Nuestros investigadores

Jon Alkorta Barragán

Publicaciones científicas más recientes (desde 2010)

Autores: Elizalde, MR; Ocaña, Ibon; Alkorta, Jon; et al.
ISSN 0263-4368  Vol. 72  2018  págs. 39 - 44
Recently, there have been significant efforts to develop micro-mechanical models for a better understanding of in-service performance of WC-Co components. However, reliable information about the mechanical properties of individual features like grains or interfaces is still lacking. In this work, micro-beam testing has been used for analyzing the fracture strength of different WC-WC interfaces in a WC-6.5 wt%Co alloy. The method is based on machining cantilever beams by using Focused Ion Beam so that a single WC-WC interface is isolated from the rest of the microstructure. This machining is carried out in order to have the selected interface at a certain distance for the fixed end and perpendicular to the cantilever axis. CSL2 boundaries and randomly oriented boundaries have been identified by means of EBSD and subsequently tested by nanoindentation until fracture. Load-displacement curves confirm that CSL2 boundaries are stronger than the others and post mortem analyses indicates that the fracture mechanisms are different depending on the orientation between adjacent WC grains. This approach could be used to investigate the intrinsic strength of other interfaces present in hardmetals (i.e. WC/Co, FCC carbides/Co, FCC carbides/WC) and how it is related with processing parameters or in-service conditions.
Autores: Isasti, Nerea; Jorge, Denis; Alkorta, Jon; et al.
Revista: JOM
ISSN 1047-4838  Vol. 68  Nº 1  2016  págs. 215 - 223
High-resolution electron backscattered diffraction (HR-EBSD) is a powerful tool to describe microstructures at the sub-micrometric scale that achieves a higher degree of angular accuracy compared with conventional EBSD. However, such an EBSD technique is time-consuming and requires data-intensive computing to save and postprocess the results obtained after each scan. In the current work, a simple strategy to transform conventional results into high-resolution results is put forward in an averaging statistical layout. This makes it possible to measure the misorientations more precisely and, subsequently, the geometrically necessary dislocations by lowering the typical noise generated from Hough transformation-based conventional EBSD. Different steel microstructures are analyzed in light of this strategy. The calculated dislocation densities for those microstructures are used as input values for evaluating the initial dislocation density contribution to the yield strength in a newly developed mechanical model.
Autores: Santamaría, J. A.; Alkorta, Jon; Gil, Javier;
ISSN 0884-2914  Vol. 30  Nº 17  2015  págs. 2593 - 2604
Highly textured, ultrafine grain pure Bi2Te3 has been obtained by applying large-strain high-pressure torsion (HPT) to hot-pressed (HP) coarse grain material. Its thermal conductivity is significantly smaller than the conductivity of HP Bi2Te3, and its crystallographic texture and mechanical properties significantly improved. The mechanical properties of both, coarse grain and ultrafine grain, samples have been assessed by compression tests of 2 mu m diameter micropillars machined by focused ion beam. The micropillars built from coarse grain samples are single crystalline, those built from ultrafine grain materials are an order of magnitude larger than their grain size. The test results put in evidence the elastic and plastic anisotropy of Bi2Te3 and the significant strengthening and toughening effect of ultrafine grain refining. For instance, after an equivalent strain of about 100, the Vickers hardness (in kg mm(-2)) increases from 60 to 120. Simultaneously, about a 40% reduction of the thermal conductivity has been measured, and a very strong basal texture is developed normal to the torsion axis. Such combination of properties looks very promising for simultaneously enhancing the thermoelectric figure of merit and the mechanical reliability of Bi2Te3-based alloys through HPT processing.
Autores: Remirez de Esparza, N.; Cocera, N.; Vazquez, L.; et al.
