Zn Ca Alloy Research Articles

Formation mechanism of Ca-deficient hydroxyapatite coating on Mg–Zn–Ca alloy for orthopaedic implant

Abstract A biodegradable Ca-deficient hydroxyapatite (Ca-def HA) coating has been directly prepared on Mg–Zn–Ca alloy by pulse electrodeposition to improve its corrosion resistance and biocompatibility. However, the formation mechanism of such a Ca-def HA coating on magnesium substrate is still not clear. In this study, the microstructure evolution of the coating was characterized using x-ray diffractometer, x-ray photoelectron spectroscopy and scanning electron microscopy. Thermodynamic and kinetic studies of the precipitation of hydroxyapatite (HA), octacalcium phosphate (OCP) and dicalcium phosphate dihydrate (DCPD) in the used electrolyte were also carried out. Theoretical analyses illustrate that the precipitation of HA, OCP and DCPD are all possible when the electrolyte pH is higher than 6 at 80 °C, and that the higher the pH value, the more favorable is the formation of HA. Nevertheless, there is mainly poor crystalline Ca-def HA on the substrate when pulse electrodeposition lasts for 5 min, and its crystallinity increases with duration time The direct formation of the Ca-def HA coating on Mg–Zn–Ca alloy is closely dependent on the phase composition and microstructure of the substrate, the deposition parameters and Mg 2+ ions substitution in HA structure.

Microstructure and bio‐corrosion behavior of Mg–Zn and Mg–Zn–Ca alloys for biomedical applications

The microstructure and bio‐corrosion behavior of binary Mg–xZn (x = 1.25, 2.5, 4) and ternary Mg–Ca–xZn (x = 1.25, 2.5, 4) alloys have been studied using scanning electron microscopy (SEM), electrochemical, and immersion tests. Microstructure analysis indicated that the binary Mg–Zn alloys are composed of primary α‐Mg matrix and Mg12Zn13 phases, while, ternary Mg–Ca–Zn alloys are composed of α‐Mg, Mg2Ca, and IM1 (Ca3MgxZn15−x) (4.6 ≤ x ≤ 12) phases or α‐Mg, IM1 and IM3 (Ca2Mg5Zn13) phases. Electrochemical results showed that Mg–4Zn alloy has lowest corrosion rate among binary alloys. At constant Ca content of 0.8 wt.%, the addition of Zn up to 1.25 wt.% decreased the corrosion rate, while further addition of Zn increased the corrosion rate of ternary alloys. Immersion tests results demonstrated that the formation of Zn oxide layer in binary Mg–Zn alloy and evolution of eutectic phase (α‐Mg + IM1 + Mg2Ca) significantly retard the bio‐degradation rate of the ternary alloys.

High-Strength Low-Alloy (HSLA) Mg–Zn–Ca Alloys with Excellent Biodegradation Performance

This article deals with the development of fine-grained high-strength low-alloy (HSLA) magnesium alloys intended for use as biodegradable implant material. The alloys contain solely low amounts of Zn and Ca as alloying elements. We illustrate the development path starting from the high-Zn-containing ZX50 (MgZn5Ca0.25) alloy with conventional purity, to an ultrahigh-purity ZX50 modification, and further to the ultrahigh-purity Zn-lean alloy ZX10 (MgZn1Ca0.3). It is shown that alloys with high Zn-content are prone to biocorrosion in various environments, most probably because of the presence of the intermetallic phase Mg6Zn3Ca2. A reduction of the Zn content results in (Mg,Zn)2Ca phase formation. This phase is less noble than the Mg-matrix and therefore, in contrast to Mg6Zn3Ca2, does not act as cathodic site. A fine-grained microstructure is achieved by the controlled formation of fine and homogeneously distributed (Mg,Zn)2Ca precipitates, which influence dynamic recrystallization and grain growth during hot forming. Such design scheme is comparable to that of HSLA steels, where low amounts of alloying elements are intended to produce a very fine dispersion of particles to increase the material’s strength by refining the grain size. Consequently our new, ultrapure ZX10 alloy exhibits high strength (yield strength R p = 240 MPa, ultimate tensile strength R m = 255 MPa) and simultaneously high ductility (elongation to fracture A = 27%), as well as low mechanical anisotropy. Because of the anodic nature of the (Mg,Zn)2Ca particles used in the HSLA concept, the in vivo degradation in a rat femur implantation study is very slow and homogeneous without clinically observable hydrogen evolution, making the ZX10 alloy a promising material for biodegradable implants.

