关键词： Magnesium alloys   Age hardening   Precipitation   Deformation  

摘要： Certain magnesium alloy systems can obtain superior mechanical strength through age hardening. However, conventional ageing heat treatments for these alloys are rarely more complex than a single isothermal hold - especially when compared to those of aluminium alloys - so age hardenable magnesium alloys may not be achieving their full strengthening potential. The use of preageing, where a lower temperature heat treatment is utilised before a hotter secondary ageing step, has proven successful in boosting the strength of magnesium alloys previously, but these trials are few in number, and the testing conditions are limited. In this work, a wide range of preageing temperatures and times were trialled on commercial and experimental magnesium alloys to determine the effectiveness of the strengthening technique. The results showed that preageing can produce a significant boost in hardness, can reduce total ageing times, and can provide a degree of mechanical property customisation through control of precipitate habit plane and morphology. However, the effectiveness of the technique is alloy-system dependent, where the fundamental precipitate nucleation and phase evolution is a key contributor to the success of preageing;as is the energy barrier to nucleation in an alloy system, whether that be controlled by alloy chemistry or thermomechanical processing. The results are discussed in the context of alloy design and industrial processing.

关键词： Biodegradable magnesium alloys   Coatings   Corrosion   Chemical conversion   Nanocrystalline zinc  

摘要： Biodegradable medical implants made of magnesium (Mg) alloys must have sufficient corrosion resistance in clinical applications. Herein, a nanocrystalline zinc (Zn) coating is electrodeposited on the WE43 Mg alloy followed by a chemical conversion treatment to produce a titania-Zn phosphate layer. The Zn coating has a rice-like nanostructure and the conversion layer is composed of ZnO, Zn-3(PO4)(2), and TiO2. The Zn coating reduces the corrosion current density of the WE43 Mg alloy from 151.1 +/- 13.8 mu A cm(-2) to 29.4 +/- 7.4 mu A cm(-2) in the simulated body fluid (SBF), while the composite coating decreases it to 4.1 +/- 0.8 mu A cm(-2). The surface morphology, pH variation, and average corrosion rates after immersion for 7 days in SBF reveal that the Zn coating in fact accelerates dissolution of the Mg substrate due to the formation of galvanic couples between the Zn coating and Mg alloy substrate. In comparison, only slight corrosion is observed from limited areas on the sample with the composite coating. The titania-Zn phosphate/nanocrystalline Zn composite coating provides long-term protection in the physiological environment and the protection mechanism is discussed.

关键词： Magnesium   Biodegradation   PEO   Corrosion   Wear  

摘要： The rapid bio-corrosion of magnesium-based alloys, the formation of hydrogen gas and, consequently, the premature loss of biomechanical functions hinder their applications as biodegradable implant materials. The corrosion becomes even accelerated, when fretting wear occurs at implant junctions, as a result of repeated disruptions of the magnesium (hydr)oxide layer formed on implant surfaces. To improve the overall performance of these materials in a bio-relevant environment, especially corrosion resistance and wear resistance, in this research, plasma electrolytic oxidation (PEO) was applied to create a coating on a magnesium alloy, ZK30. The resulting gains in corrosion resistance and wear resistance were evaluated. In vitro immersion tests in Hank's solution at 37 degrees C showed a reduction in hydrogen release from the PEO-treated alloy. The results obtained from applying the scanning vibrating electrode technique (SVET) indicated a decreased susceptibility of the PEO-treated alloy to localized corrosion, accounting for the improved corrosion resistance. In addition, PEO was found to change the surface topography and roughness, in addition to surface chemistry, which contributed to an increased but stable coefficient of friction and a decreased material removal rate, as revealed by the tribological tests with a ball-on-plate configuration. The results indicate an enlarged opportunity of magnesium-based materials for orthopedic applications, where friction and wear are involved, by applying PEO.

关键词： Fine-grained   Extrusion   Tension   Compression   Texture  

摘要： The deformation behavior of an extruded Mg-Zn-Zr alloy in the form of plate/sheet was investigated on the basis of fine-grained and initial macro-texture along various directions under uniaxial compression and tension test. The initial macro-texture revealed that 60.4% of grains have c-axes of hexagonal closed packed crystal structure along the normal direction and 17.4% grains have c-axes parallel to the transverse direction. The results showed that the flow stress and strain hardening rate under compression and tension were significantly affected by the loading directions and the initial texture. The ratio of tensile yield strength to the compressive yield strength along normal direction extruded direction, and transverse direction was found 0.85, 1.5, and 0.93, respectively. The practically symmetric yielding behavior along normal and transverse direction was caused by the fine-grained size, twinning dependencies on strain path loading, and texture. The anisotropy similar to 1.5 along extrusion direction is the cause of the large fraction of {10 (1) over bar2} <10<(1)over bar>1 > extension twinning under compression and no twinning activity in tension along ED in the early stages of deformation in large grains. The strain hardening curves under tension and compression shows almost similar behavior in the normal direction. For extrusion direction and transverse direction, strain hardening curves exhibited three and two regimes under compressive loading which leads to a large difference in ultimate compressive and tensile strength. It is also revealed that {10 (1) over bar2} < 10<(1)over bar>1> extension twinning was favorable, while {10 (1) over bar1} <10<(1)over bar>2 > contraction twinning and {10 (1) over bar1} - {10 (1) over bar2} double twinning were unfavorable for strain hardening. The underlying deformation mechanisms under different loading paths were discussed on the basis of X-ray diffraction and optical microscopic analysis. (C) 2020 Elsevier B.V. All

关键词： Dislocation climb   Vacancies   Solute softening   Magnesium alloys   Structure materials   HCP structure   Deformation   Mechanical properties   Ductility  

