and SHS products only consist of Al phase, TiB2 and TiC particulates. In addition, the morphology of TiC particulate became irregular with the increasing amount of B addition.(5) The results indicated that the granularity of reactant B4C has great influence in the composition and morphology of the SHS products when it increased to a certain level, i.e. as the granularity of B4C increased to a certain size, the SHS reaction was incomplete, and the formed TiB2 and TiC particulates were also irregular.(6) The (TiB2-TiC)p/AZ91magnesium MMCs were successfully fabricated by adding the master alloys containing in situ TiC and TiB2 particulates processed via self-propagating high-temperature synthesis (SHS) reaction in Al-Ti-B4C system under the vacuum atmosphere into molten magnesium and using the stirring casting technique. The fine TiB2 and TiC particulates were distributed homogeneously relatively in the Mg matrix. They exhibited more clearly hexagonal and near-spherical morphology, respectively, and their sizes were below ~5μm. The interface between the reinforcement and matrix is clean, and there is no interfacial reactant.(7) Compared with the AZ91 alloy, the hardness and wear resistance of...