关键词： dynamic precipitation   high strain rate rolling   mechanical properties   Mg-Al-Mn alloys   shear bands   textures  

摘要： AM60B magnesium alloy sheets with excellent mechanical properties are prepared by a double-pass high strain rate rolling (HSRR) with gradient cooling from 370 to 340 degrees C. A bimodal lamellar structure, consisting of fine dynamically recrystallized grains, shear bands and fine precipitates, is obtained after the second-pass HSRR. Formation mechanisms of shear bands are studied in depth. The results show that the formation of shear bands are closely related to the high-density intersected twin lamellae, boundaries of initial coarse grains and the extensive dynamic recrystallization (DRX), respectively. It is also clarified that the HSRR process of the coarse grained AM60B alloy is dominated in succession by the twinning deformation stage and the following shear bands flow stage. Dynamic precipitation behavior is induced in the double-pass HSRR process. The DRX is promoted by fine precipitates, resulting in local refinement of the HSRR'ed microstructure. Moreover, the weak basal texture is obtained with peak intensity less than 10. The high-density twins, DRX and shear bands are suggested as main causes for the basal texture weakening. An outstanding matching between tensile strength and ductility at room temperature are obtained owing to the bimodal lamellar structure and the weakened basal texture.

关键词： biodegradable magnesium   corrosion behavior   microgalvanic corrosion   precipitation   beta-compounds  

摘要： Microstructure analyses, immersion tests, and electrochemical measurements are performed to investigate the corrosion behavior of Mg-1.59Nd-2.91-Zn-0.05Zr-0.35Mn (NZKM) alloy under three different conditions (as-cast, heat-treated, and as-extruded) in Hanks' solution. The results indicate that the corrosion rates of the three types of NZKM alloys are all lower than 0.5 mm year(-1). The primary driving force of the NZKM alloys' corrosion is the microgalvanic coupling between alpha-Mg and beta-compounds/precipitates in Hanks' solution during the corrosion process. The lowest corrosion rates can be attributed to a low content of beta-compounds and less precipitation, a necklace-type appearance, and discontinuous distribution of the beta-compounds in the heat-treated NZKM alloy. The clustered precipitation that occurs within the as-extruded NZKM exacerbates the corrosion process due to the formation of clustered microgalvanic couple groups. These results indicate that the content of beta-compounds and precipitation of the NZKM alloys, as well as their appearances and distributions, should be controlled to allow for further application as biodegradable materials.

关键词： Conversion coating   Mg alloy   Corrosion resistance   Hydrogen induced cracking  

摘要： Cracks formed on chromate-free conversion coating (CFCC) have long been regarded as the result of dehydration. In this work, multiple forms of defects inside the phosphate-vanadium (P-V) coating and phosphate-calcium-vanadium (P-Ca-V) coating were observed and analyzed by TEM. Results show that the conversion coatings are rich with defects near the metal/coating interface. The "vena contracta" phenomenon of cracks, the trench found at the root of crack, and the isolated pores found inside the coating indicate that the cracks originate from the metal/coating interface. An entrapped hydrogen-induced coating cracking mechanism was proposed. Solutions towards governing the hydrogen-induced cracking were proposed.

关键词： AZ31B   deformation behavior   magnesium alloy sheet   pulsating loading condition  

摘要： Magnesium alloys have been known as the next generation material for lightweight body structures. Pulsating hydroforming is an effective method to improve magnesium alloy sheet forming performance, and the formed parts are characterized by lightweight, high-specific strength and stiffness. The deformation performance of magnesium alloy sheet AZ31B with a thickness of 0.6 mm under pulsating hydroforming has been investigated by means of experimental study, numerical simulation and theoretical analysis. The results show that under the same maximum hydraulic pressure, compared with simple linear loading, the magnesium alloy forming parts with pulsating hydraulic loading not only have better wall thickness uniformity and larger bulging height but also can delay the occurrence of fracture, improve the forming performance and ultimate the forming ability of magnesium alloy sheet. A new evaluation index is proposed to simplify the comprehensive forming performance of magnesium alloy parts with different amplitudes and frequencies more accurately, which can also be applied to determine the optimal forming parameters of magnesium alloy sheet AZ31B in the pulsating loading condition.

关键词： Epoxy coating   Silane   Inhibitors   EIS   EN   Pull-off adhesion  

摘要： PEO/Silane/Epoxy coating system with inorganic inhibitor (Ce(NO3)(3)) in the plasma electrolytic oxidation (PEO) layer as well as organic inhibitors in the silane layer was investigated in terms of active corrosion protection properties and adhesion strength. The results of electrochemical impedance spectroscopy (EIS), electrochemical noise (EN) and pull-off tests indicated that the presence of the 8-Hydroxyquinoline (8-HQ) organic inhibitor provides the optimal performance for coating systems. The formation of Ce(OH)(3) in the PEO coating structure and the complex chelate of Mg(HQ)(2) in the silane layer lead to the creation of active corrosion protection properties for the coating systems.

关键词： magnesium alloys   rare-earth metals   post-aging retrogression   decomposition of solid solution  

摘要： The phenomenon of post-aging retrogression in magnesium alloys containing two rare-earth metals belonging to different groups, i.e., yttrium Y (the yttrium group) and samarium Sm (the cerium group), is investigated. It is shown that retrogression may occur in alloys with different proportions of yttrium and samarium aged at 200 degrees C for maximum hardness and then annealed at somewhat higher temperatures (250 and 300 degrees C). The process is possible due to decomposition of the supersaturated magnesium solid solution. The degree of the retrogression is shown to depend on the temperature and duration of the post-aging annealing and on the yttrium- to-samarium proportion in the alloys.

