Mikhail Linderov

The authors studied the samples of as-cast ferritic-pearlitic GG15 gray cast iron with lamellar graphite (σ _u = 121 ± 7 MPa). Tension of 20 mm diameter cylindrical samples was performed on N50KT testing machine. Static crack resistance tests of cast iron were carried out at temperatures of 20, –70 and –196 °C on 10 and 20 mm thick prismatic samples using a three-point bending scheme on Instron 8802 testing machine. The authors estimated the implementation of plane strain (PS) conditions by the t /( K _Q /σ _0.2 ) ² ≥ 0.6 criterion specified in GOST 25.506 – 85, as well as by the h _max / t < 10 ^–2 criterion, where hmax is the maximum depth of the plastic zone under the fracture surface, t is the sample thickness. Depth of the plastic zones beneath the fractures surface was determined using X -ray diffraction. Tensile tests on cylindrical samples with a 10 mm diame­ter and 50 mm length were conducted at 20 °C using MK-50 tester. The authors examined the fractures surface microrelief using SIGMA scanning electron microscope (ZEISS). Static crack resistance tests of cast iron showed that at 20 °C, the PS condition according to GOST 25.506 – 85 is met only for 20 mm thick samples at –70 °C. However, according to the h _max / t criterion, the PS condition is met for all the samples, regardless of thickness and testing temperature. Therefore, the obtained static crack resistance values (20.6 ± 1.5 MPa·m ^1/2 ) correspond to K _{1 C} . To evaluate the PS conditions of GG15 cast iron, it is proposed to use the criterion t /( K _Q /σ _0.2 ) ² ≥ 0.38. In the static failure area, regardless of the sample thickness and test temperature, cast iron destruction occurred via intergranular brittle failure and cleavage mechanisms. Results from tensile tests of the samples showed that crack initiation occurred at graphite inclusions near the sample surface. Subsequent sample failure occurred at an angle to the sample cross-section by a cleavage mechanism.

Currently, there is no generally accepted standard setup for assessing the durability of biomedical magnesium alloy specimens in corrosive environments. Each research group uses its own testing system, which does not always account for the many technical factors that influence the final experimental result. This paper proposes equipment that can be assembled using a standard fatigue machine for assessing the corrosion fatigue life of bioresorbable magnesium alloys in various physiological solutions. It includes: a testing machine with a slotted plate for rigidly mounting the chamber; polyamide grips providing galvanic isolation; a peristaltic pump for pumping the medical solution; an ion meter connected to a computer to record changes in pH and temperature; a thermostat for maintaining the chamber temperature within a specified range; an additional solution fed into the chamber through a needle using a small peristaltic pump when a predetermined pH level is exceeded, maintaining the pH within an acceptable range. This setup uses corset-type specimens, although other configurations are also acceptable. A camera can be installed to visualize corrosion processes on the test specimens. The testing system achieves good repeatability and reliability of the obtained results, which has been experimentally verified and confirmed using MA14 magnesium alloy of various melts, selected as a model. Setup proposed can be used to evaluate the fatigue life of magnesium alloys for medical purposes. It is easily reproducible under standard laboratory conditions.

Biocompatibility makes magnesium alloys attractive functional materials in terms of their use as biodegradable implants. However, the technologies for manufacturing semi-finished products carry a possible diversity of the local strain rate and temperature within a rather wide range, which affects the processed material structure and properties. The purpose of the study is to determine the range of temperatures and resistance to deformation, at which there is no negative effect on the main structural characteristics of the processed material, using the example of a medical purposes alloy of the Mg–Zn–Y alloying system. The authors carried out mechanical tests of a biodegradable Mg–1Zn–2.9Y magnesium alloy at various temperatures and strain rates. The influence of temperatures in the range of 20...400 °C on the structure and properties of the Mg–Zn–Y system alloy is disclosed. Starting from a temperature of 350 °C, the process of dynamic recrystallization is accompanied both by the complete restoration (return) of the original microstructure and by coarsening of the grain size, which can adversely affect the material functional characteristics. The high thermal stability of the biodegradable Mg–1Zn–2.9Y magnesium alloy is revealed, which probably results from the presence of the LPSO phase in it. The study shows that the deformation process is accompanied by twinning. At a strain rate of 2∙10−2 s−1 over the entire temperature range, the grain size distribution slightly narrows and shifts towards smaller diameters. The application of the obtained results in technological processes for manufacturing medical semi-finished products will help to solve the issue of microstructure instability at the stage of transition from a semi-finished product to a finished product during subsequent thermomechanical treatments.

