Thermal stability of extruded Mg-Y-Nd alloy structure

Vol. 26 No. 2 2024 3 EDITORIAL COUNCIL EDITORIAL BOARD EDITOR-IN-CHIEF: Anatoliy A. Bataev, D.Sc. (Engineering), Professor, Rector, Novosibirsk State Technical University, Novosibirsk, Russian Federation DEPUTIES EDITOR-IN-CHIEF: Vladimir V. Ivancivsky, D.Sc. (Engineering), Associate Professor, Department of Industrial Machinery Design, Novosibirsk State Technical University, Novosibirsk, Russian Federation Vadim Y. Skeeba, Ph.D. (Engineering), Associate Professor, Department of Industrial Machinery Design, Novosibirsk State Technical University, Novosibirsk, Russian Federation Editor of the English translation: Elena A. Lozhkina, Ph.D. (Engineering), Department of Material Science in Mechanical Engineering, Novosibirsk State Technical University, Novosibirsk, Russian Federation The journal is issued since 1999 Publication frequency – 4 numbers a year Data on the journal are published in «Ulrich's Periodical Directory» Journal “Obrabotka Metallov” (“Metal Working and Material Science”) has been Indexed in Clarivate Analytics Services. Novosibirsk State Technical University, Prospekt K. Marksa, 20, Novosibirsk, 630073, Russia Tel.: +7 (383) 346-17-75 http://journals.nstu.ru/obrabotka_metallov E-mail: metal_working@mail.ru; metal_working@corp.nstu.ru Journal “Obrabotka Metallov – Metal Working and Material Science” is indexed in the world's largest abstracting bibliographic and scientometric databases Web of Science and Scopus. Journal “Obrabotka Metallov” (“Metal Working & Material Science”) has entered into an electronic licensing relationship with EBSCO Publishing, the world's leading aggregator of full text journals, magazines and eBooks. The full text of JOURNAL can be found in the EBSCOhost™ databases.

OBRABOTKAMETALLOV Vol. 26 No. 2 2024 4 EDITORIAL COUNCIL EDITORIAL COUNCIL CHAIRMAN: Nikolai V. Pustovoy, D.Sc. (Engineering), Professor, President, Novosibirsk State Technical University, Novosibirsk, Russian Federation MEMBERS: The Federative Republic of Brazil: Alberto Moreira Jorge Junior, Dr.-Ing., Full Professor; Federal University of São Carlos, São Carlos The Federal Republic of Germany: Moniko Greif, Dr.-Ing., Professor, Hochschule RheinMain University of Applied Sciences, Russelsheim Florian Nürnberger, Dr.-Ing., Chief Engineer and Head of the Department “Technology of Materials”, Leibniz Universität Hannover, Garbsen; Thomas Hassel, Dr.-Ing., Head of Underwater Technology Center Hanover, Leibniz Universität Hannover, Garbsen The Spain: Andrey L. Chuvilin, Ph.D. (Physics and Mathematics), Ikerbasque Research Professor, Head of Electron Microscopy Laboratory “CIC nanoGUNE”, San Sebastian The Republic of Belarus: Fyodor I. Panteleenko, D.Sc. (Engineering), Professor, First Vice-Rector, Corresponding Member of National Academy of Sciences of Belarus, Belarusian National Technical University, Minsk The Ukraine: Sergiy V. Kovalevskyy, D.Sc. (Engineering), Professor, Vice Rector for Research and Academic Aff airs, Donbass State Engineering Academy, Kramatorsk The Russian Federation: Vladimir G. Atapin, D.Sc. (Engineering), Professor, Novosibirsk State Technical University, Novosibirsk; Victor P. Balkov, Deputy general director, Research and Development Tooling Institute “VNIIINSTRUMENT”, Moscow; Vladimir A. Bataev, D.Sc. (Engineering), Professor, Novosibirsk State Technical University, Novosibirsk; Vladimir G. Burov, D.Sc. (Engineering), Professor, Novosibirsk State Technical University, Novosibirsk; Aleksandr N. Korotkov, D.Sc. (Engineering), Professor, Kuzbass State Technical University, Kemerovo; Dmitry V. Lobanov, D.Sc. (Engineering), Associate Professor, I.N. Ulianov Chuvash State University, Cheboksary; Aleksey V. Makarov, D.Sc. (Engineering), Corresponding Member of RAS, Head of division, Head of laboratory (Laboratory of Mechanical Properties) M.N. Miheev Institute of Metal Physics, Russian Academy of Sciences (Ural Branch), Yekaterinburg; Aleksandr G. Ovcharenko, D.Sc. (Engineering), Professor, Biysk Technological Institute, Biysk; Yuriy N. Saraev, D.Sc. (Engineering), Professor, V.P. Larionov Institute of the Physical-Technical Problems of the North of the Siberian Branch of the RAS, Yakutsk; Alexander S. Yanyushkin, D.Sc. (Engineering), Professor, I.N. Ulianov Chuvash State University, Cheboksary