ISSN 0002-7820  Vol. 97  Nº 12  2014  págs. 3958 - 3966
The oxidation of highly porous ceramic matrix composites (PCMCs) based on different Tyranno® fibers has been analyzed by means of thermogravimetry and electron microscopy. Both uncoated fibers and PCMC materials exhibit parabolic kinetics between 900°C and 1250°C, these being faster for Ti-doped than for Zr-doped Tyranno fibers. Oxide layers in Ti-doped fibers are porous and partially crystalline, whereas in Zr-doped materials a significant fraction of relatively coarse ß-SiC grains is still found embedded in the amorphous silica matrix. On the other hand, the CVD-SiC coatings exhibit higher oxidation rates from the outer surface than from the inner one, a phenomenon that has been associated not only with the more difficult access of oxygen to the inner face but also with the highly <111> textured structure of these coatings, for which very different oxidation rates have been published for the inward and outward directions. Cracking phenomena observed above 1100°C for long dwelling times do not lead to an acceleration of the oxidation process, which could be due to the simultaneous crystallization of the amorphous silica layers
Autores: Alkorta, Jon; García-Rosales, Carmen; et al.
ISSN 0267-0836  Vol. 30  Nº 1  2014  págs. 91 - 95
The structure and crystallographic texture of zinc strips (Zn-Cu-Ti alloy) produced by the continuous horizontal twin roll strip casting method has been characterized. In longitudinal sections normal to the transverse direction, the strips display an approximately symmetrical chevron patterned structure of columnar grains inclined about 30 degrees from the rolling direction. In association with such structure, the macroscopic texture is mainly < 1 (1) over bar 00 > 'normal' (not cyclic) fibre texture tilted approximately +/-30 degrees around the transverse direction plus a similarly tilted weak < 0001 > fibre texture. A thin layer of small equiaxed grains with a strong (0001) basal texture is present at the free surfaces. The observed structure/texture combination agrees quite well with the expected macrostructure of solidification of the alloy in the twin roll casting process.
Autores: Alkorta, Jon; Martínez, José Manuel; et al.
ISSN 0927-0256  Vol. 82  2014  págs. 314 - 319
This work presents a finite element analysis of the indentation size effect (ISE) experimentally observed in tests performed at submicron scale. A 3D model of a conical rigid surface indenting on a Nb single crystal at different depths has been developed. The bcc Nb material has been characterized within a finite-strain framework through a crystal plasticity model incorporating strain-gradient hardening. The hardness evolution for different material orientations and for different initial dislocation densities has been studied. The numerical results are compared with predictions of existing analytical models and with experimental results. (C) 2013 Elsevier B.V. All rights reserved.
Autores: Alkorta, Jon;
ISSN 0304-3991  Vol. 131  2013  págs. 33 - 38
High resolution electron backscattered diffraction (HREBSD) is a novel technique for a relative determination of both orientation and stress state in crystals through digital image correlation techniques. Recent works have tried to use simulated EBSD patterns as reference patterns to achieve the absolute orientation and stress state of crystals. However, a precise calibration of the pattern centre location is needed to avoid the occurrence of phantom stresses. A careful analysis of the projective transformation involved in the formation of EBSD patterns has permitted to understand these phantom stresses. This geometrical analysis has been confirmed by numerical simulations. The results indicate that certain combinations of crystal strain states and sample locations (pattern centre locations) lead to virtually identical EBSD patterns. This ambiguity makes the problem of solving the absolute stress state of a crystal unfeasible in a single-detector configuration. (C) 2013 Elsevier By. All rights reserved.
Autores: Isasa, M.; Pérez, N; Tavera, T.; et al.
ISSN 0040-6090  Vol. 548  2013  págs. 69 - 74
Micrometric periodical gold/silver alloy linear patterns have been prepared by thermal annealing of bilayer thin films (gold/silver) by means of laser interference metallurgy. These alloyed lines alternate with non-alloyed gold/silver thin films. A chemical attack with a nitric acid solution produces the dealloying of the annealed patterns (by preferential removal of the less noble material), and thus, the linear patterns acquire a nanoporous structure. These nanostructured lines alternate with gold thin films produced after the elimination of the silver thin film in the non-alloyed areas. (C) 2013 Elsevier B.V. All rights reserved.
