挤压镁合金AM60的腐蚀疲劳

研究了加载频率、溶液pH值、氯离子浓度和氟化物转化膜等对挤压镁合金AM60疲劳寿命的影响,讨论了疲劳机理及AlMn相的作用.结果表明:在空气中10 Hz以下,疲劳寿命与频率有一定的关系;而在10 Hz以上,疲劳寿命与频率无关.AM60在中性溶液中疲劳寿命最短,在碱性溶液中疲劳寿命最长.氯离子能明显地降低疲劳寿命,含氟转化膜提高腐蚀疲劳寿命的作用不明显.断口分析表明,挤压镁合金AM60疲劳裂纹的形成与AlMn相粒子有关.在空气中,疲劳裂纹源为AlMn相粒子;而在溶液介质中,腐蚀疲劳裂纹萌生于AlMn相粒子周围的点蚀坑.     

Grain refinement of AM60B magnesium alloy by SiC particles

AM60B alloy has been refined by SiC particles and the corresponding refining mechanism has been mainly discussed. The results indicate that the addition of 0.2 wt% SiC particles in form of mixture with Mg powder decreases the grain size from 317 μm of the not refined alloy to 46 μm. The decrease of β phase and formation of Mg₂Si and Al₄C₃ phases well demonstrate that the reactions of 3SiC + 4Al = Al₄C₃ + 3Si and 2Si + Mg = Mg₂Si occur during refining treatment. In addition, the crystal nuclei are composed of two kinds of elements, Al and C. All of these imply that the formed Al₄C₃ particles are the actual heterogeneous nucleation substrates.     

AM60B镁合金微弧氧化膜层的结构与性能研究

为了提高镁合金的耐腐蚀性能,用微弧氧化方法在AM60B镁合金表面生成了氧化物膜层.利用扫描电镜、X射线衍射分析了膜层的形貌、结构和组成.研究表明,氧化膜可分为两层,外层疏松多孔,内层结构致密,膜层主要由MgO、Mg2SiO4和少量MgAl2O4相组成,从外层到内层,Mg2SiO4相含量减少,MgO相含量增大.与镁合金基体相比,氧化物膜层表面硬度提高7～8倍. 在质量分数为3.5%NaCl溶液中的动电位极化测试表明,微弧氧化处理使镁合金的耐蚀性能得到了明显提高.     

High cycle fatigue mechanisms in a cast AM60B magnesium alloy

ABSTRACT We examine micromechanisms of fatigue crack initiation and growth in a cast AM60B magnesium alloy by relating dendrite cell size and porosity under different strain amplitudes in high cycle fatigue conditions. Fatigue cracks formed at casting pores within the specimen and near the surface, depending on the relative pore sizes. When the pore that initiated the fatigue crack decreased from approximately 110 µm to 80 µm, the fatigue life increased two times. After initiation, the fatigue cracks grew through two distinct stages before final overload specimen failure. At low maximum crack tip driving forces (K_max < 2.3 MPa√m), the fatigue crack propagated preferentially through the α‐Mg dendrite cells. At high maximum crack tip driving forces (K_max > 2.3 MPa√m), the fatigue crack propagated primarily through the β‐Al₁₂Mg₁₇ particle laden interdendritic regions. Based on these observations, any proposed mechanism‐based fatigue model for cast Mg alloys must incorporate the change in growth mechanisms for different applied maximum stress intensity factors, in addition to the effect of pore size on the propensity to form a fatigue crack.     

合金元素对AM60B镁合金性能的影响

为提高镁合金的力学性能,将元素Sr和稀土元素Y、Nd加入到AM60B中.采用X光荧光和X射线衍射对合金的化学成分和物相组成进行了分析,研究了合金元素对AM60B镁合金力学性能的影响,采用扫描电子显微镜对合金试样的断口表面进行了观察,对其断裂行为进行了探讨.研究结果表明,Sr和稀土元素Y、Nd使AM60B镁合金的力学性能得到改善,含Sr和稀土元素Nd的AM60B镁合金的断裂强度和延伸率最高,分别达到224.57 MPa和9.25%,比AM60B镁合金分别提高了32%和38%,合金的屈服强度也得到改善.稀土元素Y和Nd的加入,使AM60B镁合金表现出较大的塑性变形能力.     

