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<p>Surface & Coatings Technology 443 (2022) 128613</p><p>Available online 11 June 2022</p><p>0257-8972/© 2022 Elsevier B.V. All rights reserved.</p><p>Structural-phase transformations and changes in the properties of AISI 321</p><p>stainless steel induced by liquid carburizing at low temperature</p><p>R.A. Savrai *, P.A. Skorynina</p><p>Institute of Engineering Science, Ural Branch, Russian Academy of Sciences, 34 Komsomolskaya St., Yekaterinburg 620049, Russia</p><p>A R T I C L E I N F O</p><p>Keywords:</p><p>Austenitic stainless steel</p><p>Liquid carburizing</p><p>Structure</p><p>Phase composition</p><p>Micromechanical properties</p><p>Corrosion resistance</p><p>A B S T R A C T</p><p>Structural and phase transformations occurring due to the supersaturation of austenite with interstitial atoms</p><p>(carbon and/or nitrogen) are the key priority in the study of AISI 300 series austenitic steels. It is also very</p><p>important to achieve a greater hardening depth by nitriding or carburizing. For this purpose, a method of salt</p><p>bath carburizing at low temperature was proposed. The aim of this work is to perform a detailed analysis of</p><p>structural-phase transformations and their effect on the properties of AISI 321 austenitic steel subjected to liquid</p><p>carburizing at a temperature of 780◦С. Optical and scanning electron microscopy, optical profilometry, X-ray</p><p>diffraction analysis, energy-dispersive microanalysis, electron backscatter diffraction analysis, instrumented</p><p>microindentation and microhardness measurement are used as methods of investigation. It has been discovered</p><p>that, along with carbon-rich (up to 0.46 wt% C) austenite, chromium carbide Cr23C6, cementite Fe3C,</p><p>ε-martensite, and α-martensite are formed in the surface layer of the carburized AISI 321 steel. Carbides are</p><p>present both at the grain boundaries and within the austenite grains. Martensite formed in the carburized AISI</p><p>321 steel is induced by deformation, and the martensitic transformation path is γ → ε → α′ (two-stage trans-</p><p>formation). In its turn, plastic deformation occurs during cooling that follows carburizing, and this is a relaxation</p><p>mechanism of high thermal stresses. Liquid carburizing of AISI 321 steel also multiplies the microhardness of the</p><p>steel surface from 200 ± 7 to 890 ± 110 HV0.025, with the total hardening depth being about 500 μm. The</p><p>hardened layer is gradient and characterized by increased resistance to elastic-plastic deformation, this being</p><p>important for increasing the contact endurance and wear resistance of the steel. It has also been found that, due</p><p>to high carbon concentration, the corrosion resistance of the carburized steel does not deteriorate significantly.</p><p>1. Introduction</p><p>AISI 300 series austenitic steels are the most widely used stainless</p><p>steels. This is why so much attention is strongly focused on studying of</p><p>their structure and properties. The key priority is the study of structural</p><p>and phase transformations occurring by the supersaturation of austenite</p><p>with interstitial atoms (carbon and/or nitrogen) and by the formation of</p><p>so-called expanded austenite or the S-phase. The use of these treatments</p><p>is effective for surface hardening of steels, although carburizing has</p><p>some advantages over nitriding. In particular, it allows the formation of</p><p>gradient hardened layers, which remain paramagnetic [1]. However,</p><p>carburizing contributes to the formation of carbide phases, which can</p><p>negatively affect the range of properties of austenitic stainless steels,</p><p>including their corrosion resistance. Low-temperature carburizing per-</p><p>formed at temperatures below 550◦С provides sufficient hardening of</p><p>the steel surface and preserves corrosion resistance by decreasing the</p><p>probability of carbide formation in the surface layer [1–16]. However,</p><p>the hardening depth obtained by the low-temperature carburizing is</p><p>small, and it usually does not exceed 0.1 mm. Moreover, carbides are</p><p>still able to form after such treatments, including the ones used for the</p><p>manufacture of finished goods [2–9]. In this regard, it is important to</p><p>emphasize that the formation of some quantities of carbides does not</p><p>necessarily lead to a decrease in the corrosion resistance of the carbu-</p><p>rized steels [9,10].</p><p>Carburizing of austenitic steels may lead not only to the formation of</p><p>carbides, but also to other changes in the phase composition. In</p><p>particular, α-martensite can be formed [2–4,10]. The main reason for</p><p>the formation of martensite is considered to be a high level of stresses</p><p>caused by supersaturation of the austenite lattice with carbon. Note that</p><p>the highest experimentally observed carbon content in an austenitic</p><p>steel is 4.74 wt%, and that it is obtained after low-temperature gas</p><p>carburizing of thin foils at a temperature of 380◦С [8]. According to</p><p>* Corresponding author.</p><p>E-mail address: ras@imach.uran.ru (R.A. Savrai).</p><p>Contents lists available at ScienceDirect</p><p>Surface & Coatings Technology</p><p>journal homepage: www.elsevier.com/locate/surfcoat</p><p>https://doi.org/10.1016/j.surfcoat.2022.128613</p><p>Received 26 March 2022; Received in revised form 22 May 2022; Accepted 5 June 2022</p><p>mailto:ras@imach.uran.ru</p><p>www.sciencedirect.com/science/journal/02578972</p><p>https://www.elsevier.com/locate/surfcoat</p><p>https://doi.org/10.1016/j.surfcoat.2022.128613</p><p>https://doi.org/10.1016/j.surfcoat.2022.128613</p><p>https://doi.org/10.1016/j.surfcoat.2022.128613</p><p>http://crossmark.crossref.org/dialog/?doi=10.1016/j.surfcoat.2022.128613&domain=pdf</p><p>Surface & Coatings Technology 443 (2022) 128613</p><p>2</p><p>various studies, compressive stresses in carburized steels range from 1.6</p><p>to 5.8 GPa [1]. Stress relaxation occurs due to plastic deformation of</p><p>austenite, and characteristic signs of deformation, such as slip bands, are</p><p>observed in the steel structure [3]. It should be noted that the formation</p><p>of martensite in carburized austenitic steels has not yet been studied in</p><p>sufficient detail.</p><p>Thus, it is very important to study the possibilities to achieve a</p><p>greater hardening depth by carburizing. For this purpose, a method of</p><p>salt bath carburizing at a temperature of 780◦С was proposed [17,18].