Effect of minor content of Gd on the mechanical and degradable properties of as-cast Mg-2Zn-xGd-0.5Zr alloys

RE-containing Mg alloys used as biodegradable medical implants exhibit good promising application due to their good mechanical properties and degradation resistance. In this work, effect of Gd on the microstructure, mechanical properties and biodegradation of as-cast Mg-2Zn-xGd-0.5Zr alloys was investigated. The results showed that there were mainly α-Mg, I-phase, W-phase and MgZn2 phase in Mg-Zn-Gd-Zr alloys. With increase of the Gd content, the strength of the alloys was enhanced due to the second phase strengthening and grain refinement. The degradation resistance of Mg-2Zn-0.5Zr alloy was increased by adding 0.5%–1% Gd due to the uniformly distributed second phases which acted as a barrier to prevent the pitting corrosion. However, increasing Gd content to 2% reduced the degradation resistance of the alloy due to the galvanic corrosion between the matrix and the second phases.The good degradation resistance and mechanical properties of as-cast Mg-2Zn-1Gd-0.5Zr alloy makes it outstanding for biomaterial application.     

Development and microstructural characterizations of Mg-Zn-Ca alloys for biomedical applications

Mg-10Zn-1Ca, Mg-20Zn-1Ca and Mg-6Zn-1Ca alloys were processed from powders in an argon filled glove box. The solidification rate was varied. Fast solicitation resulted in very small grain size and continuous 3D network distribution of the secondary phase in grain boundary. The alloy processed from fast solidification had better corrosion resistance than those solidified at low cooling rates. It may be because of the increased Zn and Ca in magnesium grains when the alloy was quickly cooled down from its molten state. Liquid nitrogen quenching at the end of solution treatment also created better distribution of network shaped secondary phase than water quenched alloys. Moderate temperature in solution treatment is preferred because it did not cause too much grain growth but increased microhardness of the treated alloys. Alloy with lower Zn amount had better corrosion resistance in PBS solution in this study.     

Indirectly extruded biodegradable Zn-0.05wt%Mg alloy with improved strength and ductility: In vitro and in vivo studies

As compared to permanent orthopedic implants for load-bearing applications, biodegradable orthopedic implants have the advantage of no need for removing after healing, but they suffer from the "trilemma" problem of compromising among sufficiently high mechanical properties, good biocompatibility and proper degradation rate conforming to the growth rate of new bones. In the present work, in vitro and in vivo studies of a Zn-0.05 wt%Mg alloy(namely, Zn-0.05 Mg alloy) were conducted with pure Zn as a control. The Zn-0.05 Mg alloy is composed of a small amount of Mg_2 Zn11 phase embedded in the refined Zn matrix with an average grain size of ～20 μm. The addition of 0.05 wt% Mg into Zn significantly increases the ultimate tensile strength up to 225 MPa and the elongation to fracture to 26%, but has little influence on the in vitro degradation rate. Both Zn and Zn-0.05 Mg alloy exhibit homogeneous in vitro degradation with a rate of about 0.15 mm/year. Based on the cytotoxicity evaluation, Zn and Zn-0.05 Mg alloy do not induce toxicity to L-929 cells, indicating that they have little toxicity to the general functions of the animal. An in vivo biocompatibility study of Zn and Zn-0.05 Mg alloy samples by placing them in a rabbit model for 4.12 and 24 weeks, respectively did not show any inflammatory cells, and demonstrated that new bone tissue formed at the bone/implant interface, suggesting that Zn and Zn-0.05 Mg alloy promote the formation of new bone tissue. The in vivo degradation of Zn and Zn-0.05 Mg alloy does not bring harm to the important organs and their cell structures. More interestingly, Zn and Zn-0.05 Mg alloy exhibit strong antibacterial activity against Escherichia coli and Staphylococcus aureus. The above results clearly demonstrate that the Zn-0.05 Mg alloy could be a potential biodegradable orthopedic implant material.     