Sluggish mobility and strong icosahedral ordering in Mg–Zn–Ca liquid and glassy alloys

The dynamic properties and atomic configuration of Mg, Mg–Zn, Mg–Zn–Ca and Mg–Zn–Ca–Cu liquid and solid systems were comprehensively studied using ab initio molecular dynamics calculation. The viscosities of an Mg–Zn–Ca alloy were measured from 700K to 960K. It was found that Zn and Ca in Mg liquid induce remarkable slowing down of atoms, but Cu acted as a stimulus to the diffusion. Icosahedral short-range orders are most dominant in Mg–Zn–Ca alloys in view of the existence of a majority of 1551 type of bond pairs, 〈0,0,12,0〉 type of Voronoi polyhedra. Icosahedral medium-range orders can be formed in Mg–Zn–Ca alloys by the perfect icosahedra via the linkage of vertex-, edge-, face- and intercross-shared atoms. It is suggested that the high glass forming ability of Mg–Zn–Ca alloys is essentially related to the sluggish mobility and strong icosahedral short- and medium-range ordering in the undercooled liquid.

High strength Mg–Zn–Ca alloys prepared by atomization and hot pressing process

Mg73Zn22Ca5 alloys with the ultimate compressive fracture strength and yield stress up to 857MPa and 696MPa, respectively, and plastic strain above 4% have been prepared by hot pressing process of atomized partially glassy powders. These improved comprehensive mechanical properties for Mg–Zn–Ca alloys were obtained by proper controlling the hot pressing technologies to form a perfect interface bonding among the powders, and to make the nanoscale needle- or rod-like Ca2Mg5Zn13 phase precipitated from the magnesium matrix.

Improved plastic anisotropy of Mg–Zn–Ca alloys exhibiting high-stretch formability: A first-principles study

First-principles shear and tensile tests were performed on pure Mg, Mg–Zn–Ca, Mg–Ca and Mg–Zn models, and the generalized stacking fault energy for basal and prismatic slip systems and the surface energy of the {0001} plane were investigated to understand the origins of the high-stretch formability of Mg–Zn–Ca alloys. The calculations showed that the plastic anisotropy is reduced by the addition of Ca and Zn due to the non-linear nature of the unstable stacking fault energy for basal slip in higher-order alloying. Therefore, it is suggested that the improved plastic anisotropy gives rise to the reduced basal plane texture, resulting in the high formability of Mg–Zn–Ca alloys. The improved plastic anisotropy is caused by the charge transferred from Mg to Zn atoms, thereby compensating for the decreased charge density around the Ca atoms in the basal slip system, while no such compensation takes place in the prismatic slip system.

In vitro metal ion release and biocompatibility of amorphous Mg67Zn28Ca5 alloy with/without gelatin coating

Amorphous zinc-rich Mg–Zn–Ca alloys have exhibited good tissue compatibility and low hydrogen evolution in vivo. However, suboptimal cell–surface interaction on magnesium alloy surface observed in vitro could lead to reduced integration with host tissue for regenerative purpose. This study aims to improve cell–surface interaction of amorphous Mg67Zn28Ca5 alloy by coating a gelatin layer by electrospinning. Coated/uncoated alloys were immersed and extracted for 3days under different CO2. The immersion results showed that pH and metal ion release in the alloy extracts were affected by gelatin coating and CO2, suggesting their roles in alloy biocorrosion and a mechanism has been proposed for the alloy–CO2 system with/without coating. Cytotoxicity results are evident that gelatin-coated alloy with 2-day crosslinking not only exhibited no indirect cytotoxicity, but also supported attachment of L929 and MG63 cell lines around/on the alloy with high viability. Therefore, amorphous Mg67Zn28Ca5 alloy coated with gelatin by electrospinning technique provides a useful method to improve alloy biocompatibility.