</p><p>The reduction of the carburizing temperature, which is usually equal to</p><p>or higher than 850◦С, can significantly slow down the process. There-</p><p>fore, carburizing in a liquid agent was chosen, which ensures faster</p><p>saturation with carbon and more uniform product heating in compari-</p><p>son with other carburizing agents. This method was used for carburizing</p><p>of AISI 321 austenitic steel, and it increased the microhardness of the</p><p>steel surface up to 800 HV0.025. However, the formation of martensite</p><p>and precipitation of chromium carbide Cr23C6 were found at the</p><p>austenite grain boundaries, but no carbides were detected within the</p><p>austenite grains. It was also concluded that the martensite formed in</p><p>AISI 321 steel after carburizing was likely to be induced by deformation.</p><p>In this regard, a more detailed study of the structure and phase</p><p>composition of AISI 321 steel subjected to liquid carburizing is required.</p><p>In particular, it is necessary to clarify the thickness of the hardened</p><p>layer, the type and distribution of carbides, as well as specific features of</p><p>the martensitic transformation in the surface layer of the carburized</p><p>steel. The aim of this paper is to perform a detailed analysis of structural-</p><p>phase transformations and their effect on the properties of AISI 321</p><p>austenitic steel subjected to liquid carburizing at a temperature of</p><p>780◦С.</p><p>2. Experimental procedure</p><p>2.1. Material and specimens</p><p>A rolled sheet of AISI 321 austenitic steel with a nominal thickness of</p><p>10 mm was used in this study. The steel composition is given in Table 1.</p><p>Specimens for carburizing were cut in the form of plates with di-</p><p>mensions of 40 by 52 mm. Austenitizing of the specimens was carried</p><p>out at a temperature of 1100 ◦C for 40 min, followed by water cooling.</p><p>The surfaces of the specimens were mechanically ground and polished as</p><p>described in Ref. [19], and additionally electrolytically polished as</p><p>described in Ref.</p><p>[20].</p><p>2.2. Carburizing technique</p><p>Liquid carburizing of the polished specimens was performed in a salt</p><p>bath at a temperature of 780◦С for 15 h, followed by water cooling. The</p><p>carburizing agent consisted of 80 wt% Na2CO3 + 10 wt% NaCl +10 wt%</p><p>SiC (a mixture of salts with silicon carbide additive). An electric furnace</p><p>and a stainless steel crucible with a diameter of 100 mm and a height of</p><p>300 mm were used. In order to remove the oxide film, the carburized</p><p>specimens were electrolytically etched in a solution of 90 wt%</p><p>CH3COOH + 10 wt% H2ClO4 for 30 s.</p><p>2.3. Corrosion testing</p><p>Specimens of both the austenitized and carburized steel with di-</p><p>mensions of 7 by 7 mm and a thickness of 2 mm were prepared for</p><p>general corrosion tests. The corrosive solution consisted of a mixture of</p><p>aqueous solutions of NaCl (20 wt%) and HCl (30 wt%) in an equal ratio.</p><p>This medium gives strong corrosion attack on a material due to its ability</p><p>to destroy the passivation layer, thus providing the continuous flow of</p><p>the corrosion process. This makes it possible to conduct comparative</p><p>tests correctly. The prepared specimens were immersed into the corro-</p><p>sive solution for up to 18 h until the corrosion rate was stabilized. The</p><p>solution was not agitated. During the tests, the specimens were peri-</p><p>odically weighed. In order to remove potential corrosion products, the</p><p>specimens were carefully rinsed, wiped and dried before weighing.</p><p>Weight loss was determined on a Demcom DA-65C laboratory scale with</p><p>an accuracy of 0.01 mg. Corrosion rate km (g/m2h) was calculated by the</p><p>formula km = Δm/(S⋅τ), where Δm is weight loss, g; S is surface area of</p><p>the tested specimen, m2; τ is test time, h.</p><p>Electrochemical corrosion tests were performed using a VoltaLab 10-</p><p>PGZ100 potentiostat with the VoltaMaster 4 control software. A con-</p><p>ventional three-electrode cell consisting of a working electrode (the</p><p>specimen), a Pt counter electrode, and an Ag/AgCl reference electrode</p><p>was used. The measurements were carried out in a stagnant aqueous</p><p>solution of 3.5 wt% NaCl at room temperature. All potentials were</p><p>referred to the Ag/AgCl reference electrode. Specimens of the same di-</p><p>mensions as for general corrosion tests were used. To determine the free</p><p>corrosion potential Ecorr, the specimens were kept in the corrosive so-</p><p>lution for one hour. The cyclic potentiodynamic polarization method</p><p>was used to estimate the pitting corrosion tendency of both the auste-</p><p>nitized and carburized steel. The potential was scanned at a scan rate of</p><p>0.41 mV/s (the scan rate was adjusted by the control software) starting</p><p>from the Ecorr value in the positive direction until the current density</p><p>reached predefined maximum value jmax = 1.5 mA/cm2. Then the</p><p>scanning direction was reversed until zero current density was reached.</p><p>The pitting potential Epit and the repassivation potential Erp were</p><p>determined at the current density jmin = 0.1 mA/cm2 from the forward</p><p>and reverse potentiodynamic curves, respectively. The jmin and jmax</p><p>current densities were determined empirically.</p><p>2.4. Microstructure characterization and measurement methods</p><p>The microstructure of the austenitized steel etched in the solution of</p><p>20 cm3 HF + 10 cm3 HNO3 + 20 cm3 C3H5(OH)3 (glycerin) [21] was</p><p>examined by optical microscopy (OM) using a Neophot-21 microscope.</p><p>The microstructure, chemical and phase composition, as well as the</p><p>surface of the carburized steel, were studied by scanning electron mi-</p><p>croscopy (SEM) using a Tescan VEGA II microscope with energy-</p><p>dispersive microanalysis (EDS) and electron backscatter diffraction</p><p>analysis (EBSD) systems. The prepared cross sections were etched in a</p><p>solution of 25 vol% HNO3 + 75 vol% HCl. For EBSD analysis, the cross</p><p>sections were not etched, but additionally polished with a colloidal silica</p><p>suspension (40 nm).