Balancing the strength and ductility of Mg-6Zn-0.2Ca alloy via sub-rapid solidification combined with hard-plate rolling

In this study,we successfully prepared a Mg-6Zn-0.2Ca alloy by utilizing sub-rapid solidification (SRS)combined with hard-plate rolling (HPR),whose elongation-to-failure increases from ～17 ％ to ～23 ％without sacrificing tensile strength (～290 MPa) compared with its counterpart processed via conven-tional solidification (CS) followed by HPR.Notably,both samples feature a similar refined grain structure with an average grain size of ～2.1 and ～2.5 μm,respectively.However,the high cooling rate of ～ 150 K/s introduced by SRS modified both the size and morphology of Ca2Mg6Zn3 eutectic phase in comparison to those coarse ones under CS condition.By subsequent HPR,the Ca2Mg6Zn3 phase was further refined and dispersed uniformly by severe fragmentation.Specially,the achieved supersaturation containing exces-sive Ca solute atoms due to high cooling rate was maintained in the SRS-HPR condition.The mechanisms that govern the high ductility of the SRS-HPR sample could be ascribed to following reasons.First,refined Ca2Mg6Zn3 eutectic phase could effectively alleviate or avoid the crack initiation.Furthermore,excessive Ca solute atoms in α-Mg matrix result in the yield point phenomenon and enhanced strain-hardening ability during tension.The findings proposed a short-processed strategy towards superior performance of Mg-6Zn-0.2Ca alloy for industrial applications.     

Biodegradation mechanisms of selective laser-melted Mg–xAl–Zn alloy: grain size and intermetallic phase

Grain size and intermetallic phase were two key factors affecting the biodegradation behaviour of Mg alloys. In the paper, different grain size and intermetallic phase volume fraction were obtained by introducing Al into Mg–Zn alloy via selective laser melting. Results showed that the grain size refined while the intermetallic phase volume fraction increased with Al increasing. As Al was less than 3?wt.%, the grain refinement was the major factor affecting the degradation behaviour. The finer grain would create many grain boundaries, making the alloy passivate readily and resulted in a reduced degradation rate. However, with Al further increasing, the intermetallic phase became the main factor influencing the degradation behaviour though grain size was further refined. The large intermetallic phase volume fraction caused severe galvanic corrosion, accelerating the degradation. This work may provide guidance for balancing grain size and intermetallic phase on degradation behaviour of Mg alloys.     

高Ca/Al比的Mg-Al-Ca合金的超塑性

针对3种高Ca/Al比的Mg-Al-Ca合金(Mg-3.7Al-3.8Ca,Mg-4.4Al-4.5Ca和Mg-4.9Al-5.0Ca)的超塑性行为展开研究,研究结果表明,铸态镁合金具有二次相Al2Ca分布于晶界的枝晶结构。经挤压后,合金的晶粒被细化,二次相也被细化为更小的粒子。这些合金在400℃时表现出很高的伸长率,Mg-4.9Al-5.0Ca在400℃时3.6×10-4 s-1应变速率下获得最大伸长率572%。超塑性流变的变形机制为晶格扩散(DL)控制的晶界滑移(GBS)。对于挤压态Mg-4.9Al-5.0Ca合金,大部分高温稳定相Al2Ca粒子尺寸为80nm,对晶粒长大的抑制作用强烈,在晶界滑移时协调变形,因此在3种合金中Mg-4.9Al-5.0Ca具有最好的超塑性。     

Nd-induced honeycomb structure of intermetallic phase enhances the corrosion resistance of Mg alloys for bone implants