Vol. 26 No. 2 2024 5 CONTENTS OBRABOTKAMETALLOV TECHNOLOGY Gaikwad V., Chinchanikar S. Investigations on ultrasonic vibration-assisted friction stir welded AA7075 joints: Mechanical properties and fracture analysis........................................................................................................................ 6 Sirota V.V., Zaitsev S.V., Limarenko M.V., Prokhorenkov D.S., Lebedev M.S., Churikov A.S., Dan'shin A.L. Preparation of coatings with high infrared emissivity.......................................................................................................... 23 Babaev A.S., Kozlov V.N., Semenov A.R., Shevchuk A.S., Ovcharenko V.A., Sudarev E.A. Investigation of cutting forces and machinability during milling of corrosion-resistant powder steel produced by laser metal deposition............. 38 Dolgova S.V., Malikov A.G., Golyshev A.A., Nikulina A.A. The eff ect of laser surfacing modes on the geometrical characteristics of the single laser tracks............................................................................................................................... 57 Karlina Y.I., Kononenko R.V., Popov M.A., Deryugin F.F., Byankin V.E. Assessment of welding engineering properties of basic type electrode coatings of diff erent electrode manufacturers for welding of pipe parts and assemblies of heat exchange surfaces of boiler units............................................................................................................................. 71 Yanpolskiy V.V., Ivanova M.V., Nasonova A.A., Yanyushkin A.S. Determination of the rate of electrochemical dissolution of U10A steel under ECM conditions with a stationary cathode-tool............................................................... 95 EQUIPMENT. INSTRUMENTS Zakovorotny V.L., Gvindjiliya V.E. The study of vibration disturbance mapping in the geometry of the surface formed by turning............................................................................................................................................................................. 107 Gasanov B.G., Konko N.A., Baev S.S. Study of the kinetics of forming of spherical sliding bearing parts made of corrosion-resistant steels by die forging of porous blanks............................................................................................... 127 Gvindjiliya V.E., Fominov E.V., Moiseev D.V., Gamaleeva E.I. Infl uence of dynamic characteristics of the turning process on the workpiece surface roughness........................................................................................................................ 143 Lobanov D.V., Skeeba V.Yu., Golyushov I.S., Smirnov V.M., Zverev E.A. Design simulation of modular abrasive tool........................................................................................................................................................................................ 158 MATERIAL SCIENCE EroshenkoA.Yu., Legostaeva E.V., Glukhov I.A., Uvarkin P.V., TolmachevA.I., Sharkeev Yu.P. Thermal stability of extruded Mg-Y-Nd alloy structure.................................................................................................................................. 174 Bazaleeva K.O., Safarova D.E., Ponkratova Yu.Yu., Lugovoi M.E., Tsvetkova E.V., Alekseev A.V., Zhelezni M.V., Logachev I.A., Baskov F.A. The infl uence of technological parameters of the laser engineered net shaping process on the quality of the formed object from titanium alloy VT23......................................................... 186 Efi movich I.A., Zolotukhin I.S. Oxidation temperatures of WC-Co cemented tungsten carbides....................................... 199 Pribytkov G.A., Baranovskiy A.V., Firsina I.A., Akimov K.O., Krivopalov V.P. Study of Fe-matrix composites with carbide strengthening, formed by sintering of iron titanides and carbon mechanically activated mixtures................ 212 EDITORIALMATERIALS 224 FOUNDERS MATERIALS 235 CONTENTS

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 2 2024 Thermal stability of extruded Mg-Y-Nd alloy structure Anna Eroshenko1, a,*, Elena Legostaeva1, b, Ivan Glukhov1, c, Pavel Uvarkin1, d, Aleksei Tolmachev1, e, Yurii Sharkeev 1, 2, f 1 Institute of Strength Physics and Materials Sciences SB RAS, 2/4, pr. Akademicheskii, Tomsk, 634055, Russian Federation 2 National Research Tomsk Polytechnic University, 30 Lenin Avenue, Tomsk, 634050, Russian Federation a https://orcid.org/0000-0001-8812-9287, eroshenko@ispms.ru; b https://orcid.org/0000-0003-3684-9930, lego@ispms.ru; c https://orcid.org/0000-0001-5557-5950, gia@ispms.ru; d https://orcid.org/0000-0003-1169-3765, uvarkin@ispms.ru; e https://orcid.org/0000-0003-4669-8478, tolmach@ispms.ru; f https://orcid.org/0000-0001-5037-245X, sharkeev@ispms.ru Obrabotka metallov - Metal Working and Material Science Journal homepage: http://journals.nstu.ru/obrabotka_metallov Obrabotka metallov (tekhnologiya, oborudovanie, instrumenty) = Metal Working and Material Science. 