Improvement of mechanical properties and corrosion resistance of biodegradable Mg–Nd–Zn–Zr alloys by double extrusion

Mg–Nd–Zn–Zr alloy is a novel and promising biodegradable magnesium alloy due to good biocompatibility, desired uniform corrosion mode and outstanding corrosion resistance in simulated body fluid (SBF). However, the corrosion resistance and mechanical properties should be improved to meet the requirement of the biodegradable implants, such as plates, screws and cardiovascular stents. In the present study, double extrusion process was adopted to refine microstructure and improve mechanical properties of Mg–2.25Nd–0.11Zn–0.43Zr and Mg–2.70Nd–0.20Zn–0.41Zr alloys. The corrosion resistance of the alloys after double extrusion was also studied. The results show that the microstructure of the alloys under double extrusion becomes much finer and more homogeneous than those under once extrusion. The yield strength, ultimate tensile strength and elongation of the alloys under double extrusion are over 270 MPa, 300 MPa and 32%, respectively, indicating that outstanding mechanical properties of Mg–Nd–Zn–Zr alloy can be obtained by double extrusion. The results of immersion experiment and electrochemical measurements in SBF show that the corrosion resistance of Alloy 1 and Alloy 2 under double extrusion was increased by 7% and 8% respectively compared with those under just once extrusion.     

Optimizing mechanical property and cytocompatibility of the biodegradable Mg-Zn-Y-Nd alloy by hot extrusion and heat treatment

The Mg-Zn-Y-Nd alloy is a new type of degradable material for biomedical application. In the present study, Mg-6Zn-1.2Y-0.8Nd alloy was fabricated, and then extrusion and heat treatment were conducted to optimize its mechanical properties and cytocompatibility. The microstructure observation, mechanical property, degradation behavior and cytocompatibility tests were conducted on the Mg-Zn-Y-Nd alloy with three different states: as-cast (alloy C), as-extruded (alloy E) and extruded + heat treated (alloy EH). The results show that alloy C consists of coarse grains and continuous secondary phases. The extrusion process has caused incomplete recrystallization, and results in a mixed grain structure of elongated grains and small equiaxed grains (alloy E). The heat treatment process has promoted the recrystallization and homogenized the grain structure (alloy EH). Both the strength and ductility of the alloy has been improved by extrusion, but the following heat treatment has decreased the strength and increased the ductility. The degradation behavior of the alloy C and E alloys does not show much difference, but improves slightly in alloy EH, because the heat treatment has homogenized the microstructure and released the residual stress in the alloy. The directly and indirectly cell viability tests indicate that alloy EH exhibits the best cytocompatibility, which should be ascribed to its relative uniform degradation and low ion releasing rate. In summary, the combination of hot extrusion and heat treatment could optimize the mechanical property and cytocompatibility of the Mg-Zn-Y-Nd alloy together, which is beneficial for the future application of the alloy.     

Improvement of <Emphasis Type="Italic">in vitro</Emphasis> behavior of an Mg alloy using a nanostructured composite bioceramic coating

Magnesium (Mg) alloys as a new group of biodegradable metal implants are being extensively investigated as a promising selection for biomaterials applications due to their apt mechanical and biological performance. However, as a foremost drawback of Mg alloys, the high degradation in body fluid prevents its clinical applications. In this work, a bioceramic composite coating is developed composed of diopside, bredigite, and fluoridated hydroxyapatite on the AZ91 Mg alloy in order to moderate the degradation rate, while improving its bioactivity, cell compatibility, and mechanical integrity. Microstructural studies were performed using a transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray diffraction (XRD) analysis, and energy dispersive spectroscopy (EDS). The degradation properties of samples were carried out under two steps, including electrochemical corrosion test and immersion test in simulated body fluid (SBF). Additionally, compression test was performed to evaluate the mechanical integrity of the specimens. L-929 fibroblast cells were cultured on the samples to determine the cell compatibility of the samples, including the cell viability and attachment. The degradation results suggest that the composite coating decreases the degradation and improves the bioactivity of AZ91 Mg alloy substrate. No considerable deterioration in the compression strength was observed for the coated samples compared to the uncoated sample after 4 weeks immersion. Cytotoxicity test indicated that the coatings improve the cell compatibility of AZ91 alloy for L-929 cells.     

