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.
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