Corrosion properties and corrosion evolution of as-cast AZ91 alloy with rare earth yttrium

The corrosion resistance property and the corrosion evolution of as-cast AZ91 alloy with rare earth Y addition are investigated by using immersion tests, electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS). The results show that the proper amount of Y in the alloys can improve the corrosion resistance of AZ91 alloys effectively. With the increment of Y, the corrosion rate of the modified AZ91 alloys by Y addition was markedly less than that of AZ91 alloy. The corrosion rate of AZ91 alloy with 0.3 wt.% Y was the slightest, but further addition of Y content over 0.3 wt.% make the corrosion heavier. The XPS analysis suggests that the compound film of AZ91 alloy with 0.3 wt.% Y is mainly composed of Mg(OH)₂ and MgCO₃ without any Al(OH)₃ and Al₂O₃, in addition, Y₂O₃ phase is found in the compound film of AZ91 alloy with 0.3 wt.% Y, which benefits to stabilize the surface film.     

Reactive sputter deposition of alumina films on magnesium alloy by double cathode glow-discharge plasma technique

In order to overcome the problem of the corrosion resistance of AZ31 magnesium alloy, the nanocrystalline Al₂O₃ film was deposited on AZ31 magnesium alloy by double cathode glow-discharge plasma technique. The microstructure, chemical composition and elemental chemical state of the sputter-deposited nanocrystalline Al₂O₃ film were analyzed by means of scanning electron microscopy equipped with an energy dispersive spectroscope, X-ray diffraction), transmission electron microscope and X-ray photoelectron spectroscopy. The results indicated that the sputter-deposited nanocrystalline Al₂O₃ film consisted of single θ-Al₂O₃ phase with average grain size about 60 nm. The hardness and the elastic modulus of the as-deposited nanocrystalline Al₂O₃ film were about 17.21 GPa and 217 GPa measured by nanoindentation instrument, respectively. The corrosion behavior of the sputter-deposited nanocrystalline Al₂O₃ film in 3.5%NaCl solution was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy. The amount of porosity for the sputter-deposited nanocrystalline Al₂O₃ film calculated by two electrochemical methods was equal to 0.0086% and 0.168%, respectively. The sputter-deposited nanocrystalline Al₂O₃ film exhibited excellent corrosion resistance, which was attributed to its dense enough structure to prevent magnesium alloy from corrosion in aggressive solutions.     

Simultaneously enhanced tensile and compressive response of AZ31–NanoAl<Subscript>2</Subscript>O<Subscript>3</Subscript>–AA5052 macrocomposite

New bimetal AZ31–Al₂O₃/AA5052 macrocomposite comprising (a) Al₂O₃ nanoparticle-reinforced magnesium alloy AZ31 shell and (b) aluminum alloy AA5052 millimeter-scale core reinforcement was fabricated using solidification processing followed by hot coextrusion. Microstructural characterization revealed more rounded intermetallic particle of decreased size, reasonable Al₂O₃ nanoparticle distribution, and non-dominant (0 0 0 2) texture in the longitudinal and transverse directions in the AZ31–Al₂O₃ nanocomposite shell. Interdiffusion of Mg and Al across the core–shell macrointerface into each other was also significant. Compared to monolithic AZ31, the AZ31–Al₂O₃ shell exhibited significantly higher hardness (+33%). In tension, the presence of Al₂O₃ nanoparticles (in the AZ31 shell) and AA5052 core significantly increased stiffness (+39%), yield strength (0.2% TYS) (+9%), ultimate strength (UTS) (+19%), average failure strain (+7%), and work of fracture (WOF) (+27%) of AZ31. In compression, the presence of Al₂O₃ nanoparticles (in the AZ31 shell) and AA5052 core significantly increased yield strength (0.2% CYS) (+58%), ultimate strength (UCS) (+4%), average failure strain (+11%), and WOF (+49%) of AZ31. The effect of joint presence of (a) Al₂O₃ nanoparticles (in the AZ31 shell) and (b) AA5052 millimeter-scale core on tensile and compressive properties of AZ31 is investigated in this article.     

Influence of lanthanum on the microstructure, mechanical property and corrosion resistance of magnesium alloy

The microstructure, mechanical property and corrosion behavior of AZ91D with addition of lanthanum were investigated. With adding 1% lanthanum, the ultimate tensile strength and the elongation increased by 21.1% and by 101.2%, respectively. This mainly caused by the refinement of the β phase and the formation of Al₁₁La₃ strengthening phase. With adding 1% lanthanum in AZ91D alloy, the weight loss corrosion rate decreased to 47.2%. But when lanthanum content is over 1%, the corrosion rate increased. Small amount of lanthanum could form the small β phase similar to reticular structure, which could restrain the corrosion at some extent; large amount of lanthanum caused the depletion of aluminum in α matrix, which increased the corrosion rate.     

