**3. Results and discussion**

**Table 1** shows the mean values of the apparent specific gravity (ASG) measurements of the four samples, and their respective standard deviations. ASG values verified were around 7.5 g/cm3 on average. Through eq. 2, it was obtained about 92% densification (ASG/theoretical density), indicating that SPS sintering was effective. Density may increase as a function of sintering temperature. Thus, it is believed that we can achieve densification greater than 92% at temperatures higher than those used in this study [19, 20].


#### **Table 1.**

*Apparent specific gravity (ASG) is calculated by the Archimedes method.*

**Figure 3** shows it is possible to observe regular shaped powder particles and predominantly spherical morphology, which is a common characteristic of powders produced by atomization and may also have contributed to the good densification achieved. Evaluation of the average size of the powders revealed an average size of 74 to 149 μm.

**Figure 4** shows XRD Colmonoy-5 alloys sintered by SPS. In all sintered samples, in addition to nickel, the presence of several important phases, such as borides (CrB, Ni2B, Cr2B, Ni3B, and Cr5B3), as well as Cr7C3 chromium carbide. The phases verified in the XRD analysis of the bodies sintered by SPS are the same obtained in the diffractograms of the initial powders, which is in perfect agreement with the works [11, 21, 22].

**Figure 5** shows fine precipitated phases and homogeneous distribution. Chromium borides appear in the form of very fine dark modules and small dark blocks. Studies by Ref. [23] on NiCrSiBC alloys indicated in a transmission electron microscope (TEM) analysis that Cr2B chromium boride can be presented in the form of small dark blocks. There is also an intense formation of laths and small blocks in half-ton (lighter phase) well distributed in the matrix phase, which is probably chromium carbide.

It can be seen in **Figure 6**(a), again the presence of chromium borides (dark phase) in the form of fine needles and some small blocks. Note also a lighter phase in the form of a butterfly or wing (see circles), which is a precipitated phase rich in chromium. The EDS mapping in these regions, as shown in **Figure 6**(b), shows a high concentration of chromium in such phases, which actually indicates that it is a chromium-rich precipitate, specifically a Cr5B3 chromium boride [10]. It should be

**106 Figure 4.** *XRD diffractograms of Colmonoy-5 alloys after SPS.*

#### **Figure 5.**

*Microstructure of Colmonoy-5 alloys sintered by SPS.*

**Figure 6.** *(a) Microstructure and mapping region e (b) EDS image.*

**107 Figure 7.** *Average hardness (HV) of the Colmonoy-5 alloy for 3 samples.*

noted that it was not possible to capture boron element (B) by the EDS, due to the equipment limitation.

The average value obtained for the Vickers hardness of the Colmonoy-5 alloy sintered by SPS was 601 ± 6 HV, as shown in **Figure 7**. This result is in agreement with studies by Refs. [11, 12, 24].
