Mg/hydroxyapatite composites for potential bio-medical applications

This thesis was submitted for the degree of Master of Philosophy and awarded by Brunel University.Depending on an excellent combination of high mechanical property and fracture toughness, metallic biomaterials have been widely accepted for clinical application of bone fixation. However, some prominent disadvantages such as stress shielding effect due to their high elastic modulus, poisonous ions released by corrosion or mechanical wear [Puleo and Huh, 1995] definitely restrict their effective performance in-service. In addition, patients also have to bear the pain resulted from second surgery for removing implants. Although bio-degradable polymers and ceramics seem to solve the tough problem as promising substitutes, the low mechanical property and rapid corrosion rate make them fail to qualify against the bear-loading requirement in body. Therefore, we desperately look for a non-toxic suitable material for medical application which possesses appropriate mechanical properties, and favourable corrosion resistance

Investigation of Magnesium-Lithium-Zinc-Calcium Alloys for Biomedical Applications

Alloy design is a fundamental approach to developing Mg-based bioresorbable implant materials that possess the desired mechanical and degradation behavior required for treatment. Mg-Zn-Ca alloys have received significant interest as bioresorbable implant materials because of superior mechanical properties and lower degradation rates. However, they are prone to localized corrosion, which jeopardizes their mechanical strength and causes premature implant failure. In this study, lithium has been added to Mg-1Zn-0.5Ca to promote uniform degradation and room temperature ductility. Alloys with Li content up to 4 wt.% exhibited a hcp structure, with ~12% elongation and evidence of pitting. Alloys with 8 wt.% Li had a duplex structure, with ~30% elongation and no evidence of pitting. Alloys with 11 wt. % exhibited a single-phase bcc structure, with ~33% elongation and a lithium carbonate surface coating. The 8 and 11 wt.% Li alloys had a reduced metal dissolution, which resulted in a high viability of HUVEC cells, making them attractive for biodegradable stent materials. These simultaneous improvements in mechanical and degradation properties of these Mg-Li-Zn-Ca alloys were achieved through a reduction in precipitation of secondary phases, formation of a lithium carbonate surface coating and phase transformation from a hexagonal close packed to a duplex and body centered cubic structures. In addition, higher hardness, and elastic modulus achieved for these alloys are desirable to prevent stent recoil after deployment. Rolling, which is an important metal forming process in the manufacturing of metals and alloys, has been adopted as a thermomechanical process for these designed alloys. The effects of both cold and hot rolling on the alloys uncover their response to plastic deformation and demonstrates their formability and manufacturability