2024 vol. 26 no. 2 pp. 174–185 ISSN: 1994-6309 (print) / 2541-819X (online) DOI: 10.17212/1994-6309-2024-26.2-174-185 ART I CLE I NFO Article history: Received: 19 October 2023 Revised: 16 November 2023 Accepted: 20 March 2024 Available online: 15 June 2024 Keywords: Mg-Y-Nd alloy Extruded alloy Microstructure Phase composition Thermal stability Funding The work was performed according to the Government Research Assignment for the Institute of Strength Physics and Materials Science of the Siberian Branch of the Russian Academy of Sciences (ISPMS SB RAS), project No. FWRW-2021-0004. Experimental research was conducted using the equipment of the Common Use Center “Nanotech” at the Institute of Strength, Physics, and Materials Science, SB RAS (ISPMS SB RAS, Tomsk, Russia). Acknowledgements The authors are grateful to engineer Juergen Schmid (Department of Electrochemistry, Innovent Technology Development, Germany) and researcher. Chebodeva V.V. (IPPM SB RAS) for assistance in carrying out a number of experimental works. ABSTRACT Introduction. Today, bioresorbable magnesium alloys possessing the required physical, mechanical, corrosion, and biological properties, are promising materials for orthopedic and cardiovascular surgery. The addition of rare earth elements such as yttrium, neodymium, and cerium to magnesium alloys improves its properties. Compared to widely used titanium alloys, magnesium alloys have a number of advantages. Bioresorbable materials slowly dissolve in the body, and recurrent operation to remove the implant is not needed. Biocompatible magnesium alloys have a fairly low elastic modulus (10 to 40 GPa), approaching to that of cortical bone, that reduces the contact stress in the bone-implant system. At the same time, strength properties of magnesium alloys alloyed with rare earth elements do not always meet the requirements for medical applications. Severe plastic deformation, for example, equal channel angular pressing, torsion under quasihydrostatic pressure, uniaxial forging, extrusion, is therefore very promising technique to gain the high level of mechanical properties of metals and alloys. Severe plastic deformation of magnesium alloys improves its structural strength by 2.5 times due to the generation of an ultrafine-grained and/or fine-grained structure. The issues related to the study of heat resistance, structure and phase composition of magnesium alloys with appropriate strength are relevant. Purpose of the work is to determine the influence of thermal effects on the microstructure of the extruded Mg-Y-Nd alloy. Methodology. The extruded Mg-2.9Y-1.3Nd alloy (95.0 wt. % Mg, 2.9 wt. % Y, 1.3 wt. % Nd, ≤ 0.2 wt. % Fe, ≤ 0 wt. % Al) is investigated in this paper. The thermal stability of the alloy microstructure is studied after annealing at 100, 300, 350, 450 and 525 °С in argon for one hour. The microstructure and phase composition are investigated using optical, transmission and scanning electron microscopes and analyzed on an X-ray diffractometer. Results and discussion. The extruded Mg-2.9Y-1.3Nd alloy has the bimodal fine-grained microstructure. It is found that along with the stable α-Mg phase, the alloy structure consists of Mg24Y5 intermetallic particles and b-, b′-, and b1-phase precipitates. Annealing in the temperature range of 100–450 °С for one hour has no effect on the structure of the Mg-2.9Y-1.3Nd alloy, but promotes the growth in the linear dimensions of b-, b′-, and b1-phases precipitates. In the temperature range of 300–450 °С, the morphology of b-, b′,- and b1-phases changes, while the average grain size of the major a-phase remains unchanged. Annealing at 525 °С leads to a notable transformation of the bimodal microstructure of the alloy, which is associated with the intensive growth in the grain size of the a-phase, Mg24Y5 particles, and b-, b′-, and b1-phases precipitates. Annealing in the temperature range of 100–450 °C leads to an increase in the linear dimensions of Mg24Y5 particles, b-, b′-, and b1-phases precipitates and bimodal microstructure of the Mg-2.9Y-1.3Nd alloy remains unchanged. For citation: Eroshenko A.Yu., Legostaeva E.V., Glukhov I.A., Uvarkin P.V., Tolmachev A.I., Sharkeev Yu.P. Thermal stability of extruded Mg-Y-Nd alloy structure. Obrabotka metallov (tekhnologiya, oborudovanie, instrumenty) = Metal Working and Material Science, 2024, vol. 26, no. 2, pp. 174–185. DOI: 10.17212/1994-6309-2024-26.2-174-185. (In Russian). ______ * Corresponding author Eroshenko Anna Yu., Ph.D. (Engineering), Senior research fellow Institute of Strength Physics and Materials Sciences SB RAS, 2/4, pr. Akademicheskii, 634055, Tomsk, Russian Federation Tel.: +7 3822 28-69-11, e-mail: eroshenko@ispms.ru