Mg-5.6Zn-0.5Zr alloy (ZK60) tends to degrade too rapid for orthopedic application, in spite of its natural degradation, suitable strength and good biocompatibility. In this study, Nd was alloyed with ZK60 via laser melting method to enhance its corrosion resistance. The microstructure features, mechanical properties and corrosion behaviors of ZK60-xNd (x?=?0, 1.8, 3.6, 5.4 wt.%) were investigated. Results showed that laser melted ZK60-xNd were composed of fine ɑ-Mg grains and intermetallic phases along grain boundaries. And the precipitated intermetallic phases experienced successive changes: divorced island-like MgZn phase?→?honeycomb-like T phase?→?coarsened and agglomerated W phase with Nd increasing. It was worth noting that ZK60-3.6Nd with honeycomb-like T phase exhibited an optimal corrosion behavior with a corrosion rate of 1.56?mm?year^?1. The improved corrosion behavior was ascribed to: (I) dense surface film caused by the formation of Nd₂O₃ hindered the invasion of immersion solution; (II) the three-dimensional honeycomb structure of intermetallic phases formed a tight barrier to restrain the propagation of corrosion. Moreover, ZK60-3.6Nd exhibited good biocompatibility. It was suggested that ZK60-3.6Nd was a preferable candidate for biodegradable bone implant.     

Effects of calcium on the microstructures and tensile properties of Mg-5Li-3Al alloys

The as-cast Mg-5Li-3Al-xCa (x = 0, 0.5, 1, 1.5 wt.%) was prepared with vacuum induction melting furnace, then processed by hot extrusion. The microstructures and tensile properties were investigated. The results show that the grains of as-cast alloys were refined gradually with the increase of Ca content from 0.5 wt.% to 1 wt.%, while the Ca content increases to 1.5 wt.%, the grain size increases. The microstructures of investigated alloys were further refined after hot extrusion. Both as-cast and as-extruded Mg-5Li-3Al-0.5Ca alloys have the highest mechanical properties, which is mainly attributed to the grain refinement caused by the addition of Ca and the formation of strengthening phase, Al₄Ca. When the addition of Ca is up to 1-1.5 wt.%, the tensile properties of alloys are worsened due to the excessive (Mg, Al)₂Ca eutectic phase forming at grain boundary.     

Effect of trace HA on microstructure,mechanical properties and corrosion behavior of Mg-2Zn-0.5Sr alloy

Effect of the addition of trace HA particles into Mg-2Zn-0.5Sr on microstructure, mechanical properties, and bio-corrosion behavior was investigated in comparison with pure Mg. Microstructures of the Mg-2Zn-0.5Sr-xHA composites(x = 0, 0.1 and 0.3 wt%) were characterized by optical microscopy(OM),scanning electron microscopy(SEM) equipped with energy dispersion spectroscopy(EDS) and X-ray diffraction(XRD). Results of tensile tests at room temperature show that yield strength(YS) of Mg-2Zn-0.5Sr/HA composites increases significantly, but the ultimate tensile strength(UTS) and elongation decrease with the addition of HA particles from 0 up to 0.3 wt%. Bio-corrosion behavior was investigated by immersion tests and electrochemical tests. Electrochemical tests show that corrosion potential(Ecorr)of Mg-2Zn-0.5Sr/HA composites significantly shifts toward nobler direction from-1724 to-1660 m VSCE and the corrosion current density decreases from 479.8 to 280.8 μA cm~(-2) with the addition of HA particles. Immersion tests show that average corrosion rate of Mg-2Zn-0.5Sr/HA composites decreases from11.7 to 9.1 mm/year with the addition of HA particles from 0 wt% up to 0.3 wt%. Both microstructure and mechanical properties can be attributed to grain refinement and mechanical bonding of HA particles with second phases and α-Mg matrix. Bio-corrosion behavior can be attributed to grain refinement and the formation of a stable and dense CaHPO_4 protective film due to the adsorption of Ca~(2+)on HA particles. Our analysis shows that the Mg-2Zn-0.5Sr/0.3HA with good strength and corrosion resistance can be a good material candidate for biomedical applications.     

