Papers by Author: Rui Jorge C. Silva

Abstract: The development of ferromagnetic shape memory for practical applications needs to overpass brittleness issues, in addition to the control of the magnetoelastic domains. The Co-Ni-Ga system can provide adequate structural particularities to increase the ductility. This paper reports on structural observations of the martensitic transformation in a Co₂NiGa alloy, in the as-cast and in plastically deformed state. Characterization has been performed before and after the heat treatment, using in-situ X-ray diffraction, optical and electron microscopy, as well as DSC measurements performed on heating and cooling of the samples. The observations show a β + γ two-phase structure that can be further influenced by quenching. The structural contribution on the deformation capacity of the alloys is analyzed, based on the changes in the pattern of transformation. Severe plastic deformation by cold rolling leads to the disappearance of the thermoelastic phase transformation.

Abstract: This study concerns the elemental and microstructural characterization of proto-historic bronze rings from the southwestern Iberian Peninsula. Micro-EDXRF analyses demonstrate that the artifacts are binary bronze alloys (8–13% Sn) with arsenic and lead as the major impurities. Optical microscopy and SEM-EDS allowed the identification of common inclusions (e.g. copper sulphides) and alteration processes (redeposited copper, intergranular and intragranular corrosion). Microstructures consisting of fine dendrites, coarse and/or equiaxial grains were also identified, as well as the presence of (+) eutectoid, deformed inclusions, twinned grains and/or slip bands. The combination of these characteristics allowed establishing the metallurgical procedures (casting, forging and annealing) used in the production of the bronze rings. The identification of different thermomechanical operational sequences indicates that the metallurgical knowledge was well established in the southwestern Iberian Peninsula during those ancient times.

Abstract: Long term corrosion phenomena are generally not obtained in laboratorial corrosion experiments. Particular features, such as strong intergranular corrosion, can be an indication of the antiquity of a metallic artefact. In the present study, various corrosion features from several archaeological bronze artefacts, with ages ranging from 2 to 5 millenniums, are examined using optical and scanning electron microscopies. Elemental composition was obtained through micro-EDXRF analyses. Corrosion patterns could, in some occasions, be related to specific thermomechanical treatments performed before burial and, in others, with particular phases present in the artefact.

Abstract: Twenty brass Chinese cash coins with complex compositions were studied for a better understanding of the metallurgical cash production in China, during the 17th, 18th and 19th centuries. Elemental composition was obtained through energy-dispersive micro X-ray fluorescence spectrometry of small cleaned areas on the coins rims. Results showed that these brass alloys (Cu-Zn) frequently contain up to 3% Sn, have highly variable Pb content (from n.d. up to 14%) and Fe, Sb, and As as minor elements. Microstructures were assessed by optical microscopy, scanning electron microscopy with energy dispersive spectroscopy, and preliminary micro X-ray diffraction analysis. All the coins present typical as-cast microstructures although very fine-grained, which are supported by binary (Cu-Zn) and ternary (Cu-Zn-Sn) equilibrium phase diagrams, that explain microstructural differences due to the presence of Sn in these brasses.

Abstract: The Shape Memory Effect on Ni-Ti thin films is strongly dependent on several factors: (i) chemical composition of the matrix, (ii) presence of precipitates and (iii) preferential orientation. Ni-Ti alloys derive their unique nonlinear and anisotropic mechanical behavior from stress-induced martensitic transformations, where the resulting strains are affected by crystallographic orientation. The influence of the texture on the transformation characteristics of Ni-Ti thin films is discussed on the basis of models and experimental results of the literature. A brief review of the texture build-up on thin films obtained by different fabrication techniques (sputtering, melt spinning, diffusion treatment of ultra-fine laminates, …) is presented. Details about in situ techniques allowing the identification of the preferential orientation during the fabrication process are presented. The processing parameters that more strongly influence the preferential orientation of the Ni-Ti thin films are identified. The mechanisms for the different microstructures are summarized and a special emphasis is put on the type of preferential orientation and its evolution along the processing time.