镁合金化学镀镍层孔隙率的影响因素

根据基体元素与相应的指示剂显色原理,采用贴试纸法,测定了镁合金化学镀层的孔隙率,研究了镀液参数对孔隙率的影响．结果表明,采用铬黑T作指示剂溶液测定镁合金化学镀层孔隙率的效果最佳,镁试剂I效果次之,茜素磺酸钠指示剂最差;络合剂、缓冲剂、氟化物和稳定剂等镀液参数对化学镀层孔隙率的影响趋势为：先减小,后增加．采用贴试纸法测定化学镀层孔隙率是可行的．     

Modeling fracture properties in a die-cast AM60B magnesium alloy I—Analytical failure model

A failure model for die-cast magnesium alloys is formulated based upon a previously published critical strain model and the analysis of the stress concentration factor due to a spherical void. The failure model predicts the fracture strain during uniaxial tensile loading from the strain-hardening coefficient of the magnesium alloy and the area fraction of porosity due to a macropore in the microstructure. It was determined that this model predicts the fracture strain of 10 tensile samples with holes of different diameters drilled through the cross-section with reasonable accuracy. The predictions are further shown to agree with literature data for three magnesium-based casting alloys.     

轧制组织对镁合金AM60疲劳性能的影响

研究了AM60轧制后挤压镁合金的组织对其机械性能和疲劳裂纹扩展性能的影响。实验表明:轧制使晶粒细化,强度显著提高。沿纵轴轧制方向出现大量等轴李晶组织,而在横向原来的孪晶组织消失。对于存在大量孪晶组织的方向,其抗拉强度明显低于其它方向。轧制AM60的横向疲劳裂纹扩展速度(FCPR)明显地高于纵向。当疲劳裂纹尖端塑性区的尺寸与组织的晶粒度接近时,挤压AM60组织中晶粒大小的不均匀引起裂纹分叉,裂纹分叉和粗糙度诱发的裂纹闭合对疲劳裂纹扩展产生严重的阻滞作用在挤压镁合金AM60的疲劳裂纹扩展速度(da/dN)与应力强度因子范围(△K)的关系曲线上出现拐点(△K=64～7.5 MPa·m~(1/2))。疲劳裂纹扩展为沿晶和穿晶混合方式。     

AM60镁合金表面功能纳米有机薄膜的制备及特性研究

通过有机镀膜方法,利用一种设计合成的三氮杂嗪硫醇有机化合物钠盐在AM60镁合金表面制备了有机薄膜。采用循环伏安法和X射线光电子能谱仪(XPS)分析了镁合金表面有机镀膜过程的反应机理,使用椭圆偏振光谱仪测量了薄膜的厚度、接触角测量仪表征了薄膜的浸润性,借助极化曲线和电化学阻抗谱评价了膜层的耐腐蚀性。结果表明,该有机薄膜为纳米尺度,且使镁合金表面发生亲水到疏水特性转变;经有机镀膜后镁合金的腐蚀电流从1840nA/cm2降低到540nA/cm2、腐蚀电位从-1.454V上升到-1.340V,且电荷传递电阻从2.24kΩ·cm2提高到16.88kΩ·cm2,从而有效地提高了镁合金基体的耐腐蚀性能。     

The effect of grain size on the flow stress determined from spherical microindentation of die-cast magnesium AM60B alloy

Spherical microindentation tests were performed on samples cut from an AM60B magnesium alloy die casting to determine the effects of grain size on the local flow stress. Five samples were indented in the skin region (finer grain sizes), two samples in the core region (larger grain sizes and dendrites), and one sample was indented in both the skin and core region of the die-casting. It was determined that the Hall-Petch equation is applicable for predicting the initial yield point and the flow stress at several levels of plastic strain only in the skin region, and not the core region, of the die-casting. The Hall-Petch slope and intercept stress, determined from spherical indentation in the skin region, compare accurately with previously published results. Possible reasons for indentation results from the core region deviating from the Hall-Petch relationship are discussed. The Hall-Petch slope follows a linearly increasing relation with the square root of plastic strain; however, no conclusions can be drawn concerning this because of the small range of plastic strain tested.     