This article presents a concise overview of modern achievements and existing knowledge gaps in the area of biodegradable magnesium alloys. Hundreds of Mg-based alloys have been proposed as candidates for temporary implants, and this number tends to increase day by day. Therefore, while reviewing common aspects of research in this field, we confine ourselves primarily to the popular Mg-Zn-Ca system, taken as a representative example. Over the last decades, research activities in this area have grown enormously and have produced many exciting results. Aiming at highlighting the areas where research efforts are still scarce, we review the state-of-the-art processing techniques and summarize the functional properties attained via a wide variety of processing routes devised towards achieving a desired properties profile, including the mechanical response in terms of strength, ductility, and fatigue resistance paired with biocompatibility and bio-corrosion resistance or controlled degradability. We pay keen attention to a summary of corrosion properties and mechano-chemical interactions between an aggressive environment and loaded Mg-based structures, resulting in stress corrosion cracking and premature corrosion fatigue failures. The polemic issues and challenges practitioners face in their laboratory research are identified and discussed.

This paper evaluates the fatigue strength of ultrafine-grained (UFG) Grade 4 Ti in the low-cycle fatigue region, as well as the strength of medical implants (plates and screws) made of UFG Ti under various types of loading in comparison with the strength of products made of coarse-grained (CG) Ti. To produce a UFG state, titanium billets after annealing were processed by the ECAP-Conform technique. The fatigue of the prismatic specimens with a thickness of 10 mm from CG and UFG Ti was tested by the three-point bending method using an Instron 8802 facility. The modeling and evaluation of the stress-strain state in the ANSYS software package for finite-element analysis revealed, in particular, the localization of equivalent stresses in the area of hole edges and at fillets during the tension of the plates. The performed research has demonstrated that medical implants (plates and screws) from UFG Grade 4 Ti have a higher strength under different types of loading (tension, fatigue strength, torsion) in comparison with products from CG Ti. This opens up a possibility for the miniaturization of medical products from UFG Ti while preserving their main performance properties at an acceptable level.

Magnesium alloys are contemporary candidates for many structural applications of which medical applications, such as bioresorbable implants, are of significant interest to the community and a challenge to materials scientists. The generally poor resistance of magnesium alloys to environmentally assisted fracture, resulting, in particular, in faster-than-desired bio-corrosion degradation in body fluids, strongly impedes their broad uptake in clinical practice. Since temporary structures implanted to support osteosynthesis or healing tissues may experience variable loading, the resistance to bio-corrosion fatigue is a critical issue that has yet to be understood in order to maintain the structural integrity and to prevent the premature failure of implants. In the present communication, we address several aspects of the corrosion fatigue behaviour of magnesium alloys, using the popular commercial ZK60 Mg-Zn-Zr alloy as a representative example. Specifically, the effects of the testing frequency, surface roughness and metallic coatings are discussed in conjunction with the fatigue fractography after the testing of miniature specimens in air and simulated body fluid. It is demonstrated that accelerated environmentally assisted degradation under cyclic loading occurs due to a complicated interplay between corrosion damage, stress corrosion cracking and cyclic loads. The occurrence of corrosion fatigue in Mg alloys is exaggerated by the significant sensitivity to the testing frequency. The fatigue life or strength reduced remarkably with a decrease in the test frequency.

The tensile behaviour of the biocompatible alloy Mg-1Zn-0.2Ca (in wt.%) in the fine-grained state, obtained by severe plastic deformation via multiaxial isothermal forging, has been investigated in a wide range of temperatures (20 ÷ 300) °C and strain rates (5 × 10⁻⁴ ÷ 2 × 10⁻²) s⁻¹ with the measurements of acoustic emission (AE). The dependences of mechanical properties, including the yield stress, ultimate strength, ductility, and the strain-hardening rate, on the test temperature and strain rate, were obtained and discussed. It is shown for the first time that an acoustic emission method is an effective tool for in situ monitoring of the dynamic recrystallisation (DRX) process. The specific behaviour of the acoustic emission spectral density reflected by its median frequency as a function of strain at various temperatures can serve as an indicator of the DRX process’s completeness.

In this paper, we study the corrosion-resistant austenitic steel Fe-0.02C-18Cr-8Ni for medical applications. The microstructure and mechanical properties (tensile mechanical properties, torsional strength, impact toughness, and static and cyclic crack resistance) under different types of loading of the steel are investigated. The results are compared for the two states of the steel: the initial (coarse-grained) state and the ultrafine-grained state produced by severe plastic deformation processing via equal-channel angular pressing. It is demonstrated that the ultrafine-grained steel 0.08C-18Cr-9Ni has essentially better properties and is very promising for the manufacture of medical products for various applications that experience various static and cyclic loads during operation.