Reaction Behaviors Of Combustion Synthesis And Fabrication Of Steel Matrix Composites From Cu-Ti-C/B₄C System

Particle reinforced metal matrix composites(MMCs) combine high strength, high stiffness and high wear-resistance of the reinforcement with high ductibility and high toughness of the metal matrix, and provide excellent combination properties of strength and toughness that traditional single materials is absence, which solve preferably the contradiction of hardness and toughness, therefore, this kind of material has gained extensive application. But, However, all the progressed composites are monolithic composites, in many practical occasion, the monolithic composites reinfoced with particulates are no demand, and the abrasion of components often occur in local area, and the other part of components require excellent obdurability, Hence, in the present work, our group highlights and applies a new and potential process in which CS plus traditional casting techniques produce successfully ceramic particulate locally reinforced steel matrix composites, that is, the local easily wear region of wear casting is locally reinforced by ceramic particulates. However, it is difficult to directly research CS reaction in the steel liquid, thus, in the present work, Cu-Ti-C and Cu-Ti-B₄C systems is selected as the subject of study, and the effect of kinetic impact factor on the DTA,SHS and TE reaction is researched, and reveals the reaction mechanism and common rule of these three reactions. Based on these research results, the optimal processing parameter of CS reaction is optimized for the study of CS reaction behavior in steel liquid and the fabrication of TiC and TiC-TiBB₂ particulates locally reinforced steel matrix composites with healthy microstructure and excellent properties. The reaction process of CS in the melt is researed, and by making full use of and exerting the academic advantage of the two kind of techniques, the microstructure and properties of TiC and TiC-TiB₂ Bparticulates locally reinforced steel matrix composites is controlled reasonablly.The main research results of the present study are as follows: 1) The reaction path of ceramics formation in the Cu-Ti-C and Cu-Ti-B₄C systems under the DTA conditions and the common regularity in these two systems are given as follows:In the Cu-Ti-C and Cu-Ti-B₄C systems, TixCuy compounds (x and y depending on the Cu content) were firstly formed due to the solid diffusion between Ti and Cu; and subsequently, the Cu-Ti liquid were formed at an eutectic temperature or melting temperature with the increase of temperature, and simultaneously these liquid spreaded on the surface of C or B₄C particles; and then some C and B atoms can dissolve into the liquid to form Cu-Ti-C liquid or Cu-Ti-B-C liquid, and some TiC or TiC and TiB₂ B can be formed firstly at the locations where [C] and [B] are relatively high; with the further increase of temperature, the TixCuy compounds melt to form Cu-Ti liquid, and then more C and B atoms can dissolve into the liquid to form a large amounts of Cu-Ti-C liquid and Cu-Ti-B-C liquid, and many TiC or TiC and TiBB₂ particulates were precipitated from the liquid and grow up.2) The reaction mechanism of ceramics formation in the Cu-Ti-C and Cu-Ti-B₄C systems during SHS reaction and the common regularity in these two systems are given as follows:a) In the Cu-Ti-C system, the SHS process can be described as follows: Cu+Ti+C→TixCuy+Cu+Ti+C→Cu-Ti (liquid phase)+Ti+C→Cu-Ti-C (liquid phase)+TiC+C→TiC+Cu, the TixCuy depends on the Cu content. The SHS mechanism can be described to be dossolution -precipitation mechanism, namely, TixCuy compounds were firstly formed due to the solid diffusion between Ti and Cu; and subsequently, the Cu-Ti liquid were formed at an eutectic temperature or melting temperature with the increase of temperature, and then some C atoms can dissolve into the liquid to form Cu-Ti-C liquid, and some TiC can be formed firstly at the locations where [C] was relatively high; with the further increase of temperature, the TixCuy compounds melt to form Cu-Ti liquid, and then more C atoms can dissolve into the liquid to form a large amounts of Cu-Ti-C liquid, and many TiC particulates were precipitated from the liquid; finally, the reaction can not continue until all the C particles were consumed completely.b) In the Cu-Ti-B₄C system, the SHS process can be described as follows: Cu+Ti+B₄C→TixCuy+Cu+Ti+B₄C→Cu-Ti (liquid phase)+B₄C→Cu-Ti-B (low concentration)-C (liquid phase)+TiC+B₄C→Cu-Ti-B (high concent- ration)-C(liquid phase)+TiC+TiBB₂+B₄C→TiB₂+TiC+Cu; the TixCuy depended on the Cu content while that of TixBy depended on the concentration of C and B in the liquid. The SHS mechanism can be described to be dossolution -precipitation mechanism, namely, TixCuy compounds were firstly