**2. Materials and Methods**

A mixture of elemental Ti (<40 μm, 99.9%, Prolabo, Bankgkok, Thailand), carbon (5 μm, 99.9%, Fischer Scientific, Waltham, MA, USA) and chromium (100–300 μm, 99.9%, ACROS, Boston, MA, USA) powders was sealed into a stainless-steel vial (having a volume of 45 mL) with 5 stainless steel balls (15 mm in diameter and 14 g in mass each) in a glove box filled with purified argon to prevent oxidation. The ball to powder weight ratio was 70:1. The mechanical alloying (MA) procedure of up to 20 h was performed at room temperature using a high-energy ball mill (Fritsch Pulverisette 7 planetary mill, Weimar, Germany), as shown in Figure 1.

**Figure 1.** P7 planetary ball mill and milling media used in the experiments.

The crystalline properties of the powders were characterized by the XRD method, by using a Panalytical XPERT PRO MPD (Cambridge, UK) diffractometer with CuKα radiation. The existing phases were determined by the High Score Plus program based on the ICDD PDF2 data base, while a crystallite size was determined from acquired diffraction data by the FullProf program (Saclay, France) [23] employing the Rietveld method [24]. The morphology of powders was investigated by scanning electron microscopy (SEM, JEOL JSM-5400 model, Tokyo, Japan) coupled with energy dispersive X-ray analysis (EDX). The transmission electron microscopy (TEM) was used for the investigation of internal nanostructure of powders. The structural analysis was complemented by using FEI Tecnai G2 high-resolution TEM (Massachusetts, USA) operated at 200 kV with a spatial resolution of 1.9 Angstrom.

#### **3. Results and Discussion**

#### *3.1. XRD Characterization*

Figure 2a shows the XRD patterns of samples prepared by the MA after various milling times. The acquired XRD pattern of the un-milled powder mixture consists of Bragg diffraction peaks of Ti, Cr, and C. After 1 h 30 min of MA, all the carbon peaks disappeared totally, while the appearance of TiCr2 peak and the reflections of Ti and Cr became broad without any shift in peak positions. Consequently, it is considered that the high-energy impact leads to the transformation of crystalline graphite into amorphous carbon (which is also supported by the literature reports [25,26]). After 3 h of MA, a disappearance of Cr peaks and appearance of (110) peak related to austenite iron (α-Fe) contamination resulted from the abrasive wear between vial and balls, was observed. When the MA time was extended to 5 h, the Ti peaks disappeared completely and the diffraction peaks were broadened and shifted into lower angles as a result of the formation of (Ti,Cr)C solid solution due to refinement of the grain size and the introduction of internal stresses. An analogous behavior was also reported in [27–29]. After continuing the MA for 10 h, the (Ti,Cr)C peaks broadened further due to the decrease in the crystallite sizes and an accumulation of lattice strains caused by fracturing and cold welding of the powder particles during the milling process. Between 10 and 20 h of MA, the structure remains almost unchanged: the (Ti,Cr)C solid solution maintains the NaCl type structure with a space group of Fm-3m. These results are similar to those reported by Shon et al. [30]. They have revealed that the average crystallite size of (Ti,Cr)C powders, obtained from the mixture of Ti-Cr alloy and graphite, MA for 20 h was about 34 nm. Figure 2b presents the Rietveld refinement plot of TiCrC powder after 20 h of the MA process. The calculated profile is represented by the black line, the observed profile by the red dots, and the difference pattern by the blue continuous line. The vertical lines (green) represent the positions of all possible Bragg reflections. The figure indicates that Cr substitutes Ti in the TiC matrix and consequently lead to a decrease in lattice parameter.

**Figure 2.** XRD patterns of samples prepared by MA for different milling times (**a**). Rietveld refinement plot of the sample milled for 20 h (**b**).

The quantitative analysis of microstructural parameters derived from Rietveld refinement of XRD patterns taken from samples MAed for different times, are illustrated in Table 1. It is found that the average crystallite size of (Ti, Cr)C decreases with increasing milling time, from 18 nm after 3 h of the MA and remain nearly constant (about 10 nm) for 15 and 20 h of MA. Thus, all identified phases, namely Ti, Cr, TiCr2, (Ti,Cr)C and Fe, present the same behavior. This decrease can be explained by the refinement of structure and the creation of a large number of crystal defects, such as dislocations and stacking faults [31]. These findings are in line with reports by Zhu et al. [32] who have also connected a decrease in crystallite size with increased dislocation density; reflected as a broadening of XRD peaks. From these observations, it is evident that the reduced particle size plays significant role in inclusion of Cr atoms in TiC matrix. As it is listed in Table 1, the values of mean microstrains are higher for the (Ti, Cr)C phase as compared to other phases. Moreover, the average microstrain of (Ti, Cr)C increases with the MA time, from 0.25 ± 0.01% after 3 h of MA to reach 1.34 ± 0.04% after 20 h of MA. The increase of the microstrain with milling time is generated by the increase of dislocation density and grain boundary fraction, which due to severe plastic deformation generated from the high-energy induced during milling [33]. Furthermore, the calculated lattice parameter of (Ti, Cr)C, obtained from XRD data, decreased from 0.44169 to 0.43036 nm for the powder samples milled for 3 and 20 h, respectively. The value is consistent with that reported by Kwon et al. (a = 0.43075 nm) [34]. This suggest that the lattice parameter was decreased by the substitution of Cr because the atomic radius of Cr (166 pm) is smaller than that of Ti (176 pm) [35]. The decrease in lattice parameter can be explained by the substitution of Ti by Cr and, therefore, by the formation of the Ti0.8Cr0.2C carbide. This finding is consistent with the reported literature for other similar compounds, like TiCx, Cu-TiCx, and Cr2−xMxS3 (M = Ti, V, Sn) [36,37].