关键词： Aluminum   Coating   Cold spray   Corrosion   Hardness   Mg alloy  

摘要： Magnesium (Mg) alloys have a high strength/weight ratio, high dimensional stability, good machinability, and the ability to be recycled. However, their poor corrosion resistance in humid environments limits their usage for exterior aerospace components. This study aims to improve the corrosion resistance of two Mg alloys (AZ31B and AZ91) by using aluminum coatings. The latter have been deposited by a low pressure and temperature cold spray process. An aluminum powder (60 wt%) with a particle size ranging between 1 and 8 mu m and nickel powder (40 wt%) with a particle size of about 70 mu m were blended and used as feedstock powder. The coating thickness was about 240 mu m. Its densification was achieved by the in-situ hammering effect of the nickel particles. The shot-peening effect also resulted in an enhanced coating hardness. The microstructure, mechanical properties, and corrosion resistance of the coatings have been investigated. They showed that the aluminum had a face centered cubic structure. Potentiodynamic polarization tests were performed along with a combination of materials characterization techniques to assess the corrosion resistance of the coatings when immersed in a 3.5 wt% NaCl solution for long durations. The results revealed that the corrosion resistance increased with the immersion time because of the formation of a protective oxide layer on the surface. These results were supported by elemental and structural analyses. This study shows that cold-sprayed aluminum coatings are a promising candidate for enhancing the corrosion resistance of AZ31B and AZ91Mg alloys compared to other thermal spray processes.

关键词： fine structure of magnesium alloy   self-organizing LPSO-phases   deformation process parameters   redistribution of Zn and Y hardening elements  

摘要： Results of a study of the fine structure of an alloy of the Mg - Zn - Zr - REE system proving the presence of nanosize plates of self-organizing LPSO-phases after deformation are presented. The Zn and Y hardening elements are redistributed in the periodic rows of the LPSO phases under the effect of the deformation process parameters. The optimum deformation mode of the new alloy provides formation of periodic rows in plates of LPSO phases bearing (3.2 - 4.3) at.% Y and (3.8 - 4.2) at.% Zn.

关键词： Pulsed laser   Laser hole-cutting   Magnesium alloys  

摘要： The magnet-assisted laser hole-cutting in magnesium alloys was explored systematically with and without using water immersion, including femtosecond laser multilayer center-oriented hole-cutting and millisecond laser helical hole-cutting of both blind holes and through holes. The effect of the magnetic field and water immersion on laser hole-cutting quality, efficiency and performance is first reported for magnesium alloys with mechanism analysis and comparable discussion. It was shown that the magnetic assistance and/or the water medium improved the hole wall formation especially for underwater laser cutting of blind holes. Compared to laser hole-cutting in air, the underwater laser hole-cutting generated higher-quality holes in magnesium alloys. For laser hole-cutting in magnesium alloys in water, it is first reported that the magnetic field increases the entrance diameters for blind and through holes while decreases the blind-hole depth. The water cooling-insulating effect prevented the molten and vaporized material from re-solidifying and re-depositing onto the workpiece, and the laser-induced ignition and explosion were also prevented by water. The local laser-induced ignition and explosion were weakened when increasing the magnetic flux density. The local oxidation and carbonization reactions in air as a result of the laser-induced ignition and explosion were suppressed while the laser hole-cutting efficiency was enhanced by using the transverse magnetic assistance. The blind-hole recast layer was significantly reduced nearly without accumulated residues and carbonized or oxidized debris by using underwater laser hole-cutting, indicating that the water suppressed the laser-induced melting, vaporization, ignition and explosion and blocked the re-solidification and re-deposition surrounding the hole entrance and onto the hole wall for the recast layer formation. The average grain size number in the heat affected zone (HAZ) near the hole was improved for grain re

关键词： Hydroxyapatite Coating   Fluorinated Magnesium Alloy   Hydrothermal Growth   Biomaterial  

摘要： In order to explore the hydrothermal growth mechanism of hydroxyapatite (HA) coating on fluorinated magnesium alloy, the changes of morphology, composition and crystal phase of the calcium phosphate coating during the hydrothermal treatment were studied. And the change of electrochemical impedance spectroscopy (EIS) of the coating specimen of different hydrothermal treatment time was discussed to further understanding the change of the coating structure. The results demonstrated that calcium phosphate could rapidly nucleate on fluorinated AZ31 magnesium alloy. The crystal phase of calcium phosphate coating was mainly octacalcium phosphate (OCP) at the early stage of hydrothermal treatment. Then the content of OCP decreased and the content of HA increased with hydrothermal time. The coating consisted of only HA after hydrothermal treatment for about 4h. The HA coating composed of rod-like crystals exhibited an obvious double layer structure. The rod-like crystals of inner layer arranged into dense bundles and the rod-like crystals of outer layer arranged into loose chrysanthemum-like clusters. Fluoride conversion layer acted as an intermediate transition layer to connect magnesium alloy and HA coating into a whole. The results of immersion test in simulated body fluid demonstrated that HA crystals dissolved slowly. No peeling occurred of HA coating during the 12 days' immersion. Pitting corrosion was still the mainly corrosion mode of magnesium alloy substrate due to the electrolyte infiltration during the immersion.