F.M. Braz Fernandes

X-ray tomographic imaging of Al/SiCp functionally graded composites fabricated by centrifugal casting

Abstract The present work refers to an X-ray microtomography experiment aiming at the elucidation of some aspects regarding particle distribution in SiC-particle-reinforced functionally graded aluminium composites. Precursor composites were produced by rheocasting. These were then molten and centrifugally cast to obtain the functionally graded composites. From these, cylindrical samples, around 1 mm in diameter, were extracted, which were then irradiated with a X-ray beam produced at the European Synchrotron Radiation Facility. The 3-D images were obtained in edge-detection mode. A segmentation procedure has been adapted in order to separate the pores and SiC particles from the Al matrix. Preliminary results on the particle and pore distributions are presented.

First Results from the ESCA Microscopy Beamline on ELETTRA

The results of the first experiments with test samples, tool steel and polycrystalline Sn carried out with the scanning photoemission microscope built on the ELETTRA storage ring in Trieste are presented and discussed. The scanning photoemission microscope uses a zone plate optical system for demagnification of the photon beam to submicron dimensions. The present performance of the microscope in photoemission allows elemental and chemical mapping and small-spot photoelectron spectroscopy with lateral resolution better than 200 nm. The reported results show how photoemission microscopy can distinguish the spatial distribution of carbide phases with different Fe content at the surface of a tool steel and the correlation between inhomogeneous oxidation of a polycrystalline Sn sample and its grain structure

Shape memory effect of laser welded NiTi plates

Laser welding is a suitable joining technique for shape memory alloys (SMAs). This paper reports the existence of shape memory effect (SME) on laser welded NiTi joints, subjected to bending tests, and correlates this effect with the microstructural analysis performed with X-ray diffraction (XRD). All welded samples were able to recover their initial shape after bending to 180°, which is a remarkable result for industrial applications of NiTi involving laser welding.

In Situ Characterization of NiTi/Ti6Al4V Joints During Reaction-Assisted Diffusion Bonding Using Ni/Ti Multilayers

Reaction-assisted diffusionxa0bonding process of NiTi and Ti6Al4V was studied in situ. For this purpose, experiments were carried out at the High Energy Materials Science beamline (P-07) at PETRA-III (DESY). Ni/Ti multilayer thin films 2.5xa0μm thick with 12 and 25xa0nm modulation periods were directly deposited by magnetron sputtering onto the materials being joined. The NiTi and Ti6Al4V coated parts were placed with the films facing each other in a dilatometer equipped with Kapton windows for the x-ray beams. Microjoining was promoted by applying a 10xa0MPa pressure and inductively heating the materials, while simultaneously acquiring x-ray diffraction scans across the bond interface. Sound joints were produced at 750xa0°C. The formation of the NiTi2 phase could not be avoided.

Stability of thermal-induced phase transformations in the severely deformed equiatomic Ni–Ti alloys

Nickel–Titanium (Ni–Ti) alloys of two different near-equiatomic chemical compositions (Ni-rich and Ti-rich) are subjected to severe plastic deformation by means of high pressure torsion (HPT) by higher rotation speed and larger total number of rotations. Further, the as-received and severely deformed specimens are subjected to heat treatments at 300 and 350xa0°C. Phase transformations of the specimens under different conditions are analyzed by employing differential scanning calorimetry and by X-ray diffraction. The results obtained show that in Ti-rich Ni–Ti alloy the sequence of phase transformations is found to be stable against heat treatments and independent of previous HPT process. Also, in Ni-rich Ni–Ti alloy, when it is subjected to HPT, the sequence of phase transformations found to remain unaltered. However, with or without HPT, after the heat treatments at 300 and 350xa0°C, the sequence of the phase transformation is found to be affected.