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 2 2024 Introduction Today, bioresorbable magnesium alloys, possessing the required physical, mechanical, corrosion, and biological properties, are promising materials for orthopedic and cardiovascular surgery [1–8]. The addition of rare earth elements (REE) such as yttrium, neodymium, and cerium to magnesium alloys, improves its properties [9]. Yttrium provides the formation of stable phases with magnesium, thereby improving the alloy strength and plasticity. Neodymium and cerium improve the corrosion resistance and thermal stability of these alloys. Compared to widely used titanium alloys, magnesium alloys have a number of advantages. Firstly, bioresorbable materials slowly dissolve in the body, and recurrent operation to remove the implant is not needed [2–4]. Secondly, biocompatible magnesium alloys do not cause such negative reactions in the body as inflammatory processes, implant failure, and others. Thirdly, its elastic modulus is rather low (10–40 GPa), approaching to that of cortical bone, that reduces the contact stress in the bone-implant system [3, 4]. In this respect, severe plastic deformation, for example, equal channel angular pressing, torsion under quasi-hydrostatic pressure, uniaxial forging (abc-pressing), extrusion, is therefore a very promising technique to gain the high level of mechanical properties of metals and alloys [10–16]. Severe plastic deformation of magnesium alloys improves its structural strength by 2.5 times due to the generation of an ultrafine-grained and/or fine-grained structure. Mg-Y-Nd-based (commercial WE43, WE54) deformable alloys with the addition of yttrium and neodymium, are used in the production of units and parts for aircraft control systems [16]. Rare earth-based (neodymium, yttrium, cadmium, lanthanum) magnesium alloys are mostly used in aircraft and space equipment, since its refractoriness ranges from 250 to 300 °С [17–19]. Relevant are issues relating to the exploration of thermal stability, structure and phase composition of magnesium alloys with the appropriate strength properties. This is determined by the structural variety of magnesium alloys, both in cast and deformed states, which significantly affects its physical and mechanical properties. It is thus important to create high-strength magnesium alloys and analyze its thermal stability, structure, and phase composition. The aim of this work is to determine the thermal effect on the microstructure and phase composition of the extruded Mg-Y-Nd system alloy. Materials and research methodology The Mg-2.9Y-1.3Nd alloy (95.0 wt.% Mg, 2.9 wt.% Y, 1.3 wt.% Nd, ≤0.2 wt.% Fe, ≤0 wt.% Al) (commercial WE43 alloy) was used in experiments. The alloy was obtained by permanent mold casting [20]. The alloy specimens were subjected to severe plastic deformation (extrusion at 350 °C) for the grain refinement and enhancement of mechanical properties. The diameter of the initial bars was 60 mm, and after extrusion it decreased to 14 mm. True strain was determined by logarithm of the ratio of the initial and final thickness of the specimens. Accumulated logarithmic strain after specimen treatment was 1.46. The microstructure and phase composition of alloy specimens were studied on an Axio Observer Inverted Microscope (Carl Zeiss, Germany), a JEM-2100 (JEOL Ltd., Japan) high-resolution transmission electron microscope (TEM) combined with X-ray microanalysis, and Zeiss EVO 50 (Germany) scanning electron microscope (SEM). The X-ray phase (XRD) analysis was carried out using DRON-7 diffractometer (Burevestnik, Russia). Measurements were conducted using copper radiation (Ka). Medium sizes of grains, subgrains, fragments were detected on micrographs using the secant method [21]. The thermal stability of the alloy microstructure was studied after annealing at 100, 300, 350, 450, 525 °С in argon for one hour. According to [20-24], the thermal treatment of Mg-Y-Nd system alloys at 100 to 525 °С provides the formation of various structural and phase transformations and a complex behavior of the temperature dependence of the heat capacity.

