1,721,050 research outputs found
Laser shape setting of superelastic nitinol wires: Functional properties and microstructure
No full textShape setting is one of the most important steps in the production route of Nitinol Shape Memory Alloys (SMAs), as it can fix the functional properties, such as the shape memory effect and the superelasticity (SE). The conventional method for making the shape setting is performed at 400-500°C in furnaces. In this work, a laser beam was adopted for performing straight shape setting on commercially available austenitic Nitinol thin wires. The laser beam, at different power levels, was moved along the wire length for inducing the functional performances. Calorimetric, pseudo-elastic and microstructural features of the laser annealed wires were studied through differential scanning calorimetry, tensile testing and high energy X-ray diffraction, respectively. It can be stated that the laser technology can induce SE in thin Nitinol wires: the wire performances can be modulated in function of the laser power and improved functional properties can be obtained
CuZr Based Shape Memory Alloys: Effect of Co on the Martensitic Transformation and the Microstructure
No full textIn this work we report on the alloying of Co, in the place of both Cu and Zr, in CuZr high temperature shape memory alloy HTSMA. Characteristic temperatures of the martensitic transformation and thermal cycling stability of CuZr alloys with different Co contents were detected by calorimetric measurements. The microstructure evolution was investigated by scanning electronic microscopy
On the microstructure and superelastic evolution of laser annealed thin NiTinol wires
No full textShape setting is a crucial step of the production route of shape memory alloys (SMAs) elements for fixing their functional properties. For thin SMA wires, this peculiar setting can be performed by a laser beam scanning the wire length. In the present work the correlation among the functional properties, such as stress-strain curves, the microstructural properties and the laser power was studied on 100 μm pseudoelastic NiTi wire. A comparison between the performances of the laser treated and the commercial wires was discussed. It can be stated that the wire responses can be modulated as a function of the laser power; the optimal laser condition can induce functional properties at least comparable to the ones of the wire conventionally treated in a furnace. Laser induced superelasticity was obtained at room temperature and the corresponding microstructure suggests a texture effect associated with the directional and fast heating induced by the laser beam scan. A favorable condition for extended stress induced plateau lengths, compared to the one of the commercially available furnace treated wires, was induced by the laser scan
Morphological characterization of shape-controlled TiO2 anatase through XRPD analysis
No full textPreferential growth of anatase crystallites along different directions is known to deeply affect their photocatalytic properties, especially with respect to the exposure of the reactive {001} facets. Its extent can be easily quantified through simple geometric calculations, on the basis of crystal sizes extracted for specific directions by means of X-Ray Diffraction data analysis. Nevertheless, the actual results depend on themethod employed for such a quantification. Here we report on a comparative morphological investigation, performed by employing the Scherrer equation and the line profile from Rietveld refinements, on shapecontrolled anatase photocatalysts produced by employing HF as capping agent. Compared to the Rietveld-based method, the use of the Scherrer equation produces a systematic underestimation of crystallite dimensions, especially concerning the [100] direction, which in turn causes the percentage of exposed {001} crystal facets to be underestimated. Neglecting instrumental-related effects may further reduce the estimate
Relaxor ferroeletric behavior in S r1-x P rxTi O3: Cooperation between polar and antiferrodistortive instabilities
No full textChemical doping at the Sr and Ti sites is a feasible way to alter the quantum paraelectric state of SrTiO 3 perovskite. Doping with Pr is known to induce relaxor ferroelectricity at room temperature in the Sr1-xPrxTiO 3 solid solution. The relationship between its dielectric properties and structural phase transition has been debated, but no definitive structural argument has been proposed. Here we present a systematic structural study of Sr1-xPrxTiO 3 (0.020≤x≤0.150). We establish the structural phase diagram using high-resolution x-ray powder diffraction by finding the antiferrodistortive structural phase transitions for all the compositions studied. By using pair distribution function analysis, we show the mismatch between local and long-range structures in terms of increased local order parameters. Finally, we propose a correlation between the local structural order parameters and the emergence of hard polar modes as found by Raman spectroscopy. Our results are quantitatively consistent with recent theoretical calculations showing that the increase of local tetragonality and local octahedral tilting above a critical value in fact underlie the polar instability. This confirms that structural orders involving both polar and antiferrodistortive characters compete and cooperate at different levels, promoting ferroelectricity in Sr1-xPrxTiO 3
Structural characterization of Tb- and Pr-doped ceria
No full textThe crystallographic structure of Tb- and Pr-doped ceria is investigated through X-ray and neutron powder diffraction, combining pair distribution function and UV-vis spectroscopy. Compared to trivalent dopants, Tb and Pr show peculiar optical and crystallographic properties: whilst Tb have a mixed +3/+4 valence state, Pr is stable mostly in the +4 valence state up to ~50% dopant concentration. For larger Pr amounts, doping promotes a fluorite to monoclinic phase transformation. A straightforward method for extracting the dopant oxidation state by the cell parameter dependence on the ionic radius is also proposed