Compressive deformation behavior of Mg–Zn–Ca alloy at elevated temperature

The compressive deformation behavior at elevated temperature of the as-extruded and as-ECAPed Mg–5.3Zn–0.6Ca (wt%) alloys was investigated in the current study. The flow stress in the as-ECAPed alloy was lower than that of the as-extruded alloy when the compressive temperature was above 200°C, due to the grain boundary softening effect. The strain rate sensitivity (SRS) was remarkably enhanced with the decrease of grain size or the increase of test temperature, and the corresponding m-value was calculated as 0.06–0.13, which indicated that the deformation was mainly dominated by the climb-controlled dislocation creep. Furthermore, the grain boundary sliding (GBS) mechanism might also be activated, which contributed to the higher SRS in the as-ECAPed alloy. The true deformation activation energy (Q') modified by threshold stress was calculated as 92–98 and 70–76kJ/mol in the as-extruded and as-ECAPed alloy, respectively, which was close to the activation energy for grain boundary diffusion, and the lower Q'-value in the as-ECAPed alloy might be derived from the non-equilibrium grain boundaries.

The effect of double extrusion on the microstructure and mechanical properties of Mg–Zn–Ca alloy

The Mg–4.50Zn–1.13Ca (wt%) alloy was subjected to double extrusion, and the microstructure and mechanical properties of the extruded alloys were investigated. Double extrusion refined the microstructure significantly, resulting in the enhancement of mechanical properties of the double-extruded alloys compared with the single-extruded alloy. The yield strength of the single-extruded alloy was 173MPa, while the value was increased to 320MPa and 370MPa after subsequent second extrusion at 300°C and 250°C, respectively. The second extrusion ratio had little effect on the microstructure and mechanical properties of the double-extruded alloys, while grain size was decreased and strength was increased with the decrease of double extrusion temperature. The double-extruded Mg–Zn–Ca alloy exhibited superior elevated temperature mechanical properties, which was attributed to the fine stable Ca2Mg6Zn3 intermetallic compound particles dispersed in the alloy.

Effects of cooling rate on microstructure, mechanical and corrosion properties of Mg–Zn–Ca alloy

Mg69Zn27Ca4 alloys with diameters of 1.5, 2 and 3 mm were fabricated using copper mold injection casting method. Microstructural analysis reveals that the alloy with a diameter of 1.5 mm is almost completely composed of amorphous phase. However, with the cooling rate decline, a little α-Mg and MgZn dendrites can be found in the amorphous matrix. Based on the microstructural and tensile results, the ductile dendrites are conceived to be highly responsible for the enhanced compressive strain from 1.3% to 3.1% by increasing the sample diameter from 1.5 mm to 3 mm. In addition, the Mg69Zn27Ca4 alloy with 1.5 mm diameter has the best corrosion properties. The current Mg-based alloys show much better corrosion resistance than the traditionally commercial wrought magnesium alloy ZK60 in simulated sea-water.

Effect of Yttrium addition on microstructure and mechanical properties of Mg–Zn–Ca alloy

The effects of Y on the microstructure and mechanical properties of the solution treated Mg–6Zn–0·5Ca alloys were investigated in this study. The results indicated that the alloy was mainly composed of α-Mg solid solution and the secondary phases of MgZn2, Ca2Mg6Zn3 and Mg12ZnY. The grains size of the solution treated alloys decreased from 210 to 80 μm with the increasing of Y content up to 1·5 wt-%. The great progressional mechanical properties were observed in the prepared Mg–6Zn–0·5Ca–1Y alloy at room temperature, the ultimate tensile strength (σb), tensile yield strength (σ0·2) and elongation (ϵ) were 284 MPa, 166 MPa and 14·7% respectively, which was determined by the combined contributions of solid solution strengthening, precipitation strengthening plus grain size refinement strengthening.

