A new tri-electrode probe is presented and applied to local electrochemical impedance spectroscopy (LEIS) measurements. As opposed to two-probe systems, the three-probe one allows measurement not only of normal, but also of radial contributions of local current densities to the local impedance values. The results concerning the cases of the blocking electrode and the electrode with faradaic reaction are discussed from the theoretical point of view for a disk electrode. Numerical simulations and experimental results are compared for the case of the ferri/ferrocyanide electrode reaction at the Pt working electrode disk. At the centre of the disk, the impedance taking into account both normal and radial contributions was in good agreement with the local impedance measured in terms of only the normal contribution. At the periphery of the electrode, the impedance taking into account both normal and radial contributions differed significantly from the local impedance measured in terms of only the normal contribution. The radial impedance results at the periphery of the electrode are in good agreement with the usual explanation that the associated larger current density is attributed to the geometry of the electrode, which exhibits a greater accessibility at the electrode edge. (C) 2011 Elsevier Ltd. All rights reserved.; CAPES; Charles A. Stokes professorship
O interesse renovado das indústrias no desenvolvimento de novas ligas metálicas leves vem aumentando nos últimos anos, principalmente no setor automobilístico. As pressões econômicas tais como as reservas de petróleo e seus derivados que sofrem oscilações constantes de preço e são essenciais para os veículos, com também o anuncio recente da comunidade científica do aquecimento global com uma gestão ambiental cada vez mais efetiva no controle das emissões gasosas e processos de produção industrial. Neste contexto os fatores externos de pressão frente às indústrias fazem do magnésio e suas ligas uma alternativa de grande interesse, principalmente no setor automobilístico por apresentar entre os metais estruturais a melhor relação peso/resistência. Neste sentido, muitas ligas de magnésio vem sendo estudadas nos últimos anos, já que a liga AZ91, conhecida e produzida em alta escala, não oferece estabilidade em altas temperaturas, devido ao enfraquecimento do contorno de grão a partir da precipitação descontínua da fase β-Mg17Al12 de baixo ponto de fusão(Moreno 2003). As ligas do sistema Mg-Al-RE-Ca oferecem ótimo desempenho quando submetidas a fluência com resultados similares à liga de alumínio ADC12 utilizada em caixa de transmissão de veículos. Na literatura encontram-se trabalhos que correlacionam os parâmetros de solidificação com as propriedades mecânicas para ligas convencionais como os aços...
A liga de magnésio AZ91 tem sido extensivamente usada em várias aplicações tecnológicas, principalmente na indústria automotiva, devido às suas excelentes propriedades, tais como a baixa densidade. Entretanto, esta liga apresenta alta suscetibilidade à corrosão. No sentido de prevenir à corrosão, muitos tratamentos superficiais têm sido desenvolvidos, entre estes os filmes de silano, os quais são ambientalmente corretos. O presente estudo objetiva avaliar o comportamento eletroquímico da liga de magnésio AZ91 revestida com MTES (metiltrietóxi silano) com e sem incorporação de íons cério. O comportamento corrosivo foi avaliado em solução 0,1 mol L-1 Na2SO4 pH 6 usando as técnicas de espectroscopia de impedância eletroquímica, polarização potenciodinâmica, medidas de potencial de circuito aberto, microscopia eletrônica de varredura e espectroscopia de energia dispersiva. Verificou-se que o comportamento eletroquímico da liga não revestida é caracterizado pela formação de um filme poroso de hidróxido de magnésio, que aumenta a sua espessura com o aumento do tempo de imersão. Os produtos de corrosão formados sobre a superfície são compostos por Mg,Al, O e S os quais protegem a liga nos períodos inicias de imersão. A liga AZ 91 submetida a um pré-tratamento alcalino apresenta maior eficiência na deposição do MTES devido a um enriquecimento superficial de grupos hidroxilas...
This study analyzed the effect of high-energy milling (HEM) and subsequent isothermal annealing on the microstructural characteristics of a pre-alloyed AZ91 Mg alloy powder. To this end, the mentioned powder was milled for 14 h using a horizontal attritor. Then, the mechanically milled powder was isothermally annealed at temperatures ranging from 200 degrees C to 500 degrees C for I h up to 4 h in an Ar atmosphere. HEM caused the β-Mg₁₇Al₁₂ phase present in the microstructure of AZ91 powder particles to dissolve into their alpha-Mg solid solution matrix. In addition, the crystallite size of the alpha-Mg phase decreased to 25 nm through HEM. In contrast to the annealing time, the temperature of isothermal annealing had a significant effect on the microstructural features of the mechanically milled AZ91 powder.; The authors would like to thank the Comunidad de Madrid for their financial support of this work through the ESTRUMAT Grant S2009/MAT-1585
This study examined the hot deformation behavior and workability characteristics of A291 Mg alloy powder compacts by performing hot compression tests with a Gleeble 3800 machine. To this end, powder compacts with a relative green density of 93% were hot-compressed at temperatures ranging from 150 degrees C to 500 degrees C and at true strain rates ranging from 0.001 s(⁻¹) to 10 s(⁻¹). The true stress-true strain curves peaked at low strains, after which the flow stress increased slightly or remained constant. The work hardening rate decreased with increasing deformation temperature or strain rate. Processing maps were developed for all of the hot compression tests at strains of 0.1, 0.5, and 0.8, which represented a safe deformation domain at deformation temperatures and strain rates in the ranges of 150-300 degrees C and 0.001-0.01 s(⁻¹). Kinetic analysis of the flow stress data for the safe deformation domain yielded an activation energy of 75 kJ/mol which is lower than those previously reported for the hot compression of bulk AZ91 Mg alloy. According to the developed processing maps and the microstructures of the hot-compressed specimens, the optimum hot working window for A291 Mg alloy powder compacts was determined to lie between 275-325 degrees C and 0.001-0.01 s(⁻¹).; The authors would like to thank the Comunidad de Madrid for their financial support of this work through the ESTRUMAT Grant #S2009/MAT-1585.