Defect structure of Y-doped ceria on different length scales
No full textAn exhaustive structural investigation of a Y-doped ceria (Ce 1-xYxO2-x/2) system over different length scales was performed by combining Rietveld and Pair Distribution Function analyses of X-ray and neutron powder diffraction data. For low doping amounts, which are the most interesting for application, the local structure of Y-doped ceria can be envisaged as a set of distorted CeO2- and Y 2O3-like droplets. By considering interatomic distances on a larger scale, the above droplets average out into domains resembling the crystallographic structure of Y2O3. The increasing spread and amount of the domains with doping forces them to interact with each other, leading to the formation of antiphase boundaries. Single phase systems are observed at the average ensemble level. © 2013 American Chemical Society
Microstructure and mechanical behavior of hot-work tool steels processed by Selective Laser Melting
No full textThe present study is aimed at identifying and testing high-strength alloys for tooling applications featuring suitable processability for laser-based additive manufacturing technologies. The microstructure and mechanical properties of the H11 hot-work tool steel and a leaner version of the same alloy (L-H11) processed by Selective Laser Melting were assessed as a function of specific microstructural conditions obtained by performing different heat treatments. Tempering was performed on quenched alloys or simply from as built material. The rapidly solidified microstructures revealed able to respond directly to precipitation hardening treatment without performing any prior solution annealing. The microstructure of the as-built alloys revealed characterized by α-Fe dendritic cells decorated at boundaries by C-rich γ-Fe regions. Air quenching was responsible for the transformation of the solidification cells into lath martensitic structures and for the formation of the M3C phase, which transformed into more complex carbide species on tempering. The hardness of quenched and tempered H11 steel is similar to that obtained by processing the alloy with conventional routes, and the final hardness gap between the two SLM processed H11 and L-H11 alloys treated according to optimal tempered condition was limited to 62 HV
Development of a high strength Al–Zn–Si–Mg–Cu alloy for selective laser melting
Full text linkDespite additive manufacturing processes are already widely used in several industrial applications, there are few materials that are specifically designed and optimized for these technologies. Currently, only few Al alloys are available on the market and employed for 3D printing of structural parts. In particular, Si-Mg bearing alloys are the most common Al alloys for additive manufacturing, featuring high processability but moderate mechanical properties. By this work, we studied the effect of Si addition on the hot cracking susceptibility of a high strength Al-Zn-Mg-Cu alloy. A preliminary activity has been carried out by blending Al-Zn-Mg-Cu and Al-Si-Mg powders and analysing their microstructure and properties achieved after selective laser melting. Eventually a new Al-Zn-Si-Mg-Cu alloy has been designed, produced as powder alloy by gas atomization and tested. The microstructure and phase transformations of the new alloy has been investigated by synchrotron X-ray diffraction, differential scanning calorimetry and microscope analysis. The Al-Zn-Si-Mg-Cu alloy processed by selective laser melting featured a relative density of 99.8%, no hot cracks were noticed within the investigated microstructures. The ability of the new alloy to respond to aging starting from both as built and solution annealed conditions has been also evaluated. A good response to direct aging (directly from as built condition) was demonstrated, featuring yield strength and ultimate tensile strength of 402 and 449 MPa, respectively, and hardness of 174 HV after optimized aging at 165 degrees C for 2 h. (C) 2019 Elsevier B.V. All rights reserved
The Ba3Mo1-: X Wx NbO8.5ion conductors: Insights into local coordination from X-ray and neutron total scattering
No full textIn the last few years the Ba3Mo1-xWxNbO8.5 system has attracted increasing attention because of its potential application as an electrolyte in solid state fuel cells. The interest in the system arises also from the peculiar migration mechanism, which appears to involve hopping across partially occupied tetrahedral and octahedral sites. However, the reliability of the mechanism is under debate, as a recent single crystal study demonstrated a flaw in the structural model accepted previously, thus hampering the validity of the tetrahedral/octahedral migration mechanism. In order to verify the model, we revised the synchrotron and neutron diffraction data accounting for the correct structure. Neutron powder diffraction data revealed a clear preferential occupation of octahedral sites for the W end member, while synchrotron diffraction confirmed the increase of tetrahedral occupation at high temperature. In addition, we used the pair distribution function (PDF) to resolve the coordination environment based on the characteristic interatomic distances. The neutron PDF provided unambiguous evidence of octahedral sites in W-rich specimens, whose population decreases at high temperature. High resolution synchrotron data revealed a discontinuity with temperature, suggesting a reconstructive phase transition for all the investigated compositions. Synchrotron PDFs suggested structural inhomogeneities at the local scale. The increase of temperature induces a redistribution of O ions at the nm length scale, leading to a disordered structure more favorable for ion migration. This journal i
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