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 2 2024 Results and discussion Figure 1 presents optical and TEM images of bimodal microstructure of the extruded Mg-2.9Nd-1.3Y alloy, which can be clearly observed in Fig. 1, a. TEM observations show that the microstructure of the extruded Mg-Nd-Y alloy is based on the hexagonal close packed (HCP) α-Mg phase solid solution (see Fig. 1, b) and intermetallic compounds based on Mg, Nd, Y. Intermetallic compounds can be observed inside grains and along grain boundaries (Fig. 1, b, c, d, e, f, g). According to [25–30], in Mg-Y-Nd system alloys these phases are identified as body centered cubic Mg24Y5 particles and three types of metastable phases. These phases include a face centered cubic (FCC) eutectic equilibrium β-phase (Mg14Nd2Y) in the form of a network, an orthorhombic b′-phase (Mg12YNd) with a globular morphology, and a FCC b1-phase (Mg3NdY) lamellas. Note that metastable β or β1 phases are considered to be the main reinforcing agents, which are usually present in the annealed WE43 alloy [25–29]. According to [28], in the deformed magnesium alloy WE43 during a long-term aging the metastable β1-phase is transformed into the equilibrium β-phase. a b c d e f g Fig. 1. Optical (а) and bright field TEM images (b, c, d, f, g), energy dispersive spectrums and elemental composition (d, e) of extruded Mg-2.9Y-1.3Nd alloy. Insert: selected area diffraction pattern

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 2 2024 The optical image shows two types of structural elements, namely coarse (~17 µm) and ultra-fine (~1 µm) grains, the latter represent textured bands. The volume fraction of ultra-fine grains is 50 % of the total volume. Intermetallic particles are additionally deduced from the energy-dispersive X-ray (EDX) analysis of the elemental composition shown in Fig. 1, c, d. EDX spectra in this figure are obtained at different points of thin specimen. Mg24Y5 particles are high in yttrium (30 wt.%) and locate mostly inside grains (Fig. 1, e). In the extruded alloy, Mg24Y5 particles are mostly irregular polyhedrons with the average size of 0.6 µm. According to TEM images, the volume fraction of these particles is not over 2 %. The eutectic equilibrium β-phase is localized along the grain boundaries as a network of precipitates up to 0.3–0.4 µm thick. This phase is also irregular polyhedrons and, to a lesser extent, regular tetrahedrons presented in Fig. 1, f. The medium size of b′-phase globules is 0.2 µm. The length and width of the b1-phase vary within the range of 0.06–0.30 µm and 0.03–0.04 µm, respectively (Fig. 1, g). Note that b1-phase lamellas are positioned in the same direction. In the b-phase, yttrium and neodymium amount to (3.54–7.18) wt. % and (2.26–9.59) wt. %, respectively. In the b′-phase, the content ranges between 3.21 and 5.39 wt. % for yttrium and between 1.83 and 2.07 wt. % for neodymium. In b1-phases, yttrium and neodymium range between (3.32–5.27) wt. % and (1.75–8.46) wt. %, respectively. The b′-phase contributes to the greatest extent to the increase in the mechanical properties of Mg-Y-1Nd alloys due to its strain hardening [24]. Figure 2 presents optical images of the Mg-2.9Nd-1.3Y alloy microstructure after annealing in the temperature range of 100–450 °С. As can be seen in this figure, the bimodal microstructure does not change after annealing (Fig. 2, a-d). Finer grains of the a-phase range in size from 0.2 to 5.0 µm. After annealing, its medium size does not change and is equal to 1 µm. а b c d Fig. 2. Optical images of extruded Mg-2.9Y-1.3Nd alloy microstructure after annealing at different temperatures: a – 100 °С; b – 300 °С; c – 350 °С; d – 450 °С Note that the formation of the fine-grained structure in the extruded Mg-2.9Y-1.3Nd alloy significantly improves its yield strength and tensile strength up to 150 and 230 MPa, respectively. For the recrystallized structure obtained after 525 °С annealing for 6 hours, these parameters are 220 and 340 MPa, respectively [22]. The alloy plasticity also increases from 12 to 21 %. Figure 