Fernando Lasagni

Enhanced Young’s Modulus of Al-Si Alloys and Reinforced Matrices by Co-continuous Structures

In the present work, the elastic behavior of different hypoeutectic and hypereutectic Al-Si alloys and different Al2O3 short fiber and SiC particle reinforced materials (SFRM and PRM, respectively) is studied. The effective Young’s modulus (E) of materials was experimentally measured and compared with the different theoretical predictions of Hashin-Strikman, Tuchiniskii, shear lag, and the rule of mixtures (ROM). The unreinforced alloys present an interconnected lamellar Si structure after fast solidification, which increases the Young’s modulus up to that of the Tuchiniskii prediction for interpenetrating skeletal structures. On the other hand, alloys presenting isolated and coarse Si particles (after spheroidization treatment at 540°C) are well described by the lower bound of the ROMs. Similarly, the interconnected Si-SiC structure observed in 10 and 70 vol% SiC reinforced AlSi7Mg and AlSi7 matrices in the as cast condition is responsible for the higher stiffness of the composite, if compared with that of Al99.5 or spheroidized AlSi7 matrices. An analogous behavior is observed in the SFRMs in the as cast condition, where the Si lamellae bridge the Al2O3 fibers, increasing the Young’s modulus of the composites, if compared with the conditions of spheroidized Si. Furthermore, the primary Si particles produce an improvement in the Young’s modulus by connecting several fibers in the case of a short fiber-reinforced hypereutectic AlSi18 matrix.

Influence of Solution Treatment, Sr-Modification and Short Fibre Reinforcement on the Eutectic Morphology of Al-Si Alloys

Abstract An experimental study has been conducted on different unmodified and Sr-modified Al-Si alloys and one short fibre reinforced AlSi7 alloy (SFRM) to evaluate the morphological differences of eutectic Si in the as cast condition and after solution treatments. Al-Si alloys, with 7% and higher content of Si, present a percolating Si-network in the as-cast condition. The addition of strontium produces morphological changes in the network from lamellar Si to a fine dispersed coral fibrous like architecture. Spheroidization and coarsening of the eutectic Si architecture is studied after different exposure times at 540°C. It is observed that the integrity of these networks is very sensitive to solution treatment disintegrating into Si particles decreasing the hardness of the alloys. This effect is avoided by the embedding of alumina fibres into an AlSi7 matrix forming an Al2O3-Si-network. The morphology and rigidity of these isolated structures revealed by deep etching and hardness tests are studied after different solution treatment times.

3D Nano-characterisation of materials by FIB-SEI/EDS tomography

Focus ion beam-tomography is able to characterize materials from > 10 nm. The 3D-volumes are generated from serial FIB cross-sections, which can be imaged by the different modes of a scanning electron microscope (SEM) like secondary electrons (SE), energy-dispersive X-ray spectroscopy analysis (EDS), etc. This article reports on different examples of FIB-tomography in Al-alloys and porous Ni sample, as well as on the different methods for an adequate sectioning and imaging of the serial cross sections.

3D Microstructural Study of AlSi8Mg5 Alloy by FIB-Tomography

Abstract In the present work, a foundry AlSi8Mg5 alloy is investigated by FIB-Tomography in the as-cast condition and after semi-solid heat treatment. The different microstrutural features are characterized quantitatively using morphological shape factors. The 3D-shape of Mg2Si Chinese script as well as Si and impurity Fe containing phases are revealed in high detail. A comparative study with respect to 2D structures is presented as well.

Direct fabrication of surface architectures on polymers and copolymers using laser interference patterning

Surface pattering engineering techniques are essential to fabricate advanced topographies that can be use to modulate cell and tissue response in bio-materials. Particularly, direct laser interference patterning permits fabrication of repetitive arrays and microstructures by irradiation of the sample surface with coherent beams of light. In this study, we explore the possibilities of this technique to produce advanced architectures on several polymeric substrates. The previously calculated interference patterns using the well known interference theory could be directly reproduced on the polymeric surfaces. Moreover, the cross-section of the structured polymers changes depending on the intensity of the laser beams and number of laser pulses, and photomachinability of polymers is highly influenced by laser wavelength. High absorbance of the polymeric materials at specific wavelengths allows the reduction of the laser intensity required to achieve a determined structure depth. In addition, copolymers of methylmetacrylate-styrene were also studied showing that different types of periodic structures could be obtained depending on laser intensity. The obtained results were compared with thermal simulations by finite element methods as well as classical models.

Nano Characterization of Structures by Focused Ion Beam (FIB) Tomography

The 3D design of the micro and nanostructure of materials as well as the defects present in them, considerably determines the properties of modern materials. Therefore, a main focus in the current investigation and development trends is the direct influence of the structure of materials as well as its optimized design in continuously decreasing tolerances. However, such progress is only possible if adequate analysis techniques for the 3D representation of structures are present. The Focused Ion Beam (FIB) Tomography is a rather new technique, which allows the 3D reconstruction of structures in a range of scales and field of view which was previously, for instance through X-ray tomography or tomography in transmission electron microscopy, not available. Moreover, through the use of different signal detectors and the variety of materials that can be analyzed, it turns out to be a very versatile tool. The present chapter presents the working principles of FIB tomography, its different signal detection options, as well as different application examples. The very different examples related to aluminum alloys, multilayer coatings of different materials, oxidized nickel samples, porous materials and nanowires, intend to show the wide range of applications that these techniques can find as well as to awaken the imagination of the reader for thrilling new applications.

Dilatometry revealing Si precipitation in Al–Si-alloys

Abstract The precipitation kinetics of Si after different thermal treatments in AlSi-alloys have been studied by means of isothermal calorimetry, differential scanning calorimetry and dilatometry tests. The measured exothermic and endothermic effects produced by the precipitation and dissolution of Si are correlated with the increase and reduction in the coefficient of thermal expansion CTE(T), respectively. The deviation of the CTE(T) of the studied AlSi1.1 – 1.7 alloys with respect to pure Al correspond to the volume fraction of precipitating Si according to thermodynamic solubility limits. Two different kinetics in the growth of Si precipitates are distinguished: while Si precipitates in the range of 200 – 300 °C during heating of solution quenched samples supersaturated with vacancies, retardation into the temperature range of 300 – 420 °C is observed in samples slowly cooled after solution treatment. The presence of the eutectic Si increases the quench sensitivity of the precipitation kinetics of Si as observed in CTE(T) curves. The ripening of the eutectic Si during prolonged solution treatment retards the Si precipitation slightly.

Sample Preparation of Al-Alloys for 3D FIB Nano-Characterization

Al-Si alloys present a simple binary system with an eutectic composition close to 12.6 wt.% Si. The microstructure of these alloys depends on the Si content as well as on the casting process. Hypoeutectic alloys contain dendritic α grains surrounded by the Si-Al eutectic formed by lamellar Si separated by the α phase [3]. Si solidifies as interconnected Si networks at fast solidification rates (i.e. squeeze casting), Sample Preparation of Al-Alloys for 3D FIB Nano-Characterization