Protective coating for magnesium alloy

A phosphate–permanganate conversion coating was applied as the pretreatment process for AZ91D magnesium alloy substrate. Zn–Ni alloys were electrodeposited onto the treated AZ91D magnesium alloy from sulfate bath. The morphology and phase composition of the coatings were determined with X-ray diffraction (XRD) and Scanning Electron Microscope (SEM). The results reveal that the conversion rate depends on pH of solution and treatment time. Salt spray and the electrochemical polarization testing were applied to evaluate the corrosion performance of phosphate–permanganate and Zn–Ni coated alloys. It was found that Ni content in deposit is a function of current density and bath composition. Zn–13 wt.% Ni coating provides very good corrosion protective function to inner AZ91D magnesium alloy. Phosphate–permanganate treatment enhances the corrosion resistance of Zn–Ni coatings.     

The effect of skin characteristics on the environmental behavior of die cast AZ91 magnesium alloy

The skin characteristics of die cast AZ91 magnesium alloys and their consequent environmental behavior were evaluated in correlation with die cast wall thickness. The metallurgical examination of the skins was carried out using scanning electron microscopy (SEM), SPX-surface analyses with ESCALAB 250 apparatus, and X-ray diffraction analysis. The corrosion behavior of the skins was evaluated by immersion tests and by potentiodynamic polarization measurements in 3.5% NaCl solution saturated with Mg (OH)₂ at room temperature. The results show that the corrosion resistance of die cast specimens with relatively greater thickness was improved compared to the corrosion resistance of thin wall specimens. This was explained in light of the obtained skin characteristics, mainly in terms of aluminum content, β phase (Mg₁₇Al₁₂) quantity and morphology, and level of porosity.     

Microstructure and corrosion characteristics of laser-alloyed magnesium alloy AZ91D with Al–Si powder

Abstract

Blown-powder laser surface alloying was performed on the magnesium alloy AZ91D with Al–Si alloy powder to improve corrosion resistance. Characterization by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and x-ray diffraction (XRD) analysis revealed that intermetallic compounds (IMCs) of Mg₂Si, Al₁₂Mg₁₇ and Al₃Mg₂ were formed in the matrix of α-Mg and Al solid solutions in Al–Si alloyed layers. The anodic polarization test in 3.5% NaCl aqueous solution showed that preferential corrosion occurred in the α-Mg matrix of the AZ91D base metal. The Al–Si alloyed layers exhibited a lower corrosion rate and a higher polarization resistance than AZ91D. The compactly dispersed dendritic Mg₂Si phase, and the dendritic and angular phases of Al₁₂Mg₁₇ and Al₃Mg₂ in the alloyed microstructure were observed to be corrosion-resistant, constituting a barrier that retards corrosion. Corrosion initiated at the interface between IMCs and the solid solution matrix, and at substructures of the matrix, subsequently pervaded into the surrounding microstructure.     

Microstructure and corrosion characteristics of laser-alloyed magnesium alloy AZ91D with Al–Si powder

Blown-powder laser surface alloying was performed on the magnesium alloy AZ91D with Al–Si alloy powder to improve corrosion resistance. Characterization by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and x-ray diffraction (XRD) analysis revealed that intermetallic compounds (IMCs) of Mg₂Si, Al₁₂Mg₁₇ and Al₃Mg₂ were formed in the matrix of α-Mg and Al solid solutions in Al–Si alloyed layers. The anodic polarization test in 3.5% NaCl aqueous solution showed that preferential corrosion occurred in the α-Mg matrix of the AZ91D base metal. The Al–Si alloyed layers exhibited a lower corrosion rate and a higher polarization resistance than AZ91D. The compactly dispersed dendritic Mg₂Si phase, and the dendritic and angular phases of Al₁₂Mg₁₇ and Al₃Mg₂ in the alloyed microstructure were observed to be corrosion-resistant, constituting a barrier that retards corrosion. Corrosion initiated at the interface between IMCs and the solid solution matrix, and at substructures of the matrix, subsequently pervaded into the surrounding microstructure.     

Mechanical characterization and corrosion behavior of newly designed Sn and Y added AZ91 alloy

This paper deals the effect of Sn and Y additions on the microstructure, mechanical and corrosion properties of AZ91 alloy. It is found that by the addition of Sn, the formation and growth of discontinuous precipitate get suppressed and new intermetallic Mg₂Sn phase is formed. In the case of Y addition together with Sn, the grain size gets refined, the volume of Mg₁₇Al₁₂ gets decreased and new intermetallic Al₂Y phase is observed. Improved room and high temperature tensile properties are obtained in as-cast and aged Sn and Y added AZ91 alloy. However, maximum properties are obtained for the alloy having combined addition of 0.5 wt.% Sn and 0.9 wt.% Y. Improved corrosion resistance is also noticed with the addition of Sn and Y elements.     

