关键词： Friction stir processing   Magnesium alloy   Corrosion behavior  

摘要： The combination of rare earth (RE) alloying and friction stir processing (FSP) was demonstrated to be favorable in improving the corrosion resistance of Mg-9Al alloys. The microstructure and corrosion behavior of the modified Mg-9Al-xRE alloys were documented in this paper. Electrochemical measurements revealed that the corrosion resistance of as-cast Mg-9Al-xRE alloys in 3.5 wt% NaCl solution was weaken with the addition of RE elements. It is found that the formation of a large amount of coarse striplike Al-RE precipitates is responsible for the accelerated dissolution of anodic magnesium matrix. However, the corrosion resistance of FSPed Mg-9Al-xRE alloys was improved when the RE element was added and the improvement was further enhanced with the increasing amount of RE addition. This is mainly attributed to the alternation in the corrosion behavior of magnesium alloys owing to the refined and redistributed precipitates in alpha-Mg matrix. The refinement of cathodic precipitates enlarges the local area ratio of anode and cathode and retards the severe galvanic corrosion tendency in magnesium alloys. In addition, the presence of a large amount of fine Al-RE precipitates is favorable in accumulative growing of the corrosion product layer during the corrosion propagation, and thus improves the stability and compactness of this protective layer. (C) 2020 Elsevier B.V. All rights reserved.

关键词： Magnesium alloy   LDH-container layer   Self-repairing   Degradation retarding  

摘要： Magnesium alloys for orthopedic applications are mainly restricted by their rapid degradation and undesirable surface mineralization. A hierarchical layered double hydroxide (LDH)/bioactive-glass (Bg) coating was constructed via in-situ hydrothermal growth to obtain LDH-container layer on magnesium alloys, and then a sol-gel spin-coating method to cover an outer bioactive-glass layer. The ion-exchange capacity of LDH-container layer was studied by stimuli-triggered release experiment. The results show that alkaline environment is conducive to the ingestion of aggressive chlorides accompanied fluoride-releasing, and this ion-exchange process reached equilibrium after 40 h. Electrochemical corrosion tests manifested that the LDH-container layer delayed the corrosion onset, declined the corrosion current by two orders of magnitude and positively shifted the corrosion potential of 1.15 V. Furthermore, corrosion degradation and mineralization evolution in vitro revealed that a well-defined self-repairing effect and collaborative mineralization were obtained when the LDH-container preencapsulated with fluoride and coupled with bioactive-glass. Specifically, the self-repairing was achieved by a local re-deposition of the degradation products Mg2+ and dissolved Ca2+ with the displaced F- to form stable magnesium fluoride or fluorapatite in the erosion pits. This intelligent two-stage strategy, based on physical protective barrier and self-repairing the eroded pits by re-depositing of bone-like minerals, progressively retarded rapid degradation and simultaneously facilitated remineralization. The strategy described herein provides a versatile route for designing smart corrosion protection coatings or targeting drug delivery systems.

关键词： Micro-milling   Grey relational analysis   TOPSIS   Weight coefficient   Multi-objective optimisation   Sustainable production  

摘要： The present research attempts to maximise the overall performance of micro-milling process while machining Mg-3.0Zn-0.7Zr-1.0Cu alloy and its alumina composites. Three performance measures (surface quality, cutting forces, and tool wear) is used for the assessment. Using a Taguchi L18 orthogonal array, eighteen experiments are carried out to test the effect of three levels of spindle speed, feed per tooth, and cutting depth. Grey relational analysis (GRA) and techniques for order of preference by similarity to ideal solution (TOPSIS) are employed to optimise the parameters. In addition, the equal weight method and entropy weight method (EWM) in combination with the analytic hierarchy process (AHP) are used to assign weights to the parameters. The GRA and TOPSIS results yielded the same optimal parameter conditions for maximising the micro-milling performance while using two different weight-assigning methods. Based on the predicted closeness value results, the GRA method is the most efficient for multi-objective optimisation.

关键词： Biomedical Mg implants   Corrosion   Conversion coatings   Non-conversion coatings   Osseoconductive functionality   In vivo stability  

摘要： Several magnesium alloys are increasingly used for a variety of biomedical applications, owing to their favorable mechanical and biomedical properties. However, in vivo corrosion in aqueous environment including Cl ion-rich extracellular fluid (ECF) requires the application of protective coatings as well as the development of advanced alloy formulations with increased corrosion resistance and uniform, non-localized dissolution modes. This contribution reviews recent promising approaches towards improved biostability of Mg alloys during in vitro contact with simulated body fluid (SBF) as well as in vivo exposure to ECF. Emphasis is being put on surface coatings for corrosion protection as well as on functional osseoconductive coatings that stimulate tissue ingrowth.

关键词： Friction stir processing (FSP)   Al-Mg alloy   Microstructure   Corrosion resistance   Electrochemical behavior  

摘要： This article deals with the corrosion behavior of friction stir processed aluminum-magnesium alloys undergoing electrochemical measurements, including potentiodynamic polarization and electrochemical impedance spectroscopy. The effects of friction stir processing (FSP) critical parameters such as the rotational tool speed (w in the range of 800-1400 rpm) and traverse velocity (v in the field of 50-200 mm/min) along with varying cooling rates imposed using submerged cooling medium (air, water/dry-ice mixture, and liquid nitrogen) on the microstructural features and mechanical properties of Al-Mg alloys in two tempering states of annealed and wrought were characterized. Then, by assessing the electrochemical behavior of the friction stir-modified stirred regions produced using varying cooling rate and initial temper, a correlation between the microstructural details with the mechanical and chemical properties was established. The experimental results revealed the significant grain refinement down to similar to 1 um following FSP treatment by controlling the processing conditions, which had a detrimental impact on the electrochemical behavior due to the activation of grain boundary corrosion phenomenon. The primary tempering state of alloy in the form of stored strain energy played a critical role by controlling the fraction of sub-boundaries and recrystallized grains upon FSP modification. The inferior electrochemical property reported for the FSP treated annealed Al-Mg alloy under processing conditions of w= 800 rpm and v = 200 mm/min, as the less noble structure with the highest corrosion rate (CR) of 0.01814 mm/year.