新型Mg-8Li-5Al-5Ca合金的微观组织、力学及耐腐蚀性能

设计了新型高钙铝比Mg-8Li-5Al-5Ca合金,通过常温拉伸、失重法、pH测定和电化学测试等方法研究了合金的常温力学性能和耐腐蚀性能。采用扫描电镜(SEM)和X射线衍射(XRD)分析了基体和腐蚀产物相结构、合金显微组织以及腐蚀形貌。研究结果表明,这种镁锂合金形成Al2Ca相包围双基体(α-Mg+β-Li)的结构,挤压后基体组织和第二相粒子均明显细化。Mg-8Li-5Al-5Ca合金的耐腐蚀性能优于一般镁锂合金,且随着挤压比的增大进一步提升。该合金的力学性能协调了镁锂合金的优良塑性和高钙铝比镁合金的高强度,拥有较高的抗拉强度(222 MPa)和延伸率(8.3%)。     

Microstructure and Properties of As-Cast Mg–2.0Sn–1.0Zn–1.0Y–0.3Zr Alloys at Different Extrusion Ratios

Herein, the effect of hot extrusion with different extrusion ratios (λ = 6, 8, 10, and 12) on the microstructure evolution and properties of as-cast Mg–2.0Sn–1.0Zn–1.0Y–0.3Zr magnesium alloys, using optical microscopy (OM), scanning electron microscopy (SEM), immersion corrosion and electrochemical corrosion experiment, and tensile testing, is investigated. The results show that the Mg₁₄SnY and Mg₆SnY precipitated phases exist in the alloy before and after extrusion. After hot extrusion, the second phase of the alloy is broken into particles along the extrusion direction, whereas the grain size is significantly reduced, and dynamic recrystallization and deformed grains exist in the microstructure. The mechanical properties of the extruded alloy improve, but the corrosion resistance weakens. When the extrusion ratio is λ = 10, the extruded alloy exhibits relatively good mechanical properties and corrosion resistance. The corrosion behaviors of the extruded alloys are affected by both the grain size and galvanic corrosion. In the initial stage of corrosion, intergranular corrosion plays a major role in reducing the corrosion resistance of the extruded alloys. With prolonged corrosion time, galvanic corrosion has a more significant effect on weakening the corrosion resistance of the extruded alloys.     

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.     

挤压温度对Mg-5.3Gd-2.6Y-1.1Nd-0.3Zr合金的力学性能和耐生物腐蚀性能的影响

在不同温度下对均匀化处理后的Mg-5.3Gd-2.6Y-1.1Nd-0.3Zr(质量分数/%)镁合金进行挤压。利用光学显微镜和拉伸实验对比研究固溶态铸锭、不同挤压温度合金的显微组织和力学性能,采用析氢法、失重法和极化曲线实验综合测试合金在Hank’s人体模拟液中的耐生物腐蚀性能,利用扫描电子显微镜观察腐蚀后合金的表面腐蚀形貌。结果表明:随着挤压温度的降低,合金的晶粒不断细化,强度及塑性不断提高,其中390℃温度下挤压得到的合金屈服强度达到223.4MPa,较固溶态铸锭(139.8MPa)提升约60%,且挤压后的合金在Hank’s体液中具有更高的耐腐蚀性,随着挤压温度的降低,合金的耐蚀性先升高后降低,450℃挤压的棒材耐生物腐蚀性能最优,腐蚀速率为0.74mm/a。     

Improved properties of Ti-6Al-4V by combining isothermal equal-channel angular processing and hot-extrusion

The aim of this work was to study effects of hot extrusion on the microstructure of Ti-6Al-4V (wt-%) alloy processed by ECAP. Firstly, an isothermally Ti–6Al–4V alloy processed by Equal channel angular pressing(ECAP) was preheated at 950°C for 6?min and then hot extruded at 900°C. The hot extrusion minimised the grain size and maximised the mechanical strength. Therefore, it was demonstrated that hot extrusion of Ti-6Al-4V alloys that processed by ECAP could be performed without compromising any mechanical properties. Therefore, it is possible to use the ability to apply a reduced cross-section in hot extrusion for an Ti-6Al-4V processed by ECAP without concern about the reduction of properties.     