Microstructure, mechanical and bio-corrosion properties of Mn-doped Mg–Zn–Ca bulk metallic glass composites

The effects of Mn substitution for Mg on the microstructure, mechanical properties, and corrosion behavior of Mg69−xZn27Ca4Mnx (x=0, 0.5 and 1at.%) alloys were investigated using X-ray diffraction, compressive tests, electrochemical treatments, and immersion tests, respectively. Microstructural observations showed that the Mg69Zn27Ca4 alloy was mainly amorphous. The addition of Mn decreases the glass-forming ability, which results in a decreased strength from 545MPa to 364MPa. However, this strength is still suitable for implant application. Polarization and immersion tests in the simulated body fluid at 37°C revealed that the Mn-doped Mg–Zn–Ca alloys have significantly higher corrosion resistance than traditional ZK60 and pure Mg alloys. Cytotoxicity test showed that cell viabilities of osteoblasts cultured with Mn-doped Mg–Zn–Ca alloys extracts were higher than that of pure Mg. Mg68.5Zn27Ca4Mn0.5 exhibits the highest bio-corrosion resistance, biocompatibility and has desirable mechanical properties, which could suggest to be used as biomedical materials in the future.

Effects of Ca on microstructure, mechanical and corrosion properties and biocompatibility of Mg–Zn–Ca alloys

Zn and Ca were selected as alloying elements to develop an Mg-Zn-Ca alloy system for biomedical application due to their good biocompatibility. The effects of Ca on the microstructure, mechanical and corrosion properties as well as the biocompatibility of the as-cast Mg-Zn-Ca alloys were studied. Results indicate that the microstructure of Mg-Zn-Ca alloys typically consists of primary α-Mg matrix and Ca₂Mg₆Zn₃/Mg₂Ca intermetallic phase mainly distributed along grain boundary. The yield strength of Mg-Zn-Ca alloy increased slightly with the increase of Ca content, whilst its tensile strength increased at first and then decreased. Corrosion tests in the simulated body fluid revealed that the addition of Ca is detrimental to corrosion resistance due to the micro-galvanic corrosion acceleration. In vitro hemolysis and cytotoxicity assessment disclose that Mg-5Zn-1.0Ca alloy has suitable biocompatibility.

Effect of extrusion ratio on microstructure, texture and mechanical properties of indirectly extruded Mg–Zn–Ca alloy

Indirect extrusion of as-cast Mg–5.25 Zn–0.6 Ca (wt%) alloy was performed at 300°C with different extrusion ratios (ER, 20:1 and 10:1), and the microstructure, texture and tensile properties of the as-extruded alloys were investigated in the current study. With the increase of ER, the volume fraction of DRXed grains was remarkably increased, but the average DRXed grains size was almost unchanged. The basal texture weakening phenomenon occurred in the as-extruded alloy with higher ER, which resulted from the higher volume fraction of the DRXed grain. Intensive basal fiber texture was observed in the unDRXed grains, but the basal planes of the DRXed grains were inclined to ED from 0° to 35°. The ER had only little effect on the tensile properties of the as-extruded alloys; due to the similar strengthening effect from the unDRXed and DRXed grains, a competitive mechanism exists between the fine grain strengthening and texture weakening effect.

The Y-doped MgZnCa alloys with ultrahigh specific strength and good corrosion resistance in simulated body fluid

The microalloying effects of Y on the microstructure, mechanical properties and bio-corrosion resistances of Mg69Zn27Ca4 (at.%) alloy were investigated by X-ray diffraction, compressive tests and electrochemical treatments, respectively. The Y-bearing Mg–Zn–Ca alloys were found to possess an ultrahigh compressive strength above 1000MPa as well as high specific strength of 3.44×105Nmkg−1. The enhanced mechanical properties can be attributed to the ductile dendrite phase, Mg12YZn, which is dispersed in the matrix with the Y addition. Furthermore, the Y-doped Mg–Zn–Ca alloys studied in this work have much better corrosion resistance than traditional ZK60 and pure Mg alloys in simulated body fluid (SBF) at 37°C, which could be good candidates to be used as biomedical materials.

