While developing materials for degradable implants based on magnesium, we have given a special regard to mechanical properties (incl. Young’s modulus E) and degradation rates over decades. These are indeed crucial and important properties. They are influenced by chemical composition, microstructural features like intermetallic phases, grain size etc. Additionally, the different processing steps (casting/solidification, heat treatments, wrought processing, machining etc.) also have impact. However, numerous implants and medical devices are rods with a high aspect ratio (length l to area of cross section A) like needles, nails, K-wires, clips, cannulas etc. For these types of devices, it is also about geometry and therefore about moments of inertia as reported by Euler and standard textbooks regarding civil engineering.

A relation between the tensile flow stress and grain size i.e., so-called an empirical Hall-Petch (H-P) law, for ultrafine-grained (UFG) pure Zn was evaluated experimentally for the first time. In reality it's problematic to assess this prediction using experimental results because of a low recrystallization temperature of a pure Zn, which undergoes grain growth at the room testing temperature. Three Zn bulk materials with the intercept grain size (dl) ranging from 0.61 to 1.11 µm, stabilized with a small portion of nanoscale ZnO dispersoids positioned at high angle grain boundaries, were fabricated from fine gas atomized pure Zn powders. The material with the finest grain size of 0.61 µm, ever reported for unalloyed Zn, showed the highest ultimate tensile strength and 0.2% strain offset yield stress (YS0.2), ever reported for unalloyed Zn. At the same time, the strengths were accompanied with a reasonably high ductility. The experimental data were compared to a theoretical model for the deformation behavior of UFG metals based on grain boundary sliding (GBS) through dislocation glide. We validated, that the conventional linear H-P relation, following YS0.2=40.8+104.8 dl^-0.5, cease to exist in the range ~400–0.6 µm. We disproved a grain refinement softening in UFG region predicted by the theoretical model. The effect of the refined dl and increased ZnO content on the corrosion and in-vitro biological responses was elaborated. From a practical point of view, the obtained data will be of help to design of new high-strength, especially bioresorbable, Zn-based materials.

Magnesium-lithium (Mg-Li) alloys are unique given their dual-phase structure (HCP α-Mg, BCC β-Li), with formation of a BCC structure at Li contents above 11.7wt.%. In this research, we investigate the underlying effect of rapid solidification on microstructure formation in a dual-phase (α+β) Mg-Li alloy, with the aim of tailoring microstructure to ultimately improve the control of material properties. To pursue this aim, cast dual-phase Mg-6wt.%Li plates were used as substrates in a laser-powder bed fusion machine, where laser tracks were deposited at a constant speed and at varying powers (i.e., 15 to 100W). Microstructural characterisation by scanning electron microscopy revealed a profound underlying influence of laser power, whereby melt pool microstructure was found to transform from planar/cellular to cellular/dendritic morphology, and finally to the formation of fine, nano-scale eutectic lamellae with increasing laser power. Further characterisation of nano-lamellae by transmission electron microscopy confirmed the presence of amorphous Li coupled with that of HCP α-Mg. The ability to tailor microstructure in Mg-6wt.%Li through controlled rapid solidification processing is promising for future design of degradable implants. Most significant is the formation of nano-scale lamellae consisting of layered HCP α-Mg and amorphous Li, as this can enable enhanced mechanical strength, along with improved corrosion resistance due to amorphous Li structure.

Simultaneously improving the strength, plasticity and corrosion properties has become the key to the wider clinical applications of Bio-Mg alloys. Besides the alloying technology, the microstructure optimization design is an effective method to improve the comprehensive properties of Mg alloys. However, it is not clear that what kind of microstructure can simultaneously improve the strength and corrosion properties of biomedical Mg alloys.In the present work, Mg-Nd-Zn-Zr alloy (denoted as JDBM) was chosen as one candidate of the biodegradable vascular stent materials. The samples with three typical microstructures were prepared successfully, i.e., fine equiaxed grains, coarse equiaxed grains and bimodal microstructure. Mechanical properties and corrosion behaviors of these samples were studied to uncover the best microstructure design strategy, including grain size, distribution and orientation relationship. The present findings will facilitate the researchers in optimizing their study strategy and achieving a better combination of mechanical and corrosion properties when designing and processing biodegradable Mg-based alloys. Samples with fine and uniform equiaxed grains are recommended in most cases for the application of biodegradable Mg alloys, which will provide excellent mechanical properties and corrosion resistance simultaneously.