Bio-corrosion characterization of Mg–Zn–X (X = Ca, Mn, Si) alloys for biomedical applications

The successful applications of magnesium-based alloys as biodegradable orthopedic implants are mainly inhibited due to their high degradation rates in physiological environment. This study examines the bio-corrosion behaviour of Mg–2Zn–0.2X (X = Ca, Mn, Si) alloys in Ringer’s physiological solution that simulates bodily fluids, and compares it with that of AZ91 magnesium alloy. Potentiodynamic polarization and electrochemical impedance spectroscopy results showed a better corrosion behaviour of AZ91 alloy with respect to Mg–2Zn–0.2Ca and Mg–2Zn–0.2Si alloys. On the contrary, enhanced corrosion resistance was observed for Mg–2Zn–0.2Mn alloy compared to the AZ91 one: Mg–2Zn–0.2Mn alloy exhibited a four-fold increase in the polarization resistance than AZ91 alloy after 168 h exposure to the Ringer’s physiological solution. The improved corrosion behaviour of the Mg–2Zn–0.2Mn alloy with respect to the AZ91 one can be ascribed to enhanced protective properties of the Mg(OH)₂ surface layer. The present study suggests the Mg–2Zn–0.2Mn alloy as a promising candidate for its applications in degradable orthopedic implants, and is worthwhile to further investigate the in vivo corrosion behaviour as well as assessed the mechanical properties of this alloy.     

生物医用镁合金腐蚀行为的研究进展EI北大核心CSCD

近年来,镁合金作为生物可降解材料受到了越来越多研究者的关注,由于其具有良好的生物相容性、力学性能及可降解吸收等特点,被誉为一种“革命性的生物材料”。然而,由于腐蚀速率过快和存在局部腐蚀的缺点,目前的生物镁合金仍达不到临床应用的要求。本文从高纯化、合金化、热处理工艺、表面改性等方面综述了最近几年生物镁合金在提高腐蚀性能方面的研究进展,并从添加无毒性合金元素,适当的表面涂覆,先进的制备技术及热处理工艺方面,对如何研制出腐蚀性能更好的生物可降解材料进行了展望。     

ECAP变形对Mg2Si增强耐热镁合金组织及性能的影响

为了提高镁合金的耐热性能,在Mg-Zn合金中加入Si,形成Mg-Zn-Si镁合金.采用ECAP工艺在变形温度为573 K和挤压路径为Bc条件下对Mg-Zn-Si镁合金进行不同道次的变形.运用金相显微镜(OM)、X射线衍射仪(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)等手段对变形后的Mg-Zn-Si镁合金进行了组织表征,对变形后的合金进行了室温拉伸和高温蠕变等力学性能测试.结果表明:随着挤压道次增加,α-Mg基体、Mg Zn相及Mg2Si相均得到细化且分布趋于均匀.1道次挤压后部分基体α-Mg细化,4道次挤压后α-Mg的尺寸减小为5~10μm,且晶粒大小趋于均匀;2道次挤压后Mg2Si相枝晶在原位置破碎为颗粒状,6、8道次挤压后Mg_2Si相呈弥散分布.4道次挤压后合金的屈服强度和抗拉强度均提高120%,伸长率提高353%;8道次挤压后合金的抗拉强度和伸长率与4道次相比变化不大,但屈服强度进一步提高了19%.随着挤压道次增加,高温抗蠕变性能提高,8道次后高温稳态蠕变速率降低5倍.Mg2Si相细化机理为受剪切而机械碎断.     

