S.F. Santos

Materials selection as an interdisciplinary technical activity: basic methodology and case studies

The technical activity known as Materials Selection is reviewed in its concepts and methodologies. Objectives and strategies are briefly presented and two important features are introduced and discussed; (i) Merit Indices: a combination of materials properties, which maximises the objectives chosen by the designer and (ii) Materials Properties Maps: a bi-dimensional space whose coordinates are pairs of properties in which materials can be plotted and compared directly in terms of their merit indices. A general strategy for the deduction of these indices is explained and a formal methodology to establish a ranking of candidate materials when multiple constraints intervene is presented. Finally, two case studies are discussed in depth, one related to materials substitution in the context of mechanical design and a less conventional case linking material selection to physical comfort in the home furniture industry.

Fracture and resistance-curve behavior in hybrid natural fiber and polypropylene fiber reinforced composites

This article presents the results of a combined experimental and theoretical study of fracture and resistance-curve behavior of hybrid natural fiber- and synthetic polymer fiber-reinforced composites that are being developed for potential applications in affordable housing. Fracture and resistance-curve behavior are studied using single-edge notched bend specimens. The sisal fibers used were examined using atomic force microscopy for fiber bundle structures. The underlying crack/microstructure interactions and fracture mechanisms are elucidated via in situ optical microscopy and ex-situ environmental scanning microscopy techniques. The observed crack bridging mechanisms are modeled using small and large scale bridging concepts. The implications of the results are then discussed for the design of eco-friendly building materials that are reinforced with natural and polypropylene fibers.

Mechanical and Reactive Milling of a TiCrV BCC Solid Solution

Nowadays, many efforts have been concentrated in research and development of hydrogen absorbing materials due to a possible application as electrode for rechargeable batteries, on board hydrogen storage systems, getters, catalysts, etc. Novel technologies for materials processing have been used to generate new alloys with metastable structures, such as amorphous and/or nanocrystalline alloys. In this context, mechanical milling or mechanical alloying is a very attractive way to produce this alloys, specially when carried out under hydrogen atmosphere (reactive milling). In this work, we have studied the structural evolution of a TiCrV bcc solid solution during mechanical and reactive milling. The process parameters analyzed were: milling atmosphere (argon and hydrogen), milling time and gas pressure into the vials (in the case of reactive milling). Structural evolution was investigated by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM). Hydrogen contents of the reactive milled alloys were determined using a Leco analyzer. Differential scanning calorimetry (DSC) was used to determine the hydrogen desorption temperature and the stability of the alloys.

Microstructures and electrode performances of Mg50Ni(50-X)Pdx alloys

AbstractMagnesium and several of its alloys can absorb large amounts of hydrogen. This feature is desirable for several technological applications such as solid state hydrogen storage tanks and anodes of nickel-metal hydride (Ni-MH) batteries. For the latter, Mg-Ni alloys are considered very promising due to their high discharge capacities. Conversely, the low stability of Mg-Ni alloys in alkaline electrolytes have hindered their practical use. In the present manuscript, the effects of palladium black addition on the structure and electrochemical properties of the Mg50Ni50 (in at.%) alloy were investigated. The studied ternary alloys have general composition of Mg50Ni(50-x)Pdx, with 0 ≤ x ≤ 5 (in at.%). These alloys were synthesized by mechanical alloying from pure elements. The alloy powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAS) while their electrode performances were evaluated by galvanostatic cycles of charge and discharge. The investigated alloys have multi-phase structures composed of amorphous and nanocrystalline phases, with nano-grain sizes of nearly 5 nm. Concerning the electrode performance, the best results were attained by the Mg50Ni47.5Pd2.5 alloy, which kept a high discharge capacity and improved the cycling stability.

Two Routes to Process the Mg-5at.%Nb Nanocomposite

In this work, we report the processing of MgH2 + 5at.% Nb nanocomposite by two different routes. In the first one, MgH2 and Nb powders were milled under argon for 20h, using a shaker mill. In the second route, Mg chips and Nb powders were milled under hydrogen atmosphere for 48h, using a centrifugal mill. The phase transformations and microstructural changes were studied by transmission electron microscopy (TEM), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Significant differences in the morphology and phase formation were observed between the two processing routes. The nanocomposite obtained under hydrogen atmosphere showed a slight improvement in the hydrogen desorption properties as compared with the nanocomposite obtained under inert atmosphere. Moreover, we have observed that the addition of Nb improves the kinetics of Mg hydriding.

Selection methodologies of materials and manufacturing processes

The present work discusses some principles of materials selection and examines a few selection criteria and contexts in which this technical activity takes place. Emphasis is given to the concept of Merit Indices, and to the methodology employed for its deduction. The Materials Properties Charts and their integration with the Merit Indices are introduced and illustrated by the former of the two case studies here presented whilst the second associates the methodology of materials selection to process selection.