3 contains TEM images of the alloy microstructure after annealing at different temperatures. On bright field images, one can see four types of intermetallic inclusions after 100 °С annealing, namely Mg24Y5 particles (Fig. 3, а) and b-, b′- and b1-phases (Fig. 3, b), as in the extruded structure. Unlike the extruded structure, the medium size of Mg24Y5 particles grows up to 0.9 µm, and there is a slight increase in the width of the subgrain b-phase boundary, which varies in the range (0.4–0.5) µm (Fig. 3, a, b). Linear dimensions of secondary b′- and b1-phase precipitates do not change. The temperature growth up to 300 °С leads to a further increase in the medium size of Mg24Y5 particles from 0.9 to 1.2 µm and morphology transformation of some particles from irregular polyhedrons to regular tetrahedrons, as presented in Fig. 3, c. This indicates the occurrence of recrystallization process. The microstructure in Fig. 3, d–f, includes all types of secondary non-equilibrium phases described above. The network width of the grain boundary increases up to 1.2–1.7 µm and consists of the eutectic β-phase (Fig. 3, d). In Fig. 3, e, f, one can see b′-phase globules and b1-phase lamellas. A significant growth in b1-

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 2 2024 phase lamellas is observed with coarsening Mg24Y5 particles, i.e., the length grows from 0.3 to 0.8 µm and the width expands from 0.04 to 0.07 µm (Fig. 3, e). The average cross-sectional dimension of b′-phase precipitates is 0.2 µm (Fig. 3, f). It is notable that the length and width of smaller b1-phase lamellas localizing nearby the grain boundaries, vary from 0.3 to 0.8 µm and from 0.02 to 0.08 µm, respectively. It is worth noting that according to [24], the formation of b1-phase lamellas is determined by the presence of b′-phase globules. TEM observations [23] revealed b′-, and b1-phases precipitates in the Mg-Y-Nd system alloy after annealing at 250 °С. The latter led to the formation of metastable b′-, and b1-phases followed by the formation of the equilibrium β-phase. It is shown that the b1-phase represented b′-phase nuclei and could generate much shear energy and shear energy accommodation. During a long-term annealing, e.g., aging at 250 °С, the b1-phase transferred to the equilibrium β-phase. Annealing at 350 °С provides the width expansion of continuous subgrain boundary in the range of 0.8 to 1.7 µm (Fig. 3, g). The average length and width of b1-phase lamellas are respectively 0.6 and 0.03 µm. The average size of b′-phase precipitates is 0.2 µm, as it is demonstrated in Fig. 3, h. Annealing at 350 °С promotes aggregation of Mg24Y5 intermetallic particles. Annealing at 450 °С results in widening of the eutectic b-phase network up to 2 µm. In this case, most of Mg24Y5 particles are tetrahedrons having the medium size of 1.3 µm. Localized regions consist of typiFig. 3. Bright field TEM images with corresponding microdiffraction patterns of extruded Mg-2.9Nd-1.3Y alloy microstructure after annealing at: a, b – 100 °С; c–f – 300 °С; g, h – 350 °С; i, j – 450 °С a b c d e f g h i j

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 2 2024 cal b′-phase globules and b1-phase lamellas. The average size of b′-phase precipitates is 0.2 µm, while the length and width of the b1-phase lamellas vary from 0.6 to 0.7 and from 0.02 to 0.05 µm, respectively (Fig. 3, i). Eutectic β-phase particles are well-defined rhombs, as it is shown in Fig. 3, j. Annealing at 450 °С provides further coarsening of Mg24Y5 intermetallic particles. At higher annealing temperatures, b′-, and b1-phases particles become larger or are replaced by the stable β-phase according to b′ → β or b1→ β phase transformations [24, 27–30]. After annealing at 525 °С, the alloy microstructure undergoes transformation. According to optical and SEM microscopy the structure becomes more homogeneous (Fig. 4, а, b). The average size of the base a-phase reaches 32 µm. There are no textured bands consisting of magnesium fine grains, which indicates intensive recrystallization processes. Bright field