Development and evaluation of a magnesium–zinc–strontium alloy for biomedical applications — Alloy processing,microstructure, mechanical properties,and biodegradation

A new biodegradable magnesium–zinc–strontium (Mg–Zn–Sr) alloy was developed and studied for medical implant applications. This first study investigated the alloy processing (casting, rolling, and heat treatment), microstructures, mechanical properties, and degradation properties in simulated body fluid (SBF). Aging treatment of the ZSr41 alloy at 175 °C for 8 h improved the mechanical properties when compared to those of the as-cast alloy. Specifically, the aged ZSr41 alloy had an ultimate tensile strength of 270 MPa, Vickers hardness of 71.5 HV, and elongation at failure of 12.8%. The mechanical properties of the ZSr41 alloy were superior as compared with those of pure magnesium and met the requirements for load-bearing medical implants. Furthermore, the immersion of the ZSr41 alloy in SBF showed a degradation mode that progressed cyclically, alternating between pitting and localized corrosion. The steady-state average degradation rate of the aged ZSr41 alloy in SBF was 0.96 g/(m²·hr), while the pH of SBF immersion solution increased. The corrosion current density of the ZSr41 alloy in SBF solution was 0.41 mA/mm², which was much lower than 1.67 mA/mm² for pure Mg under the same conditions. In summary, compared to pure Mg, the mechanical properties of the new ZSr41 alloy improved while the degradation rate decreased due to the addition of Zn and Sr alloying elements and specific processing conditions. The superior mechanical properties and corrosion resistance of the new ZSr41 alloy make it a promising alloy for next-generation implant applications.     

Bond strength and corrosion resistance of bioglass coated magnesium alloy fabricated by uniaxial pressing and microwave hybrid heating

Bioactive ceramics coated magnesium alloys with a combination of suitable mechanical strength and adjustable corrosion resistance are desired for biodegradable implants. In this study, a dense bioglass coated magnesium alloy was fabricated by uniaxial pressing and microwave hybrid heating technique. The microstructure, bond strength and corrosion behavior of the samples were evaluated by means of scanning electron microscopy, X-ray diffraction, tensile bond test, electrochemical and immersion test. It was shown that uniaxial pressing conducted at the glass transition temperature significantly densified the sol–gel derived bioglass coating, which was free of pores and micro-cracks. The compact coating structure combined with mild interfacial stress not only improved the cohesion/adhesion strength (25.8 ± 2.6 MPa) but also enhanced corrosion resistance by retarding the penetration of corrosive solution. Furthermore, the formed CaP precipitates on the surface of the coating would provide another protection for the magnesium alloy to some extent.     

Study of vanadium-based chemical conversion coating on the corrosion resistance of magnesium alloy

In this study, magnesium alloy (AZ61) was immersed in vanadium containing bath with various conditions, such as the vanadium concentration, immersion time and bath temperature. The results indicate that increase of both vanadium concentration and immersion time produces a thicker conversion layer. However, when immersion time is too long, it will worsen the corrosion resistance due to the increasing of the crack density. The experimental parameter of bath temperature has no significant effect on corrosion resistance. Our results demonstrated that the better corrosion resistance coating can be obtained when the samples are submitted to an immersion in the conversion bath containing NaVO₃ with concentration of 30 g l⁻¹ for 10 min at 80 °C. The presented conversion treatment has its potential to replace the chrome-based conversion coating treatment.     