</p><p>The roughness and surface topography features of the steel before</p><p>and after carburizing were investigated by optical profilometry using a</p><p>Wyko NT-1100 profilometer. The arithmetic average of the profile Ra</p><p>and the average maximum height of the profile Rz were determined.</p><p>Vickers microhardness was measured at a load of 0.245 N, a loading</p><p>speed of 40 μm/s and holding under the load for 15 s using a Shimadzu</p><p>HMV-G21DT hardness tester. Microhardness distribution over the</p><p>thickness of the carburized layer was studied on the cross section.</p><p>Microindentation characteristics were determined in accordance with</p><p>ISO 14577 at the maximum force P ranging from 9.8 mN (1 gf) to 1960</p><p>mN (200 gf), force application time of 20 s, with holding under the load</p><p>for 15 s and force removal period of 20 s using a Fischerscope HM2000</p><p>XYm instrumented indentation system with a Vickers indenter and the</p><p>WIN-HCU control software. The maximum indentation depth hmax, the</p><p>Table 1</p><p>Chemical composition (wt%) of AISI 321 steel.</p><p>C Cr Ni Ti Mn Si Mo Co Nb Cu P S Fe</p><p>0.05 16.80 8.44 0.33 1.15 0.67 0.26 0.13 0.03 0.31 0.036 0.005 Bal.</p><p>R.A. Savrai and P.A. Skorynina</p><p>Surface & Coatings Technology 443 (2022) 128613</p><p>3</p><p>permanent indentation depth hp, the indentation hardness measured at</p><p>the peak load HIT, the Martens hardness HM, the contact elastic modulus</p><p>E*, the elastic deformation work of indentation We and the total me-</p><p>chanical work of indentation Wt were determined. In order to estimate</p><p>the ability of the steel to resist elastic-plastic deformation, the measured</p><p>characteristics were used to calculate the ratio HIT/E* [22], elastic re-</p><p>covery Re = (hmax − hp)/hmax × 100% [23,24], the power ratio HIT</p><p>3 /E*2</p><p>[25] and plasticity characteristic δA = 1 − (We/Wt) [26].</p><p>X-ray diffraction (XRD) phase analysis was performed using a DRON-</p><p>3 diffractometer with CuКα radiation. Volume fraction of the α-phase Vα</p><p>(vol%) was calculated by the formula Vα = 100/{1 + 1.45 × (I(111)γ/</p><p>I(110)α)} [27], where I(111)γ and I(110)α are integral intensities of the</p><p>(111)γ and (110)α lines, respectively. Lattice parameter of austenite aγ</p><p>(Å) was calculated for the (111)γ plane by the formula aγ = dhkl⋅</p><p>̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅(</p><p>h2 + k2 + l2</p><p>)√</p><p>[28], where dhkl is interplanar spacing, Å; h, k, l are</p><p>plane indices. Carbon content was determined by optical emission</p><p>spectrometry (OES) using a Spectromaxx F analyzer.</p><p>3. Results and discussion</p><p>3.1. Microstructure and phase composition</p><p>Prior to carburizing, the heat treatment of AISI 321 steel has formed</p><p>a structure comprising polyhedral austenite grains (γ-phase) and in-</p><p>clusions of titanium carbide TiC (Fig. 1a). The XRD phase analysis has</p><p>shown no α-phase in the structure of the austenitized steel (Fig. 2a).</p><p>Electrolytic polishing of the specimens resulted in the formation of a</p><p>smooth surface with typical etch pits and the roughness characteristics</p><p>Ra = 0.04 μm and Rz = 2.06 μm (see Fig. 1b).</p><p>After liquid carburizing at a temperature of 780◦С, the XRD phase</p><p>analysis has shown that, along with the γ-phase, there are the α-phase</p><p>and magnetite Fe3O4 in the surface layer of AISI 321 steel (see Fig. 2b).</p><p>The high intensity of Fe3O4 peaks indicates that the formed oxide film is</p><p>sufficiently dense, and it can impede the identification of the phases. In</p><p>some areas of the steel surface, the thickness of the oxide film can reach</p><p>10–15 μm (Fig. 3). It is important to emphasize that chromium oxide</p><p>Cr2O3 has not been detected (see Fig. 2b). This is due to the destruction</p><p>of the protective oxide layer, which is observed on chromium‑nickel</p><p>steels at high temperatures in corrosive environments. In this case, the</p><p>passivation behavior is absent [29]. Besides, the carburizing agent used</p><p>in laboratory conditions is not optimal from the point of view of pro-</p><p>tection against oxidation. To reduce the thickness of the oxide layer, the</p><p>carburizing agent of</p><p>a different composition should be used, and this is</p><p>easy to implement in industrial conditions.</p><p>Short-term electrolytic etching has made it possible to remove the</p><p>oxide film almost entirely (Fig. 4) and to identify the phase composition</p><p>of the carburized steel more accurately. In particular, chromium carbide</p><p>Cr23C6 and cementite Fe3C have been additionally found (see Fig. 2c).</p><p>Besides, the intensity of the α-phase peak has increased (see Fig. 2c). It</p><p>should be noted that in [17] the chromium carbide Cr23C6 was found in</p><p>the carburized steel, although the steel surface was not cleaned from</p><p>oxides. This may be due to the fact that the penetration depth of CrКα</p><p>radiation used in that study is approximately twice that of CuКα radia-</p><p>tion used in the present work. Nevertheless, no cementite was found in</p><p>Ref. [17], and this could point to its high dispersity.</p><p>The removal of the oxide film has also allowed a more detailed ex-</p><p>amination of the surface of the carburized AISI 321 steel. It is evident</p><p>from Fig. 4a that slip bands are formed in the austenite grains. Moreover,</p><p>the calculated lattice parameter of austenite is a0 = 3.606 Å prior to</p><p>carburizing and aγ = 3.588 Å after carburizing, that is, its reduction is</p><p>observed. This indicates that microstrain has occurred in the material</p><p>[30]. It is known that high residual stresses take place in the surface</p><p>layers of carburized steel and that they can reach 2000 MPa or even</p><p>higher values [1,31]. Residual stresses are a consequence of thermal</p><p>stresses. In austenitic steels, the relaxation of thermal stresses can be</p><p>produced by plastic deformation during cooling that follows</p><p>Fig. 1. Microstructure (a) and surface appearance (b) of AISI 321 steel prior to carburizing.</p><p>Fig. 2. X-ray diffraction patterns for AISI 321 steel prior to carburizing (a),</p><p>after liquid carburizing (b), and after carburizing and removal of the oxide</p><p>film (c).