Combustion Synthesis Reaction Mechanism Of Ni-Ti-C/B₄C Systems

In-situ particulate reinforced metal matrix composites (MMCs) possess excellent combination properties of high strength and toughness which the single-phase materials are usually short of. So far, however, most of the practical MMCs have been reinforced monolithically. Under many practical conditions, the part may need optimal combination of many properties, i.e. the place where endures the strong abrasion, high-temperature application and tends to be fatigue failure should be distributed by a high volume fraction of ceramic particulates, while the place where hardly loads strong action should be the metal matrix with a high toughness. Hence, in the present study, our group fabricated a high volume fraction of ceramic particulate locally reinforced steel matrix composites by using the combustion synthesis during the casting process. The properties of the ceramic locally reinforced steel matrix composites depend on the microstructure of the local reinforced region, while the work life relies on the intensity of the interface bond between the reinforced region and metal matrix. However, the combustion synthesis in the metal melt is a very complex process which is characteristic by the high temperature, high reaction rate, non-linear and non-equilibrium transport including energy, mass and momentum transports. And simultaneously the complex process is full of many physical and chemical phenomena. It is very difficult to study the reaction directly in the steel melt. Therefore, it is the critical theory issue that constructing the common regularity and exploring the individual difference of the combustion synthesis reaction mechanism and reaction behaviors between the inside and outside of the steel melt, which is significant to control the combustion synthesis reaction and provide a guide for the fabrication of the ceramic locally reinforced steel matrix composites.Hence, Ni-Ti-C and Ni-Ti-B4C systems were selected as the objective of the present study. The study mainly focused on the exploration of the reaction path and the thermodynamic and dynamic conditions for the ceramic formation under different reaction modes (DTA, SHS, TE and metal melt) and different atmospheres (air and Ar), as well as the common regularity and individual difference in the reaction mechanism and reaction behaviors. It is expected that these present results can provide an optimal technical parameter range and control the microstructure, the properties and work life of the locally reinforced composites. The main results of the present study can be described as follows:1) The common regularity and individual difference in the DTA (the exothermic reaction at a low heating rate), SHS and TE reaction mechanism for the formation of TiC and TiC-TiB2 in the Ni-Ti-C/B4C system are opened out and can be concluded as follows:①Ni-Ti-C system: firstly, Ni reacted with Ti to form Ti2Ni; after reaching the eutectic temperature for the Ti2Ni and Ti, Ni-Ti liquids were formed; then C dissolved into the Ni-Ti liquid to form Ni-Ti-C liquid. [Ti] reacted with [C] in the liquid to form TiC.②Ni-Ti-B4C system: firstly, Ni reacted with Ti and B4C to form NixTiy and NixBy intermetallic compounds, and subsequently the Ni-Ti and Ni-B liquids were formed at the eutectic point for the Ti2Ni and Ti, Ni2B and Ni4B3. Meanwhile, some C atoms from the reaction between Ni and B4C can dissolve into Ni-Ti liquid to form Ni-Ti-C liquid. Ni-Ti, Ni-B and Ni-Ti-C liquids mixed together to form Ni-Ti-C-B liquid. [Ti] reacted with [C] and [B] in the liquid to form TiC and TiB2.2) The ignition temperatures of TE reaction from the Ni-Ti-C?B4C systems under air are much lower than those under Ar.①Thermodynamic calculation results indicated that: under air, the heat released from the reactions of Ni, Ti, C and B4C with O2 and N2 were much higher than those from the reactions between Ni with Ti and Ni with B4C. In particular, the heat released from the reactions which Ti, C and B4C reacted with O2 and N2 were much higher than those from the reactions which Ti reacted with C and B4C.