Combustion Synthesis Of TiB₂/TiC Locally Reinforced Steel Matrix Composites In Molten Steel Using Cu-Ti-B/B₄C Systems

With the development of modern industry technology, more and morecomponents serve under the high-temperature, high-velocity and abrasionconditions. Generally, wear of a small part of component will lead to the end of theservice life of the whole component. If the steel component was incorporated withhard and thermal stable particulates on the specific surface or part, the service lifeof the component will be extremely extended. Therefore, in recent years, the steelmatrix composites locally reinforced with reinforcing particulates have beenbecoming one of the hot topics in the research of the metal matrix composites(MMCs).With the progress of study, lost preparation methods presented. These methodscould be classified into two species, the in-situ method and the ex-situ method, bythe incorporation mode of the reinforcing particulates. Compared with theconventional MMCs fabricated by ex-situ methods, the in situ MMCs exhibit thefollowing advantages: (i) the in situ formed reinforcement is thermodynamicallystable at the matrix, leading to the less degradation in elevated-temperature service;(ii) the reinforcement-matrix interface is clean, resulting in a strong interfacialbonding;and (iii) the in situ formed reinforcing particulate is finer in size and theirdistribution in the matrix is more uniform, yielding better mechanical properties.Consequently, in-situ hard particulates reinforced steel MMCs are one of the bestselection and the optimum point of arrest from a long-term interests of industryapplications.During the last twenty years, combustion synthesis (CS), or theself-propagating high-temperature synthesis (SHS), is one of the new methods tofabricate in-situ particulate reinforced MMCs. The advantages of this method arethat the CS reaction need no more exterior energy once the reaction started, and theproducts is very pure. Combined with the conventional fabricating methods, thecombustion synthesis was expected not only to avoid the porosity but also tofabricate the locally particulates reinforced steel matrix composites. Recently, akind of method, which utilizes the heat of the liquid steel during casting to ignitethe SHS reaction of the preform to yield in-situ ceramic particles local reinforcedsteel matrix composites, has get more and more attention. However, because thatthe steel liquid possesses of high temperature and density, which will lead to thedifficulty in controlling the combustion synthesis reaction in the steel liquid, the CSsystems are not enough for cosmically industrial application. Hence, exploring newCS systems and propagating techniques have become the key problem.Based on the above standpoint and statement, the Cu-Ti-B and Cu-Ti-B4Csystems are selected for the CS systems. And the TiBB2 and (TiB2+TiC) ceramicparticulates locally reinforced steel matrix composites were successfully fabricated.The reaction mechanisms of the two systems and the effect of Cu content on themicrostructure and abrasion resistance had been studied. The major research effortsof the present study are as follows:(1) The mechanisms of the combustion synthesis reaction of Cu-Ti-B andCu-Ti-B4C systems were as follows: at first, CuxTiy forms between thereaction of Cu and Ti. Subsequently, CuxTiy melts immediately, and then, itreacts with B and B4C respectively to form TiBB2 and TiB2B/TiC.(2) It was found by the experiment that with the increasing of Cu content theaverage size of ceramic particles decreasing, and the ceramic particlesdistribute more and more uniformly.(3) The steel matrix and reinforced region combine very well by a widetransition region forming between them. The content of ceramic particlesdisperses gradingly in the transition region. And the adhesive phasesbetween the ceramic particles are the Fe phase solid-dissolved a little Cuand the Cu phase solid-dissolved a little Fe.(4) It was found by the experiment that the amount of ceramic particles, theobdurability of the adhibit phases and the wettability of adhibit phases andceramic particles have important effect on the abrasion resistance ofcomposites. The more amounts of ceramic particles, the better obdurabilityof the adhibit phases and the better wettability of adhibit phases and ceramicparticles, the higher abrasion resistance.(a) The abrasion resistance of the composite with 30wt.% Cu is betterthan that of the composite with 40wt.% Cu in the Cu-Ti-B system.Under low load condition, their abrasion resistances arrive at 5 and 3times of the abrasion resistance of steel matrix, respectively.(b) The abrasion resistance of the composite with 10wt.% Cu is the bestamong Cu-Ti-B4C system. Under low load condition, the abrasionresistances of the composites with 10wt.%, 30wt.%, 40wt.% and60wt.% Cu arrive at 3.6, 2, 1.6 and 2.2 times of the abrasion resistanceof steel matrix, respectively.