The development of micro-alloyed magnesium-zinc based ternary alloy

Magnesium (Mg) alloys have been widely studied for applications in 3C and transport areas. However, the intrinsically high susceptibility to corrosion and the inadequate ductility at room temperature largely limit their wider practical applications. The poor creep resistance of commonly commercial magnesium-aluminum (Mg-Al) based alloys at elevated temperature drives the demand for Al-free Mg alloys, among which magnesium-zinc system (Mg-Zn) shows great potential for the development of low-cost Mg alloys with higher strength and good corrosion performance. In this thesis, low-Zn containing Mg-Zn alloys micro-alloyed with different ternary alloying elements were developed and comprehensively studied, aiming at achieving a good combination of corrosion performance and mechanical properties. By investigating the influence of ternary alloying elements on the microstructures and corrosion behavior of Mg0.5Zn0.2X and Mg4Zn0.2X alloys (in wt.%), Mg0.5Zn0.2Ca, Mg0.5Zn0.2Ge and Mg4Zn0.2Sn alloys were identified as promising alloys with possible good combination of corrosion performance and mechanical strength. Afterwards, hot extrusion at different speed was applied to the three alloys to further improve corrosion resistance and strength. The extrusion speed showed little influence on the corrosion resistance of the three optimized alloys because of the slight alternation of the microstructures. Affected by the chemistry of the bulk materials, the corrosion rates of Mg0.5Zn0.2Ge and Mg4Zn0.2Sn alloys were reduced after extrusion while that of Mg0.5Zn0.2Ca alloy was not clearly affected. The corrosion mechanism of Mg0.5Zn0.2Ge alloy changed from localized corrosion to uniform corrosion after extrusion owing to the refined microstructure and the increased participation of Zn in the corrosion product layer. In comparison, both as-cast and extruded Mg0.5Zn0.2Ca alloys revealed uniform corrosion, while Mg4Zn0.2Sn alloys in both conditions suffered from localized corrosion in corrosive electrolytes due to the heterogeneous microstructures. Deionized water based sodium chloride (NaCl) solutions at different concentrations did not affect the corrosion mechanism of the alloys, while artificial tap water based NaCl solution significantly enhanced the corrosion resistance of the alloys owing the formation of an additional calcium carbonate layer on the top of the primary oxide/hydroxide layer. The stronger textures of Mg0.5Zn0.2Ge and Mg4Zn0.2Sn alloys conferred higher tensile strength but higher mechanical anisotropy on the alloys compared with Mg0.5Zn0.2Ca alloy. The tensile properties of all alloys deteriorated with exposure time in salt spray because of corrosion, especially when localized corrosion happened. However, the variation tendency of the tensile properties was closely related to the corrosion resistance of the alloys in salt fog. The fatigue behavior (S-N curves) of the optimized alloys deviated from near-linear trend in air. In the presence of corrosive electrolytes, the fatigue lives and fatigue limits of the alloys decreased. Again, the corrosion fatigue behavior of the alloys were strongly related to the corrosion behavior in different solutions, especially for Mg4Zn0.2Sn alloy. The susceptibility of the alloys to stress corrosion cracking (SCC) in four different electrolytes were studied by constant load tests. Mg0.5Zn0.2Ca and Mg4Zn0.2Sn alloys were resistant to SCC in all environments. While Mg0.5Zn0.2Ge alloy exhibited susceptibility to SCC in all environments, especially in deionized water. This was because of the different corrosion products/substrate interfaces formed in different solutions, which influenced the development of cracks. The results emphasized the influence of corrosion on the fatigue behavior and mechanical properties of Mg-Zn alloys, and also highlighted the importance of the investigation of the overall properties (corrosion, mechanical, fatigue and stress corrosion properties) of Mg alloys during practical alloy development

Towards reducing tension-compression yield and cyclic asymmetry in pure magnesium and magnesium-aluminum alloy with cerium addition

In this study, we report the effect of cerium (Ce) addition on the tension-compression yield and cyclic asymmetry in commercially pure magnesium (Cp-Mg) and Mg-Al alloy at room temperature (RT). The materials examined include extruded and annealed Cp-Mg, Mg-0.5Ce, and Mg-3Al-0.5Ce alloys. Incorporation of 0.5wt.% Ce in pure Mg results in the weakening of its basal texture, uniform distribution of Mg12Ce precipitates, and refinement of the grain size. Consequently, the tensile yield strength and ductility of pure Mg increase, and tension-compression yield asymmetry is eliminated. However, the presence of 3wt.% Al in Mg suppresses the beneficial effects of Ce addition. The formation of complex precipitates, such as Mg-Al-Ce and Al11Ce3, limits the weakening of the basal texture, reduction in grain size, improvement in ductility, and elimination of tension-compression yield asymmetry observed in Mg-0.5Ce. Nevertheless, Al contributes to the solid solution strengthening in Mg, resulting in the highest tensile yield strength of Mg-3Al-0.5Ce. Finally, the addition of 0.5wt.% Ce enhances the cyclic strength, stabilizes cyclic stress response, reduces inelastic strain, and minimizes cyclic asymmetry in both pure Mg and Mg-Al alloy while maintaining a comparable fatigue life. Overall, Ce addition positively impacts the microstructure and mechanical behavior of pure Mg and its investigated alloy. The reasons for these improvements are discussed in detail

The effect of equal-channel angular pressing on microstructure, mechanical properties, and biodegradation behavior of magnesium alloyed with silver and gadolinium

The effect of equal channel angular pressing (ECAP) on the microstructure, texture, mechanical properties, and corrosion resistance of the alloys Mg-6.0%Ag and Mg-10.0%Gd was studied. It was shown that ECAP leads to grain refinement of the alloys down to the average grain size of 2–3 μm and 1–2 μm, respectively. In addition, in both alloys the precipitation of fine particles of phases Mg$_{54}$Ag$_{17}$ and Mg$_{5}$Gd with sizes of ~500–600 and ~400–500 nm and a volume fraction of ~9% and ~8.6%, respectively, was observed. In the case of the alloy Mg-6.0%Ag, despite a significant grain refinement, a drop in the strength characteristics and a nearly twofold increase in ductility (up to ~30%) was found. This behavior is associated with the formation of a sharp inclined basal texture. For alloy Mg-10.0%Gd, both ductility and strength were enhanced, which can be associated with the combined effect of significant grain refinement and an increased probability of prismatic and basal glide. ECAP was also shown to cause a substantial rise of the biodegradation rate of both alloys and an increase in pitting corrosion. The latter effect is attributed to an increase in the dislocation density induced by ECAP and the occurrence of micro-galvanic corrosion at the matrix/particle interfaces