Fabrication of SiC particles-reinforced magnesium matrix composite by ultrasonic vibration

Magnesium matrix composites reinforced with two volume fractions (1 and 3%) of SiC particles (1 μm) were successfully fabricated by ultrasonic vibration. Compared with as-cast AZ91 alloy, with the addition of the SiC particles grain size of matrix decreased, while most of the phase Mg₁₇Al₁₂ varied from coarse plates to lamellar precipitates in the SiCp/AZ91 composites. With increasing volume fraction of the SiC particles, grains of matrix in the SiCp/AZ91 composites were gradually refined. The SiC particles were located mainly at grain boundaries in both 1 vol% SiCp/AZ91 composite and 3 vol% SiCp/AZ91 composite. SiC particles inside the particle clusters may be still separated by magnesium. The study of the interface between the SiC particle and the alloy matrix suggested that SiC particles bonded well with the alloy matrix without interfacial reaction. The ultimate tensile strength, yield strength, and elongation to fracture of the SiCp/AZ91 composites were simultaneously improved compared with that of the as-cast AZ91 alloy.     

镁合金微弧氧化膜结构及耐蚀性的初步研究

空气中由于AZ91D镁合金耐腐蚀性差,影响实际应用.为了弄清腐蚀原因,增加应用效果,作者利用扫描电镜和X射线衍射分析了AZ91D镁合金表面微弧氧化膜的形貌、结构和相组成,并对氧化膜的耐蚀性作了初步试验分析.研究表明,AZ91D微弧氧化膜呈3层结构,外层氧化膜存在一些孔洞;中间层氧化膜疏松、具有较大厚度;内层氧化膜与基体金属结合紧密.氧化膜主要由MgO,MgSiO3,MgAl2O4,Mg3(PO4)2组成.经1周3%NaCl溶液浸泡试验,结果表明微弧氧化膜可以较大程度地提高AZ91D镁合金的耐蚀性,但氧化膜表面富含Si,P的颗粒是易发生腐蚀的电化学活性点,导致氧化膜发生局部腐蚀.     

Nanoparticle interactions with the magnesium alloy matrix during physical deformation: Tougher nanocomposites

This study is aimed at understanding the toughness enhancing function of nanoparticles in magnesium nanocomposites, focussing on experimentally observed nanoparticle–matrix interactions during physical deformation. Al₂O₃ nanoparticles were selected for reinforcement purposes due to the well known affinity between magnesium and oxygen. AZ31/AZ91 (hybrid alloy) and ZK60A magnesium alloys were reinforced with Al₂O₃ nanoparticles using solidification processing followed by hot extrusion. In tension, each nanocomposite exhibited higher ultimate strength and ductility than the corresponding monolithic alloy. However, the increase in ductility exhibited by ZK60A/Al₂O₃ (+170%) was significantly higher than that exhibited by AZ31/AZ91/Al₂O₃ (+99%). The previously unreported and novel formation of high strain zones (HSZs, from nanoparticle surfaces inclusive) during tensile deformation is highlighted here as a significant mechanism supporting ductility enhancement in ZK60A/Al₂O₃ (+170% enhanced) and AZ31/AZ91/Al₂O₃ (+99% enhanced) nanocomposites. Also, ZK60A/Al₂O₃ exhibited lower and higher compressive strength and ductility (respectively) compared to ZK60A while AZ31/AZ91/Al₂O₃ exhibited higher and unchanged compressive strength and ductility (respectively) compared to AZ31/AZ91. Here, the previously unreported nanograin formation (recrystallization) during room temperature compressive deformation as a toughening mechanism in relation to nanoparticle stimulated nucleation (NSN) ability is also highlighted.     

Characterization and wear resistance of laser surface cladding AZ91D alloy with Al + Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

The laser surface cladding of AZ91D magnesium alloy with Al + Al₂O₃ powders was investigated. X-ray diffraction (XRD) was used to identify the phases in the laser cladding layer, and the growth morphology of the boundary zone between the laser surface cladding layer and AZ91D substrate was observed by optical microscope and scanning electron microscope (SEM). The elements mapping scanning analysis on the boundary zone were carried out with energy-dispersive spectrum (EDS). The results showed that the distribution of the Al₂O₃ particles was homogeneous in the laser surface cladding layer; the growth morphology of the boundary zone was found to be in a unique parallel-branching feature. The temperature gradient, the liquidus temperature, the rate of dendrite growth and the rate of pool solidification on the growing fronts affected its formation. Furthermore, compared with the AZ91D substrate, the wear resistance of the laser cladding coatings was improved.     

