Effect of deformation processing of the dilute Mg-1Zn-0.2Ca alloy on the mechanical properties and corrosion rate in a simulated body fluid

Magnesium and its alloys have several competitive advantages due to their low density and the highest specific strength among modern structural metallic materials. However, the relatively low ductility and poor corrosion resistance hinder their broader use in industry. Unlike many engineering applications, the ability of magnesium alloys to dissolve in chlorine-containing media is attractive for their applications as temporary implants. In the present work, the influence of thermomechanical processing on mechanical properties and corrosion resistance of the alloy Mg-1Zn-0.2Ca intended for biomedical applications is investigated. The low content of alloying elements permitted grain boundary hardening to be realised to a large extent during deformation processing. Severe plastic deformation through the multi-axis isothermal forging at relatively high homological temperatures in combination with isothermal rolling gave rise to the significantly refined to the micrometre scale homogeneous microstructure with an excellent balance of the tensile strength and ductility (the yield stress and ultimate tensile strength are in excess of 210 and 260 MPa, respectively, and the elongation at break is over 20 %) and corrosion resistance in the simulated body fluid (SBF) in vitro. With the pH value of the SBF maintained at 7.4 throughout the test, the corrosion rate assessed by the hydrogen evolution and gravimetric methods was found to be nearly constant without signatures of saturation for the deformation-processed specimens. The rate of hydrogen desorption of 0.5 ml / cm2 / day was found to be far below the amount that could be accommodated by the human body without adverse effects

On the Corrosion Fatigue of Magnesium Alloys Aimed at Biomedical Applications: New Insights from the Influence of Testing Frequency and Surface Modification of the Alloy ZK60

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

Monitoring Dynamic Recrystallisation in Bioresorbable Alloy Mg-1Zn-0.2Ca by Means of an In Situ Acoustic Emission Technique

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