Corrosion of experimental magnesium alloys in blood and PBS: A gravimetric and microscopic evaluation

Corrosion tests for medical materials are often performed in simulated body fluids (SBF). When SBF are used for corrosion measurement, the open question is, how well they match the conditions in the human body. The aim of the study was to compare the corrosion behaviour of different experimental magnesium alloys in human whole blood and PBS^minus (phosphate buffered saline w/o Ca and Mg) as a simulated body fluid by gravimetric weight measurements and microscopic evaluation. Eight different experimental magnesium alloys, containing neither Mn nor other additives, were manufactured. With these alloys, a static immersion test in PBS^minus and a dynamic test using the Chandler-loop model with human whole blood over 6 h were performed. During the static immersion test, the samples were weighed every hour. During the dynamic test, the specimens were weighed before and after the 6 h incubation period in the Chandler-loop. From both tests, the total mass change was calculated for each alloy and the values were compared. Additionally, microscopic pictures from the samples were taken at the end of the test period. All alloys showed different corrosion behaviour in both tests, especially the alloys with high aluminium content, MgAl9 and MgAl9Zn1. Generally, alloys in PBS showed a weight gain due to generation of a microscopically visible corrosion layer, while in the blood test system a more or less distinct weight loss was observed. When alloys are ranked according to corrosion susceptibility, the results differ also between the test systems. The MgAl9 alloy, showing the most pronounced corrosion in PBS, was one of the least corroding alloys under simulated in vivo conditions in blood. Thus, the ranking concerning clinical suitability of the magnesium alloys tested in this study is different, depending on the used electrolyte and the kind of method. For a possible clinical use, the alloy MgAl9Zn1 might be preferable for further investigations.     

The effect of SMAT-induced grain refinement and dislocations on the corrosion behavior of Ti–25Nb–3Mo–3Zr–2Sn alloy

A nanocrystalline layer, which consists of pure β phase with high density of dislocations on Ti–25Nb–3Mo–3Zr–2Sn alloy, was fabricated by surface mechanical attrition treatment (SMAT). The corrosion behavior of the as-SMATed sample, together with the solution-treated coarse-grained and 200 °C annealed SMATed samples, was investigated by potentiodynamic polarization and electrochemical impedance spectroscope (EIS) techniques in physiological saline and simulated body fluid (SBF) solutions. The results demonstrate that the corrosion resistance of the studied alloy in both of the solutions considerably increased as the grain size decreased from microscale to nanoscale, which is ascribed to the dilution of segregated alloying elements at grain boundaries and the formation of more stable and much thicker passive protection films on the nanograined samples. Although the SMAT-induced grain refinement and dislocations both have positive effects on the corrosion behavior of the studied alloy, our post annealing experimental results indicate that the improved corrosion resistance is mainly due to the grain refinement.     

Potentiostatic pulse-deposition of calcium phosphate on magnesium alloy for temporary implant applications — An in vitro corrosion study

In this study, a magnesium alloy (AZ91) was coated with calcium phosphate using potentiostatic pulse-potential and constant-potential methods and the in vitro corrosion behaviour of the coated samples was compared with the bare metal. In vitro corrosion studies were carried out using electrochemical impedance spectroscopy and potentiodynamic polarization in simulated body fluid (SBF) at 37 °C. Calcium phosphate coatings enhanced the corrosion resistance of the alloy, however, the pulse-potential coating performed better than the constant-potential coating. The pulse-potential coating exhibited ~ 3 times higher polarization resistance than that of the constant-potential coating. The corrosion current density obtained from the potentiodynamic polarization curves was significantly less (~ 60%) for the pulse-deposition coating as compared to the constant-potential coating. Post-corrosion analysis revealed only slight corrosion on the pulse-potential coating, whereas the constant-potential coating exhibited a large number of corrosion particles attached to the coating. The better in vitro corrosion performance of the pulse-potential coating can be attributed to the closely packed calcium phosphate particles.     

