| Combustion synthesis (CS) has been widely utilized to fabricate ceramic particulate reinforced steel matrix composites (RSMCs) due to its process simplicity, low cost, high efficiency and pure products. However, the RSMCs fabricated solely by the CS technique are often monolithic. From the point of view of the application, only is the local region of the casting required to possess high abrasion resistance, while other regions must possess excellent obdurability. As a result, it is desirable that the local region rather than the whole casting is reinforced by ceramic particulates. Therefore, we highlighted a potential in situ process that combines the CS technique with the traditional casting method, by which the local RSMCs were successfully fabricated. The abrasion resistance of the local region reinforced with ceramic phases was significantly enhanced. In the case of fabricating the local RSMCs by combining the CS technique with the traditional casting method, only when we get to know the CS thermodynamics, formation mechanism of the reinforced phases and the kinetic reaction behaviors in the molten steel, is it possible to control the CS reaction and thus produce high quality RSMCs. Unfortunately, a theoretical method for investigating the CS reaction mechanism in the molten steel has not been developed. Moreover, it is scarce of the theoretical instruction with regard to control the reaction behaviors and products in the molten steel. Due to the high temperature and the opacity of the molten steel, it is quite difficult to directly investigate the CS reaction in the steel molten. In the present study, author took an Al-Ti-B4C system as the research object, studying the formation mechanisms of TiC and TiB2, respectively, in the heating reaction in the DTA and the combustion reaction in the glove box, the thermal explosion furnace and the steel molten, showing the common and particular regulations of the formation mechanisms of the TiC and TiB2 phases between out of the molten steel and in the molten steel. Furthermore, through the study of effects of dynamics factors on the CS reaction behaviors and products out of the steel molten, the ranges of the CS processing parameters which is suitable for producing the locally RSMCs were optimized. Therefore, the TiC-TiB2 local RSMCs with healthy microstructure and excellent properties were successfully fabricated. The main conclusions are given as follows:1) Changes in the standard Gibbs free energy (ΔG0) in the Al-Ti-B4C system show that Al3Ti, Al4C3 and AlB2 phases in the product are metastable, while TiC and TiB2 phases are stable. Theoretical estimation of adiabatic temperatures (Tad) shows that the value of Tad is more than 1800K for the presence of less than 52.2 wt.% Al in the reactants, under which the combustion reaction in the Al-Ti-B4C system could be self-sustainable once it is initiated. 2) The common and particular regularities of the formation mechanisms of TiC and TiB2 inthe heating reaction in the DTA, the SHS reaction in the glove box and the TE reaction in the thermal explosion furnace in the Al-Ti-B4C system are given as follows.①Common regularity: Under above mentioned three conditions, the formation mechanisms of TiC and TiB2 are same. Namely, after the formation of the Al-Ti liquid, B4C decomposes and dissolves into the liquid to form Al-Ti-B-C liquid, finally, the reactions among [Ti], [C] and [B] in the Al-Ti-B-C liquid yield the TiC and TiB2 phases.②Particular regularity: (a) For the DTA reaction, due to the small heating rate and the large heat loss, the formation rate of Al-Ti liquid is small, which postpones the Al-Ti-B-C liquid formation, as a result, the time of the reaction forming TiC and TiB2 in the Al-Ti-B-C liquid is much longer. (b) For the SHS reaction in the glove box and the TE reaction in the thermal explosion furnace, after the reaction of Al with Ti, a large amount of Al-Ti liquid immediately forms, subsequently, a large amount of Al-Ti-B-C liquid forms, which initate the combustion reaction rapidly, and thus TiC and TiB2 precipitate from the Al-Ti-B-C liquid, the combusition reaction is quickly complete.3) The common and particular regurities of the formation mechanisms of TiC and TiB2 in the SHS reaction in the glove box and the TE reactions in the thermal explosion furnace and the molten steel in the Al-Ti-B4C system are given as follows.①Common regularity: When the B4C particle size is less than 40μm, the formation mechanisms of TiC and TiB2 in the combustion reactions under the above three conditions are same. Namely, Al-Ti liquid forms first, then B4C decomposes and dissolves into the Al-Ti liquid, forming the Al-Ti-B-C liquid, subsequently, the reactions in the Al-Ti-B-C liquid occur into the formation of TiC, TiB2 and Al.②Particular regularity: When the B4C particle size is more than 40μm, particular regularities in the formation mechanisms of TiC and TiB2 in the combustion reactions under the above three conditions comes into being as follows.(a) In the SHS reaction in the glove boxe and the TE reaction in the thermal explosion furnace, the Al-Ti-B-C liquid forms and thus reacts into the formation of TiC, TiB2, TixAly and a small quantity of Al, the combustion reaction is a little incomplete.