Tungsten inert gas (TIG) welding of Ni-rich NiTi plates: functional behavior

It is often reported that, to successfully join NiTi shape memory alloys, fusion-based processes with reduced thermal affected regions (as in laser welding) are required. This paper describes an experimental study performed on the tungsten inert gas (TIG) welding of 1.5 mm thick plates of Ni-rich NiTi. The functional behavior of the joints was assessed. The superelasticity was analyzed by cycling tests at maximum imposed strains of 4, 8 and 12% and for a total of 600 cycles, without rupture. The superelastic plateau was observed, in the stress–strain curves, 30 MPa below that of the base material. Shape-memory effect was evidenced by bending tests with full recovery of the initial shape of the welded joints. In parallel, uniaxial tensile tests of the joints showed a tensile strength of 700 MPa and an elongation to rupture of 20%. The elongation is the highest reported for fusion-welding of NiTi, including laser welding. These results can be of great interest for the wide-spread inclusion of NiTi in complex shaped components requiring welding, since TIG is not an expensive process and is simple to operate and implement in industrial environments.

Texture Development and Phase Transformation Behavior of Sputtered Ni-Ti Films

It is essential to identify and control the preferential orientation of Ni-Ti shape memory alloy (SMA) films since it is a crucial factor in determining the shape memory behavior. In the present work, in situ studies by synchrotron radiation scattering enabled to identify the different steps of the structural evolution of Ni-Ti films during co-sputtering deposition. For micro-electromechanical systems (MEMS) integration, there is a need for an electrically and thermally insulating or sacrificial layer. Widening the scope of previous experiments concerning the influence of the deposition parameters on the structure of the Ni-Ti films, the incorporation of a TiN buffer layer has been tested. Here, it is established a relationship between the TiN substrates and Ni-Ti texture development (B2 phase). Ni-Ti films mainly containing grains with (110) or (211) planes of the B2 phase parallel to the film surface could be produced using TiN buffer layers with distinct thickness values. The electrical resistivity measurements performed during temperature cycling have shown that the crystallographic orientations of the Ni-Ti films influence their phase transformation characteristics. The resistivity increase during R-phase transformation, especially visible on cooling, is higher for Ni-Ti films with a higher fraction of grains of the B2 phase with (211) parallel to the film surface.

The transformation behaviour of bulk nanostructured NiTi alloys

The phase transformation behaviour of bulk nanostructured NiTi shape memory alloys, produced by an innovative approach called MARESxa0(mechanically activated reactive extrusion synthesis), was investigated using in situ x-ray diffraction and differential scanning calorimetry measurements. For the experimental conditions used, a suitable adjustment of the NiTi matrix composition was achieved after ageing at 500u2009°C for 7xa0h. The aged materials showed a homogeneous dispersion of Ni4Ti3 precipitates embedded in a B2-NiTi matrix. Under this condition the B2-NiTi matrix has undergone a two-stage phase transformation. This was attributed to the complex microstructural evolution during MARES processing, i.e. formation of large-scale and small-scale heterogeneities. Transmission electron microscopy investigations of the solution-treated materials showed the existence of equiaxed nanocrystals in the nanocrystalline NiTi matrix.

In Situ Phase Evolution of Ni/Ti Reactive Multilayers

Joining shape-memory alloys (SMA) to other materials is strongly required in order to enlarge their fields of application. Fusion welding induces strong compositional and microstructural changes that significantly affect the shape-memory effect and the superelastic behavior of these alloys. The exothermic and in some cases self-propagating character of some nano-multilayer reactions is explored in this study as an alternative for joining SMA. To follow these very fast reactions, high brilliance sources, such as synchrotron radiation, are required. In situ high-resolution x-ray diffraction data, giving the phase evolution sequence with temperature of the Ni/Ti multilayer thin films under study, are presented. A correlation between the multilayer design and the tendency for the sequence of phase formation is established.

Role of oxygen and nitrogen in mechanical alloying mechanism of Ni–Ti powder mixtures

Abstract In this study, the critical milling behaviour of Ni–Ti powder mixtures was evaluated in relation to the effect of atmospheric gases, more specifically to oxygen and nitrogen. Within the experimental conditions used, it is shown that both gases play an important role in the alloying process and that not only oxygen gas reacts with the mechanically alloyed powders but also nitrogen. The most effective mixing occurred for the mixtures with the highest contaminant contents.