Effect Of Different Reaction Conditions On Self-propagating High-temperature Synthesis (SHS) Reaction Of Ni-Ti-B System

With the development of modern science and technology, fine engineering ceramic materials have been applied in the leading field of national defence and industry more and more widely, and the requests towards combination properties of the engineering ceramic materials tend to be stricter and stricter. Among most kinds of ceramic materials, TiBB2 combines many good physical and chemical properties, such as high melting temperature (3225℃), high specific strength(25-35 GPa at room temperature) and stiffness (530 GPa), high fracture toughness, high heat conductivity (37-122 WmK at 300K), low density (4.45 gcm), low resistance(0.13μ?m), good thermal stability (thermal expansion coefficient 8.1×10 K at 20~1000℃) and chemical stability. Therefore, TiB2-1-1-3-6-1B ceramics and composites have been paid special attention by the areas of aerospace, national defence, crude oil, chemical industry, putrescence, machinery and electronic industry etc, and TiBB2 ceramics and composites have the broad application future in high-temperature structured, corrosion resistant and electronic materials. However, it is difficult to fabricate TiBB2 ceramics with high purity and low cost through the traditional ceramic fabrication techniques and, hence, it is difficult to meet the requests stemming from stricter and stricter techniques and engineering, and the wide applications of TiB2B ceramics have also been limited. Therefore, aiming at the deficiencies of traditional fabrication techniques, TiBB2 ceramics have been fabricated through new synthesis techniques, such as self-propagationhigh-temperature synthesis (SHS), exothermic dispersion (XD), sintering, mechanical alloying, flux assisted synthesis (FAS) and so forth. Among these techniques, SHS has been emerged distinctly based on its relative simple process and low cost, and become one of the research focuses. There are several attractive characteristics of SHS technique, such as synthesizing products with high purity, small size, homogeneous distribution as so on, and the trend of the maturity of this processing technique provides the new ways of producing TiB2TMB ceramics and composites with high performance on a large scale.One of the most notable characters of SHS reaction of Ti-B system is the high exothermicity, which is because of the large formation heat release of TiBB2. The high exothermicity resulting from the formation of TiB2B benefits the process of SHS reaction, however, it can also cause some serious problems. Therefore, it is necessary to add metal elements as diluents to Ti-B system to weaken SHS reactions and synthesize finer TiBB2 particulates, and make use of metal as the bonding agent to improve the mechanical properties of products simultaneously.The wettability between TiBB2 and Ni is very good, and at the same time, the research on Ni-Ti-B system is rather limited. Therefore, the SHS reaction in Ni-Ti-B system is chosen as the research object in this study. First, the thermodynamics of the SHS reactions in this system have been calculated, and then the reaction mechanism of Ni-Ti-B system has been investigated based on the result of differential thermal analysis (DTA); finally, the SHS reactions of Ni-Ti-B system ignited by two methods, namely, glove box and arc furnace, have been studied, and so have the influence factors of the SHS reactions in the two ways systematically. The principle findings of the present study are as follows:(1) It has been shown based on the standard Gibbs free energy calculation of Ni-Ti-B system that the reactions of synthesizing TiB2, TiB, NiB, Ni4BB3, Ni3B and Ni3Ti are all favorable thermodynamically, and Ni and TiB2 are the thermodynamically stable phases in the products. Furthermore, it has been shown according to the standard formation enthalpy calculation of Ni-Ti-Bsystem that the reaction of synthesizing TiB2 has the highest exothermicity due to the most negative enthalpy of this reaction. The calculated result of adiabatic combustion temperature shows that the Tad decreases with Ni content increasing except the phase transition ranges (horizontal line), which indicates that Ni serves as a diluent. In addition, the content of Ni to maintain the SHS reaction automatically in Ni-Ti-B system without any preheating corresponds to 72.20 wt.% according to the experiential criterion proposed by Mezhanov.(2) The reaction process of Ni-Ti-B system under DTA condition has been shown according to the DTA result of Ni-Ti-B system: first, Ni-Ti and Ni-B compounds can be formed through solid state diffusion reactions between Ni-Ti and/or Ni-B. When temperature increases further, Ni-Ti-B melt can be formed, and then TiB2 can be precipitated out from the melt mainly through diffusion-dissolution-precipitation mechanism. In addition, TiB2 can also be formed through solid state diffusion reaction between Ti and B.(3) It can be found that large amounts of hexagonal prismatic TiB2 particulates have been synthesized during the process of the SHS reactions in Ni-Ti-B system ignited by glove box. Besides:(i) Ni content has little effect on the phase compositions of the SHS products of Ni-Ti-B system, including stable phases Ni and TiB2 and transient phases Ni4BB3, Ni3B, NiB and Ni3Ti. The average sizes of normal TiB2 particulates decrease dramatically from 4~6μm at 30 wt.% Ni to to 0.6μm or less at 70 wt.% Ni.(ii) The research on the effect of Ti particle sizes on the SHS products of Ni-Ti-B system indicates that the phase compositions of 30 wt.% Ni-Ti-B system include stable phases Ni and TiB2 and transient phases Ni3Ti and Ni3B, and the contents of transient phases decrease dramatically with thedecrease of Ti particle size. The average sizes of TiB2 are 4μm. While the sizes of Ti particles have little effect on the products of 60 wt.% Ni-Ti-B system.(iii) The research on the effect of Ni particle sizes on the SHS products of Ni-Ti-B system shows that the average sizes of TiB2 particulates are about 15 and 5μm at 30 wt.% Ni-Ti-B system, respectively, when the sizes of Ni particles are 5μm and 325 mesh, respectively, which indicates the particle sizes of Ni particles have some effects on products at low Ni content. While the sizes of Ni particles have little effect on products at high Ni content (60 wt.%).(4) It can be found during the process of the SHS reactions in Ni-Ti-B system ignited by arc melting:(i) The average particle sizes of TiB2 decrease from 400μm at 50 wt.% Ni to 50μm at 70 wt.% Ni.(ii) The value of B/Ti has little effect on the phase compositions of products which including stable phases Ni and TiB2 and transient phases Ni3B, Ni4BB3, Ni2Ti and Ni3Ti; while it has obvious effect on the morphology of products. The morphology of products is irregular at B/Ti (mol.) = 4.0.(iii) The ex situ and in situ TiBB2 have obvious effects on the phase compositions and morphologies of products during the remelting process. When TiB2B is ex situ, the phase compositions of products include stable phases Ni and TiB2 and transient phases Ni2Ti, Ni3Ti, Ni4Ti3 and Ni3B, and the morphology of products is irregular. When TiB2 is in situ, the phase compositions of products include stable phases Ni and TiB2 andtransient phases Ni2Ti, Ni3Ti, Ni3B and Ni4BB3, and TiB2 particles are well-like, with the average sizes of 300μm.(5) The formation mechanism of TiB2 during the process of the SHS reactions of Ni-Ti-B system ignited by glove box is mainly of dissolution-reaction-precipitation mechanism. Its growth process mainly includes dissolution of Ti and B into Ni-Ti-B melt, the formation of TiB2 growth primitive, the transport of TiB2 growth primitive at the TiB2 crystal interface, the formation of TiB2 crystal nucleus, the accumulation of TiB2 growth primitive at the interface, the process of TiB2 crystal growth interface and finally the growth of TiB2 crystal.To sum up, TiBB2 ceramics have the wide application future and development space based on the good combination properties. Therefore, it is expected that the present study can establish some foundation for the in situ synthesis of TiB2B ceramics both experimentally and theoretically.