TEM images demonstrate four types of intermetallic inclusions, namely Mg24Y5 particles (Fig. 4, c) and b-, b′-, and b1-phases (Fig. 4, d, е) similar to those observed after annealing at 450 °С. The medium size of Mg24Y5 particles is 1.4 µm, and its shape is a regular tetrahedron. The width of the subgrain b-phase boundary expands and ranges between 0.6 and 1.2 µm. The length and width of b1-phase lamellas vary from 1.1 to 6.2 and from 0.4 to 1 µm, respectively. The average size of b′-phase precipitates is 0.3 µm. c d e Fig. 4. Optical, SEM and TEM images of Mg-2.9Y-1.3Nd alloy microstructure after annealing at 525°С: a, b – optical and SEM images; c–e – bright field TEM images with corresponding microdiffraction pattern a b The dependence of the average size of structural elements of various phases on the annealing temperature is shown in Fig. 5. When annealing in the temperature range 100–450 °C, the average grain size of the a-phase does not change, but at the same time there is a slight increase in the particle size of Mg24Y5 particles and precipitates of b-, b′-, and b1-phases, which indicates its thermal instability at the above temperatures. At a temperature of 525 °C, there is a noticeable increase in both the grain size of the matrix a-phase of magnesium, and Mg24Y5 particles, and precipitates of b-, b′-, and b1-phases. Note that the Mg24Y5 particles and precipitates of b-,b′-, and b1-phases are present at fairly high temperatures, up to 525 °C. It should be noted that Mg24Y5 particles and b-, b′-, and b1-phases in the Mg-Nd-Y system alloy, are thermally stable at rather high annealing temperatures reaching 525 °С. In Fig. 6, one can see fragments of XRD patterns of the extruded Mg-2.9Y-1.3Nd alloy and annealed at temperatures of 100–525 °С.

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 2 2024 Fig. 5. The dependences of the average size of structural elements of phases on the annealing temperature Fig. 6. Fragments of XRD patterns of extruded Mg-2.9Y-1.3Nd alloy and after annealing According to XRD patterns of the extruded Mg-2.9Y-1.3Nd alloy, high intensity peaks belong to the HCP α-Mg phase solid solution. After annealing at 100–450 ℃, XRD patterns do not change, whereas at a temperature of 525 ℃, the X-ray line width of the α-phase notably decreases and the intensity redistribution occurs in (100), (101) and (101) directions, indicating to intensive recrystallization processes accompanied by the grain growth. In addition to the major α-Mg phase, TEM investigations show the presence of fine-grained Mg24Y5 intermetallic particles and b-, b′-, and b1-phases precipitates in the alloy, which cannot be identified by the X-ray spectroscopy. Thus, we can conclude, that the bimodal microstructure of the Mg-2.9Y-1.3Nd alloy does not change after one hour annealing in the temperature range of 100–450 °С, although a slight growth is observed for fine-grained Mg24Y5 particles and b-, b′-, and b1-phases precipitates with its morphology transformation. Conclusions In conclusion, this study has shown the formation of the bimodal microstructure in the extruded Mg2.9Y-1.3Nd alloy. The microstructure consisted of a a-phase and textured bands with an average grain size of 17 µm and 1 µm, respectively. It was found that along with the major stable α-Mg phase, the alloy microstructure comprised Mg24Y5 intermetallic particles and precipitates of the of three types phases. Annealing within 100–450 °С for 1 hour had no effect on the general structure of the Mg-2.9Y-1.3Nd alloy, but promoted the growth in the linear dimensions of precipitates of b-, b′-, and b1-phases. It is shown that within 300–450 °С, the morphology of b-, b′-, and b1-phases changed while maintaining the average size of the α-phase. Annealing at 525 °С resulted in a significant transformation of the bimodal microstructure, conditioned by the recrystallization process and intensive grain growth of the major phase, Mg24Y5 intermetallic particles, and secondary precipitates of b-, b′-, and b1-phases.

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