Ultrafine-grained Zn-0.45Li alloy with enhanced mechanical property,degradation behavior and cytocompatibility prepared by hot extrusion and multi-pass drawing

Biodegradable Zn-0.45Li alloys with high strength and ductility were successfully fabricated by hot extrusion and multi-pass drawing to obtain ultrafine-grained microstructures with a secondary phase of fine LiZn₄ participates. The mechanical properties, degradation behavior and cytocompatibility of the alloys were subsequently investigated. Results showed that grain refinement could be achieved in the alloys after hot extrusion and multi-pass drawing. The yield strength, ultimate tensile strength, and elongation to failure of the ultrafine-grained Zn-0.45Li alloys reached 416 MPa, 567 MPa and 55.4 %, respectively. Enhancements in both strength and elongation could be attributed to interactions between LiZn₄ and matrix dislocations, the pinning effect of LiZn₄ on grain boundaries, and grain refinement. Immersion tests and MTT cytotoxicity assay indicated that the Zn-0.45Li alloys have a corrosion rate and cytocompatibility similar to the values reported for biomedical implants.     

Effects of icosahedral phase on mechanical anisotropy of as-extruded Mg-14Li (in wt%) based alloys

Through investigating and comparing microstructure and crystallographic texture of as-extruded Mg-14Li and Mg-14Li-6Zn-1Y(in wt%) alloys,the differences in their mechanical anisotropy were investigated.It revealed that the formation of I-phase(Mg_3Zn_6Y,icosahedral structure) can effectively refine grain size.Moreover,compared with Mg-14Li alloy,the texture type of Mg-14Li-6Zn-1Y alloy changed slightly,but its texture intensity decreased remarkably.As a result,the stronger texture contributed to the "normal" mechanical anisotropy of Mg-14Li alloy with higher tensile strength and a lower elongation ratio along transverse direction(TD) than those along extrusion direction(ED).However,for Mg-14Li-6Zn-1Y alloy,the zonal distribution of I-phase particles along ED caused "abnormal" mechanical anisotropy,i.e.higher tensile strength and better plasticity along ED.     

Microstructures and mechanical properties of Mg–Zn–Zr–Dy wrought magnesium alloys

Microstructures and phase compositions of as-cast and extruded ZK60–xDy (x?= 0–5) alloys were analysed by optical microscope, scanning electron microscope, X-ray diffraction and differential scanning calorimetry. Meanwhile, the tensile mechanical property was tested. With increasing Dy content, Mg–Zn–Dy new phase increases gradually, while MgZn₂ phase decreases gradually to disappear. As-cast microstructure is refined gradually; meanwhile extruded one is refined further with decreasing average grain size to 1 μm for ZK60–4·32Dy alloy. Second phase, tending to distribute along grain boundary by continuous network in as-cast state, breaks and distributes dispersedly in extrusion state. As-cast tensile mechanical property remains almost unchanged at ambient temperature; however, extruded ones are enhanced significantly at ambient and elevated temperatures, respectively. Tensile strength at 298 and 473 K increases gradually from 355 and 120 MPa for ZK60 alloy to 395 and 171 MPa for ZK60–4·32Dy alloy, respectively. Extruded tensile fractures exhibit a typical character of ductile fracture.     

High strength Al–Zn–Mg–Cu–Ni–Si alloy with improved casting properties

Abstract

The casting properties of high strength Al-7Zn-7Mg-1Cu-3Ni-3Si(wt-%) alloy are described. Compared with common Al-Zn-Mg-Cu alloys, an improvement of casting properties has been achieved by adding elements (Ni, Mg, Si) that form eutectic phases, thus reducing the solidification interval of the alloy. A comparison of thermal cooling curves, castability and hot tearing tendency has been carried out for three alloys: Al-7Zn-2Mg-1Cu (structure consists mainly of solid solution), quasi-ternary eutectic alloy Al-7Zn-7Mg-1Cu-3Ni-3Si and the common casting alloy Al-10Si. In addition, the effect of melt protection against oxidation on castability has been evaluated. It is shown that the casting properties of the protected quasi-ternary eutectic alloy are significantly better than those of the common Al-7Zn-2Mg-1Cu alloy and that they achieve a level close to that of Al-10Si alloy.     