Corrosion behavior of friction stir welded AZ31B magnesium alloy with plasma electrolytic oxidation coating formed in silicate electrolyte

A protective ceramic coating of about 50 μm thick on a friction stir welded (FSW) joint of AZ31B magnesium alloy was prepared by plasma electrolytic oxidation (PEO) in silicate electrolyte. Electrochemical corrosion behavior of uncoated and coated FSW joints was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The equivalent circuits of EIS plots for uncoated and coated FSW magnesium alloy were suggested. The corrosion resistance of FSW magnesium alloy depended on microstructure of the FSW joint. The heat-affected zone with severe grain growth was more susceptible to corrosion than the stir zone and base metal. The PEO coating consisted of a porous outer layer and a dense inner layer. The inner layer of PEO coating played a key role on corrosion protection of the FSW joint of magnesium alloy. Meanwhile, corrosion potential, corrosion current density and impedance at different zones of coated FSW joint were almost the same. The PEO surface treatment significantly improved the corrosion resistance of FSW joints of AZ31B magnesium alloy.     

AZ80镁合金表面冷喷涂铝/微弧氧化复合涂层耐蚀性能

采用冷喷涂技术在 AZ80 镁合金表面制备一层纯铝涂层,然后通过微弧氧化技术在纯铝涂层表面成功制备纯铝/氧化铝复合涂层.使用扫描电镜(SEM)、能谱仪(EDS)、X 射线衍射仪(XRD)分析涂层的表面和截面形貌、成分、相结构,并利用动电位扫描技术和电化学阻抗谱研究涂层在 3.5%NaCl(质量分数)溶液中浸泡不同时间(30 min和 7 天)的腐蚀行为.结果表明:浸泡 30 min后,纯铝涂层和纯铝/氧化铝复合涂层的腐蚀电流密度分别为 3.7×10－6 ,8.0×10－7 A·cm－2 ;浸泡 7 天后,腐蚀电流密度分别为 9.0×10－6 ,1.8×10－6 A·cm－2 ,纯铝/氧化铝复合涂层和冷喷涂铝涂层均能有效延缓镁合金基体腐蚀.其中,微弧氧化复合涂层的耐蚀性约为冷喷涂纯铝涂层的 5 倍,耐蚀性的进一步提高归因于微弧氧化陶瓷层优异的物理屏障作用.     

Formation of a novel nanocrystalline coating on AZ31 magnesium alloy by bias sputtering

A novel protective coating was deposited on AZ31 magnesium alloy by bias sputtering with a Ti/Al composite target in this study. It was determined by the analysis of EDS and XRD that the coating was mainly composed of Ti₃Al phase. The calculation, based on the Scherrer formula, further revealed the formation of nanocrystalline structure in the coating. SEM and AFM observations showed that the coating was compact and no obvious columnar structure occurred. In corrosion tests, it was found that the coating was more cathodic than the AZ31 substrate and had a good corrosion resistance in a 3.5 wt.% NaCl solution. Accordingly, the AZ31 substrate was protected by this obtained coating.     

固溶处理对AM60B+XRE及AZ91D+XRE 镁合金性能的影响

研究了添加少量富铈混合稀土的AM60B+xRE及AZ91D+xRE合金(x=0.4、0.8、1.2、1.6和2.0%,质量分数)固溶处理后的显微组织与机械性能.结果表明,添加混合稀土能显著提高合金的抗拉强度σb和屈服强度σ0.2,固溶处理明显提高AZ91D+xRE合金的强度;AM60B+xRE及AZ91D+xRE合金的铸态组织由α(Mg)固溶体、杆状Al11RE3相、颗粒状Al10Ce2Mn7相以及网状Mg17Al12相组成,经过固溶处理后,网状Mg17Al12相完全溶解,只剩下热稳定性较高的Al11RE3相和Al10Ce2Mn7相,随固溶时间的延长,其形态略有改变.AM60B+xRE合金拉伸试样断口呈带局部韧窝的准解理断裂形式,而AZ91D+xRE合金则呈现沿晶断裂+解理断裂的混合断口形态.     

固溶处理对AM60B＋xRE及AZ9lD＋xRE镁合金性能的影响

研究了添加少量富铈混合稀土的AM60B xRE及AZ9lD xRE合金(x＝0．4、0．8、1．2、1．6和2．0％，质量分数)固溶处理后的显微组织与机械性能．结果表明，添加混合稀土能显著提高合金的抗拉强度σb和屈服强度σ0.2，固溶处理明显提高AZ9lD xRE合金的强度；AM60B xRE及AZ9lD xRE合金的铸态组织由α(Mg)固溶体、杆状Al11RE3相、颗粒状Al10Ce2Mn7相以及网状Mg17Al12相组成，经过固溶处理后，网状Mg17Al12相完全溶解，只剩下热稳定性较高的Al11RE3相和Al10Ce2Mn7相，随固溶时间的延长，其形态略有改变．AM60B xRE合金拉伸试样断口呈带局部韧窝的准解理断裂形式，而AZ9lD xRE合金则呈现沿晶断裂 解理断裂的混合断口形态．     