摘要： Active room temperature solute induced diffusion-less climb of the < a > edge dislocations in model Mg-Al alloys was observed using molecular dynamics simulations. Dislocations on prismatic and pyramidal I planes climb through the basal plane to overcome solute obstacles. This out-of-plane dislocation motion softens the high resistance pyramidal I glide and significantly reduces the anisotropy of dislocation mobility, and could help improve the ductility of Mg. The flow stress scales linearly with solute concentration, c(Al). Dislocations climb predominantly in the negative direction, with climb angle on the order of 0.01c(Al), producing very high vacancy concentration on the order of 10(-4). This climb behavior was rationalized using an energy analysis by comparing the in-plane and out-of-plane motions between different Mg slip planes. The ease of climb depends on the strength of the solute-dislocation interaction and the compactness of the dislocation core. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

关键词： Mg alloy   Strength   Ductility   Corrosion   Precipitate  

摘要： It is always challenging to synchronously improve the strength, ductility and corrosion resistances, in the development of magnesium alloys. Herein, a concept called hierarchical precipitate embedded fine grain microstructure is proposed and implemented in an Mg-13Al (wt%) alloy by equal channel angular pressing and aging. Compared to the as-cast alloy, the modified alloy with the designed microstructure has high strength, good ductility, and improved corrosion resistance. The enhancement of mechanical properties is mainly attributed to the synergistic effect of fully recrystallized fine grains (similar to 3.7 mu m) and hierarchical beta phase (Mg17Al12) precipitates. Although the formation of the hierarchical beta phase precipitates in fine-grain Mg matrix can accelerate the initial corrosion in 3.5 wt% NaCl solution due to the galvanic corrosion, the ultimate corrosion resistance is successfully improved by the consumption of exposed Mg anodes and the synergistic barrier effect from the newly-formed corrosion products and corrosion-resistant beta phases. (C) 2020 Elsevier B.V. All rights reserved.

关键词： Magnesium alloys   Plasticity   Texture   EBSD   Cryogenic deformation  

摘要： In this work, a commercial cast magnesium alloy AZ31 was compressed at room and cryogenic temperatures (RT and CT) to study how the twins and dislocations affect the flow stress, plastic strain and strain hardening. Cryogenic deformation suppresses the dislocation activities to enable the effects of dislocation slip and twinning on plasticity to be separately interrogated. A quasi-in-situ Electron Backscatter Diffraction (EBSD) method was used to trace the evolution and distribution of twins and geometrically necessary dislocation (GND) density manifested as kernel average misorientation (KAM) in the same group of grains before and after plastic deformation, at RT and CT respectively. Compared to the RT deformation, higher flow stress, higher strain hardening rate and moderately reduced strain to failure were observed in the CT deformation. These interesting findings were rationalised by the subsequent EBSD analysis. It is found that the increased fraction of finer twins and twin twin interactions rather than dislocation density leads to higher stress and higher strain hardening at CT. This suggests that the hardening effect induced by twin boundary and twin-twin interactions is considerably more effective than dislocation density hardening.

关键词： USSP   Amorphous   Corrosion   Hydrogen   Nanograins  

摘要： The influence of ultrasonic shot peening (USSP) treatment on the microstructure, corrosion behavior and hydrogen evolution of AZ31 magnesium alloy is investigated. An Fe-rich composite in amorphous state is introduced on the surface layer. Microstructure results indicate that nanoscale grains are formed on the surface layer after USSP treatment. Comparing to the untreated AZ31, the charge transfer resistance of the USSP-treated sample decreases by similar to 410 times and the hydrogen evolution rate increases by similar to 64 times. The acceleration of corrosion rate is attributed to the micro-galvanic interaction between the nanocrystalline Mg matrix and amorphous Fe-rich composite. (c) 2021 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

关键词： Magnesium alloys   Multi directional forging   Corrosion   Polarization   Mechanical strength  

摘要： An attempt was made to develop a corrosion-resistant, high-strength Mg alloy using multi-directional forging (MDF). An extruded alloy as well as pure Mg was MDFed at temperatures between 220 and 340 degrees C with 40 degrees C intervals. Grains, second phase distribution, and Volta-potential difference between second phases and matrix were measured. Moreover, tensile and compression tests were used to assess mechanical properties. The corrosion behavior was characterized using electrochemical, hydrogen evolution, and mass loss techniques. A corrosion-resistant Mg alloy with a corrosion rate less than even high purity Mg obtained which has not been previously reported for a light Mg alloy. It was found that the lowest corrosion rate can be obtained for the alloy MDFed at 300 degrees C. (C) 2020 Published by Elsevier B.V.

关键词： Low cycle fatigue   Magnesium alloy   In situ neutron diffraction   Deformation mode transitions  

摘要： This work investigates the microscopic deformation mechanisms of an extruded, precipitation-strengthened AZ80 magnesium (Mg) alloy subjected to strain-controlled low-cycle fatigue using in situ neutron diffraction measurements. Results demonstrate that the plastic deformation during cyclic loading is dominated by the alternating {10.2} extension twinning and detwinning mechanisms. The observed deformation mode is strongly texture and precipitate dependent. For the initial texture, the tested material has two major texture components which result in the occurrence of extension twinning during both compression and reverse tension in the first two cycles. The prolonged detwinning process in the following cycles is proposed to relieve the shear stress field of {00.2} grains, leading to the disappearance of twinning. The precipitation strengthening results in an increase of the critical resolved shear stress (CRSS) by similar to 33 MPa for the extension twinning in this AZ80 alloy. The synergistic effects of the initial texture, precipitation strengthening, and load sharing of various grain families and phases contribute to the complicated evolution of dominant deformation mechanisms, among which elevated dislocation activities are believed to be responsible for the relatively poor low-cycle-fatigue lifetime when compared to other Mg alloys.