**4. Conclusions**

The Fe-18Cr-18Mn-N nickel-free austenitic stainless steel powder was synthesized by mechanical alloying under argon for the first 30 h followed by nitrogen for up to 150 h. The formation of the nanocrystalline Fe-fcc phase involved the bcc to fcc phase transition induced by MA, which was studied via the analysis of powder diffraction data. After the first 30 h of MA in argon, an XRD examination revealed solubility of the main alloying elements (manganese and chromium) in the iron matrix and the mixture of fcc and bcc phase structure. Nitrogen incorporation into the powder intensified the phase transformation. The final morphology of the powder was globular but flattened, with a bimodal distribution. A greater increase in the hardness of the powder was measured after changing the milling atmosphere from argon to nitrogen.

The XRD line profile analysis was illustrated herein by means of a typical case of an experimental study of plastically deformed metal powders subjected to intense ball milling. It was demonstrated that the classic Williamson–Hall method can be relevantly enhanced by incorporating the anisotropic contrast factor, but there is no doubt that WPPM provides a far more detailed description of complex phenomena occurring during MA, and, in general, microstructure evolution. However, despite entirely different methodology of processing the XRD data, our comparison shows that results obtained by MWH and WPPM are coincident in terms of the domain size evolution trend, as both of them indicate that the final value of *d* halved during the MA cycle. The agreement between the grain size trend in WPPM and MWH certainly contributes to validating the calculations. However, each domain size value produced by MWH is generally twice higher compared to the results obtained by WPPM; however, both of these methods still indicate very fine, nanocrystalline sizes of crystallites (~10 nm or below). The source of the discrepancies in the *d* estimation might be numerous but is most likely caused by the fact that neither shape nor distribution of the domain size was taken into account in the MWH procedure. Thus, MWH should mostly be used for the preliminary analysis of diffraction lines and quantitative interpretation of its results should be discouraged.

Our simulations proved that the WPPM algorithm can be successfully adopted to quantify the microstructural parameters of more complex, multi-phase patterns when overlapping of line profiles occur. WPPM delivered detailed information on the evolution of the crystallite size and average defect density. Microstructural information attained by WPPM is useful in terms of optimizing and controlling the phase transformation during MA.

The SEM-EDS analysis of the powder revealed that manganese and chromium content continuously increases up to 90 h of MA; however, after this time, the concentration of these elements is almost unaffected, which suggests that the process of MA can be considered finished. Due to the gentle MA process parameters applied in this work in comparison to the literature, no amorphous phase of the steel powder was detected.

**Author Contributions:** Conceptualization, E.R.-R. and K.N.; methodology, E.R.-R., K.N. and B.S.; formal analysis, E.R.-R., K.N. and B.S.; investigation, E.R.-R. and K.N., data curation, K.N. and E.R.-R.; writing—original draft preparation, E.R.-R., K.N. and B.S.; writing—review and editing, E.R.-R. and B.S.; visualization, E.R.-R. and K.N.; supervision, E.R.-R. and B.S.; project administration, E.R.-R.; funding acquisition, E.R.-R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Institute of Biomedical Engineering, Faculty of Mechanical Engineering, Bialystok University of Technology, project no. WZ/WM-IIB/2/2020.

**Acknowledgments:** The authors are grateful to Zbigniew Oksiuta for the constructive suggestions during the planning and development of this research.

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

#### **References**

