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Neutron investigation of Nitinol stents and massive samples before and after PIRAC coating
Rogante, M. ; Buhagiar, J. ; Cassar, G. ; Debono, M. ; Lebedev, V. ; Mikula, Pavol ; Ryukhtin, Vasil
Nitinol, a thermoelastic Ni-Ti Shape Memory Alloy (SMA) with approximately 50 at. % Ti, is adopted in a wide range of medical equipment and devices used in interventional radiology, orthopaedics, neurology and cardiology, in particular as a smart material for stents. In this work, NiTi real stents and massive samples before and after different Powder Immersion Reaction Assisted Coating (PIRAC) treatments have been investigated by using two neutron techniques: (1) Small and Ultra-Small Angle Neutron Scattering (SANS, USANS) for nano- and micro-scale characterization, obtaining information on structure and the effects due to the coating treatment, and (2) High-Resolution Neutron Diffraction (HRND), evaluating the macrostrain components resulting from angular shifts of diffraction peaks and the micro-strains in the plastically deformation region by means of profile-broadening analysis. The obtained results contribute: improving knowledge of defects and other key features of the materials complementary to those achieved by using traditional examination techniques. helping to better understand the functional characteristics of Nitinol parts and predict the material's mechanical behaviour.
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On a possible use of neutron three axis diffractometer for studies of elastic and plastic deformation of polycrystalline materials
Mikula, Pavol ; Ryukhtin, Vasil ; Rogante, M.
Feasibility of using a high-resolution three axis neutron diffractometer performance for elastic and plastic deformation studies of metallic polycrystalline samples is presented. The method consists of unconventional set up employing bent perfect crystal (BPC) monochromator and analyzer with a polycrystalline sample in between. After the realization of focusing conditions in real and momentum space at the neutron wavelength of 0.162 nm, a high angular resolution up to FWHM(d/d)=2x10-3 was achieved on the standard Fe(110) sample (2 mm diameter) which then opened the possibility for the measurements of small lattice parameter changes of samples. The feasibility of the instrument for macro-and microstrain as well as grain size studies is demonstrated on the polycrystalline samples of low carbon shear deformed steel wires and the NiTi plates subjected to heat treatment.
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Residual stress determination by neutron diffraction in low-carbon steel wires with accumulated shear deformation
Rogante, M. ; Mikula, Pavol ; Strunz, Pavel ; Zavdoveev, A.
Modern methods of severe plastic deformation (SPD) currently allow obtaining the ultrafine-grained (UFG) structure nearly in any material. In the manufacturing process of wire with UFG structure, a main restriction is the continuous drawing scheme in which this process occurs, thus it is important to consider the factors affecting the drawing efficiency. Despite the data on SPD steels such as strength, plasticity and conductivity, obtained by classical methodologies, further investigations are needed: a key information is the residual stresses (RS) status, and RS determination is an essential issue to improve knowledge of SPD effects. In this work, 15 wires samples made of low-alloyed quality structural steel Grade 08G2S GOST 1050 with accumulated shear deformation - as result of rolling with shear of the metal ingot and conventional wire drawing - have been investigated by neutron diffraction (ND). Results provide substantial data helping to evaluate the effect of shear deformation on RS of the considered steel, as well as additional support to complement the information already achieved by using the other characterization methodologies. Knowledge of the RS status can help developing a lowcarbon wire drawing technology with needed manufacturability and efficiency, playing a decisive role in the debugging of material selection and engineering design requirements.
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Residual Stresses Assessment in Coated Materials: Complementarity between Neutron and X-Ray Techniques
Rogante, M. ; Mikula, Pavol ; Vrána, Miroslav
The residual stress (RS) status induced in the substrate of coated materials by the coating process plays frequently a major role in lending the component’s characteristics. RS assessment can give, thus, a substantial contribution in justifying different cases of failure or else bad performance of coated components due to e.g. the coating delamination or to other occurrences which are not simply interpretable via the conventional mechanical tests or microstructure analyses. The adoption of both neutron diffraction (ND) and X-ray diffraction (XRD) techniques has revealed its usefulness in assessing the RS values in proximity of the coating interface area, respectively, without any layer removal or hole drilling at the extreme surface.
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