## 3 Results and Discussion
### Discussion  
The results obtained in the course of this investigation into the structure of composite SHS materials are in agreement with the data provided in research works [4; 5; 6; 7; 30] in the body of literature. The authors of these research works have used self-propagating high-temperature synthesis to fabricate composites where each composite comprises a matrix with ceramic particles distributed throughout its volume. The physical and mechanical properties and features of the deformation behaviour of the obtained composite material NiTi-TiB2 are described in article [31]. In particular, the results show that the strength properties of the TiNi matrix cannot provide the strength values that were obtained for real specimens of the cermet composite fabricated by direct laser deposition from the same powders (TiNi-TiB2) that were synthesised by using SHS. The study of the structure of the TiNi matrix showed that the matrix material is saturated with nanosized TiB2 particles (up to 30-40% of the volumetric content), which significantly (1.5 times) increases the mechanical characteristics of the TiNi matrices. Thus, the developed metal-ceramic composite TiNi-TiB2 implements multilevel hardening with respect to  
Figure 6: SEM image of a local area of a composite SHS powder particle (Figure 5, area 3) and maps of the distribution of chemical elements in a local area of a composite particle. Figure 7: **(a)** SEM image of a local area of a composite SHS powder particle, and (b) titanium diboride particle distribution by size. (1) nanosized TiB2 particles, which are uniformly distributed over the entire metal matrix, and (2) reinforcing particles of micron size. This work is largely devoted to the study of combustion in the SHS process and the formation of a structure in NiTi-TiB2 particles. From the results obtained in the course of this investigation into the combustion of samples of the system consisting of 63.5 wt.% NiB + 36.5 wt.% Ti and the phase composition and structure of the combustion products, we can make suppositions about the mechanism of the formation of the NiTi-NiTi2-TiB2 composite. Figure 8 shows a diagram of the formation of SHS materials in the course of combustion of a Ti-NiB powder mixture in a certain local area. After the top-side surface of the sample has been heated by a molybdenum spiral, the igniting layer is ignited. The dissipated heat is transferred into the main layer of the initial mixture (Figure 8, area 1) via the mechanism of conduit and convective heat transfer. A heating zone is formed, in which incongruent melting of NiB particles occurs. During melting, liquid-phase particles of nickel and boron are deposited, which are distributed in the melt together with titanium particles (Figure 8, region 2). As shown in the Ni-Ti and Ti-B phase diagrams [9; 10; 11], titanium particles start dissolving in liquid nickel before boron particles do. A Ni-Ti melt is formed, and B particles continue dissolving in this melt (Figure 8, area 3). After the Ni-Ti melt is saturated with boron, an exothermic reaction takes place whereby titanium diboride particles are formed. At this moment, a large amount of heat is released, which is characterised by a sharp jump in temperature, with the formation of a peak in the thermogram (Figure 3, area 1). In this case, part of the heat is transferred to the next region of the sample, forming a heating zone there, and on the thermogram, this heat loss is characterised by region 2. As the synthesis products cool down, the Ni-Ti melt crystallises and NiTi and NiTi2 phases are deposited in the process (Figure 8, area 4). These phases work as a matrix for titanium diboride particles that are distributed inside it. It can be noted that the size of the TiB2 particles in the NiTi-TiB2 composite obtained by SHS from the NiB-Ti powder mixture is by an order of magnitude smaller than that of the materials synthesised from the Ni-Ti-B mixture [26; 27].