Effects of Sr on the microstructure,mechanical properties and corrosion behavior of Mg-2Zn-<Emphasis Type="Italic">x</Emphasis>Sr alloys

The effects of the addition of Sr on the microstructures, mechanical properties and bio-corrosion properties of Mg-2Zn alloys were investigated. Examination of the microstructures indicates that Sr addition promoted grain refinement and the formation of secondary phases. The results for the mechanical and corrosion property analyses show that the Mg-2Zn-0.2Sr alloy exhibited the best mechanical properties and bio-corrosion resistance. Notably, fine grain structure can promote strength and ductility, while the excessive secondary phases Mg₁₇Sr₂ resulted in the degradation of mechanical performance. Moreover, corrosion resistance could be improved by reducing the impurity concentrations and forming a denser corrosion product layer, while the secondary phases accelerated the corrosion process by forming micro-galvanic couples with the Mg matrix. It is considered that the addition of Sr (0.2%) represents the main contributing factor to these improved properties. The Mg-2Zn-0.2Sr alloy provides excellent strength and corrosion resistance for biomedical applications.

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Effect of Gd on physics properties of the dual-phase Ni–Mn–Ga-based alloys

The microstructure, martensitic transformation (MT) and shape memory effect (SME) of the dual-phase Ni₅₈Mn₂₅Ga_17?xGd_x (x?=?0, 0.1, 0.2, 0.5, 1) alloys have been investigated. The results show that the refined the grain size and adjust the distribution of γ phase by added rare earth Gd in the Ni₅₈Mn₂₅Ga_17?xGd_x alloys. With increasing Gd content, the MT temperatures of Ni₅₈Mn₂₅Ga_17?xGd_x alloys first gradually decrease and then increase with the increasing content of Gd, reaching their minimum values when the content of Gd is 0.5?at.-%. In addition, the Ni₅₈Mn₂₅Ga_16.9Gd_0.1 has a SME of 5.1% owing to the favourable γ phase distribution, which is mainly attributable to the γ phase grain refined and weaken resistance of reverse MT.     

Microstructure and mechanical properties of as-cast Mg-4Zn-0.5Zr-0.2Cu-0.2Ce alloy

In this study, an approach is proposed to improve the microstructure and mechanical properties of Mg-4Zn-0.5Zr alloy by combining trace Cu and rare earth Ce addition. The results showed that Cu and Ce additions led to obvious grain refinement and the formation of Mg-Zn-Cu and Mg-Zn-Ce phases. The Mg-Zn-Ce phase was identified to have an orthorhombic structure. The length of the [0001]_α rods in the Cu-containing alloys remarkably decreased. The yield strength increased slightly after Cu and Ce co-addition, which was attributed to grain refinement and precipitation strengthening. The coarse Mg-Zn-Ce phase distributed at the grain boundaries would reduce the ductility by promoting crack propagation during tensile processes.     

Effects of Nd on microstructures and properties of extruded Mg-2Zn-0.46Y-xNd alloys for stent application

The microstructures, mechanical and corrosion properties of three extruded Mg-2Zn-0.46Y-xNd alloys (x = 0.0, 0.5, 1.0 wt%) were studied by optical microscopy, scanning electronic microscopy (SEM), electrochemical measurements and tensile tests. Microstructural observations indicated that Nd led to the uniformity and the variation of morphology of major second phase; tensile tests showed that Nd can improve the ductility at moderate amount (0.5 wt%) and will be detrimental up to 1.0%; Mg-2Zn-0.46Y-0.5Nd alloy exhibited excellent mechanical properties (σ_b, 269.0 MPa, σ_0.2, 165.6 MPa and elongation, 24%); electrochemical tests revealed that Nd can enhance the corrosion resistance and Mg-2Zn-0.46Y-1.0Nd alloy had lowest corrosion current density, which was reasoned that the line-shape and rodlike NdZn₂ phase might serve as corrosion barriers and the dissolved Nd can raise the electrode potential of the matrix.     

