This paper presents a comprehensive study of the effect of the processing by high-pressure torsion (HPT) on the corrosion behavior in Ringer’s solution for two popular bioresorbable magnesium alloys—Mg-1Ca and Mg-1Zn-0.2Ca. Three states were studied for each alloy—the initial homogenized state, the as-HPT-processed state and the state after subsequent annealing at 250 and 300 °C. It is shown that HPT processing results in a very strong grain refinement in both alloys down to a mean grain size of about 210 nm for the Mg-1Ca alloy and 90 nm for the Mg-1Zn-0.2Ca alloy, but their corrosion resistance values differ significantly (by an order of magnitude). The conducted precision scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction studies demonstrate that such a difference in the corrosion behavior is conditioned by a difference in the morphology and origin of the nano-sized particles of second phases, as well as by a change in the electrochemical properties of the “particle–α-Mg” pair. The obtained results are discussed from the perspective of the innovative applications of biodegradable Mg alloys for the manufacture of advanced medical implants and products.

The development of high-performance biodegradable alloys with controllable corrosion rates to be used for manufacturing advanced implants is a hot topic of modern materials science and biomedicine. This work features the changes in microstructure, corrosion behavior and mechanical properties of the Mg-2 wt.%Sr alloy progressively induced by equal-channel angular pressing, high-pressure torsion and annealing. We show that such processing leads to significant microstructure refinement including diminishing grain size, defect accumulation and fragmentation of the initial eutectics. We demonstrate that the application of severe plastic deformation and heat treatment is capable of considerably enhancing the mechanical and corrosion performance of a biodegradable alloy of the Mg-Sr system. The best trade-off between strength, plasticity and the corrosion resistance has been achieved by annealing of the Mg-Sr alloy subjected to combined severe plastic deformation processing.

It is known, that magnesium-based alloys are the appropriate materials to be used as biodegradable metals to produce new-generation medical implants. Magnesium can decompose in the human body during the healing process. If dissolution is controlled, there is no need in additional operation for implant removal after healing completion. Particularly, Mg-Zn-Ca alloys are considered the most appropriate biodegradable metal implants due to their biocompatibility. In the Mg-Zn-Ca alloys, the addition of Zn and Ca as alloying elements can improve the mechanical properties and increase the corrosion resistance compared to pure Mg without affecting biocompatibility. The work covers the study of the structure and mechanical properties of the magnesium Mg-1%Zn-0.2%Ca alloy after severe plastic deformation (SPD). The research of the structure was carried out using scanning and transmission electron microscopy. The study of mechanical properties was carried out by measuring microhardness and tension tests. The study shows that applying the equal channel angular pressing (ECAP) method and additional treatment with the severe plastic deformation (SPD) method to the Mg–1%Zn–0.2%Ca alloy leads to the formation of the ultra-fine grain (UFG) structure with the average grain size of less than 1 micron. The authors identified that, as a result of strong refinement of the magnesium alloy grain structure, the ultimate strength increases twice up to 283 MPa compared to the homogenized state, when the ultimate strength is 125 MPa. At the same time, in the UFG state, the plasticity significantly decreases up to 3 %.

The formation of ultrafine-grained structure is very desirable in the microstructural design of magnesium alloys, in particular Mg-Zn-Ca medical alloy, for a substantial increase in their strength and corrosion resistance. However, conventional processing of these alloys by equal channel angular pressing is not easily applicable due to their low deformability, which often leads to rapid fracture of billets. In this paper, computer simulation data and principles of physical mesomechanics are used to demonstrate that preliminary deformation of Mg alloy billets by reduction at high temperatures and low strain rates significantly increases their deformation capacity and enables equal channel angular pressing at lower temperatures, resulting in billets with ultrafine-grained structure. Consideration is given to the physical nature of the established effect.

В данной работе представлены результаты исследований влияния ультрамелкозернистой (УМЗ) структуры на прочностные и коррозионные свойства магниевого сплава Mg–1%Zn–0,2%Ca. Особое внимание уделено изучению влияния УМЗ-структуры на скорость коррозии. Методами растровой электронной микроскопии (РЭМ) и просвечивающей электронной микроскопии (ПЭМ) изучены особенности структуры после коррозионных испытаний в растворе Рингера. Методами рентгеноструктурного анализа определены кристаллографические плоскости, наиболее подверженные коррозионному воздействию. Обсуждается различие в коррозионном поведении крупнозернистых и УМЗ-образцов.

The paper considers the structure and corrosion properties of ultrafine-grained magnesium alloys Mg-1Zn and Mg-1Zn-0.2Ca processed by high pressure torsion (HPT). The average grain size and phase composition of particles were determined by electron microscopy and X-ray diffraction. The corrosion rate in the studied samples in the homogenized state and after nanostructuring by HPT was calculated by the gravimetric method. It is shown that the presence of Ca in the Mg-1Zn-0.2Ca alloy leads to the formation of cathode-type Ca2Mg6Zn3 particles, which have a positive effect on corrosion resistance in the homogenized samples compared to the Mg-1Zn alloy. The HPT samples of the Mg-1Zn-0.2Ca alloy subjected to additional heat treatment at a temperature of 300 °C also showed better corrosion resistance compared to the Mg-1Zn alloy subjected to similar treatment. The difference in corrosion properties is discussed from the point of view of the formation of galvanic pairs between the matrix and cathode particles, which are observed in the ternary Mg-1Zn-0.2Ca alloy and are absent in the binary Mg-1Zn alloy due to the complete solubility of zinc in magnesium at a Zn content of less than 1 wt%. В работе рассмотрены структура и коррозионные свойства ультрамелкозернистых магниевых сплавов Mg-1Zn и Mg-1Zn-0,2Са, полученных методом интенсивной пластической деформации кручением (ИПДК). Методами электронной микроскопии и рентгеноструктурного анализа определены средний размер зерен и фазовый состав частиц. Гравиметрическим методом рассчитана скорость коррозии в исследованных образцах в гомогенизированном состоянии (КЗ) и после наноструктурирования методом ИПДК. Показано, что присутствие Ca в сплаве Mg-1Zn-0,2Са ведет к образованию частиц Ca2Mg6Zn3 катодного типа, которые положительно влияют на коррозионную стойкость в гомогенизированном состоянии по сравнению со сплавом Mg-1Zn. ИПДК образцы сплава Mg-1Zn-0,2Ca, подвергнутые дополнительной термической обработке при температуре 300оС, также проявили лучшую коррозионную стойкость по сравнению со сплавом Mg-1Zn, подвергнутом аналогичной обработке. Различие в коррозионных свойствах обсуждается с точки зрения образования гальванических пар между матрицей и катодными частицами, которые присутствуют в тройном сплаве Mg-1Zn-0,2Са, и отсутствуют в бинарном сплаве Mg-1Zn вследствие полной растворимости цинка в магнии при содержании Zn менее 1 вес.%.