Analysis of the dislocation activity of Mg-Zn-Y alloy using synchrotron radiation under tensile loading.

An understanding of deformation behavior and texture development is crucial for the formability improvement of Mg alloys. X-ray line profile analysis using the convolutional multiple whole profile (CMWP) fitting method allows the experimental determination of dislocation densities separately for different Burgers vectors up to a high deformation degree. A wider use of this technique still requires exploration and testing of various materials. In this regard, the reliability of the CMWP fitting method for Mg-Zn-Y alloys, in terms of the dislocation activity during tensile deformation, was verified in the present study by the combined analysis of electron backscatter diffraction (EBSD) investigation and visco-plastic self-consistent (VPSC) simulation. The predominant activity of non-basal 〈a〉 dislocation slip was revealed by CMWP analysis, and Schmid factor analysis from the EBSD results supported the higher potential of non-basal dislocation slip in comparison with basal 〈a〉 dislocation slip. Moreover, the relative slip activities obtained by the VPSC simulation also show a similar trend to those obtained from the CMWP evaluation.

Degradation Behavior and Mechanical Integrity of a Mg-0.7Zn-0.6Ca (wt.%) Alloy: Effect of Grain Sizes and Crystallographic Texture.

The microstructural characteristics of biodegradable Mg alloys determine their performance and appropriateness for orthopedic fixation applications. In this work, the effect of the annealing treatment of a Mg-0.7Zn-0.6Ca (ZX11) alloy on the mechanical integrity, corrosive behavior, and biocompatibility-osteoinduction was studied considering two annealing temperatures, 350 and 450 °C. The microstructure showed a recrystallized structure, with a lower number of precipitates, grain size, and stronger basal texture for the ZX11-350 condition than the ZX11-450. The characteristics mentioned above induce a higher long-term degradation rate for the ZX11-450 than the ZX11-350 on days 7th and 15th of immersion. In consequence, the mechanical integrity changes within this period. The increased degradation rate of the ZX11-450 condition reduces 40% the elongation at failure, in contrast with the 16% reduction for the ZX11-350 condition. After that period, the mechanical integrity remained unchanged. No cytotoxic effects were observed for both treatments and significant differentiation of mesenchymal stem cells into the osteoblast phenotype was observed.

Excellent age hardenability with the controllable microstructure of AXW100 magnesium sheet alloy.

Age-hardenability and corresponding improvement of the mechanical properties of Mg-1Al-0.7Ca and Mg-1Al-0.7Ca-0.7Y alloy sheets are addressed with respect to the microstructure and texture evolution during thermomechanical treatments. A fine grain structure and weak texture with the basal pole split into the sheet transverse direction are retained in the Mg-1Al-0.7Ca-0.7Y sheet even after the homogenization at 500 °C, due to the grain boundary pinning by Y-containing precipitates possessing a high thermal stability. Contrarily, the Mg-1Al-0.7Ca sheet shows a coarse microstructure and basal-type texture after the homogenization. The peak-aged condition is attained after the aging at 250 °C for 1800 s of both homogenized sheets, while the Y-containing sheet shows a higher hardness than the Mg-1Al-0.7Ca sheet. TEM analysis and thermodynamic calculation show the formation of metastable precipitates composed of Al, Ca, Y and Mg in the Mg-1Al-0.7Ca-0.7Y sheet at the homogenized and peak-aged conditions. A significant increase in the yield strength is obtained in the peak-aged condition from 162 MPa after the homogenization to 244 MPa, which arises from the increased size and number density of the precipitates. The high age-hardenability of the Mg-1Al-0.7Ca-0.7Y sheet attributes to the superior mechanical properties with an improved ductility promoted by the weak texture.

Alloying and Processing Effects on the Microstructure, Mechanical Properties, and Degradation Behavior of Extruded Magnesium Alloys Containing Calcium, Cerium, or Silver.

Magnesium alloys attract attention as degradable implant materials due to their adjustable corrosion properties and biocompatibility. In the last few decades, especially wrought magnesium alloys with enhanced mechanical properties have been developed, with the main aim of increasing ductility and formability. Alloying and processing studies allowed demonstrating the relationship between the processing and the microstructure development for many new magnesium alloys. Based on this experience, magnesium alloy compositions need adjustment to elements improving mechanical properties while being suitable for biomaterial applications. In this work, magnesium alloys from two Mg-Zn series with Ce (ZE) or Ca (ZX) as additional elements and a series of alloys with Ag and Ca (QX) as alloying elements are suggested. The microstructure development was studied after the extrusion of round bars with varied processing parameters and was related to the mechanical properties and the degradation behavior of the alloys. Grain refinement and texture weakening mechanisms could be improved based on the alloy composition for enhancing the mechanical properties. Degradation rates largely depended on the nature of second phase particles rather than on the grain size, but remained suitable for biological applications. Furthermore, all alloy compositions exhibited promising cytocompatibility.

In vitro evaluation of the ZX11 magnesium alloy as potential bone plate: Degradability and mechanical integrity.

Considering the excellent biocompatibility of magnesium (Mg) alloys and their better mechanical properties compared to polymer materials, a wrought MgZnCa alloy with low contents of Zn (0.7 wt%) and Ca (0.6 wt%) (ZX11) was developed by twin roll casting (TRC) technology as potential biodegradable bone plates. The degradability and cell response of the ZX11 alloy were evaluated in vitro, as well as the mechanical integrity according to tensile tests after immersion. The results revealed a slightly higher degradation rate for the rolled ZX11, in comparison to that of the annealed one. It was mainly caused by the deformation twins and residual strain stored in the rolled alloy, which also seemed to promote localized degradation, thereby leading to a relatively fast deterioration in mechanical properties, especially the fracture strain/elongation. In contrast, after the annealing treatment, the alloy showed relatively lower strength, yet a lower degradation rate and quite stable elongation during the initial weeks of immersion were observed. More importantly, the ZX11 alloy, regardless of the annealing treatment, showed good in vitro cytocomopatibility regarding human primary osteoblasts. The assessment indicates the rolled alloy as a good choice for implantation sites where relatively high mechanical strength is needed during the early implantation, while the annealed alloy is a potential candidate for the sites which demand stable mechanical integrity during service. STATEMENT OF SIGNIFICANCE: The development of magnesium alloys as bone implants demands low degradation rate to gain not only a slow hydrogen evolution, but also a stable mechanical integrity during service. The present study develops a micro-alloyed MgZnCa alloy via twin roll casting (TRC) technology. It exhibited limited cytotoxicity, fairly low degradation rate and comparable strength to the reported Mg-1Zn-5Ca alloy which has been used as bone screws in clinical trials, indicating the great potential application as biodegradable bone implants. Furthermore, it showed good mechanical integrity during immersion to support the defect healing. Our results can aid other researchers to evaluate the mechanical integrity of biodegradable materials and to pay more attention to the effect of degradation behaviour on mechanical integrity of materials.