关键词： Neutron diffraction   Stress localisation   Slip systems   Self-consistent modelling   Elastoplastic deformation   Magnesium alloy  

摘要： The main purpose of this work is to develop neutron diffraction methodology in order to determine stresses localised in polycrystalline grains during elastoplastic deformation, directly from experiment. As a result, for the first time, the von Mises stress for chosen grain orientations and Critical Resolved Shear Stresses (CRSS) for active slip systems were unambiguously measured without the help of crystallographic models, which introduce different theoretical assumptions. The stresses measured for groups of grains and the determined CRSS values are important characteristics of a material, which allow to study plastic deformation in textured material at different scales: slip system and grain, which play a key role in mechanical properties and formability of the material. The new method was successfully tested and applied to textured AZ31 alloy subjected to tensile deformation and the components of stress tensor were for the first time determined from measured lattice strains corresponding to chosen orientations of crystallite lattice. The obtained results positively verified hypotheses that, during plastic deformation, a large difference in the hardness as well as in the localised stresses occurs for grains having different lattice orientations. It was found directly from experiment that, the activation of basal glide, having small CRSS, does not lead to significant plastic deformation, and the activation of other non-basal systems (with higher CRSS) induces the development of plasticity at the macroscopic scale. The early plastic deformation occurring due to slip on basal system is small but it can destructively affect fatigue life, limiting applicability of the material for structural components. Finally, the comparison of experimental results with a modified version of Elastic-Plastic Self-Consistent (EPSC) model showed its capability to simulate the mechanical behaviour of such materials.

关键词： Magnesium alloy   Torsion   Swift effect   Strain hardening   Twinning  

摘要： The mechanical response and microstructure evolution in a rolled AZ31B magnesium alloy were experimentally characterized using companion thin-walled tubular specimens under free-end monotonic torsion. The tubular specimens were made with their axes along the normal direction of the rolled magnesium plate. The shear stresss-hear strain response shows a subtle sigmodal shape that is composed of four distinctive stages of strain hardening. Basal slips and tension twinning are operated throughout the shear deformation. Both tension twinning and compressing twinning are favored. Growth and interaction of tension twins with multiple variants lead to formation of twin-twin boundaries (TTBs). The collective hardening effects by twin boundary (TB) and TTB result in a unique rise of the strain hardening rate in Stage II and III. In addition to primary twins, tension-compression double twins and tension-compression-tension tertiary twins with detectable sizes are observed in the tension-twin favorable grains whereas compression-tension double twins are detected in the tension-twin unfavorable grains;all of which become more observable with the increasing shear strain. During Stage IV deformation where TTB formation exhausts, non-basal prismatic slips become more significant and are responsible for the progressive decrease in strain hardening rate in this stage. Swift effect, which is commonly observed in textured materials, is evidenced under free-end torsion. The origin of Swift effect is confirmed to be dislocation slips at a shear strain less than 5% but is predominantly due to tension twinning at a larger plastic strain.

关键词： Coatings   Polymers   Electrochemistry   Corrosion inhibiters   Polyaniline   Praseodymium   Corrosion inhibition   AZ31 magnesium alloy  

摘要： Purpose The purpose of this study was to evaluate the possibility of using conductive polymers such as polyaniline (PANI) as corrosion inhibitors for metals. Design/methodology/approach In this study, the effect of the addition of praseodymium (Pr3+) cations on the corrosion inhibition performance of PANI for AZ31 magnesium alloy was appraised through electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests. Findings The results of EIS and potentiodynamic polarization tests indicated the improvement of corrosion resistance of AZ31 during different immersion times. Research limitations/implications This anti-corrosion ability of PANI/Pr(3+)composite applies as non-toxic environmentally friendly corrosion inhibitor on the self-healing corrosion protection properties. Practical implications The conductive polymers are interested for many industries. The reported data can be used by the formulators working in the R&D departments. Social implications The anti-corrosion ability of PANI/Pr(3+)composite present a novel and high effective route against metal corrosion besides application of toxic corrosion. Originality/value The application of titanium dioxide coating in the field of architectural heritage is a great challenge. Therefore, the main objective of this study is to study the synthesis, characterization and corrosion inhibition performance of Pr(3+)cations doped PANI nano-fibers as an anti-corrosion additive for AZ31 magnesium alloy in 3.5 Wt.% NaCl solution.

关键词： Semi-solid extrusion   Magnesium alloys   Mechanical properties   Microstructure   Texture  

摘要： The microstructure evolution and mechanical properties of AZ31 magnesium alloy during semi-solid extrusion (SSE) has been studied and compared with traditional extrusion (TE). The results show that non-equilibrium crystallization and plastic deformation occur simultaneously in the semi-solid extrusion process. Throughout the SSE process, the nucleation rate of new fine grains is significantly higher than that of the TE, which is the result of combined nucleation of crystallization and dynamic recrystallization. Twinning can be hardly activated during the SSE process, because the restraint between deformed grains is largely reduced due to the good fluidity of the semi-solid Mg alloy. The Mg alloys extruded by the SSE is characterized for homogeneous microstructure with fine grain size, weakened texture and improved mechanical properties.