Improving the corrosion resistance of MgZn1.2GdxZr0.18 (x =0, 0.8, 1.4, 2.0) alloys via Gd additions

Funding Information: This research was financially supported by the National Key Research and Development Program of China (Grant No. 2016YFB0301101 ), the National Natural Science Foundation of China (Grant No. 51971054 ) and the Fundamental Research Funds for the Central Universities (Grant Nos. N180904006 and N2009006 ). Publisher Copyright: © 2020 Elsevier LtdEffects of Gd addition on microstructure, corrosion behavior and mechanism of cast and extruded MgZn1.2GdxZr0.18 alloys are investigated through microstructure observation, weight loss and electrochemical tests. Increasing Gd from 0 to 2.0 at.%, grains are refined, MgZn2 phase, W-phase and X-phase are formed successively, and basal texture intensity is decreased. The significantly decreased grain size by extrusion and Gd addition induces formation of protective Gd2O3 and MgO layer. The extruded MgZn1.2Gd2.0Zr0.18 alloy shows decreased corrosion rate of 3.72 ± 0.36 mm/year, owing to fine and homogeneous microstructure, dual-role (micro-anode and barrier) of X-phase, compact oxidation layer and basal crystallographic texture.Peer reviewe

Processing and Characterization of Magnesium-Based Materials

Due to their lightweight and high specific strength, Mg-based alloys are considered as substitutes to their heavier counterparts in applications in which corrosion is non-relevant and weight saving is of importance. Furthermore, due to the biocompatibility of Mg, some alloys with controlled corrosion rates are used as degradable implant materials in the medical sector. The typical processing route of Mg parts incorporates a casting step and, subsequently, a thermo–mechanical treatment. In order to achieve the desired macroscopic properties and thus fulfill the service requirements, thorough knowledge of the relationship between the microstructure, the processing steps, and the resulting property profile is necessary. This Special Issue covers in situ and ex situ experimental and computational investigations of the behavior under thermo–mechanical load of Mg-based alloys utilizing modern characterization and simulation techniques. The papers cover investigations on the effect of rare earth additions on the mechanical properties of different Mg alloys, including the effect of long-period stacking-ordered (LPSO) structures, and the experimental and computational investigation of the effect of different processing routes

Extrusion temperature impacts on biometallic Mg-2.0Zn-0.5Zr-3.0Gd (wt%) solid-solution alloy

AbstractTo obtain ideal implant materials, we hot extruded Mg-2.0Zn-0.5Zr-3.0Gd solid-solution alloys, and studied extrusion temperature impacts on materials properties. Fine dynamic recrystallized (DRXed) grains (∼5 μm) and elongated coarse un-dynamic recrystallized (unDRXed) deformed grains turned out at the range of 470–490 °C, but changed to bigger ones (∼8 μm) and abnormal growth (30–40 μm) at 490–510 °C. Precipitated phases consist of rod-like (Mg, Zn)₃Gd particles and newly precipitated Mg₂Zn₁₁ rectangles. The alloy extruded at 490 °C meets all mechanical and anticorrosive requirements for biomaterials, thanks to evenly distributed second phases via the solid solution, and the grain refinements through the hot extrusion.Abstract To obtain ideal implant materials, we hot extruded Mg-2.0Zn-0.5Zr-3.0Gd solid-solution alloys, and studied extrusion temperature impacts on materials properties. Fine dynamic recrystallized (DRXed) grains (∼5 μm) and elongated coarse un-dynamic recrystallized (unDRXed) deformed grains turned out at the range of 470–490 °C, but changed to bigger ones (∼8 μm) and abnormal growth (30–40 μm) at 490–510 °C. Precipitated phases consist of rod-like (Mg, Zn)₃Gd particles and newly precipitated Mg₂Zn₁₁ rectangles. The alloy extruded at 490 °C meets all mechanical and anticorrosive requirements for biomaterials, thanks to evenly distributed second phases via the solid solution, and the grain refinements through the hot extrusion