formed due to the solid diffusion between Ti and Cu; and subsequently, the Cu-Ti liquid were formed at an eutectic temperature or melting temperature with the increase of temperature, and simultaneously these liquid spread on the surface of B₄C particles; and then some C atoms escaped from B₄C particles can dissolve into the liquid to form Cu-Ti-C liquid, and some TiC can be formed firstly at the locations where [C] was relatively high; therefore, TiC formed earlier than TiB₂Subsequently, with the continuous escape and dissolution of C and B, a large amounts of Cu-Ti-B-C liquid were formed and many TiC and TiB₂ particulates were precipitated from the liquid.c) During the SHS process form the Cu-Ti-C and Cu-Ti-B₄C systems, a common regularity is given: TixCuy compounds were firstly formed due to the solid diffusion between Ti and Cu; and subsequently, the Cu-Ti liquid were formed at an eutectic temperature or melting temperature with the increase of temperature, and simultaneously these liquid spread on the surface of C or B₄C particles. Some C and B atoms from B₄C particles can dissolve into the liquid to form Cu-Ti-C and Cu-Ti-B-C liquids, and ceramics particulates can be formed. The SHS reaction begined with the formation of liquid. Compared with the process in the DTA apparatus, for the same system, the SHS reaction can be initiated without many intermediate reactions once the liquid were formed.3) The effect of kinetic impact factor on the SHS reaction kinetics of Cu-Ti-C and Cu-Ti-B₄C systems is revealed: with the increase of Cu content, the combustion temperature and wave velocity decreased while the ignition time exhibited a first increase and then a significant decrease with the minimum value at 20wt.%. Moreover, the ceramic particulates size decreased with the increase of Cu content; With the increase of C and B₄C particle sizes, the reaction became difficult to ignite and the combustion temperature and wave velocity decreased. Furthermore, the reaction became incomplete and the ceramic particulates sizes became smaller with the increase of C and B₄C particle sizes. Compared with the C and B₄C particle sizes, Ti particle sizes exhibited a slight influence.4) The effect of kinetic impact factor on the TE reaction kinetics of Cu-Ti-C and Cu-Ti-B₄C systems is revealed: with the increase of Cu content, the combustion temperature decreased while the ignition temperature increased; Moreover, the ceramic particulates size decreased with the increase of Cu content; With the increase of C and B₄C particle sizes, the combustion temperature decreased while the ignition temperature increased and the reaction became incomplete. Furthermore, the ceramic particulates sizes became smaller with the increase of C and B₄C particle sizes. Compared with the C and B₄C particle sizes, Ti particle sizes exhibited a slight influence. Compared with the DTA and SHS process, the heat from the pre-reaction can increase the temperature to the ignition temperature. Therefore, the formation of liquid was later than the ignition reaction.5) Using the optimized processing parameter of CS reaction of Cu-Ti-C and Cu-Ti-B₄C systems outside steel liquid, TiC and TiC-TiB₂ particulates locally reinforced steel matrix composites are successfully fabricated by CS reaction and casting techniques. The sizes of TiC and TiB₂ Bparticulates in reinfoced region are fine, and their distribution is relatively uniform, and the interface between the reinforcement and matrix is clean, and the bonding of transition region between reinforce and matrix area is good; compared with the matrix region, the hardness values of the reinforced region of two systems are increased about 1.3².4 times, and the wear resistance is increased 1.1³.1 and 1.7³.2 times; however, with increasing Cu content, the wear resistance of the composite is increased firstly and then decreased. The hardness and wear resistance of the reinforced region of TiC-TiB₂ particulates locally reinforced steel matrix composites are higher obviously than those of TiC particulate locally reinforced steel matrix composite; meanwhile, the optimal processing parameter of CS reaction with hardness and wear resistance is optimized. In the Cu-Ti-C system, Cu contents 30⁴0wt.%, Cu particle size ⁶μm, Ti particle size ²5μm, C particle size ¹Î¼m^<sup>38μm; In the Cu-Ti-B₄C system, Cu contents 31⁴0wt.%, Cu particle size ³Î¼m, Ti particle size ³8μm, B₄C particle size³.5μm^<sup>45μm.6) It is found that the influencing law of the impact factor of reaction kinetics (Cu content, C, granularity of B₄C powder) on DTA, SHS, TE and CS reaction kinetics (igniting process, combustion temperature, the size and morphology of product) of Cu-Ti-C and Cu-Ti-B₄C systems outside steel liquid is coincident, their differences only lie in different reaction velocity and digree.