Improvement of mechanical property and corrosion resistance of Mg-Zn-Nd alloy by bi-direction drawing

Mg-Zn-Nd alloy is a promising biodegradable metal material for surgical staples during the reconstruc-tion of digestive tract due to its good biocompatibility and suitable mechanical properties.However,its deformation property and corrosion resistance should be improved to make better safety and effective-ness of staples.In the present study,bi-direction drawing was adopted to maintain the initial texture characteristics,and improve mechanical property and corrosion resistance of Mg-2Zn-0.5Nd alloy.The results showed that the microstructure after bi-direction did not change too much,but the texture could maintain its initial characteristics.The ductility of the alloy with 60 ％ accumulative area reduction after bi-direction drawing was increased by 70 ％,indicating that an outstanding deformation property of Mg-Zn-Nd alloy can be obtained by bi-direction drawing.The corrosion resistance was also improved after bi-direction drawing compared with that under single direction drawing.     

Structure,mechanical and corrosion properties of extruded Mg-Nd-Zn,Mg-Y-Zn and Mg-Y-Nd alloys

Rare earth elements are known to improve both mechanical and corrosion properties. However, it highly depends on the final microstructure conditions of prepared material. During extrusion, intermetallic phases may be redistributed, partially dissolved or on the contrary, precipitated. The knowledge of the impact of extrusion on the individual alloys is therefore essential for their application. In this work, three magnesium alloys (Mg-4Y-3RE, Mg-2Y-1Zn, Mg-3Nd-0.5Zn) were prepared by an extrusion process. Microstructure, mechanical and corrosion properties were compared with extruded pure Mg. The advantages and disadvantages of individual alloys are discussed. Based on the obtained results, the Mg-4Y-3RE alloy seems to exert the best mechanical and corrosion properties. Other materials were characterised with anisotropy of mechanical properties and much higher corrosion rate.     

Ti based biomaterials,the ultimate choice for orthopaedic implants – A review

The field of biomaterials has become a vital area, as these materials can enhance the quality and longevity of human life and the science and technology associated with this field has now led to multi-million dollar business. The paper focuses its attention mainly on titanium-based alloys, even though there exists biomaterials made up of ceramics, polymers and composite materials. The paper discusses the biomechanical compatibility of many metallic materials and it brings out the overall superiority of Ti based alloys, even though it is costlier. As it is well known that a good biomaterial should possess the fundamental properties such as better mechanical and biological compatibility and enhanced wear and corrosion resistance in biological environment, the paper discusses the influence of alloy chemistry, thermomechanical processing and surface condition on these properties. In addition, this paper also discusses in detail the various surface modification techniques to achieve superior biocompatibility, higher wear and corrosion resistance. Overall, an attempt has been made to bring out the current scenario of Ti based materials for biomedical applications.     

Mn和Zr对Al-Zn-Mg-Cu铝合金各向异性的影响

采用光学显微镜(OM)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)以及室温拉伸、剥落腐蚀、晶间腐蚀等测试方法,研究了微量的Mn和Zr对Al-Zn-Mg-Cu铝合金的组织和性能各向异性的影响。结果表明,在Al-Zn-Mg-Cu-Ti合金中,分别添加微量的Mn和Zr,合金中对应析出细小弥散的Al6Mn和Al3Zr相,这两相均能抑制基体再结晶,促使合金的晶粒纵横比增大。合金的力学性能、抗晶间腐蚀和剥落腐蚀性能提高,但性能各向异性增大。同时,结果显示Zr对合金的组织和性能各向异性的影响显著大于Mn。     

硼对铜合金组织和性能的影响

研究了加硼铜合金的组织以及力学、腐蚀、腐蚀磨损和冲蚀性能。结果表明：硼能明显细化铜合金的组织，提高其强度、硬度，改善其耐蚀、耐腐蚀磨损及耐冲蚀能力；并确定硼在铜合金中的最佳含量范围。     