In vivo degradation and tissue compatibility of ZK60 magnesium alloy with micro-arc oxidation coating in a transcortical model

Magnesium alloys were studied extensively as a class of biodegradable metallic materials for medical applications. In the present study, ZK60 magnesium alloy was considered as a candidate and the micro-arc oxidation (MAO) treatment was adopted in order to reduce the degradation rate of the alloy. The in vivo degradation behaviors and biological compatibilities of ZK60 alloys with and without MAO treatment were studied with a transcortical model in rabbits. The implant and the surrounding bone tissues were characterized by CT, SEM and histological methods at 2, 4 and 12 weeks after the implantation. The results demonstrated that both the bare and MAO-coated ZK60 alloys completely degraded within 12 weeks in this animal model. The MAO coating decreased the degradation rate of ZK60 alloy and enhanced the response of the surrounding tissues within the first 2 weeks. After then, an acceleration of the degradation of the MAO-coated ZK60 alloy was observed. It was found that the alloy could be degraded before the complete degradation of the MAO coating, leading to the local peeling off of the coating. An in vivo degradation mechanism of the MAO-coated ZK60 alloy was proposed based on the experimental results. The severe localized degradation caused by the peeling off of the MAO coating was the main reason for the acceleration of the degradation of the MAO-coated ZK60 alloy.     

Corrosion fatigue behavior of extruded AZ80, AZ61, and AM60 magnesium alloys in distilled water

Rotary bending fatigue tests were conducted in laboratory air and distilled water using three extruded magnesium (Mg) alloys AZ80, AZ61, and AM60 with different chemical compositions. In laboratory air, the fatigue strengths at high stress levels were similar in all alloys because cracks initiated at Al-Mg intermetallic compounds, whereas AZ80 with the largest Al content exhibited the highest fatigue strength at low stress levels, which was attributed to the crack initiation due to cyclic slip deformation in the matrix microstructure. In distilled water, fatigue strengths were considerably decreased due to the formation of corrosion pits in all alloys, and the difference of fatigue strength at low stress levels among the alloys disappeared, indicating that the addition of Al that improved the fatigue strength in laboratory air was detrimental to corrosion fatigue. __________ Translated from Problemy Prochnosti, No. 1, pp. 141–145, January–February, 2008.     

AM60B镁合金在不同pH值酸雨中的腐蚀行为

重庆是我国镁合金产业化的重要基地,酸雨对镁合金的腐蚀非常严重.采用极化曲线和电化学交流阻抗法研究了不同pH值条件下AM60B镁合金在模拟酸雨中的腐蚀行为,用扫描电镜(SEM)观察了镁合金的腐蚀形貌,用x射线衍射仪(XRD)分析了腐蚀产物的组成.结果表明:随着溶液pH值的下降,AM60B镁合金的自腐蚀电位变负,腐蚀速率增加,同一腐蚀电位下阳极极化电流变大;电化学阻抗谱显示其容抗减小,膜电容下降,镁合金表面的保护膜被破坏;AM60B镁合金在模拟酸雨中的腐蚀以点蚀为主,形成了较深的腐蚀坑,腐蚀程度随着溶液pH值下降而增大,腐蚀产物主要由MgO和MgAl2(SO4)4·2H2O组成.     

Effect of friction stir processing on mechanical and microstructural properties of AM60B Magnesium alloy

Some AM60B magnesium alloys sheets produced by High Pressure die Casting were Friction Stir Processed and the mechanical and microstructural features are presented in the present study. The mechanical properties of the FSP material were analyzed in longitudinal direction respect to the processing one and compared with those of the base material. Tensile tests were performed at room and high temperature and different strain rates in the nugget zone, in order to analyse the superplastic properties of the recrystallized material and to observe the differences respect to the base material after the strong grain refinement effect due to the Friction Stir Process. The high temperature behaviour of the material was studied, in longitudinal direction, by means of tensile tests in the temperature and strain rate ranges of 175–250°C and 10⁻²–10⁻⁴ s⁻¹ respectively.