| CS (Combustion synthesis) technology has been systematically investigated by Merzhanov et al. since the 1960s and has gradually become one of potential technologies for the materials-fabricating. CS method is essentially based on the sufficiently and self-sustainingly (or thermal explosion) chemical exothermic reaction of starting reactant mixtures once ignited by external energy supply to synthesize the designed products. As an in-situ method of materials fabrication, CS has been widely applied to synthesize intermetallic compounds, functional gradient materials, composites and ceramic particles. However, the mechanical properties and applications of in-situ materials have been paid more and more attention, while few work or report has been focused on its formation mechanism, especially its growth mode. So, in this paper, TiC and ZrC ceramic particulates were designed and fabricated by the CS technology. The reaction characteristics, crystal morphologies, formation mechanisms and the growth kinetics modes of the in-situ TiC and ZrC ceramics were investigated in detail by using the modern analyzing methods such as differential thermal analyzer (DTA), X-ray diffractometer (XRD) and micro-diffractometer, field emission scanning electron microscopy (FE-SEM), energy dispersive spectrometer (EDS), as well as transmission electron microscopy (TEM).The thermodynamic analysis showed that TiC and ZrC are the most thermodynamically stable and favorable phase in Al-Ti-C and Al-Zr-C reaction systems, respectively. The experimental results exhibited that when fabricating TiC ceramic by SHS (Self-propagating high-temperature synthesis), with Al contents increasing in Al-Ti-C mixtures, the adiabatic temperature Tad, reaction temperature Tc and combustion wave rate Vc all reduced evidently, and also the synthesized particles size became more finer. Once the Al content exceeds 50 mass.%, SHS reaction will become insufficient and the intermediate phase will retain in the final products. The formation mechanism of TiC was studied by combining the reaction characteristic of SHS with the DTA experiment and quenching experiment. After being preheated, the solid-state reaction between Al and Ti initially occurred and produced a few TiAl3 phase, and released some heat to make Al melt. Owing to some Al melting, the drastic solid-liquid reaction took place to produce more TiAl3 compound and to liberate a mass of heat, and thus leading the temperature abruptly to rise and the formed TiAl3 to melt. Subsequently, C dissolved into the Al-Ti melt and reacted with it to synthesize the thermodynamically stable TiC phase. As a result, the TiC phase nucleated in the melt, precipitated and grew as the TiC grains with various morphologies.When synthesizing ZrC by SHS in the Al-Zr-C powder mixtures, with Al contents increasing, the combustion temperature, reaction wave rate, as well as the ZrC particles size all decreased obviously. But once Al content is higher than 50 mass.%, the combustion temperature was so low (1460 K) that the ZrC-forming reaction was hardly induced. It is noted that once Al exceeds 30 mass.%, the nano-scaled ZrC particles (≤150 nm) were produced successfully, especially the ZrC size is less than 50 nm for 40 mass.% Al addition. The combustion process of SHS, DTA analysis and combustion wave front quenched experiment all indicate that, the Al additive in mixtures serves not only as a diluent to reduce the reaction temperature and to inhibit the ZrC particle from growing and coarsening, but importantly as an intermediate reactant to participate in the total SHS process and thus to control the formation mechanism and growth morphologies of ZrC grains. It was also found that, among all Zr-Al compounds, the SHS-synthesized ZrAl3 phase was the most favorable and only phase, which is greatly influenced by its three lattice structures such as L12 and D022 metastable structures and D023 stable structure, and also the similarity of lattice structure between D023-ZrAl3 and fccα-Al is responsible for.When synthesizing TiC and ZrC by TE(Thermal explosion), because TE processing has more rapid reaction rate and cooling rate than SHS processing, the total reaction proceeded insufficiently and resulted in more intermediate phase such as Al4C3, ZrAl3 and Zr3Al3C5 retaining in the products. Moreover, the TE-synthesized ZrC and TiC particles sizes are more finer, especially the nano-sized TiC particles were synthesized during TE processing in 10 mass.%Zn-Ti-C mixtures. However, the TE-synthesized TiC and ZrC crystals were of rather insufficient growth.TiC and ZrC crystals exhibit the strong faceting and smooth growth tendency. It showed that Al contents, ignition method, as well as the diluent species have evident influence on the growth morphology rather than on the growth kinetics mechanisms of TiC and ZrC crystals, namely both growth mechanisms are of the layer-by layer growth mode through the two-dimensional (2D) nucleation method. Due to the high supersaturation and undercooling, TiC and ZrC nuclei have different solidification, precipitation and growth rate, and hence resulting in the diverse growth morphologies of TiC and ZrC crystals. In particular, the hollow and square ZrC grains were synthesized by TE when 10 mass.%Zn was added into 10 mass.%Al-Zr-C mixtures. Once Al content in Al-Ti-C compact exceeds 40 mass.%, the SHS-synthesized TiC crystal appears as a regular octahedral morphology, and its unfolded eight planes are of the {111} facets with the biggest face density and the lowest surface attachment energy. The growth cell of TiC crystal is Ti-C6 octahedron unit. The Ti-C6 units continuously enter into the {111} planes and generate the new steps through 2D nucleation mode, and thus the octahedron units grew as the big and regular TiC octahedron through this layer-by-layer growth mode. TiC octahedra are linked through an edge-shared manner in order to favorably meet the geometry symmetry and energy stability.For Al-Zr-C mixtures, when Al content is 20 mass.%, the SHS-synthesized ZrC crystal grew as a well-developed hexagonal morphology with the {111} basal planes. According to the analysis, for the monolayer ZrC crystal, the rate in the [111] direction was suppressed and the rate in the [110] direction was prompted. As a result, the ZrC growth units in the (111) facet grew as a monolayer hexagonal platelet via 2D growth mode along [110] direction, and then the thick ZrC hexagonal multilayers were formed through the layer-by-layer growth mechanism of monolayer ZrC hexagonal platelets.
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