</p><p>R.A. Savrai and P.A. Skorynina</p><p>Surface & Coatings Technology 443 (2022) 128613</p><p>4</p><p>carburizing. For example, the formation of slip bands and strain-induced</p><p>martensite was observed in the surface layer of AISI 304L steel subjected</p><p>to low-temperature carburizing [3]. Austenite grains in a polycrystalline</p><p>material are oriented differently with respect to acting stresses. There-</p><p>fore, some grains demonstrate only single slip bands, whereas other</p><p>grains have more such bands since more slip systems are activated (see</p><p>Fig. 4a). The oxide-free surface of the carburized AISI 321 steel has a</p><p>higher surface roughness (see Fig. 4b) in comparison with the electro-</p><p>lytically polished surface (see Fig. 1b). In particular, Ra has increased</p><p>from 0.04 to 0.54 μm, and Rz has increased from 2.06 to 6.57 μm. There</p><p>are several factors that may affect the surface roughness of carburized</p><p>steels, in particular, plastic deformation. However, oxidation and sub-</p><p>sequent removal of the oxide layer also increases the surface roughness.</p><p>This makes it difficult to assess the contribution of each individual</p><p>factor.</p><p>Fig. 5 illustrates the structure of the carburized AISI 321 steel. It can</p><p>be seen from Fig. 5a that a continuous layer with nonuniform etchability</p><p>locates at a depth of up to 130–150 μm. At a depth ranging from</p><p>130–150 μm to 500–600 μm, nonuniform etchability is mostly intrinsic</p><p>to austenite grain boundaries and adjacent areas (see Fig. 5a). A more</p><p>detailed examination of the microstructure of the layer with nonuniform</p><p>etchability has revealed the presence of particles precipitated at the</p><p>austenite grain boundaries, as well as clusters of dispersed particles in</p><p>the vicinity of these boundaries (see Fig. 5b). Within the austenite</p><p>grains, the formation of a large number of acicular crystals (indicated</p><p>with arrows A in Fig. 5c,d) is observed. It is evident that there is a</p><p>crystallographic relationship of these crystals with austenite since they</p><p>have a similar orientation within an austenite grain and different ori-</p><p>entations within the adjacent grains (see Fig. 5b-d). Along with the</p><p>acicular crystals, both highly dispersed (about 100 nm) particles (indi-</p><p>cated with arrows B in Fig. 5c,d) and coarser particles that form linear</p><p>clusters characterized by a certain orientation (indicated with arrows C</p><p>in Fig. 5c,d) are observed within the austenite grains. In the underlying</p><p>layers (at a depth below 130–150 μm), only particles that have precip-</p><p>itated at the austenite grain boundaries are observed, but there are no</p><p>structural changes within the austenite grains (see Fig. 5e).</p><p>In accordance with the data of the EDS analysis of the carburized</p><p>steel (Table 2), the particles at the austenite grain boundaries (indicated</p><p>Fig. 3. SEM cross-sectional view (a) and the corresponding EDS oxygen map (b) for the oxide film on the surface of the carburized AISI 321 steel.</p><p>Fig. 4. Surface appearance (SEM and optical profilometry images) of the carburized AISI 321 steel after the removal of the oxide film.</p><p>R.A. Savrai and P.A. Skorynina</p><p>Surface & Coatings Technology 443 (2022) 128613</p><p>5</p><p>with arrows 1 and 4 in Fig. 5b,e) have an increased carbon and chro-</p><p>mium content, and, taking into account the data of the XRD phase</p><p>analysis (see Fig. 2c), they can be identified as chromium carbides</p><p>Cr23C6. However, due to the high dispersity of the structure, the EDS</p><p>analysis reveals no essential variations in the chemical composition</p><p>between the structural elements within the austenite grains (see Table 2,</p><p>points 2 and 3, respectively). Carbon distribution over the thickness of</p><p>the surface layer (see Fig. 5f) testifies that, at a depth of up to 90 μm, the</p><p>carbon content is high and averages 4.0 wt%. At greater depths, the</p><p>carbon content gradually decreases, and at a depth of 150 μm, it is 0.7 wt</p><p>%. At a depth exceeding 150 μm there is a non-uniform distribution of</p><p>carbon, and the peaks correspond to the austenite grain boundaries and</p><p>adjacent areas (see Fig. 5a). In particular, the peak at a depth of 220 μm</p><p>corresponds to the chromium carbides shown in Fig. 5e.</p><p>EBSD analysis was carried out in order to clarify the phase compo-</p><p>sition and structure features of the carburized AISI 321 steel (Fig. 6). It</p><p>can be seen from Fig. 6a that the particles at the austenite grain</p><p>boundaries (indicated with arrows 1 and 4 in Fig. 5b,e) are indeed the</p><p>Fig. 5. SEM cross-sectional images of the microstructure (a-e) and distribution of carbon (EDS) in the surface layer (f) for the carburized AISI 321 steel. Squares 1, 2,</p><p>and 3 in (a) indicate areas used for (b, c), (d), and (e) images, respectively. Arrow in (a) indicates the linescan, and arrows 1, 2, 3 in (b) and 4 in (e) indicate the points</p><p>for quantitative EDS analysis. Arrows A, B, and C in (c, d) indicate the characteristic areas of the microstructure.</p><p>R.A. Savrai and P.A. Skorynina</p><p>Surface & Coatings Technology 443 (2022) 128613</p><p>6</p><p>chromium carbide Cr23C6, and the clusters of dispersed particles in the</p><p>vicinity of the boundaries (see Fig. 5b) are mostly cementite. The pre-</p><p>cipitation of coarser particles of the chromium carbide Cr23C6 at the</p><p>grain boundaries (see Fig. 5b,e and 6a) is caused by a higher diffusion</p><p>rate and diffusion mobility of atoms. The carbon content in the chro-</p><p>mium carbide Cr23C6 is slightly lower than that in cementite Fe3C and</p><p>equals to 5.68 and 6.67 wt%, respectively. This is why the particles of</p><p>chromium carbide</p><p>Cr23C6 with the lower carbon content are coarser</p><p>under conditions of sufficient diffusion mobility of the chromium atoms.</p><p>It should also be noted that the morphology of the grain-boundary</p><p>chromium carbides Cr23C6 varies to a significant extent (see Fig. 5b,e).</p><p>This is due to the fact that the morphology of carbides is closely related</p><p>to the misorientation of the grain boundaries. At the boundary where the</p><p>density of coincidence points between two adjacent grains is high</p><p>(special boundary) lamellar carbides are mostly observed. As the</p><p>misorientation between the two adjacent grains increases, angular car-</p><p>bides (mostly triangular in shape) begin to prevail [32].