Alexey I. Tolmachev

УСТАНОВЛЕНИЕ ВЛИЯНИЯ УМЗ СТРУКТУРЫ В СПЛАВЕ Ti – Nb НА УСТАЛОСТНУЮ ПРОЧН

Introduction. One of the most promising fields for the application of magnesium alloys is medicine. Their key advantages are bioresorbability and a low elastic modulus, comparable to that of human cortical bone (up to 30 GPa). Biocompatible Mg-Zn-Zr-Ce (MA20) system alloys are among the most promising for medical applications. Due to their relatively low mechanical properties, the development of severe plastic deformation (SPD) techniques for forming an ultrafine-grained (UFG) state in bulk billets of the Mg-Zn-Zr-Ce alloy to achieve optimal functional properties requires further research. Analyzing the conditions for forming a high-strength UFG state necessitates considering various strengthening mechanisms, including well-known ones related to the effect of UFG structures. Identifying the deformation and strain hardening mechanisms in magnesium alloys subjected to SPD is also highly relevant. The purpose of this work is to establish the mechanisms of strain hardening and to investigate the influence of heat treatment on the structure and properties of the MA20 magnesium alloy after combined SPD. Research methods. The study object was the MA20 alloy in a UFG state (wt. %: Mg – 98.0; Zn – 1.3; Ce – 0.1; Zr – 0.1; O – 0.5). The UFG state was achieved via a combined SPD process involving ABC-pressing followed by multi-pass rolling in grooved rolls. To study the effect of annealing on the microstructure and mechanical tensile properties, samples were annealed in air at temperatures of 200, 250, 300, and 500 °C for 24 hours. The microstructure and phase composition of the samples were investigated using optical and transmission electron microscopy. Results and discussion. It was established that applying a combined SPD method (ABC-pressing and multi-pass rolling) to the MA20 alloy results in the formation of an ultrafine-grained structure with an average grain size of about 1 μm. This leads to a significant increase in yield strength (σ0.2) to 250 MPa and ultimate tensile strength (σu) to 270 MPa, while simultaneously reducing ductility to 3%. Annealing at 200 °C was found to preserve the UFG state in the MA20 alloy and to lead to a 100% increase in ductility, with an 8% decrease in σ0.2 and a 4% decrease in σu compared to the initial UFG state (non-annealed). Conclusions. It was revealed that the grain boundary (σgrain = 202 MPa) and dislocation (σdis = 69 MPa) strengthening contributions provide the most significant increase in the strength of the UFG MA20 magnesium alloy. For the magnesium alloy in the UFG and fine-grained (FG) states, a critical grain size interval of (1–7) μm was identified, corresponding to a sharp increase in the intensity of change for the calculated contributions of dislocation (dσdis/ dd), grain boundary (dσgrain/ dd), overall strengthening (dσtotal/dd), and dislocation density (dρ/dd). For the coarse-grained (CG) state of the alloy in the grain size range (7–40) μm, these parameters stabilize.

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.

Porous calcium phosphate coatings (C-P) with ZnO nanoparticles were obtained via the micro-arc oxidation method on a titanium substrate. ZnO nanoparticles were added to the C-P coatings to change the zeta potential and improve the coatings’ bioactivity and antibacterial properties. The samples with coatings were studied via scanning electron microscopy (SEM), X-ray diffraction, energy dispersive microanalysis, potentiodynamic polarization, and zeta potential measurement. The coatings modified with ZnO nanoparticles showed improved physical, electrochemical, and electrical properties, compared to the initial unmodified coatings. Modification with ZnO nanoparticles contributed to an increase in zeta potential from −60 mV to −53 mV. Functionalization of the coatings with ZnO nanoparticles allowed us to increase the anticorrosion characteristics by about 30%. The biological studies showed that the coatings had no cytotoxic effect on L929 fibroblast cells. The antibacterial activity of the coating rose by 99% after the addition of ZnO nanoparticles against the bacterium Staphylococcus aureus.

Magnesium-based alloys hold potential for medical applications, but face challenges like rapid bioresorption and limited mechanical strength during early bone healing. In our study, we present a novel Mg–Zn–Zr–Ce alloy with low cerium content (up to 0.1 wt.% Ce) processed using two severe plastic deformation (SPD) techniques. Through an innovative combination of multiaxial forging and multipass rolling, we have achieved a fine-grained structure with an average grain size of the primary α-Mg phase of 1.0 μm. This refined microstructure exhibits improved mechanical properties, including a substantial increase in yield strength (σ_YS) from 130 to 240 MPa, while preserving ductility. The alloy’s composition includes α-Mg grains, cerium and zinc hydrides, and intermetallic phases with cerium and zinc elements. Tensile testing of the fine-grained alloy demonstrates an enhancement in yield strength (σ_YS) to 250 MPa, marking a 2.8-fold improvement over the conventional state (σ_YS = 90 MPa), with a modest 2-fold reduction in ductility. Crucially, electrochemical tests conducted in physiological solutions highlight substantial advancements in corrosion resistance. The corrosion current was reduced from 14 to 2 μA/cm², while polarization resistance decreased from 3.1 to 8.1 kΩ∙cm², underlining the alloy’s enhanced resistance to biodegradation. Our results show that the novel Mg–Zn–Zr–Ce alloy, after combined SPD, demonstrates mitigated bioresorption and enhanced mechanical properties. Our findings highlight the fact that the introduction of this innovative alloy and the application of SPD represent significant steps towards addressing the limitations of magnesium-based alloys for medical implants, offering potential improvements in safety and effectiveness.

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.

In the present work, the surface of a biodegradable Mg alloy was modified to create porous diatomite biocoatings using the method of micro-arc oxidation. The coatings were applied at process voltages in the range of 350–500 V. We have studied the influence of the addition of ZrO₂ microparticles on the structure and properties of diatomite-based protective coatings for Mg implants. The structure and properties of the resulting coatings were examined using a number of research methods. It was found that the coatings have a porous structure and contain ZrO₂ particles. The coatings were mostly characterized by pores less than 1 μm in size. However, as the voltage of the MAO process increases, the number of larger pores (5–10 μm in size) also increases. However, the porosity of the coatings varied insignificantly and amounted to 5 ± 1%. It has been revealed that the incorporation of ZrO₂ particles substantially affects the properties of diatomite-based coatings. The adhesive strength of the coatings has increased by approximately 30%, and the corrosion resistance has increased by two orders of magnitude compared to the coatings without zirconia particles.