Papers by Author: Francisco Manuel Braz Fernandes

Abstract: This study presents the result of the researches regarding the obtaining of NiTi alloy through powder metallurgy (PM) as a possible alternative to present technologies (melting through induction in vacuum—MIV and re-melting with electric arch in vacuum—VAR). The researches made by the authors have aim at the obtaining of Ni-Ti materials with fine grain or ultrafine grain through powder metallurgy techniques, starting from ordinary metallic powders of Ti, Ni, Cu, with grain size less than 100 micrometers, and also using processing through severe plastic deformation (HPT — high pressure torsion). The fabrication through PM has an important advantage because a product requires low processing subsequent considering that it can get with sizes and shape very similar to the final ones, which is not negligible if one takes into account that the alloys Ni-Ti do not excel on cutting processability. Cylindrical samples were produced by cold uniaxial compression, at the specific pressure of 600 MPa, dosed in a proportion of 52.5 % Ni + 43.5 % Ti + 4.0 % Cu, mass composition. The compressed samples, after the sintering in vacuum and severe plastic deformation have been characterized by X-ray diffraction (XRD) , differential scanning calorimetry (DSC) and optical microscopy.

Abstract: The demand of emerging joining techniques for shape memory alloys (SMA) has become of great importance, as their functional properties, namely shape memory effect (SME) and superelasticity (SE) present unique solutions for state-of-the-art applications. Literature shows that significant efforts have been conducted on laser welding of these alloys, although very limited results concerning mechanical properties are repeatedly achieved. A better understanding of the mechanical behaviour of these welded joints may be got through a detailed analysis of the structural characteristics of the material from the base metal to the weld bead. Such studies have been carried out on a series of Ni-rich Ni-Ti SMA laser welded plates using synchrotron radiation.

Abstract: The Nickel-Titanium (Ni-Ti) alloys are the most attractive amongst shape memory alloys (SMA) due to their good functional properties coupled with high strength and good ductility. The transformation temperatures in Ni-Ti SMA can be altered by chemical composition and thermal and/or mechanical treatments adequate to obtain reversible martensitic transformation in one or more steps. The goal of the present work is to investigate the evolution of texture in Ni-Rich Ni-Ti (50.8at%Ni-Ti) SMA showing different phase transformation temperatures as a result of different thermal/mechanical history: straight-annealed (as-received condition) and subsequent thermal treatment at 500°C for 30 minutes in air. The microstructural and textural results were obtained by Electron Backscattering Diffraction on Scanning Electronic Microscopy (ESBD/SEM) and by X Ray Diffraction (XRD) at room temperature. Mechanical properties were measured by Vickers micro hardness tests at room temperature.

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: Shape Memory Alloys (SMA) belong to a special group of metallic materials, which are capable of returning to a pre-determined shape or size when submitted to an appropriate thermal cycle. Generally, these alloys can be easily deformed at a relatively low temperature and, when exposed to a higher temperature, can return to their original shape, that is, to the shape they had before the mechanical deformation was imposed. Although there are a vast variety of materials that can achieve the Shape Memory Alloy effect, only those in which it is possible to have a significant recovery of the deformation – or in which it is possible to generate an important force during the shape's change – can be functionally and commercially interesting. The Ni-Ti alloys, which are one type of SMA, were numerically modelled through Ansys software and these studies are presented in this paper. These special alloys, among many others applications, could be used, for instance, in the form of wires (fibres), in smart composites, as actuators to recover partially the structural integrity of a matrix with cracks. Some 2D and 3D cracked plates were modelled through finite elements and the Stress Intensity Factor, in Mode I, KI, at each crack tip, was determined and compared with the result obtained through the analytical solution. Wires of Ni-Ti, with very small diameters and with different length/diameter ratios, were modelled and mechanical load cycles, at different temperatures, were applied and the material’s behaviour/response was obtained. The thermo-mechanical behaviour of the material was defined based on results published by other authors. Also, a composite material with Ni-Ti fibres embedded was modelled and a mechanical load was applied to it.

Abstract: In shape memory alloys (SMA), the texture can be an interesting factor influencing the anisotropic physical and mechanical characteristics during the phase transformations. It is well known that the texture significantly influences the stress-strain curve and shape memory strain of NiTi SMA. The aim of the present experiment was to analyze the textural modifications in the Ti-rich Ni-Ti SMA after annealing at moderate (500°C for 30 min) and subsequent low level of cold work reduction (10% thickness reduction). The textural results were obtained by X-Ray Diffraction (XRD) during thermal cycling in three points: (i) at room temperature (B19’ phase, after cold work), (ii) at 180°C (B2 phase), and (iii) at room temperature (B19’ phase, after cooling from 180°C). The phase transformations were characterized by Differential Scanning Calorimetry (DSC) and XRD.

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: In the present study, equiatomic powder blends of Ni and Ti were mechanically activated for a short period of time in a planetary ball mill using different levels of energy input. The characterization of the mechanically activated materials was achieved by scanning electron microscopy, X-ray diffraction, differential thermal analysis and chemical analysis (oxygen and nitrogen measurements). During mechanical activation no phase transformation was induced and the high temperature reaction between Ni and Ti elemental powders was shifted to lower temperatures. Moreover, the temperature and the intensity of the exothermic reaction, i.e. the reactivity observed in the powder blends, decreased with the increase in the level of milling energy input. A maximum oxygen content of 0.39 wt% was measured after mechanical activation.

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.