Control of degradation rate of bioabsorbable magnesium by anodization and steam treatment

The control of degradation rate of bioabsorbable magnesium devices is crucial for their biomedical applications. In this study, the influence of anodizing voltages and autoclaving on the degradation behavior of anodized pure magnesium was examined by immersion tests in a culture medium for 14 d. The anodization and autoclaving varied the morphology of surface film. Porous films were formed at 7 V and 100 V, and non-porous films were formed at 2 V and 20 V. The microscopic appearance of the anodized films did not change by autoclaving. The degradation rate on Day 1 was the highest and subsequently decreased to a quasi-steady state within the initial 3–5 d. The 7 V- and 100 V-anodized specimens showed the highest and the lowest quasi-steady degradation rate, respectively. The autoclaving significantly retarded the degradation of the anodized specimens. These facts revealed that anodization and autoclaving are useful for the control of the degradation rate of magnesium and its alloys. The porous anodized films showed local corrosion, whereas the non-porous anodized film formed at 20 V did not show apparent local corrosion. The local corrosion was prevented by autoclaving. These results suggest that the occurrence of local corrosion depends on the porous morphology of surface film.     

Fabrication and formation of bioactive anodic zirconium oxide nanotubes containing presynthesized hydroxyapatite via alternative immersion method

Hydroxyapatite (HA) coating has been widely applied on metallic biomedical implants to enhance their biocompatibility. It has been reported that HA coating can be formed on annealed zirconium with anodic zirconium oxide nanotubular arrays after immersion in simulated biological fluid (SBF) for about 14 days. In the present study, we apply an alternative immersion method (AIM) to form presynthesized HA on ZrO₂ nanotubes. The AIM-treated specimen was then moved to the SBF to evaluate the capability for the formation of HA on it. The HA coating formed after only 2 days immersion and thickened after 5 days in the SBF. The HA coating is the carbonated HA with a ratio of Ca to P of about 1.4, similar to the physiological HA containing other minor elements such as Mg and Na. The results demonstrate that the AIM treatment is indeed suitable for the zirconium oxide nanotubes and highly accelerates the formation of HA coating in comparison with the existing methods, i.e. the annealing of the as-formed zirconium oxide nanotubular arrays.     

Factors influencing the deposition of hydroxyapatite coating onto hollow glass microspheres

Hydroxyapatite (HA) and HA coated microcarriers for cell culture and delivery have attracted more attention recently, owing to the rapid progress in the field of tissue engineering. In this research, a dense and uniform HA coating with the thickness of about 2 μm was successfully deposited on hollow glass microspheres (HGM) by biomimetic process. The influences of SBF concentration, immersion time, solid/liquid ratio and activation of HGM on the deposition rate and coating characteristics were discussed. X-ray diffraction (XRD) and Fourier transform infrared spectrum (FTIR) analyses revealed that the deposited HA is poorly crystalline. The thickness of HA coating showed almost no increase after immersion in 1.5SBF for more than 15 days with the solid/liquid ratio of 1:150. At the same time, SBF concentration, solid/liquid ratio and activation treatment played vital roles in the formation of HA coating on HGM. This poorly crystallized HA coated HGM could have potential use as microcarrier for cell culture.     

Bioactivity of calcium phosphate coatings prepared by electrodeposition in a modified simulated body fluid

The purpose of this study was to investigate bioactivity of calcium phosphate coatings prepared by electrodeposition in a modified simulated body fluid (SBF). Calcium phosphates were electrodeposited on commercially pure titanium substrates in the modified SBF at 60 °C for 1 h maintaining the cathodic potentials of − 1.5 V, − 2 V, and − 2.5 V (vs. SCE). Subsequently, the calcium phosphate coatings were transformed into apatites during immersion in the SBF at 36.5 °C for 5 days. The apatites consisted of needle-shaped crystallites distributed irregularly with different grain sizes. As the coatings were electrodeposited at higher cathodic potential, the crystallite of the apatites got denser and the grain sizes of the apatites became bigger during subsequent immersion in the SBF. However, as the coatings were electrodeposited at higher cathodic potential, the coatings were transformed into apatites with lower crystallinity and the Ca/P atomic ratio of the apatites got higher than 1.67, that of stoichiometric hydroxyapatite, after subsequent immersion in the SBF. In addition, CO₃²⁻ ions contained in the modified SBF were incorporated in the calcium phosphate coating during electrodeposition and had an influence on transforming the calcium phosphate into bonelike apatite during subsequent immersion in the SBF showing that CO₃²⁻ incorporated in the apatites disturbed crystallization of the apatites. These results revealed that the coating electrodeposited at − 2.0 V (vs. SCE) in the modified SBF containing CO₃²⁻ ions was the most bioactive showing transformation into carbonate apatite similar to bone apatite.     

