Comparison between mechanochemical effect on die cast and extruded magnesium alloys

Abstract

The mechanochemical effect (MCE) of several magnesium based alloys, obtained by both die casting and extrusion methods, was studied by potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) techniques. The mechanochemical behaviour of each alloy was evaluated as a function of die cast parameter, environment, and alloy composition. Electrochemical tests were performed in a buffer solution of sodium tetraborate, (Na₂B₄O₇) with and without magnesium hydroxide (Mg(OH)₂). The MCE was correlated with the microstructure of the Mg alloys.     

Influence of elevated temperatures on the cyclic deformation behaviour of the magnesium die-cast alloys AZ91D and MRI 230D

The magnesium alloys AZ91D and MRI 230D were investigated in form of die-cast specimens with a cast skin. The fine-grained microstructure consists of a dendritic magnesium solid solution and interdentritic precipitates. The cyclic deformation behaviour was characterised in stress-controlled load increase tests and constant amplitude tests by means of mechanical stress–strain hysteresis measurements at room temperature and at T = 150 °C. The MRI alloy leads to higher plastic strain amplitudes and nevertheless higher lifetimes for both temperatures. Load increase tests allow a reliable short-time estimation of the endurance limit under both, room and elevated temperatures. With the physically based fatigue life calculation method “PHYBAL” the lifetime of the magnesium alloys can be calculated on the basis of cyclic deformation data determined in one load increase test and two constant amplitude tests in excellent agreement with the conventionally determined S–N curve.     

电流密度对Mg_(97)Y_2Zn_1-0.5Al合金阳极氧化及膜层性能的影响

为制备出绿色环保且耐蚀性能更好的镁合金阳极氧化膜,以无害、无污染的氢氧化钠、硅酸钠、碳酸钠和柠檬酸钠为电解液主要成分,以不同的电流密度(5~20 m A/cm~2)对Mg_(97)Y_2Zn_1-0.5Al合金进行阳极氧化,通过扫描电镜观察阳极氧化膜的表面形貌,利用动电位极化曲线和3.5%Na Cl水溶液浸泡试验检测膜层的耐腐蚀性能,研究了电流密度对膜层表面形貌与性能的影响。结果表明:恒流条件下不同电流密度时的阳极氧化过程均分为氧化膜快速生长、缓慢生长及生长相对平稳3个时期;随着电流密度的增加,氧化膜上的孔洞数目逐渐减少,孔径逐渐增大,膜层的耐蚀性能先增强后减弱;15 m A/cm~2电流密度下所得膜层的孔洞与裂纹较少,孔径较小,自腐蚀电流密度最小,腐蚀失重最小,耐蚀性最好。     

H_2O_2对AZ91D镁合金表面铈转化膜形成及耐腐蚀性能的影响研究

对AZ91D镁合金表面铈转化膜的制备条件及膜的结构进行了研究,结合SEM、EDS、Tafel、EIS等分析手段,在室温(24±3)℃,硝酸铈浓度0.02mol/L,成膜促进剂H2O2的添加量为5mL/L,成膜时间10min时,可获得耐腐蚀性较好的转化膜。对此条件下制备的铈转化膜的结构进行了分析,研究认为,转化膜分3层结构:(1)蜂窝状结晶层,主要含有Mg、Al、Ce的氧化物或氢氧化物;(2)封孔致密层,主要含有大颗粒状Mg、Ce的氧化物或氢氧化物;(3)类凝胶层,主要含有类凝胶状Mg、Ce的氧化物及氢氧化物。     

流变成形压力对Al2Y/AZ91镁基复合材料摩擦磨损行为的影响

采用销盘式摩擦副,在转速为100 r/min干摩擦条件下,结合OM、SEM结果,考察了不同载荷与成形压力对流变成形Al2Y/AZ91镁基复合材料(质量分数2%Y)摩擦磨损性能的影响,并探究耐磨性与材料显微组织、力学性能之间的关系.研究表明:在相同的实验载荷下,随着制备复合材料流变成形压力的增加,材料的磨损质量和摩擦系数减少,本实验条件下最大成形压力为100 MPa时磨损量和摩擦系数最小,摩擦磨损性能较佳;对于在相同成形压力下制备的镁基复合材料,磨损质量随着载荷的提升而增大,而摩擦系数有所降低.当载荷较小时,Al2Y/AZ91镁基复合材料的磨损机制以磨粒磨损为主;随着载荷的增大,磨损机制逐步发生转变;当载荷较大时,磨损机制以剥层磨损为主.     

In-situ hydrothermal crystallization Mg(OH)2 films on magnesium alloy AZ91 and their corrosion resistance properties

Mg(OH)₂ films have been fabricated on magnesium alloy AZ91 substrates by an in-situ hydrothermal method. AZ91 alloy substrates act as both the source of Mg²⁺ ion and the support for the Mg(OH)₂ film in synthetic process. The effect of pH value and hydrothermal treatment time on the morphologies and corrosion resisting properties of Mg(OH)₂ film is studied. The obtained Mg(OH)₂ films are uniform and compact. The adhesion between the films and the substrate is strong due to the in-situ growth process, which enhances their potential for practical applications. Potentiodynamic polarization measurements showed that the Mg(OH)₂ films obtained at pH 10, 3 h exhibits the highest increase in corrosion potential at −0.7097 V and lowest i_corr, which suggests that it is the best effective film in improving the corrosion resistance of AZ91in all obtained films.     

Effect of Y and Zn substitution on elastic properties of 6H-type ABCBCB LPSO structure in Mg97Zn1Y2 alloy

Theoretical investigations of the effect of Y and Zn atom substitution on elastic properties of 6H-type ABCBCB LPSO structure in Mg₉₇Zn₁Y₂ alloy have been performed from density function theory. Elastic properties, including elastic constants and elastic modulus were investigated, and the influence of Y and Zn substitution were discussed in detail. Elastic anisotropies were analyzed by several methods, and the results show that the anisotropy in compression is almost negligible, whereas the anisotropy in shear is relatively large. Furthermore, the shear anisotropy becomes larger with Zn substitution than Y substitution. The electronic characteristics indicate that the Mg-Y and Mg-Zn bonds exhibit covalent feature due to hybridization, so the interactions of Mg with Y and Zn are enhanced.     

Atomic-resolution studies on reactions between basal dislocations and { 10(1)2 } coherent twin boundaries in a Mg alloy

Reactions between basal 〈a60〉 dislocations and {1012} coherent twin boundaries(CTBs) in a Mg alloy were studied with atomic resolution. Individual dislocation-CTB reactions produced steps with residual dislocations and multiple t winnin g dislocations(TDs) gliding away, consequently resulting in TB migration. Reactions between {1012} CTBs and low-angle grain boundaries composed of basal 〈a60〉 dislocations created either basal-prismatic/prismatic-basal interfaces or asymmetric tilt grain boundaries, depending on whether TDs gliding away or not. Not only the emission of TDs by dislocation-TB reactions may drive TB migration, but also the resultant steps or facets along TBs can act as TD sources to facilitate TB migration. Our results indicate that roughness or severe loss of local coherency induced by dislocation-TB reactions does not intrinsically impede TB migration in Mg alloys. Dislocation-TB reactions may provide another feasible strategy to improve the ductility of Mg alloys, in addition to other techniques like grain refinement and texture modification.