This study examined the hot deformation behavior and workability characteristics of nanocrystalline AZ91 Mg alloy by performing hot compression tests with a Gleeble-3800 machine. To this end, a nanocrystalline alloy powder with a crystallite size of 25 nm was synthesized via mechanical milling of a pre-alloyed AZ91 Mg alloy powder for 14 h. The mechanically milled (MM) AZ91 powder was subsequently cold pressed at 600 MPa into cylindrical compacts measuring 10 mm in diameter and 12 mm in height. Then, the powder compacts with a relative green density of 91% were hot-compressed at temperatures ranging from 150 ºC to 500 ºC and at true strain rates ranging from 0.001 s⁻¹ to 10 s⁻¹.The true stress-true strain curves peaked at low strains, after which the flow stress increased moderately. Processing maps were developed for all of the hot compression tests at strains of 0.1, 0.5, and 0.8, which represented a safe deformation domain at deformation temperatures and strain rates in the ranges of 250-350 ºC and 0.1-10 s⁻¹. The crystallite size of the nanocrystalline AZ91 Mg alloy hot-compressed within the aforementioned domain was measured to be 140 nm, which is considered very fine for Mg alloys and resulted in a high hardness value of 133 HV for the hot-compressed alloy.; The authors would like to thank the Comunidad de Madrid for their financial support of this work through the ESTRUMAT Grant #S2009/MAT-1585.
Aluminum (Al) and magnesium (Mg) alloys have received considerable attention over the past decades as potential materials of choice to achieve significant weight savings. Although the market for Al and Mg products continues to grow, many opportunities remain untapped because of the low stiffness and strength of Al and Mg alloys in comparison with ferrous alloys and some other structural materials. On the other hand, grain refinement is considered an effective way to increase the strength of metallic materials. In recent years,
high-energy milling has been widely exploited for the production of nanostructured materials, and grain sizes with nanometer dimensions have been observed in almost all high-energy milled pure metals, metallic alloys, and intermetallic compounds. Although the processing
and characterization of nanostructured Al alloys and composites by mechanical alloying has been the subject of many studies, studies on the high-energy milling of Mg alloys are scarce. It should also be considered that to be suitable for structural applications, the nanocrystalline
Al and Mg powders produced by mechanical alloying should then be consolidated to bulk materials. The consolidation route can have significant effects on the microstructure and mechanical properties of the processed bulk product. However...
The objective was to study the mechanical properties of a magnesium alloy welded by a CO2 laser. Residual stresses were measured by X-ray diffraction. They were calculated by the classic sin2Y method in the isotropic zones by using the orientation distribution function (ODF) in the textured zones. The results demonstrated that laser welding results in the formation of a number of different zones with different microstructural and mechanical properties. Welding principally leads to a reduction in grain size and a new distribution of phases. The most remarkable observation was that of a superficial layer on the surface of the welded zone. This layer has the peculiarity of a marked crystallographic texture, a reduction in the level of aluminium as well as an elevated microhardness. These characteristics disappear after a depth of 200μm under the welded zone. These modifications can be explained by the nature of the solidification which occurs under nonequilibrium conditions resulting in an equiaxial columnar transition. This transition is also evident within the profile of residual tensile stresses which are at their maximum at the interface between the superficial layer and the rest of the welded zone. These results are explained by the anisotropic properties of the textured layer in relation to the plasticity.
Magnesium alloys are highly desirable for a wide range of lightweight structural components. However, rolling Mg alloys can be difficult due to their poor plasticity, and the strong texture yielded from rolling often results in poor plate forming ability, which limits their further engineering applications. Here we report a new hard-plate rolling (HPR) route which achieves a large reduction during a single rolling pass. The Mg-9Al-1Zn (AZ91) plates processed by HPR consist of coarse grains of 30–60 μm, exhibiting a typical basal texture, fine grains of 1–5 μm and ultrafine (sub) grains of 200–500 nm, both of the latter two having a weakened texture. More importantly, the HPR was efficient in gaining a simultaneous high strength and uniform ductility, i.e., ~371 MPa and ~23%, respectively. The superior properties should be mainly attributed to the cooperation effect of the multimodal grain structure and weakened texture, where the former facilitates a strong work hardening while the latter promotes the basal slip. The HPR methodology is facile and effective, and can avoid plate cracking that is prone to occur during conventional rolling processes. This strategy is applicable to hard-to-deform materials like Mg alloys...
The influence of Ca and Y on the microstructure and corrosion resistance of
vacuum die casting AZ91 alloy is investigated using optical microscope,
electron scanning microscope, weight-loss test and electrochemical corrosion
test. The results indicate that the microstructure of AZ91 alloy can be
refined, amount of Mg17Al12 phases is reduced, making Mg17Al12 phases transform
from banding to reticular, and stringer Al2Ca phases and block Al2Y phases are
formed through adding both Ca and Y. The corrosion resistance of AZ91 magnesium
alloy can be increased greatly by adding both Ca and Y. The corrosion rate of
AZ91-1.5Ca-1.0Y alloy is dropped to 16.2% of that of AZ91 alloy immersed in
3.5% NaCl aqueous solution for 24 hours. The corrosion current density of
AZ91-1.5Ca-1.0Y alloy is dropped by one order of magnitude.; Comment: 9 pages, 7 figuers, 4 tables