Abstract: Ni-Ti Shape Memory Alloy thin films are suitable materials for microelectromechanical devices. During the deposition of Ni-Ti thin films on Si substrates, there exist interfacial diffusion and chemical interactions at the interface due to the high temperature processing necessary to crystallize the film. For the present study, Ni-Ti thin films were prepared by magnetron cosputtering from Ni-Ti and Ti targets in a specially designed chamber mounted on the 6-circle goniometer of the ROssendorf BeamLine (ROBL-CRG) at ESRF, Grenoble (France). The objective of this study has been to investigate the interfacial structure resulting from depositions (at a temperature of ≈ 470°C) on different substrates: naturally oxidized Si(100), Si(111) and poly-Si substrates. A detailed High-Resolution TEM analysis of the interfacial structure has been performed. When Ni-Ti is deposited on Si(100) substrate, a considerable diffusion of Ni into the substrate takes place, resulting in the growth of semi-octaeder A-NiSi2 silicide. In the case of Ni-Ti deposited on Si(111), there appears an uniform thickness plate, due to the alignment between substrate orientation and the [111]-growth front. For Ni-Ti deposited on poly-Si, the diffusion is inhomogeneous. Preferential diffusion is found along the columnar grains of poly-Si, which are favourably aligned for Ni diffusion. These results show that for the Ni-Ti/Si system, the morphology of the diffusion interface is strongly dependent on the type of substrates.

Abstract: Ni-Ti SMA are smart materials undergoing first order martensitic transformations driven by temperature and/or stress. In the form of film they are very attractive candidates for microelectro- mechanical system (MEMS) applications. Future directions include the production of functionally graded films by changing deliberately the ratio Ti/Ni across their thickness. However, for the successful development of this type of films, it is important to characterize, model and control the variations in composition, crystalline structure and transformation temperatures. Our approach is in-situ XRD study of the actual growth of the films of varying composition along the thickness carried out using a deposition chamber installed at a synchrotron radiation beamline. These studies were complemented with ex-situ analysis techniques. The results achieved on a Ni-Ti film co-sputtered from Ni-Ti and Ti targets on a TiN buffer layer are presented in this paper. The deposition started by using optimised parameters for a near equiatomic composition. After 1 h (≈330 nm thick film), the Ti power was increased from 20 to 25 W, leading to the precipitation of Ti2Ni. The evolution of the lattice parameter values of the B2 phase, calculated from the corresponding XRD data, is clearly linked with the increase of the Ti power. The depth profile of the atomic concentrations determined by Auger Electron Spectroscopy (AES) is in agreement with the in situ XRD results. The temperature dependence of the electrical resistivity was used to monitor phase transformations, Scanning Electron Microscopy (SEM) has shown the presence of twinned martensite on the film’s surface at room temperature.

Abstract: In the present paper, a selection of micrographs showing some typical microstructures and corrosion layers developed in copper and bronze alloys from different archaeological contexts will be discussed. Metallurgic interpretation of the microstructures observed, based in the proper binary equilibrium phase diagrams is presented. Micro-EDXRF and SEM-EDS analysis were carried out to assess the alloy composition and to contribute to the understanding of the corrosion processes occurred during the long periods of burial of the archaeological metals. Examples of surface decuprification, strong intergranular corrosion, cuprite under green corrosion layers and copper redeposition indicate particular corrosion processes. The presence of copper oxides, sulphides inclusions, lead globules and porosities is also documented.

Abstract: Current technology provides means of fabrication of spherical micro-particles, either hollow or compact, for all engineering materials. Such spherical particles can be further embedded into another material to build-up either random dispersions or close-packed arrays, according to the production route and the degree of anisotropy intended for the ultimate composite material. In this study, a simple analytical formula for the composite stiffness is derived from an early micromechanics model, to describe the actual reinforcement of ductile matrices by a random dispersion of uniform spherical ceramic particles. Predictions from this model are checked against some other relevant models, and specific features arising from its theoretical derivation are pointed out.

Abstract: A sputter deposition chamber inserted into the six-circle Huber diffractometer of the materials research station of the ROssendorf BeamLine (ROBL-CRG) at ESRF allowed to perform in-situ experiments during film growth of Ni-Ti. It is equipped with Kapton windows for X-Ray Diffraction (XRD) and specular Reflectivity (XRR) measurements. By following in situ the evolution of the structure of the growing film, we reveal intermediate “states” which cannot be seen/revealed ex situ, because those states occurred only during the growth but were no longer visible after deposition. Vertical Bragg-Brentano large-angle scattering geometry was employed to study the different trends of structural transformations taking place during deposition. Ni-Ti films exhibiting a non-uniform phase content across the film thickness could be produced by varying the power of co-sputtering Ni-Ti plus Ti. A significant decrease of IB2{110}/IB2{200} was observed when a bias of -45 V was applied.