Significantly improved corrosion resistance of Mg-15Gd-2Zn-0.39Zr alloys: Effect of heat-treatment

The effects of heat-treatment on corrosion behavior of Mg-15Gd-2Zn-0.39 Zr alloys were investigated through microstructure characterization, corrosion tests, and scanning Kelvin probe force microscope (SKPFM) analysis. In long-term corrosion experiments, the corrosion rates of Mg-Gd-Zn-Zr alloys were mainly determined by the effects of micro-galvanic corrosion. During heat-treatment, the β-(Mg,Zn)₃Gd eutectic phase in as-cast alloys transformed into a long-period stacking ordered (LPSO) phase, coupled with the precipitation of small precipitates. As heat-treatment proceeded, the local potential and the volume fraction of the LPSO phases reduced gradually compared with the eutectic phase, which resulted in a remarkable decrease of the micro-galvanic effect between the second phase and Mg matrix. As a result, the corrosion resistance of heat-treated alloys improved significantly.     

Predicting the degradation behavior of magnesium alloys with a diffusion-based theoretical model and in vitro corrosion testing

Magnesium alloys have shown great potential for their use in the medical device field, due to the promising biodegradability. However, it remains a challenge to characterize the degradation behavior of the Mg alloys in a quantitative manner. As such, controlling the degradation rate of the Mg alloys as per our needs is still hard, which greatly limits the practical application of the Mg alloys as a degradable biomaterial. This paper discussed a numerical model developed based on the diffusion theory, which can capture the experimental degradation behavior of the Mg alloys precisely. The numerical model is then implemented into a finite element scheme, where the model is calibrated with the data from our previous studies on the corrosion of the as-cast Mg-1Ca and the as-rolled Mg-3Ge binary alloys. The degradation behavior of a pin implant is predicted using the calibrated model to demonstrate the model’s capability. A standard flow is provided in a practical framework for obtaining the degradation behavior of any biomedical Mg alloys. This methodology was further verified via the comparison with enormous available experimental results. Lastly, the material parameters defined in this model were provided as a new kind of material property.     

固溶体Mg-5Al-xGd合金的制备及其压缩性能EI北大核心

利用Gd(0.00%,0.25%,0.50%,0.75%,1.00%,质量分数,下同)元素和凝固压力(3 GPa)调控Mg-5Al合金凝固组织结构,对合金样品进行压缩测试,并研究组织结构与室温压缩性能相关性。结果表明:常压石墨型铸造下,仅Mg-5Al-0.75Gd合金获得了晶界无共晶相生成、粒状Al2Gd相弥散分布在基体上、晶粒平均尺寸约为85μm的固溶体组织,其抗压强度和最大压缩应变分别为379 MPa和33.46%,高于存在晶间第二相的合金。3 GPa高压下,Gd含量≤0.25%合金的凝固组织为单一固溶体,Gd含量≥0.50%合金的晶界(枝晶间)有共晶Al_(2)Gd相生成。固溶体Mg-5Al-0.25Gd合金的平均晶粒尺寸是74μm,抗压强度是402 MPa,最大压缩应变是33.61%。Mg-5Al-0.75Gd合金的平均晶粒尺寸是38μm,抗压强度和最大压缩应变分别是341 MPa和25.12%,其性能低于Mg-5Al-0.25Gd合金。可见,晶间Al,Gd元素的存在形式是影响铸造Mg-Al合金力学性能的重要因素。