E. V. Legostaeva

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 quasi-hydrostatic 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 -, -, and 1-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 -, -, and 1-phases precipitates. In the temperature range of 300–450 °С, the morphology of -, ,- and 1-phases changes, while the average grain size of the major -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 -phase, Mg24Y5 particles, and -, -, and 1-phases precipitates. Annealing in the temperature range of 100–450 °C leads to an increase in the linear dimensions of Mg24Y5 particles, -, -, and 1-phases precipitates and bimodal microstructure of the Mg-2.9Y-1.3Nd alloy remains unchanged.

The microstructural investigation, mechanical properties, and accumulation and dissipation of energies of the magnesium alloy Mg-2.9Y-1.3Nd in the recrystallized state and after severe plastic deformation (SPD) by extrusion are presented. The use of SPD provides the formation of a bimodal structure consisting of grains with an average size 15 µm and of ultrafine-grained grains with sizes less than 1 µm and volume fractions up to 50%, as well as of the fine particles of the second Mg₂₄Y₅ phases. It is established that grain refinement during extrusion is accompanied by an increase of the yield strength, increase of the tensile strength by 1.5 times, and increase of the plasticity by 1.8 times, all of which are due to substructural hardening, redistribution of the phase composition, and texture formation. Using infrared thermography, it was revealed that before the destruction of Mg-2.9Y-1.3Nd in the recrystallized state, there is a sharp jump of temperature by 10 °C, and the strain hardening coefficient becomes negative and amounts to (−6) GPa. SPD leads to a redistribution of thermal energy over the sample during deformation, does not cause a sharp increase in temperature, and reduces the strain hardening coefficient by 2.5 times.

Выполнено исследование влияние экструзии на механические свойства биомедицинских сплавов на основе систем Mg-Y-Nd и Mg-Ca. Сплавы деформировали методом горячей обратной экструзии, накопленная деформация составила e=1,46. Показано, что в результате экструзии в сплавах формируются дисперсно-упрочненные частицами Mg24Y5 и Mg2Caбимодальные структуры, состоящие из зерен a-фазы магния со средним размером 14 и 9 мкм соответственно для Mg-Y-Nd и Mg-Ca и зерен a-фазы с размерами менее 1 мкм для обоих сплавов, объемная доля которой составила 22 и 50 %, соответственно. Бимодальная структура в сплавах Mg-Y-Nd и Mg-Ca обеспечивает повышение условного предела текучести, предела прочности в 1,5 и 2,5 раза, а пластичности – в 1,8 и 6,3 раза, соответственно, что связано с субструктурым упрочнением, перераспределением фазового состава и формирующейся текстурой. Показано, что обработка экструзией сплава Mg-Y-Nd не приводит к ухудшению коррозионной стойкости сплава в физиологическом растворе (раствор 0,9 % NaCl в дистиллированной воде, pH = 7) в отличие от сплава Mg-Ca, в котором присутствие большей объемной доли ультрамелких зерен и межзеренных границ способствует значительному увеличению скорости коррозии.

Introduction. The interest of modern medical materials science is focused on the development of beta-alloys of ternary systems (TNZ) based on titanium, niobium and zirconium with the low Young’s modulus, which is comparable with the elastic modulus of the bone. A wide application of the above alloys in medicine is limited by its insufficiently high strength properties, such as yield strength, ultimate strength, fatigue strength, fatigue life, etc. The formation of bulk ultrafine-grained structure in the alloys via deformation processing, including severe plastic deformation, ensures a considerable increase in the mechanical properties of alloys without toxic alloying elements. The aim of the work is to analyze the influence of deformation (multipass rolling and abc-pressing in combination with rolling) on the microstructure and mechanical properties of the alloy of the Ti-Nb-Zr system. The research methods. The Ti-42Nb-7Zr alloy cast blanks were made from pure titanium, niobium, and zirconium iodides by arc melting with a tungsten electrode in the protective argon atmosphere. It is shown that the cast blanks obtained have a high degree of uniformity in the distribution of niobium and zirconium alloying elements. To form an ultrafine-grained (UFG) structure, the cast blanks were subjected to deformation according to two schemes: 1) multipass rolling and 2) a combined method of severe plastic deformation, consisting in abc-pressing and subsequent multipass groove rolling. Results and discussion. As a result of deformation processing by rolling, an ultrafine-grained (UFG) structure is formed, which is represented by non-equiaxed -subgrains with cross-sectional dimensions 0.2…0.8 µm and length 0.2…0.7 µm, dispersion strengthened nanosized ω-phase, as well as subgrains of the -phase. Application of combined severe plastic deformation has promoted formation of a more dispersed UFG (+ω)-structure with an average size of structural elements equal to 0.3 μm. The UFG structure formed as a result of two-stage SPD has provided a high level of mechanical properties: yield strength – 480 MPa, ultimate strength – 1.100 MPa, microhardness – 2.800 MPa, with a low modulus of elasticity equal to 36 GPa.

The effect of extrusion on the mechanical properties of biomedical alloys based on Mg–Y–Nd and Mg–Ca systems is studied. The alloys are deformed by hot backward extrusion, accumulated deformation is e = 1.46. It is shown that the extrusion leads to the formation of bimodal structures that are disperse-strengthened by Mg₂₄Y₅ and Mg₂Ca particles and consist of α-phase magnesium grains with mean sizes of 14 and 9 μm for Mg–Y–Nd and Mg–Ca alloys, respectively, and α-phase grains with a size of less than 1 µm for both alloys (the volume fractions are 22 and 50%, respectively). The bimodal structure in the Mg–Y–Nd and Mg–Ca alloys provides an increase in the yield strength by factors of 1.5 and 2.5, respectively, and an increase in ductility by factors of 1.8 and 6.3, respectively, due to substructure strengthening, redistribution of phase composition, and emerging texture. It is shown that the extrusion treatment of the Mg–Y–Nd alloy does not result in worsening of corrosion resistance of the alloy in physiological solution (0.9% NaCl solution in distilled water, pH 7) unlike the Mg–Ca alloy for which the presence of a larger volume fraction of ultrafine grains and intergranular boundaries leads to a significant increase in corrosion rate.