(b) In the TE reaction in the molten steel, Al-Ti-B-C liquid forms first, a large amount of TiC and TiB2 form and precipitate from the Al-Ti-B-C liquid. Subsequently, the molten steel infiltrates into the reacted sample, which brings the formation of the Al-Fe-Ti-B-C liquid (low C and B concentration), the unreacted B4C dissolves into the Al-Fe-Ti-B-C liquid, and again reacts to form TiC and TiB2. The reaction mechanism is as follows: (1) Al + Ti + B4C→(2) Al-Ti (liquid) + B4C→(3) TiC + TiB2 + Al-Ti-B-C (liquid) + B4C→(4) TiC + TiB2 + Fe-Al-Ti-C-B (liquid) + B4C→(5) TiC + TiB2 + Fe-Al-Ti-C-B (liquid)→(6) TiC + TiB2 + Fe-Al-B (liquid)→(7) TiC + TiB2 + (FeAlx + Fe2B), the FeAlx and Fe2B phases are fomed during cooling.4) It is found that the initiation and the degree of conversion in the combustion reaction are mainly dependent on the formation of Al-Ti-B-C liquid.①The proper addition of Al to the Ti-B4C reactants makes the Al-Ti-B-C liquid form easily, promoting the ignition reaction.②With an increase in the B4C particle size, the B4C particle more and more difficultly dissolves into the Al-Ti liquid to form Al-Ti-B-C liquid, which retards the initiation of the combustion rection and decreases the degree of conversion.③The addition of Fe easily forms the Al-Fe-Ti liquid. Moreover, the enhanced natural convection of the Al-Fe-Ti liquid and its role on the decomposition of B4C make the Al-Fe-Ti-B-C liquid form easily, which increases the degree of conversion and considerably decreases the dependent extent of combustion reaction on the B4C powders size.④The heating rate in the steel molten is higher than that in the thermal explosion furnace, and the heat loss in the steel molten is less than that in the glove box. The Al-Ti-B-C liquid forms more easily in the steel molten and the combustion reaction occurs more completely.5) It is found that the sizes of TiC and TiB2 particulates synthesized from Al-Ti-B4C system are dependent on the combustion temperature, which is also controlled by the rate of the instantaneous concentration change for [C] and [B] in the Al-Ti-B-C liquid, namely the rate of heat release in the instantaneous reaction.①With increasing Al content and B4C particle size, the instantaneous concentration of [C] and [B] in the Al-Ti-B-C liquid decreases, so does the rate of heat release in the instantaneous reaction, as a result, the combustion temperature decreases, the ceramic particle size decreases.②Ti particle size does't influence the change rate of the instantaneous concentration of [C] and [B] in the Al-Ti-B-C liquid, therefore, the effect of Ti particle size on the combustion temperature and ceramic particle size is very limited.③The addition of Fe increases the instantaneous concentration of [C] and [B] in the Al-Fe-Ti-B-C liquid, namely, the rate of heat release in the instantaneous reaction increases, therefore, the combustion temperature increases, so does the ceramic particle size.④The combustion temperature of TE reaction in the thermal explosion furnace is higher than that of SHS reaction in the glove box, therfore, the ceramic particle size considerably increases.6) It is found that the microstructures in the reinforced region could be effectively controlled by varying the reactant composition, the green density and the reactant particle size, through which the sizes of TiC and TiB2 particulates are fine (~1-10μm), the particulate distribution is relatively uniform, the interface between the reinforcement and matrix is clean, and the bonding of the transition region between the locally reinforced region and the steel matrix is excellent. It reveals that when the B4C powders with a wide particle size distribution of≤50μm is used, the macroscopic holes in the reinforced region disappear, the bonding between the reinforced region and the steel matrix is perfect, the distribute of ceramic particles become more homogeneous, and a few coarse ceramic particulates appear at the local matrix of the reinforced region. A good combustion reaction processing parameters for producing the TiC-TiB2 locally RSMCs with the excellent integrate performance is given as follows: wAl : 10-30wt.%,η: 65-75%, Ti:B4C=3:1, Al particl size ~29μm, Ti particle size 48μm, B4C particle sizes 3.5μm and≤50μm.7) The TiC-TiB2 particulate locally RSMCs were successfully fabricated by the process of combining the CS technique with the traditional casting method. The hardness and abrasion resistance of the locally reinforced region are, respectively, 2.0~2.8 and 1.2~5.1 times higher than those of the steel matrix. The abrasion mechanism is mainly"plough". It is found that the addition of Fe powders replacing some Al powders in the reactant could markedly eliminate the influence of the coare B4C particle size on bringing a large amount of macroscopic holes, which is significantly in favor of increasing the wear resistance. In sum, in this study, through studying the common and particular regularities of the TiC and TiB2 formation mechanisms in the combustion reactions under different reaction conditions, the kinetics law that influences the combustion reaction yielding TiC and TiB2 phases was revealed, which at some extent provides a theoretical groundwork in controling the microstructures of the local reinforced region and exploring the fabrication process of TiC-TiB2 particulate locally RSMCs. Moreover, this study provides the scientific research route and approach as well as is of great theoretical instruction for producing the particulate locally reinforced metal matrix composites.
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