</p><p>The EBSD analysis has also shown that, along with the grain</p><p>boundary carbides, there are dispersed particles of the chromium car-</p><p>bide Cr23C6 and cementite Fe3C within the austenite grains. The amount</p><p>of cementite is noticeably larger, its particles are coarser and they can</p><p>form clusters (see Fig. 6a). Thus, the observed highly dispersed particles</p><p>(indicated with arrows B in Fig. 5c,d) are the chromium carbide Cr23C6,</p><p>and the coarser particles that form linear clusters (indicated with arrows</p><p>C in Fig. 5c,d) are cementite. Within the austenite grains, substitutional</p><p>diffusion is constrained, and the saturation of austenite with carbon</p><p>atoms makes this process even more difficult. Under these conditions,</p><p>cementite forms more easily than the chromium carbide Cr23C6. It</p><p>should also be noted that the carbides form either at the grain bound-</p><p>aries or next to the areas of localized plastic deformation (see Fig. 6a).</p><p>This is caused by the fact that the defects in the crystal structure are the</p><p>preferred nucleation sites, and plastic deformation, which has occurred</p><p>in the carburized steel and resulted in the formation of a large number of</p><p>defects, facilitates the formation of carbides [2]. The observed orienta-</p><p>tion of the linear clusters of carbides (indicated with arrows C in Fig. 5c,</p><p>d) can also be associated with plastic deformation occurring in well-</p><p>defined crystallographic planes. However, this requires further</p><p>investigation.</p><p>Thus, the high (approximately 4.0 wt%) carbon content in the sur-</p><p>face layer at a depth of up to 90 μm (see Fig. 5f) is caused not only by the</p><p>saturation of austenite with carbon, but also by the presence of a large</p><p>number of dispersed carbides within the austenite grains (see Fig. 5c,</p><p>d and 6a). In order to determine the carbon content (XC) in austenite,</p><p>equations like aγ = a0 + kXC are often used. However, as noted above,</p><p>plastic deformation has occurred in the surface layer of the carburized</p><p>AISI 321 steel, and the lattice parameter of austenite has decreased due</p><p>to deformation. Therefore, the use of such an equation is incorrect in this</p><p>case. For this purpose, the OES analysis of the carburized AISI 321 steel</p><p>specimen was performed. Note that the depth of the analyzed layer does</p><p>not exceed 100 μm. The analysis has shown that the average carbon</p><p>content in the carburized steel is 0.46 wt%. This amount can be taken as</p><p>the upper limit of carbon content in austenite.</p><p>The carburized AISI 321 steel also contains α-martensite in the</p><p>amount of 20 vol% according to the XRD data (see Fig. 2c). It can be seen</p><p>from Fig. 6a that martensite is non-uniformly distributed in the surface</p><p>layer of the steel, and its amount in the adjacent grains varies to a sig-</p><p>nificant extent. As noted above, the adjacent austenite grains deform</p><p>differently (see Fig. 4a) due to their different orientations with respect to</p><p>the acting stresses, this being consistent with the EBSD data (see Fig. 6b).</p><p>Therefore, the amount of the formed martensite should also vary, and</p><p>this is observed (see Fig. 6a). This indicates that the martensite in the</p><p>carburized AISI 321 steel is induced by deformation. In its turn, plastic</p><p>deformation occurs during cooling that follows carburizing. According</p><p>to classical concepts, the intersections of slip bands are the preferred</p><p>nucleation sites for strain-induced martensite (the double shear theory)</p><p>[33,34]. However, it follows from Fig. 6 that the formation of martensite</p><p>occurs within individual (non-intersecting) slip bands. Modern studies</p><p>of austenitic steels, including chromium‑nickel ones, have shown that</p><p>the key factor determining the specific features of martensite formation</p><p>is the stacking fault energy (SFE) of steel. The intersection of slip bands</p><p>is a prerequisite for the formation of strain-induced martensite only for</p><p>steels with high SFE [35–37]. In steels with low SFE (including AISI 321</p><p>steel), strain-induced nucleation occurs as follows. In an f.c.c. lattice,</p><p>slip occurs on {111}γ planes, and stacking faults are formed on the same</p><p>planes [38] (Fig. 7). Therefore, deformation is accompanied by the</p><p>formation of stacking faults within slip bands (actually, an individual</p><p>slip band introduces the first shear). An increase in the density of</p><p>stacking faults leads to the formation of an intermediate ε-phase, which,</p><p>in turn, is the embryo of α′-martensite. In this case, acting shear stresses</p><p>provide sufficient mechanical driving force for the ε → α′ martensitic</p><p>transformation [36]. A schematic illustration of the γ → ε and ε → α′</p><p>transformations [39] is shown in Fig. 8.</p><p>The presence of the ε-phase, which is located next to the</p><p>α-martensite, is confirmed by the EBSD analysis (see Fig. 6a). Note that</p><p>the ε-phase has not been found by the XRD method (see Fig. 2). This is</p><p>caused by its low residual content, as well as by the possible complete</p><p>transformation of the ε-phase into the α-phase directly on the steel</p><p>surface due to the highest level of stresses. The pole figures shown in</p><p>Fig. 6e indicate that there is a certain orientation relationship among the</p><p>lattices of austenite, ε-phase, and α-martensite, namely {111}γ‖</p><p>{0001}ε, {0001}ε‖{110}α and 〈2− 1− 10〉ε‖〈111〉α. This relationship is</p><p>characteristic of the strain-induced martensitic transformation with the</p><p>formation of the intermediate ε-phase [39]. Note that the Kurdju-</p><p>mov–Sachs orientation relationship is also maintained (see Fig. 6e).</p><p>Thus, the martensite formed in AISI 321 steel after liquid carburizing at</p><p>a temperature of 780◦С is actually caused by deformation, and the</p><p>martensitic transformation path is γ → ε → α′ (two-stage trans-</p><p>formation). Note also that this mechanism of martensitic transformation</p><p>is observed for steels with higher nickel content (10 wt% or more) and,</p><p>consequently, higher SFE compared to AISI 321 steel [36]. However, the</p><p>saturation of austenite with carbon should lead to a certain increase in</p><p>the SFE of AISI 321 steel, which compensates for the lack of nickel. It is</p><p>known that carbon has a strong influence on the SFE of austenitic steels</p><p>and contributes to its increase [40,41].