Effect of Zr on the microstructure, mechanical properties and corrosion resistance of Mg–10Gd–3Y magnesium alloy

The influence of Zr on the microstructure, mechanical properties and corrosion resistance of Mg–10Gd–3Y (wt.%) magnesium alloy was investigated. The grain size of alloys decreased with Zr content from 0% to 0.93% (wt.%). The addition of Zr greatly improved the ultimate tensile strength (UTS) and the elongation (EL), while slightly improved the tensile yield strength (TYS). The UTS and the EL of the alloy containing 0.93% Zr increased by 125.8 MPa and 6.96% compared with base alloy, respectively. The corrosion resistances were found to decrease with Zr content from 0% to 0.42% and then increase from 0.42% to 0.93%. The differences in the sizes and distributions of the Zr-rich particles have significant effects on the corrosion behaviors. The alloy with 0.42% Zr addition revealed the optimum combination of mechanical properties and corrosion resistance.     

Comparison of biodegradable behaviors of AZ31 and Mg–Nd–Zn–Zr alloys in Hank's physiological solution

The main challenge for the application of magnesium and its alloy as degradable biomaterials lies in their high degradation rates in physiological environment. In the present work, the biodegradable behavior of a patent magnesium alloy Mg–Nd–Zn–Zr (JDBM) and a reference alloy AZ31 was systematically investigated in Hank's physiological solution. The corrosion rate of JDBM (0.28 mm/year) was much slower than that of AZ31 (1.02 mm/year) in Hank's solution for 240 h. After corrosion products were removed, smooth surface of the JDBM was observed by SEM observation compared to many deep pits on the surface of AZ31. Open-circuit potential and potentiodynamic polarization results manifested that pitting corrosion did not occurred on the surface of JDBM at the early period of immersion time due to the formation of a more protective and compact film layer suggested by electrochemical impedance spectroscopy study. The corrosion rate of magnesium alloys was found to slow down in dynamic corrosion in comparison with that in the static corrosion. This provided the basis for scientific evaluation of in vitro and in vivo corrosion behavior for degradable biomagnesium alloy. The present results suggest that the new patent magnesium alloy JDBM is a promising candidate as degradable biomaterials and is worthwhile for further investigation in vivo corrosive environment.     

Dry sliding wear of an AZ31 magnesium alloy processed by equal-channel angular pressing

A magnesium AZ31 alloy was processed by equal-channel angular pressing (ECAP) for up to 8 passes to reduce the grain size to ~1.0 μm. Following ECAP, microhardness measurements were taken to evaluate the mechanical properties of the material. Ball-on-disc dry sliding tests were conducted to compare the wear behaviour of the as-received alloy and the alloy processed by ECAP. The surface topography and volume loss were recorded for all samples. The results show that the fluctuations and average values of the coefficient of friction are improved after processing by ECAP. In addition, there is a decrease in the wear depth and volume loss with increasing numbers of ECAP passes. The ECAP-processed alloy has a higher wear resistance than the unprocessed alloy and it is a suitable candidate material for use in industrial applications.     

Effects of extrusion and equal channel angular pressing on the microstructure,tensile and fatigue behaviour of the wrought magnesium alloy AZ80

The microstructure, mechanical properties, fatigue life and fatigue crack propagation rate of Mg‐8Al‐0.5Zn‐0.3Mn (AZ80) magnesium alloy were investigated after extrusion and equal channel angular pressing (ECAP). The highest ultimate and yield strengths and a large enhancement in the fatigue lifetime were obtained after two passes of ECAP. These were decreased with further pressing, although the grain size became finer. There was a correlation between the fatigue and ultimate strengths of AZ80 alloy. The transition from twinning to dislocation slip has also occurred at an average grain size of 7.9 μm. Simultaneous influences of the grain size and the yield strength caused an almost the same threshold of the stress intensity ratio for different process conditions. Moreover, the enhanced ductility of the ECAPed alloy resulted in an increase in the crack growth resistance because of its better ability to accommodate plastic strains during cycling.