Effects of Milling pH and Hydrothermal Treatment on Formation of Nanostructured Boehmite Binder for Alumina Extrusion

The in situ formation of nanostructured aluminum hydroxides on the surface of alumina particles, which can work as inorganic binder, was reported in this paper. The effect of the suspension pH during milling of alumina powder and subsequent hydrothermal treatment for the hydroxide formation and microstructure was depicted. Under acidic pH condition, the formation of hydroxides was not observed. When the pH of suspension changed from acidic to basic during milling, bayerite [Al(OH)3] nanoparticles were formed, but only a fraction of this hydroxide was converted to boehmite (AlOOH) during subsequent hydrothermal treatment. The aluminum hydroxide and oxyhydroxide formed in this condition improved the smoothness of extruded rods and the strength of presintered segments. For the powder milled under basic pH condition, the mechanochemically formed bayerite was completely converted into boehmite nanoparticles during the hydrothermal treatment. The presence of boehmite nanoparticles contributed to improving plasticity during extrusion, which allowed the reduction of organic binder and increased the strength of presintered alumina rods.

Microstructural Characterization of As-Quenched and Heat Treated Al-Si-Mg Melt-Spun Ribbons

Al-Si based alloys are the most important and widely used among the family of aluminum cast alloys. With small additions of magnesium these alloys become heat treatable, increasing the mechanical strength with toughness. In this work, the commercial A359 alloy was rapidly quenched by using melt spinning process, using rotating speeds of 22 and 56 m/s. The asmelt spun ribbons were characterized by a combination of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The microhardness of the ribbons was also measured. It was found that the quenched of the ribbon (wheel side) presents a featurless zone and a second zone composed by dendritic-cellular grains. Silicon particles, with nanometer size were found to preferentially precipitate along the grain boundaries. During heat treatment the over-saturated silicon precipitated with morphology spherical. The microstructures changes as well as the increasing of hardness with the increasing of cooling rate might be attributed to the supersaturated solid solutions of Si in Al matrix and to the refinement of the microstructure. Introduction Aluminum-Silicon alloys are widely used in the aluminium foundry due to their excellent castability and high strength-to-weight ratio. Magnesium is frequently added to these alloys to promote an increase of the strength and hardenabilty of the cast part [1]. On the other hand, due to the low solid solubility of Fe, Si and Mg in Al, during conventional casting, precipitates of Al5FeSi, Al8Si6Mg3Fe, Mg2Si and others with more complex structures are formed. However, a substantial increase of solid solubility of those elements in aluminum might be achieved by non-conventional techniques such as the rapid solidification, and consequently to suppress the formation of those precipitates. The objective of the present study is to analyze the influence of rapid solidification process on the microstructure in melt-spun of the Al-9Si-0.7Mg (A359 type) alloy. Experimental Procedure The chemical composition of the Al-9Si-0.7Mg (A359-type) as-cast ingots is shown in Table 1. Approximately 4 g of the molten alloy with 250°C above the liquidus temperature was ejected with an overpressure of 200 mbar onto a rotating Cu-Be wheel. For rotating speed of 56 m/s, the melt-spun ribbon was produced with 2.7 mm in width and 28-34 μm of thickness. On the other hand, for 22 m/s the ribbon was obtained with 3.4 mm in width and 45-55 μm of thickness. The samples have been characterized by using a combination of optic, MO, scanning, SEM, and transmission electron microscopy, TEM, energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and by Vickers microhardness testing. The samples were prepared by standard metallographic procedures. The XRD measurements were carried out in a Siemens D5005 using CuKα radiation. Samples for TEM (M120-Philips with EDS) were thinned by ion milling using dual gun system operating at 5kV with 800 μA per gun. The ribbons were heat treated at three temperatures: 200°C, 350°C and 450°C for 12 hours in air. After each heat treatment, measurements of Vickers hardness were performed with a load of 10gf. Journal of Metastable and Nanocrystalline Materials Online: 2004-08-01 ISSN: 2297-6620, Vol. 22, pp 103-108 doi:10.4028/www.scientific.net/JMNM.22.103

Prospective on the Use of Nanostructured Magnesium Alloys as Anode Materials for Ni–MH Rechargeable Batteries

New developments on portable electronics, electrical vehicles, and hybrid electrical vehicles drive the demand for secondary batteries with optimized performances, such as larger energy densities, specific energies, and power densities. In this scenarios the development of nickel—metal hydride (Ni–MH) rechargeable batteries with improved performances is mandatory to achieve the present needs and also to face the tough competition with other technologies such as Li-ion batteries and low-temperature fuel cells. The production of such improved batteries is closely related to the development of novel hydrogen storage materials which can be successfully achieved through the incorporation of new finds of nanotechnology. In this chapter, some fundamental aspects concerning Ni–MH cells and their component materials are introduced. Recent developments on anode materials, such as novel alloy compositions, non-conventional processing routes, and optimized microstructures are depicted. Furthermore, special emphasis is given for the nanostructured Mg alloys which are promising candidates for this application.

Synthesis of magnetic microtubes decorated with nanowires and cells

Antiferromagnetic and ferrimagnetic microtubes decorated with nanowires have been obtained during thermal oxidation process, which was assisted by in situ electrical resistivity measurements. The synthesis route including heat treatment and electrical current along with growth mechanism are presented. This simple method and the ability to tune in the magnetic moment of the obtained microtubes going from a nonmagnetic-like to a large magnetization saturation open an avenue for interesting applications. In vitro experiments involving adherence, migration, and proliferation of fibroblasts cell culture on the surface of the microtubes indicated the absence of cytotoxicity for this material. We have also calculated both torque and driving magnetic force for these microtubes with nanowires and cells as a function of external magnetic field gradient which were found to be robust opening the possibility for magnetic bio micro-robot device fabrication and application in biotechnology.