**Table 1.** The results of quantitative analysis of microstructural parameters derived from Rietveld refinement for XRD patterns of samples MA for different times.


#### *3.2. SEM Characterization*

Figure 3 shows SEM micrographs of the Ti0.8Cr0.2C powders after the MA for 5, 10, and 20 h. After 5 h of the MA, the grains are formed by an assembly of large particles exhibiting an irregular shape and having an average particle size of about 1–2 μm (Figure 3a). When the MA time was increased to 10 h, we observed significant particle refinement. The initial powders were transformed into fine particles with an average particle size of about 0.4 μm (Figure 3b). The MA up to 20 h results in a further particle size reduction and a formation of homogenous powders with fine and agglomerated particles having the average particle size less than 1 μm (Figure 3c). It is well accepted that the MA process leads to continuous cold welding, fracturing, and rewelding of powder particles [35]. Figure 3d shows an exemplary EDX spectrum acquired from the powder milled for 20 h. The results indicate a presence of major elements (i.e., Ti, Cr, and C). In addition, the results of EDX analyzes revealed trace amounts of Fe, probably coming from the applied milling materials of balls and vial. Furthermore, the results of EDX analyses indicate that the carbide contains 47. 59 C, 40.49 Ti, 9.95 Cr, and 1.97 Fe (at.%). Thus, the alloying ratio is Ti:Cr:C = 0.82:0.201:1, which indicates that carbides may be Ti0.8Cr0.2C.

**Figure 3.** SEM micrographs of samples after MA for5h(**a**); 10 h (**b**); and 20 h (**c**). (**d**) An exemplary EDX spectrum taken form the sample milled for 20 h (zone marked with X).

#### *3.3. TEM Characterization*

Figure 4 shows a TEM micrograph and selected area electron diffraction (SAED) pattern of the sample after the MA for 20 h. Figure 4a reveals that the particle powders were irregular in shape, while the average size was refined to about 15 nm and the agglomeration was high. Therefore, the figure reveals the existence of many grains (i.e., the polycrystalline structure) in nanoscale powder particles. This result was in a good agreement with those obtained by the XRD method. Figure 4b shows the indexed SAED showing only the presence of cubic (Ti,Cr)C solid solution phase in the 20 h milled sample.

To summarize: based on our original experimental findings coming from the pioneering systematic studies, we confirm that a nanostructured (Ti,Cr)C solid solution having an adequate microstructural features and morphology to be applied in the SPS process, can be successfully obtained after 20 h of MA of Ti, Cr, and C elemental powders. The as-obtained nanopowders can be further consolidated through SPS in order to obtain a dense bulk (Ti,Cr)C nanocarbide with enhanced properties compared to TiC carbide. Thus, three parameters in terms of crystallite size, microstrain, and lattice parameter are obtained from XRD line profiles. The values are typical to these of batch powders for the SPS process [36]. Thus, the milling time should be carefully selected taking to account the particle size and shape of the prepared powders. It can be observed that both crystallite size and lattice parameter decrease with increasing milling time, whereas the microstrain increases. These results are in a good agreement with the reference work [38]. As shown in the SEM images, the morphological evolution during the MA process includes three typical steps for a mechanical synthesis of multicomponent intermetallic/interstitial compounds, i.e., (i) a cold welding, (ii) a fracturing, and (iii) a rewelding between particles for longer milling times (between 15 and 20 h). Both applied diffraction-based techniques (TEM/SAED and XRD) confirmed the morphology and structure of MAed powders.

**Figure 4.** (**a**) HRTEM image and (**b**) SAED patterns of the powder after MA for 20 h.

#### **4. Conclusions**

Based on the results of the present study supported by a proper literature data, the following conclusions can be drawn:


**Author Contributions:** M.M. have designed and performed the experiments. M.M. and W.P. have discussed and analyzed the data. M.M has written the draft version of manuscript. W.P. has edited the draft and prepared the final version of manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** Raw experimental data are available on demand from M.M.

**Acknowledgments:** The authors would like to thank to all who have contributed to this work.

**Conflicts of Interest:** The authors declare no conflict of interest.