②The reason for that the ignition temperatures of TE reaction from the Ni-Ti-C?B4C systems under air are much lower than those under Ar is the chemical oven mechanism.a)The chemical oven mechanism of Ni-Ti-C system under air can be described as follows: at 400℃, Ni on the surface of the compact reacted with O2 to form NiO; with the heat released from the reaction, the temperature increased to 500℃quickly and Ti reacted with N2 to form TiN0.3; meanwhile, C reacted with O2 to form the oxides of carbon. The heat released from the above reactions made the temperature of the system increase greatly and promoted the formation of Ti2Ni from the solid-state reaction between Ni and Ti. Subsequently, Ni-Ti liquids were formed and C dissolved into the Ni-Ti liquid to form Ni-Ti-C liquid. [Ti] reacted with [C] in the liquid to form TiC. b) The chemical oven mechanism of Ni-Ti-B4C system under air can be described as follows: the exothermic reactions which Ni reacted with O2 and Ti reacted with N2 were the same as those in the Ni-Ti-C system. The difference was that B4C reacted with O2 to form the oxides of C and B. The heat released from these reactions promoted Ni to react with Ti and B4C to form Ni-Ti and Ni-B intermetallics. Subsequently, Ni-Ti and Ni-B liquids were formed quickly, which made these liquids mix together to form Ni-Ti-C-B liquid. [Ti] reacted with [C] and [B] in the liquid to form TiC and TiB2.c)The chemical oven mechanism of the TE reactions from Ni-Ti-C/B4C systems under air were extracted, i.e. the heat released from the exothermic reactions which Ni reacted with O2, Ti with N2, C and B4C with O2 promoted Ni to react with Ti to form Ti2Ni and then form Ni-Ti liquid or promoted Ni to react with Ti and B4C to form Ti2Ni and Ni-B and then form Ni-Ti and Ni-B liquids. Moreover, the reaction mechanism after the formation of Ni-Ti and Ni-B liquids were the same as those under Ar.3) The DTA products from the DTA reactions in the Ni-Ti-C/B4C systems mainly consisted of the oxides of Ni and B and the nitrides and oxides of Ti. TiC and TiB2 were hardly formed.4) The controlled factors for the Ni-Ti-C liquid formation rate of Ni-Ti-C system with different C particles were different.①When the fine C particle (<1μm) was used, C dissolved into Ni-Ti liquid to form Ni-Ti-C liquids. Therefore, the dissolution rate of C was the controlled factor of Ni-Ti-C liquid formation.②When the coarse C particle (>38μm) was used, the low stoichiometric TiCx layers were formed firstly at the interface between Ni-Ti liquid and C particle. Therefore, the diffusion rate of C through TiCx layer was the controlled factor of Ni-Ti-C liquid formation.5) In the Ni-Ti-C/B4C systems under Ar, the ignition difficulty and reaction completeness depended on the difficulties of the Ni-Ti-C/-B-C liquid formation difficulty.①Under DTA, because the heat rate was low and the heat dissipation was very great, the formation rates of the Ni-Ti-C/-B-C liquids were very slow. The duration time of the reactions for that [Ti] reacted with [C] and [B] in the liquid to form TiC and TiB2 were long, and thus the reaction is incomplete. Under SHS and TE reaction, the heating rate is high and the heat dissipation was very small, therefore, a large number of Ni-Ti-C/-B-C liquids can be formed quickly, which initiated the occurrence of combustion synthesi. TiC and TiC-TiB2 can be formed in a very short time.②The addition of Ni into Ti-C/B4C system favored the formation of Ni-Ti-C/-B-C liquids, which lowered the ignition difficulty and the reaction incompleteness.③With the increase of C and B4C particle size, the formation of Ni-Ti-C/-B-C liquids became difficult, which increased the ignition difficulty and the reaction incompleteness.④Ni and Ti particle sizes only affected the formation difficulties and rates of Ni-Ti and Ni-B liquids. Therefore, they only affected the ignition difficulty, and hardly affected the formation difficulties of Ni-Ti-C/-B-C liquids.6) The ignition time and ignition temperature of the TE reaction from Ni-Ti-C?B4C systems under air and Ar depended on the formation difficulties of Ni-Ti-C/-B-C liquids, while the formation of Ni-Ti-C/-B-C liquids were controlled by the formation difficulties of Ni-Ti and Ni-B liquids.①Under Ar, with the increase of Ni and Ti particle size, the rate of solid-state reaction for Ni and Ti lowered which made the formation time of Ni-Ti or Ni-Ti and Ni-B liquids long and caused the increase of ignition time.②Under air, increasing Ni particle size and decreasing C particle size can strengthen the role of chemical oven, which made the formation temperature of Ni-Ti or Ni-Ti and Ni-B liquids low. And thus, the ignition temperature became low and the ignition time became short.7) Using the CS reaction during casting, TiC and TiC-TiB2 particulates locally reinforced steel matrix composites are successfully fabricated. The sizes of TiC and TiB2 particulates in reinforced 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 steel matrix, wear resistance of TiC and TiC-TiB2 particulates reinforced region increased by 1.3-3.2 and 1.8-3.8 times, respectively. Wear resistance of TiC-TiB2 particulates reinforced region was much better than TiC particulates reinforced region. The optimal processing parameter of CS reaction in the Ni-Ti-C/B4C systems which can be used for preparing the composites can be given: Ni contents: 20~40wt.%, Ni particle size: ~45μm, Ti particle size: ~25μm, C particle size: ~1μm-~38μm, B4C particle size: ~3.5μm~~45μm, the molar ratio of Ti and B4C: 3:1.