Effects of calcium on texture and mechanical properties of hot-extruded Mg–Zn–Ca alloys

The effects of calcium on microstructure, texture and mechanical properties of hot-extruded Mg–Zn–Ca alloys were studied. The results showed that calcium elements can weaken the strong basal textures and refined the grain size of extruded Mg–Zn–Ca alloys. The weakening of extrusion textures with increasing content of Ca is related to the particle stimulated nucleation of recrystallization (PSN) and the particle retarded the growth of the dynamic recrystallization grains. The increase of the tensile elongation is attributing to the weakened extrusion textures and refined grains.

Fabrication of chitosan/magnesium phosphate composite coating and the in vitro degradation properties of coated magnesium alloy

A composite coating was fabricated on home-developed Mg–Zn–Ca alloy to improve its biocorrosion resistance in simulated body fluid (SBF, Kokubo solution). The coating was prepared by micro arc oxidation (MAO) of the alloy, followed by dip-coating in chitosan solution. The coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM). The electrochemical performance of the MAO, chitosan and chitosan/MAO coated alloys was evaluated by anodic polarization measurements in SBF. After being coated by composite coating, anticorrosion performance was significantly enhanced, while single MAO or chitosan coating only improved anticorrosion properties within a limited range. The barrier effect and interface condition of composite coating were analyzed and related with the improvement in electrochemical performance. The present result indicates that fabrication of composite coatings is a significant strategy to slow down the degradation of Mg alloys, thus enhancing the potential of using Mg alloys as bio-implants.

Biocorrosion of coated Mg–Zn–Ca alloy under constant compressive stress close to that of human tibia

Besides common immersion tests, it's essential to investigate biocorrosion of degradable magnesium alloys in physiological environment under stress that bone implants bear in vivo. Using self-designed immersion method simulating in vivo conditions and conventional immersion test, corrosion morphologies and pH variations of the coated samples with or without compressive stress and the substrate alloy were studied. Results showed that the applied stress can change the degradation mode of coated samples and promote corrosion process, causing peeling-off locations on the coating and continuous increase of pH value. Meanwhile, biocompatibility of the coated samples was not apparently affected by the compressive stress.

In vivo degradation and bone response of a composite coating on Mg–Zn–Ca alloy prepared by microarc oxidation and electrochemical deposition

Composite coatings with hydroxyapatite (HA), octacalcium phosphate (OCP) in electrochemical deposition (ED) layers and MgO, Mg3 (PO4 )2 in microarc oxidation (MAO) layers were prepared by ED and MAO on Mg-Zn-Ca alloy to improve the corrosion resistance and bone response. Substrates and coated samples were implanted in the femur shaft of rabbits to observe in vivo degradation behavior during 50 weeks. Results showed that the degradation rate of the substrates was much faster than the coated at 8, 12 weeks and became close to the coated at 18 weeks postoperatively. The composite coatings prevented the substrate from rapid release of magnesium ions at the interface and gradually degraded at the same time. The composite coatings induced more newly formed bone tissue and faster bone response. Overall, reduced degradation rate and improved bone response were achieved by the composite coatings. Thus, the composite coatings on Mg-Zn-Ca alloy are promising for clinical application in the future.

Composite coating prepared by micro-arc oxidation followed by sol–gel process and in vitro degradation properties

A Mg phosphate coating was prepared on home-developed Mg–Zn–Ca alloy to improve its anticorrosion performance in simulated body fluid (SBF, Kokubo solution). The coating was prepared by micro-arc oxidation (MAO) method at the working voltage of 120–140V. Evident improvement of anticorrosion was obtained even through the surface was porous. To further diminish the contact with SBF, a TiO2 layer was coated on the porous MAO layer by sol–gel dip coating followed by an annealing treatment. The coatings were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS). The electrochemical performance of the MAO and TiO2/MAO coated alloys was evaluated by anodic polarization measurements. The pores on Mg phosphate layer provided accommodation sites for the subsequent TiO2 sol–gel coating which sealed the pores and hence significantly enhanced the anticorrosion while single MAO coating only improve anticorrosion within a limited range. The present result indicates that fabrication of composite coatings is a significant strategy to improve the corrosion resistance of Mg–Zn–Ca alloy and other alloys, thus enhancing the potential of using Mg alloys as bio-implants.