Autores: Santamaria, J.A.; Alkorta, Jon; Gil, Javier;
ISSN 0366-3175  Vol. 52  Nº 3  2013  págs. 137 - 142
Bismuth telluride, Bi2Te3, is the main thermoelectric material currently in use for commercial cooling devices or for energy harvesting near room temperature. Because of its highly anisotropic layered structure, Bi2Te3 is very brittle, failing by cleavage along its basal plane. Refining its grain size is expected to increase its toughness with the advantage that, simultaneously, its thermoelectric "figure of merit" results increased. In this work, powders of the compound have been compacted by conventional methods as well as by severe plastic deformation under high pressure (3 GPa) using high pressure torsion (HPT, one turn at room temperature). Near-theoretical density has been achieved. The hardness and toughness of the compacts have been assessed by micro and nano-indentation.
Autores: Alkorta, Jon; Gil, Javier;
ISSN 0884-2914  Vol. 27  Nº 1  2012  págs. 45-52
The elastic anisotropy of cementite (Fe3C) is still under discussion. Recent theoretical (ab initio) calculations predict a very high elastic anisotropy for this iron carbide, and a few published experiments suggest that prediction could be true. This work presents a first attempt of using nanoindentation for assessing the elastic anisotropy of such an important component of steels. Our nanoindentation results show that the elastic anisotropy of Fe3C is high but smaller than predicted by ab initio calculations. The elastic modulus is obtained from the load-penetration curves before the first pop-in indicative of plasticity nucleation is detected. The tests thus provide information on the plastic anisotropy of cementite. Surprisingly, the mean indentation pressure or the maximum shear stress under the indenter at the onset of plasticity has been observed to be nearly independent of the crystalline orientation of the indented surface.
Autores: Alkorta, Jon; Martínez, José Manuel; Gil, Javier;
ISSN 1478-6435  Vol. 91  Nº 42620  2011  págs. 1400 - 1408
Unloading stiffness is a critical magnitude when extracting elastic modulus in instrumented indentation. Any phenomenon which interacts with its measurement may affect the final calculation of the modulus. Analytical and numerical calculations have been carried out to determine the influence of thermal drift and creep response on its measurement, and the predictions were in good agreement with experimental results. Since the influence of thermal drift is depth-dependent, it determines the effective resolution of an indentation device for a given material. In contrast, indentation creep significantly alters unloading stiffness even for weakly rate-sensitive materials (sensitivity exponent, m < 0.05) but its effect could be smoothed down due to measurement artefacts (unloading curve fitting strategy). For instance, for an ultra-fine grained (UFG) pure niobium at room temperature (m similar to 0.015 and H/E(r) similar to 0.02), the error in the measurement of elastic modulus with a typical nanoindentation procedure (5 s of holding time and 65 s of unloading time) can be as high as 15%. This paper proposes simple rules for a reliable experimental procedure to avoid both thermal drift and creep effects on the measurement of elastic modulus, which are especially relevant for the new generation of high temperature instrumented indentation facilities.
Autores: Schnell, M.; Garcia-Etxarri, A.; Alkorta, Jon; et al.
ISSN 1530-6984  Vol. 10  Nº 9  2010  págs. 3524 - 3528
We demonstrate that the local near-field vector and polarization state, On planar antenna structures and in nanoscale antenna gaps can be determined by scattering-type near-field optical microscopy (s-SNOM). The near-field vector is reconstructed from the amplitude and phase images of the in- and our-of-plane near-field components obtained by polarization-resolved interferometric detection. Experiments with a mid-infrared inverse bowtie antenna yield a vectorial near-field distribution with unprecedented resolution of about 10 nm and in excellent agreement with numerical simulations. Furthermore, we provide first direct experimental evidence that the nanoscale confined and strongly enhanced fields at the antenna gap are linearly polarized. s-SNOM vector-field mapping paves the way to a full near-field characterization of nanophotonic structures in the broad spectral range between visible and terahertz frequencies, which is essential for future development and quality control of metamaterials, optical sensors, and waveguides.