Microstructure,mechanical property and corrosion behaviors of interpenetrating C/Mg-Zn-Mn composite fabricated by suction casting

A novel interpenetrating C/Mg-Zn-Mn composite was fabricated by infiltrating Mg-Zn-Mn alloy into porous carbon using suction casting technique. The microstructure, mechanical properties and corrosion behaviors of the composite have been evaluated by means of SEM, XRD, mechanical testing and immersion test. It was shown that the composite had a compact structure and the interfacial bonding between Mg-Zn-Mn alloy and carbon scaffold was very well. The composite had an ultimate compressive strength of (195 ± 15) MPa, which is near with the natural bone (2–180 MPa) and about 150-fold higher than that of the original porous carbon scaffold, and it still retained half of the strength of the bulk Mg-Zn-Mn alloy. The corrosion test indicated that the mass loss percentage of the composite was 52.9% after 30 days′ immersion in simulated body fluid (SBF) at 37 ± 0.5 °C, and the corrosion rates were 0.043 mg/cm²h and 0.028 mg/cm²h after 3 and 7 days′ immersion, respectively. The corrosion products on the composite surface were mainly Mg(OH)₂ and hydroxyapatite (HA).     

Microhardness and corrosion properties of hypoeutectic Al–7Si alloy processed by high-pressure torsion

High-pressure torsion (HPT) was used to produce hypoeutectic Al–7Si alloy samples having a range of microstructures to investigate the effect of the grain refinement on its corrosion behavior in 3.5 wt.% NaCl solution for the first time. Optical microscopy measurements reveal that with the HPT processing increased from 1/4 to 10 revolutions under an applied pressure of 6.0 GPa, brittle coarse silicon particles and intermetallic phases were effectively broken into ultrafine-grained particles and redistributed homogeneously into the Al-rich matrix. Open-circuit potential and polarization curves results exhibit that corrosion resistance of the Al–7Si alloy in NaCl solution was significantly enhanced upon high torsion strains, with corrosion rate reduced from 7.41 μm y⁻¹ for the as-received sample to 1.68 μm y⁻¹ for the 10-turn processed sample. Electrochemical impedance spectroscopy analysis combined with characterization of the corroded samples using scanning electron microscopy and energy dispersive X-ray spectroscopy indicates that the enhancement in corrosion performance of the Al–7Si alloy is due to the breakage of coarse silicon particles and intermetallic phases, the microstructure homogeneity and the increased HPT-induced active sites. It is demonstrated that microstructure refinement through HPT processing can significantly improve both microhardness and corrosion properties of the Al–7Si alloy.     

Effect of sintering parameters on microstructure,mechanical properties and electrochemical behavior of Nb–Zr alloy for biomedical applications

Despite the importance of Nb–Zr alloys as candidate materials for biomedical applications, little attention has been given to their processing and the development of new or improved structures. Here, we explore the viability of synthesizing a nano/sub-micron grain structured Nb–Zr alloy through the use of mechanical alloying (MA) and spark-plasma sintering (SPS). The sintered samples were characterized through measurements of densification, Vickers hardness (HV), X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The effect of the SPS parameters on the microstructure and mechanical properties of the sintered alloys was also investigated. Moreover, electrochemical corrosion analyses were performed by a means of a conventional three-electrode cell to assess the corrosion resistance of the developed alloys in Simulated Body Fluids (SBF) medium. A nano/sub-micron grain structured Nb–Zr alloy with an average grain size of between 100 and 300 nm was produced using the MA-SPS techniques. A maximum hardness and relative density of 584 HV and 97.9% were achieved, respectively. Moreover, the nano/sub-micron grain structured Nb–Zr alloy exhibited higher corrosion resistance in SBF medium, which makes this alloy is a promising candidate for use in biomedical applications.     

