Influence of the chemical composition of the matrix on the structure and properties of monolithic SHS composites

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 3 2021 Besides that, the values of the friction coefficient ( K ) were calculated using the following formula (5) and relative wear ( ε ) using the following formula (6): K = F fr /N ; (5) ε = Δ m/ Δ m ben , (6) where F fr – friction force, N; N – normal pressure force, N; Δ m – sample weight loss, Δ m ben – benchmark weight loss (a sample made out of steel 40Cr was selected as a benchmark). Wear-surface after the trials was explored using optical interferometer-profilometer Wyko, which was also used to determine the surface roughness Ra . Results and its further discussion Heating conditions and geometric size of the samples with different powder mixture compositions were identical. Hot compaction immediately after the completion of the synthesis allows us to obtain dense and non-porous composites. In all the studied composites, reinforcing phases are carbide particles TiC and di- broid titanium TiB 2 . Those phases are formed as a result of exothermic reactions (1), which one can imagine as a combination of chemical reactions: Ti + C → TiC + Q; (7) 3Ti + B 4 C → TiC + 2TiB 2 + Q (8) Grey-colored TiC particles are sized between 0.5 and 2 µm and resemble a shape of a sphere (Fig. 2, a ). Black-colored TiB 2 particles are sized between 2–10 μm and resemble a shape of a cube (Fig. 2, b ). In all studied composites, the distribution of reinforcing particles is uneven in volume: some areas contain mostly TiC particles, some others – TiB 2 . It is obviously associated with heterogeneous carbon (soot) and boron carbide B 4 C distribution in the volume of the original powder mixture, despite thorough mixing of the ini- tial powders before filling a steel casing. The reason for this is the tendency for multi-component powder mixtures, containing different particle shapes, dispersion and bulk density, for mechanical segregation. It is caused by external forces mostly. In our case, gravity is the main force (the process of pouring the powder mixture from the mixer into the pipe container) and pressure power compaction on a hydraulic press). In the areas with predominant soot content a chemical reaction, takes place (7), but in the areas with higher boron carbide concentration the following chemical reaction, takes place (8). Under the same conditions of formation of SHS -composites, different chemical matrix composition affects the structure and nature of the distribution by volume of the material obtained. Depending on the chemical composition, matrix of composites consists of different phases. Composite Fe-Ti-C-B (obtained from powder mixture 1 in table 1) contains an austenitic matrix and reinforcing TiC and TiB 2 particles, which is characterized by hardness of 63-6 HRC and density of R bm 30 = 800 MPa. The microstructure of the composite is shown in the Figure 3, a . Nickel addition to the matrix composition (powder mixture No. 2 in table 1) led to the formation of separate areas with eutectic structure γ-Fe + Fe 2 B in the composite with a particular skeletal structure (Figure 3, b ). Conditions for the formation of such a structure were earlier described [25]. Dendritic axes are enriched in iron and contain boron. Interdendritic spaces are austenite (Figure 4 and table 3). Separate micropores sized no more than 5 μm are fixed in areas with eutectics γ-Fe + Fe 2 B. Perhaps the relatively low bending strength of this composite R bm = 620 MPa is associated with the formation of eutectic structures, the destruction of which requires a bit of work (see Table 2). The basis of the metal matrix of the Fe-Ni-Cr-Ti-C-B composite system formed from a powder mix- ture of composition 3 (see Table. 1), is austenite (indicated by the number 1 in Fig. 5, a ). In addition,