</p><p>The EBSD data also show that the martensite formed in the surface</p><p>layer of the carburized AISI 321 steel has a complex structure (see</p><p>Fig. 6c). In particular, three different orientations of martensite crystals</p><p>with misorientation angles of about 90◦ are observed within an indi-</p><p>vidual slip band (see Fig. 6d). It also follows from Fig. 5d that the</p><p>martensite crystals (indicated with arrows A in Fig. 5d) have three</p><p>different orientations and form a kind of a triangle. This indicates that</p><p>the deformation occurs in three systems simultaneously (see Fig. 7). It is</p><p>also important to emphasize that some martensitic areas, which are</p><p>observed both near the austenite grain boundaries and within the</p><p>austenite grains (see Fig. 6c), are not related to the slip bands. The</p><p>presence of slip bands indicates that large deformation has occurred in</p><p>the steel, and adjacent austenitic areas are also deformed, although to a</p><p>lesser extent. In some cases, the accumulated strain may be sufficient to</p><p>initiate the martensitic transformation in such areas.</p><p>Table 2</p><p>EDS microanalysis results of the carburized AISI 321 steel for the points illus-</p><p>trated in Fig. 5b,e.</p><p>Point no. Content, wt%</p><p>C Ti Cr Mn Fe Ni Si</p><p>1 7.20 0.44 29.04 1.20 55.22 6.35 0.56</p><p>2 3.33 0.30 18.48 1.06 68.30 7.94 0.59</p><p>3 3.49 – 20.33 1.07 66.38 8.14 0.60</p><p>4 8.47 0.58 24.69 1.23 57.66 6.56 0.44</p><p>R.A. Savrai and P.A. Skorynina</p><p>Surface & Coatings Technology 443 (2022) 128613</p><p>7</p><p>(caption on next page)</p><p>R.A. Savrai and P.A. Skorynina</p><p>Surface & Coatings Technology 443 (2022) 128613</p><p>8</p><p>3.2. Microhardness and micromechanical properties</p><p>Liquid carburizing of AISI 321 steel multiplies the microhardness of</p><p>the steel surface by more than four times, namely from 200 ± 7 to 890</p><p>± 110 HV0.025 (Fig. 9). This is comparable to the hardness of the steel</p><p>subjected to surface strain hardening, when the martensite content in</p><p>the surface layer is close to 100 vol% [20,42–44]. As noted above, the</p><p>surface layer of the carburized steel contains about 20 vol% of</p><p>martensite. Therefore, the increased microhardness of the carburized</p><p>AISI 321 steel is caused by the following factors: a) solid solution</p><p>hardening resulting from the saturation of austenite with carbon, b) the</p><p>formation of strain-induced martensite (see Figs. 5b-d and 6a,c), c) the</p><p>increase in the density of structural defects, which accompanies plastic</p><p>deformation, and d) dispersion hardening due to the precipitation of the</p><p>chromium carbide Cr23C6 and cementite Fe3C within the austenite</p><p>grains (see Figs. 5c,d and 6а). The surface layer of the carburized steel is</p><p>characterized by a pronounced negative hardness gradient in the layer</p><p>up to 150 μm deep, with the total hardening depth being about 500 μm</p><p>(see Fig. 9). The decrease in the microhardness of the steel with</p><p>increasing depth is due to the gradually decreasing amount of the car-</p><p>bide phase, α′-martensite and the carbon content in austenite (see</p><p>Fig. 5c,d,f). Note that the hardening depth was overestimated in [17].</p><p>The analysis has shown that this is due to the formation of surface</p><p>martensite during the preparation of the metallographic specimen since</p><p>the steel is characterized by strain-induced instability. Careful prepa-</p><p>ration of the metallographic cross section made it possible to avoid the</p><p>formation of surface martensite and to determine the thickness of the</p><p>hardened layer more accurately.</p><p>Fig. 10 shows the microindentation data obtained for AISI 321 steel</p><p>before and after carburizing. It can be seen that in the whole range of</p><p>maximum indentation forces, the carburizing of the steel results in</p><p>decreased values of hmax, hp, Wt and increased values of HM, HIT, We, this</p><p>being due to the hardening of the material [20,42,49]. The contact</p><p>elastic modulus E* also increases, and the reason for this is the saturation</p><p>of austenite with carbon and the increasing density of structural defects,</p><p>which accompanies plastic deformation. The structural mechanism of</p><p>increasing the elastic modulus is the formation of a structure with</p><p>limited mobility of dislocations [51], as well as an increase in the</p><p>strength of interatomic bonds [52]. It can be seen from Fig. 10c-e that</p><p>the characteristics HM, HIT, and E* decrease with the increasing</p><p>maximum indentation force for the carburized AISI 321 steel, whereas</p><p>they are stable for the non-carburized steel. As noted above, the</p><p>Fig. 6. EBSD phase map (a), IPF-Z orientation maps of austenite (b) and α-martensite (c), preferred orientations of martensitic crystals within an individual slip band</p><p>(d) and local pole figures of phases within an individual slip band (e) for the carburized AISI 321 steel. Arrow in (a) indicates the individual slip band used for (d) and</p><p>(e) images. The dashed lines in (b, c) indicate the austenite grain boundaries. Red and green circles in (e) indicate parallel planes and directions, respectively. (For</p><p>interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</p><p>Fig. 7. An example of slip and twin systems in an f.c.c. lattice.</p><p>Fig. 8. Schematic representation of the γ → ε and ε → α′ transformations [39].</p><p>Fig. 9. Distribution of microhardness HV0.025 in the surface layer of the</p><p>carburized AISI 321 steel. The microhardness of the steel prior to carburizing is</p><p>indicated by the dashed line.</p><p>R.A. Savrai and P.A. Skorynina</p><p>Surface & Coatings Technology 443 (2022) 128613</p><p>9</p><p>hardened layer is gradient, and this causes the observed changes in HM,</p><p>HIT, and E*. Note that as the maximum indentation force increases, the</p><p>characteristics hmax, hp, Wt, and We also increase (see Fig. 10a,b,f,g) since</p><p>they are directly proportional to the indentation force.