Induction of hydroxycarbonate apatite formation on polyethylene or alumina substrates by spherical vaterite particles deposition

Hydroxycarbonate apatite (HCA) layers were formed on polyethylene (PE) or alumina substrates by depositing spherical sub-micron vaterite particles and then immersing in simulated body fluid (SBF). HCA formation on vaterite‐coated PE was faster than that on coated alumina (3 days for PE and 7 days for alumina). The adsorption of phosphate ions on the vaterite particles in SBF was studied by monitoring changes in the concentration of phosphorous in SBF and the surface charges of vaterite during the SBF immersion. The phosphorous concentration of SBF in which a vaterite-coated PE was immersed for 1 h was lower than that in which a vaterite-coated alumina was immersed. Zeta potential of the vaterite surface deposited on PE drastically decreased after 1 h immersion in SBF. The vaterite particles deposited on each substrate immediately adsorbed phosphate ions in SBF. The amount of ions adsorbing on the vaterite surfaces deposited on PE was larger than that on alumina. This was attributed to differences in the surface charges between PE (? 16 mV) and alumina (+ 38 mV). The phosphate adsorption was predominantly electrostatic therefore related to the surface charge of vaterite particles. The surface charges of substrates may affect the charge of vaterite particles.     

Electrophoretic bilayer deposition of zirconia and reinforced bioglass system on Ti6Al4V for implant applications: An in vitro investigation

The physical, chemical and biological properties of the bioglass reinforced yttria-stabilized composite layer on Ti6Al4V titanium substrates were investigated. The Ti6Al4V substrate was deposited with yttria stabilized zirconia — YSZ as the base layer of thickness ≈ 4–5 μm, to inhibit metal ion leach out from the substrate and bioglass zirconia reinforced composite as the second layer of thickness ≈ 15 μm, which would react with surrounding bone tissue to enhance bone formation and implant fixation. The deposition of these two layers on the substrate was carried out using the most viable electrophoretic deposition (EPD) technique. Biocompatible yttria-stabilized zirconia (YSZ) in the form of nano-particles and sol gel derived bioglass in the form of micro-particles were chosen as precursors for coating. The coatings were vacuum sintered at 900 °C for 3 h. The biocompatibility and corrosion resistance property were studied in osteoblast cell culture and in simulated body fluid (SBF) respectively. Analysis showed that the zirconia reinforced bioglass bilayer system promoted significant bioactivity, and it exhibited a better corrosion resistance property and elevated mechanical strength under load bearing conditions in comparison with the monolayer YSZ coating on Ti6Al4V implant surface.     

Research on the microstructure,fatigue and corrosion behavior of permanent mold and die cast aluminum alloy

Permanent mold (PM) and high pressure die cast (HPDC) AlMg5Si2Mn are employed to investigate the microstructure, fatigue strength and corrosion resistance. Results indicated that the mechanical properties (R_m, R_0.2 and δ) of HPDC specimens (314 MPa, 189 MPa and 7.3%) are significantly better than those of PM specimens (160 MPa, 111 MPa and 2.5%) due to the finer grain size and less cast defects. Fatigue cracks of PM samples dominantly initiated from shrinkage pores and obscure fatigue striations are observed in crack growth region. Corrosion and pitting potentials of PM and HPDC AlMg5Si2Mn alloy are around −1250 mV, −760 mV and −1220 mV, −690 mV respectively. Numerous pits are observed around the grain boundaries because the corrosion potential of Mg₂Si is more anodic than that of α-Al matrix. In addition, the superior corrosion resistance of HPDC samples can be attributed to the fine grain size and the high boundary density which improved the formation of oxide layer on the surface and prevented further corrosion.     