Nd对AZ91镁合金显微组织和耐腐蚀性能的影响

用金相显微镜、扫描电镜和静态质量损失法对AZ91-xNd镁合金(x=1.1%,1.4%,1.9%)的微观组织和腐蚀性能进行表征,研究了Nd对AZ9l镁合金显微组织和耐腐蚀性能的影响.结果表明:稀土Nd的添加明显细化了合金的组织,使半连续网状β(Mgl7Al12)相变为细小的长条状,且分布更加均匀.在合金中还生成了颗粒状...     

Effect of Ca, RE elements and semi-solid processing on the microstructure and creep properties of AZ91 alloy

The effects of calcium and rare earth elements (RE) on the microstructure and creep properties of as-cast and thixoformed AZ91 magnesium alloy have been investigated. It has been shown that the amount of β(Mg₁₇Al₁₂) intermetallic compound decreases by adding Ca and RE elements into AZ91 alloy and new Al₁₁RE₃ and Al₂Ca intermetallic compounds form in the microstructure. The coarsening of primary α(Mg) particles in AZ91 alloy and in the Ca and RE containing (AZRC91) alloy takes place by Ostwald ripening mechanism. Adding Ca and RE elements into AZ91 alloy results in a decrease in the coarsening rate of solid particles in semi-solid slurry, whereas it has no visible effect on the shape factor of the solid-particles. The creep properties of AZ91 alloy are improved by adding Ca and RE elements particularly, in the thixoformed condition.     

Characterisation and formation of boundary in laser cladding AZ91D with Al + Al2O3

Abstract

The laser surface cladding of AZ91D magnesium alloy with Al + Al₂O₃ powders was investigated. The growth morphology of the boundary zone between the laser surface cladding layer and AZ91D substrate, and the distribution of Al₂O₃ particles in the laser surface cladding layer, were observed by optical microscope and scanning electron microscope. Furthermore, the element line and mapping scans analyses on the boundary zone were carried out with energy dispersive spectrum. The phases in the laser cladding layer and the substrate of AZ91D magnesium alloy were identified by X-ray diffraction. The results showed that the distribution of the Al₂O₃ particles was homogeneous in the laser surface cladding layer and the growth morphology of the boundary zone was in a unique parallel branching feature. The formation of the parallel branching dendrite is mainly related to the temperature gradient, the ultimate rate of dendrite growth and the rate of pool solidification on the growing fronts.     

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.     

Improving the wear resistance of AZ91D magnesium alloys by laser cladding with Al-Si powders

To improve the wear resistance of AZ91D magnesium alloy, laser surface cladding with Al and Si powders was investigated using a Nd:YAG pulsed laser. With appropriate processing parameters and the suitable weight ratio of Al to Si in powders, a modified surface layer free of cracks and pores was formed by reaction synthesis of Mg with Al and Si. X-ray diffractometry (XRD) confirmed the main phases in the layer to be Mg₂Si and Mg₁₇Al₁₂. The surface hardness increased from 35 HV for as-received magnesium alloy to more than 170 HV for laser treated sample. Accompanying the increase in hardness, the wear resistance of the clad layer increased more than 4 times that of the substrate.     

Study on microstructure of squeeze casting AZ91D alloy

Abstract

The present paper reports the investigation of the microstructure distribution of squeeze cast AZ91D alloy. The microstructure of squeeze cast AZ91D alloy is not uniform and is composed of four zones, which are chilled layer, segregation zone, pressured crystallisation area and hot spot area respectively. Moreover, in the pressured crystallisation area, the microstructure sequence in the transverse section from the outside to the inside could be divided into four sublayers, such as fine equiaxial dendrite area, dendrite area with a high directivity, confusion dendrite area and disorder dendrite area. The volume fraction of the intermetallic compound Mg₁₇Al₁₂ also varied with the location. The volume fraction of Mg₁₇Al₁₂ in the pressured crystallisation area is the largest except in the segregated zone.     

An analysis of microstructure and impression creep response of squeeze-cast AZ91–xBi–ySr alloys

ABSTRACT

The present investigation deals with the microstructural modification following the Bi?+?Sr additions to the squeeze-cast AZ91 alloy and its effect on impression creep response. The Bi?+?Sr additions form the Al₄Sr and Sr₂Bi phases besides the α-Mg and β-Mg₁₇Al₁₂ phases, and improves creep resistance of the AZ91 alloy. The AZ91?+?1.0Bi?+?0.5Sr alloy reveals the best creep resistance among the alloys. The stress exponent and the activation energy values of all the alloys are in the range of 4–7 and 100.2–112.7?kJ?mol^?1, respectively, depicting the pipe diffusion-controlled dislocation creep is the governing creep mechanism. The post-creep microstructural study confirms several dislocations pile-ups around the Al₄Sr and Sr₂Bi phases resulting in improved creep resistance of the modified AZ91 alloys.