</p><p>The increased parameters Re, HIT/E*, and HIT</p><p>3 /E*2 for the carburized</p><p>AISI 321 steel (see Fig. 10h-j) testify that liquid carburizing at a tem-</p><p>perature of 780◦С increases its resistance to elastic-plastic deformation</p><p>[22–25] and can result in its better wear resistance and contact</p><p>endurance [20,45–50]. It can be seen from Fig. 10h that the parameter</p><p>Re increases with the increasing maximum indentation force for the non-</p><p>carburized AISI 321 steel. This may be due to the formation of strain-</p><p>induced martensite during indentation, with a higher amount of</p><p>martensite at higher loads. As the maximum indentation force increases,</p><p>the parameters HIT/E* and HIT</p><p>3 /E*2 decrease for the carburized AISI 321</p><p>steel (see Fig. 10i,j). This is caused by the observed decrease in HIT and</p><p>E* (see Fig. 10d,e). The decreased parameter δA for the carburized steel</p><p>Fig. 10. Dependence of the maximum indentation depth hmax (a), permanent indentation depth hp (b), Martens hardness HM (c), indentation hardness measured at</p><p>the peak load HIT (d), contact elastic modulus E* (e), elastic deformation work of indentation We (f), total mechanical work of indentation Wt (g), elastic recovery Re</p><p>(h), the ratio HIT/E* (i), the power ratio HIT</p><p>3 /E*2 (j) and plasticity characteristic δA (k) on the maximum indentation force P for AISI 321 steel prior to carburizing (1)</p><p>and after the liquid carburizing (2).</p><p>R.A. Savrai and P.A. Skorynina</p><p>Surface & Coatings Technology 443 (2022) 128613</p><p>10</p><p>(see Fig. 10k) testifies to reduced plasticity and results from surface</p><p>hardening. Thus, the hardened layer is gradient and characterized by</p><p>increased resistance to elastic-plastic deformation, this being important</p><p>for increasing the contact endurance and wear resistance of the steel.</p><p>3.3. Corrosion behavior</p><p>The results of general corrosion tests are shown in Figs. 11, 12 and</p><p>13. It can be seen that the corrosion rate of the carburized steel is even</p><p>lower than that of the austenitized steel (see Fig. 11), and that there is a</p><p>fairly uniform general corrosion of the carburized steel without signif-</p><p>icant localized corrosion (see Fig. 12b). The appearance of the austeni-</p><p>tized AISI 321 steel specimen also indicates sufficiently intense</p><p>corrosion (see Fig. 12a). The average corrosion rate at the steady stage is</p><p>0.32 and 0.40 g/m2h, respectively, despite the precipitation of the</p><p>chromium carbide Cr23C6 at the grain boundaries (see Figs. 5b,e and 6a)</p><p>and the formation of martensite (see Figs. 5c,d and 6). This is due to the</p><p>fact that interstitial atoms, in particular carbon, stabilize the electronic</p><p>structure of iron, thus increasing its corrosion resistance. Along with</p><p>this, the oxyanions formed by these elements, such as HCO3</p><p>− and CO3</p><p>2− ,</p><p>are effective inhibitors to suppress anodic corrosion [53]. Moreover,</p><p>recent studies have shown that the active dissolution of α-martensite is</p><p>suppressed by the presence of interstitial carbon. In particular, the</p><p>electron density of the states (DOS) of iron atoms near and at the Fermi</p><p>level decreased with an increase in the carbon concentration. This in-</p><p>dicates that the dissolved carbon enhances the electrochemical stability</p><p>of b.c.t. martensite. Thus, the interaction between iron and carbon</p><p>atoms ensures high stability of the electronic structure of martensite,</p><p>and this contributes the suppression of its active dissolution [54].</p><p>The study of the cross section convincingly shows that the carburized</p><p>layer is preserved after the corrosion tests (see Fig. 13a). Note that the</p><p>cross section was additionally etched to reveal the carburized layer. It</p><p>can be seen that the thickness of the carburized layer has not changed</p><p>significantly. This is consistent with the obtained corrosion rate, which</p><p>indicates that the thickness of the carburized layer has decreased by</p><p>about 1 μm during the tests. The microhardness of the carburized AISI</p><p>321 steel decreases from 890 to 410 HV0.025 after the corrosion tests</p><p>(see Figs. 9 and 13b). As noted above, the surface layer of the carburized</p><p>steel is characterized by a pronounced negative hardness gradient (see</p><p>Fig. 9). Therefore, even a slight decrease in its thickness should be</p><p>accompanied by a decrease in microhardness. However, it follows from</p><p>Fig. 9 that a decrease in the thickness of the carburized layer by 1 μm</p><p>should not lead to such a significant decrease in the surface micro-</p><p>hardness. It is the corrosive environment that loosens the steel surface,</p><p>thus reducing its hardness further. At a depth of several microns, the</p><p>hardness is much higher (see Fig. 13a), and, at greater depths, it de-</p><p>creases approximately the same as before the corrosion tests (see Figs. 9</p><p>and 13b).</p><p>The results of electrochemical corrosion tests are shown in Fig. 14</p><p>and Table 3. It can be seen that the pitting potential Epit for the carbu-</p><p>rized steel is lower than that of the austenitized steel. This is due to the</p><p>formation of a heterophase structure in the carburized layer, in partic-</p><p>ular, the precipitation of carbides (see Figs. 2, 5 and 6). On the one hand,</p><p>chromium carbide Cr23C6 and cementite have high corrosion resistance</p><p>[55,56], which exceeds that of AISI 321 austenitic steel. In particular,</p><p>the formation of a highly corrosion-resistant film on the cementite phase</p><p>is reported [56]. Along with the effect of interstitial carbon, this also</p><p>contributes to the good corrosion resistance of the carburized AISI 321</p><p>steel during general corrosion tests (see Fig. 11). On the other hand,</p><p>these carbides facilitate the pit initiation [57–59]. It is reported that</p><p>instances of pitting corrosion easily occur near chromium-rich carbides,</p><p>in particular, Cr23C6 [57]. Therefore, the carburized steel is more sus-</p><p>ceptible to initiation of localized corrosion. However, the potential</p><p>difference ΔE for the carburized steel is significantly lower (see Table 3).</p><p>A narrow hysteresis loop indicates that repassivation occurs more easily.</p><p>This is due to the fact that the oxyanions formed by carbon are effective</p><p>inhibitors and that they suppress anodic corrosion [53]. Thus, the</p><p>corrosion resistance of the carburized steel does not significantly dete-</p><p>riorate. It can be expected that a decrease in the temperature of liquid</p><p>carburizing will reduce the amount of the carbide phase, and this will</p><p>increase the resistance of the carburized steel to localized corrosion.