Influences of magnetic annealing on the grain growth in a cryoECAPed 1050 aluminum alloy

The evolutions of hardness, grain microstructures and dislocations were investigated in cryoECAPed 1050 aluminum alloy after annealing at 423–673 K for 1 h without and with a magnetic field of 12 T. Compared to those of samples annealed without the magnetic field, the hardness of samples annealed with the magnetic field is lower at 423–573 K, and then higher at > 573 K. The high magnetic field accelerates the formation of dislocation cells or sub-grains. The interface dislocations frequently observed in materials deformed at high temperature were detected in samples annealed with the magnetic field at ≥ 423 K. The fraction of low angle boundaries in sample annealed with the field is 49% and 34% in sample annealed without the field even at 673 K. No abnormal grain growth occurs in samples annealed with the magnetic field due to the rapid consumption of the stored distortion energy during recovery.     

Improvement of mechanical properties in severely plastically deformed Ni–Cr alloy

This study was carried out to evaluate the grain refining and mechanical properties in alloys that undergo severe plastic deformation (SPD). Conventional rolling (CR) and cross-roll rolling (CRR) were introduced as methods for SPD, and a Ni–20Cr alloy was selected as the experimental material. The materials were cold rolled to 90% thickness reduction and subsequently annealed at 700 °C for 30 min to obtain the fully recrystallized microstructure. The annealed materials after cold rolling were assessed through electron backscattered diffraction (EBSD) analysis to investigate the grain boundary characteristic distributions (GBCDs). The CRR process was more effective than the CR process in developing grain refinement; the grain size decreased from 70 μm in the initial material to 4.2 μm (CR) and 2.4 μm (CRR), respectively. The grain refinement affected mechanical properties such as microhardness, yield, and tensile strength, which were significantly increased relative to the initial material.     

The study of tribological and corrosion behavior of plasma nitrided 34CrNiMo6 steel under hot and cold wall conditions

This paper reports on a comparative study of tribological and corrosion behavior of plasma nitrided 34CrNiMo6 low alloy steel under modern hot wall condition and conventional cold wall condition. Plasma nitriding was carried out at 500 °C and 550 °C with a 25% N₂ + 75% H₂ gas mixture for 8 h. The wall temperature of the chamber in hot wall condition was set to 400 °C. The treated specimens were characterized by using scanning electron microscopy (SEM), X-ray diffraction (XRD), microhardness and surface roughness techniques. The wear test was performed by pin-on-disc method. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests were also used to evaluate the corrosion resistance of the samples. The results demonstrated that in both nitriding conditions, wear and corrosion resistance of the treated samples decrease with increasing temperature from 500 °C to 550 °C. Moreover, nitriding under hot wall condition at the same temperature provided slightly better tribological and corrosion behavior in comparison with cold wall condition. In consequence, the lowest friction coefficient, and highest wear and corrosion resistance were found on the sample treated under hot wall condition at 500 °C, which had the maximum surface hardness and ε-Fe_2–3N phase.     

Characterisation of the bioactive behaviour of sol–gel hydroxyapatite–CaO and hydroxyapatite–CaO–bioactive glass composites

The fabrication and characterization of sol–gel derived hydroxyapatite–calcium oxide (HAp–CaO) material is investigated focusing on the effect of the addition of a bioactive glass on the material bioactive behaviour through the fabrication of a novel HAp–CaO (70 wt.%)–bioactive glass (30 wt.%) composite material. The bioactive behaviour of the materials was assessed by immersion studies in Simulated Body Fluid (SBF) and the alterations of the materials surfaces after soaking periods in SBF were characterized by Scanning Electron Microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). A brittle and weakly crystalline carbonate hydroxyapatite (HCAp) layer was found to develop on the surface of all samples, few hours after immersion in SBF, confirming the high bioactivity of the material. Alterations of the morphology of the developed HCAp layer, which led to a more compact structure, were observed on the surface of composite samples after 7 days of immersion in SBF. The presence of the CaO phase seems to accelerate the formation of HCAp, while the bioactive glass affects both the morphology and cohesion of the developed layer.