</p><p>However, this requires further investigation.</p><p>4. Conclusions</p><p>A detailed analysis of structural-phase transformations and their ef-</p><p>fect on the properties of AISI 321 austenitic steel subjected to the liquid</p><p>carburizing at a temperature of 780◦С has been performed. The</p><p>following key conclusions can be drawn.</p><p>1. After liquid carburizing, chromium carbide Cr23C6 and cementite</p><p>Fe3C are formed in AISI 321 steel. Relatively coarse (micron-sized)</p><p>chromium carbides precipitate at the austenite grain boundaries, and</p><p>the clusters of dispersed (submicron-sized) cementite particles are</p><p>observed in the vicinity of the boundaries. Along with the grain</p><p>boundary carbides, there are dispersed particles of chromium car-</p><p>bide and cementite within the austenite grains, while the amount of</p><p>cementite is noticeably larger, its particles are coarser and they can</p><p>form linear clusters.</p><p>2. EDS analysis has shown that the carbon content in the surface layer</p><p>of the carburized AISI 321steel is high and it averages 4.0 wt% at a</p><p>depth of up to 90 μm. This is caused not only by the saturation of</p><p>austenite with carbon, but also by the presence of a large number of</p><p>dispersed carbides within the austenite grains. At greater depths, the</p><p>carbon content gradually decreases. According to the OES data, the</p><p>average carbon content in the carburized steel is 0.46 wt%. This</p><p>amount can be taken as the upper limit of carbon content in</p><p>austenite.</p><p>3. Plastic deformation occurs in the surface layer of the carburized AISI</p><p>321 steel. This is evidenced by the formation of slip bands in the</p><p>austenite grains and a decrease in the lattice parameter of austenite</p><p>from a0 = 3.606 Å (prior to carburizing) to aγ = 3.588 Å (after</p><p>carburizing). Plastic deformation occurs during cooling that follows</p><p>carburizing, and this is a relaxation mechanism of high thermal</p><p>stresses.</p><p>4. The carburized AISI 321 steel also contains α-martensite in the</p><p>amount of 20 vol%. The formation of martensite occurs mostly</p><p>within individual (non-intersecting) slip bands. The presence of the</p><p>ε-phase, which is located next to α-martensite, has also been found.</p><p>The reason for the appearance of the ε-phase is deformation by</p><p>Fig. 11. General corrosion rate for AISI 321 steel prior to carburizing (1) and</p><p>after liquid carburizing (2).</p><p>R.A. Savrai and P.A. Skorynina</p><p>Surface & Coatings Technology 443 (2022) 128613</p><p>11</p><p>twinning, which is due to the low stacking fault energy of AISI 321</p><p>steel. There is a certain orientation relationship among the lattices of</p><p>austenite, ε-phase, and α-martensite, namely {111}γ‖{0001}ε,</p><p>{0001}ε‖{110}α and 〈2− 1− 10〉ε‖〈111〉α. This indicates that</p><p>martensite formed in the carburized AISI 321 steel is induced by</p><p>deformation, and that the martensitic transformation path is γ → ε →</p><p>α′ (two-stage transformation).</p><p>5. Martensite formed in the surface layer of the carburized AISI 321</p><p>steel has a complex structure. According to the EBSD data, three</p><p>different orientations of the martensitic crystals with misorientation</p><p>angles of about 90◦ are observed within an individual slip band. This</p><p>is consistent with the results of metallographic analysis and indicates</p><p>that the deformation occurs in three systems simultaneously.</p><p>Fig. 12. Surface appearance (SEM) of the austenitized (a) and carburized (b) AISI 321 steel specimens after general corrosion testing for 18 h.</p><p>Fig. 13. SEM cross-sectional image (a) and distribution of microhardness HV0.025 in the surface layer (b) for the carburized AISI 321 steel specimen after general</p><p>corrosion testing for 18 h.</p><p>Fig. 14. Cyclic potentiodynamic polarization curves for AISI 321 steel prior to</p><p>carburizing (1) and after liquid carburizing (2).</p><p>Table 3</p><p>The corrosion characteristics of AISI 321 steel measured through cyclic poten-</p><p>tiodynamic polarization tests.</p><p>Processing Ecorr, mV Epit, mV Erp, mV ΔE = |Epit – Erp|, mV</p><p>Austenitizing − 472 − 411 − 476 65</p><p>Carburizing − 513 − 470 − 500 30</p><p>R.A. Savrai and P.A. Skorynina</p><p>Surface & Coatings Technology 443 (2022) 128613</p><p>12</p><p>6. Liquid carburizing of AISI 321 steel multiplies the microhardness of</p><p>the steel surface from 200 ± 7 to 890 ± 110 HV0.025. This is caused</p><p>by solid solution hardening resulting from the saturation of austenite</p><p>with carbon, by the formation of strain-induced martensite, by the</p><p>increase in the density of structural defects, which accompanies</p><p>plastic deformation, and by dispersion hardening due to the pre-</p><p>cipitation of carbides. The total hardening depth is about 500 μm.</p><p>The hardened layer is gradient and characterized by increased</p><p>resistance to elastic-plastic deformation, this being important for</p><p>increasing the contact endurance and wear resistance of the steel.</p><p>7. The corrosion resistance of the carburized steel does not significantly</p><p>deteriorate. This is due to the fact that interstitial carbon stabilizes</p><p>the electronic structure of iron (both austenite and martensite), thus</p><p>increasing its corrosion resistance. Along with this, the oxyanions</p><p>formed by carbon are effective inhibitors and they suppress anodic</p><p>corrosion.</p><p>Data availability</p><p>The raw/processed data required to reproduce these findings cannot</p><p>be shared at this time as the data also forms part of an ongoing study.</p><p>Declaration of competing interest</p><p>The authors declare that they have no known competing financial</p><p>interests or personal relationships that could have appeared to influence</p><p>the work reported in this paper.</p><p>Acknowledgments</p><p>This work was done within the state order for IES UB RAS, reg. no.</p><p>АААА-А18-118020790147-4. Scanning electron microscopy, optical</p><p>profilometry and microhardness measurements were performed in</p><p>Collective Use Center “Plastometriya” of the Institute of Engineering</p><p>Science UB RAS. The authors are also grateful to Yu.M. Kolobylin for the</p><p>assistance in preparing the specimens.</p><p>References</p><p>[1] F. 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