1,558 research outputs found
Twin-induced plasticity of an ECAP-processed TWIP steel
The TWIP steels show high strain hardening rates with high ductility which results in high ultimate tensile strength. This makes their processing by equal channel angular pressing very difficult. Up to now, this has only been achieved at warm temperatures (above 200 °C). In this paper, a FeMnCAl TWIP steel has been processed at room temperature and the resulted microstructure and mechanical properties were investigated. For comparison, the material has also been processed at 300 °C. The TWIP steel processed at room temperature shows a large increase in yield strength (from 590 in the annealed condition to 1295 MPa) and the ultimate tensile strength (1440 MPa) as a consequence of a sharp decrease in grain size and the presence within the grains of a high density of mechanical twins and subgrains. This dense microstructure results also in a loss of strain hardening and a reduction in ductility. The material processed at 300 °C is more able to accommodate deformation and has lower reduction in grain size although there is a significant presence of mechanical twins and subgrains produced by dislocation activity. This material reaches an ultimate tensile strength of 1400 MPa with better ductility than the room temperature material.Postprint (published version
Stress-strain response and microstructural evolution of a FeMnCAl TWIP steel during tension-compression tests
© 2016.
The stress-strain response of a Fe-17.5Mn-0.7C-2Al TWIP steel during cyclic loading has been investigated by means of tension-compression tests within the strain limits of ±2%, ±5% and ±10%. In addition, the microstructural evolution during the ±5% cyclic test has also been studied. The difference between the forward and reverse stress for each pre-strain has been analyzed at 0.2% offset strain and at the strains in which forward and reverse curves were parallel in order to study the Bauschinger effect (BE) and permanent softening, respectively. The evolution of the BE with pre-strain for this steel is similar to other FeMnC TWIP steels, that is, increasing values of BE are obtained as the pre-strain increases. However, its absolute values are half those reported in the literature on other FeMnC steels. This diminution of the BE is related to the lower activity of mechanical twinning in FeMnCAl TWIP steels at the pre-strains herein investigated, which promotes less polarized stresses in the matrix due to the lower dislocation storage capacity.Regarding permanent softening, the evolution is similar to that of the BE and the same analysis can be applied. During reverse compression, a slight increase of twin thickness and twin spacing with respect to the first tensile stage took place. This fact might be linked to the lower flow stress observed in the permanent softening period during reverse straining.Peer ReviewedPostprint (author's final draft
Study of the nanometric grain size distribution in iron compacts obtained by mechanical milling
Postprint (published version
3D printing of Inconel 718 by MEAM (Metal Extrusion Additive Manufacturing)
In this study the production of 3D-printed pieces made from Inconel 718 using Metal Extrusion Additive Manufacturing of highly-filled polymers has been investigated. The printer used was a Prusa I3 MK3 printer modified with a gravity fed-pellets screw extruder. The feedstock was pellets composed of metal powder and polymeric binders. Different types of geometries and sizes were printed during the study with the aim to determine both the limits of the technique and the final mechanical properties of the pieces after the whole process (Fig 1). After printing, debinding, and sintering some of the samples were cut and observed by SEM to determine the presence of remaining porosity inside the pieces. The samples for tensile tests were subjected to a final precipitation heat-treatment to check the mechanical properties. As a general conclusion of the study, better results were obtained in small pieces with a small thickness. As thickness increases porosity and the amount of large defects created during the printing or debinding steps rises. On the other hand, pieces with regular geometries show less warping. This warping comes from the large shrinkage experimented by the samples during sintering. The hardness and yield strength of the tensile samples are within the expected range for this material, but ductility is easily affected by the presence of defects
Unitary evolution and uniqueness of the Fock representation of Dirac fields in cosmological spacetimes
We present a privileged Fock quantization of a massive Dirac field in a
closed Friedmann-Robertson-Walker cosmology, partially selected by the criteria
of invariance of the vacuum under the symmetries of the field equations, and
unitary implementation of the dynamics. When quantizing free scalar fields in
homogeneous and isotropic spacetimes with compact spatial sections, these
criteria have been shown to pick out a unique Fock representation (up to
unitary equivalence). Here, we employ the same criteria for fermion fields and
explore whether that uniqueness result can be extended to the case of the Fock
quantization of fermions. For the massive Dirac field, we start by introducing
a specific choice of the complex structure that determines the Fock
representation. Such structure is invariant under the symmetries of the
equations of motion. We then prove that the corresponding representation of the
canonical anticommutation relations admits a unitary implementation of the
dynamics. Moreover, we construct a rather general class of representations that
satisfy the above criteria, and we demonstrate that they are all unitarily
equivalent to our previous choice. The complex structures in this class are
restricted only by certain conditions on their asymptotic behavior for modes in
the ultraviolet sector of the Dirac operator. We finally show that, if one
assumes that these asymptotic conditions are in fact trivial once our criteria
are fulfilled, then the time-dependent scaling in the definition of the
fermionic annihilation and creation-like variables is essentially unique.Comment: 24 page
Uniqueness of the Fock quantization of Dirac fields in 2+1 dimensions
We study the Fock quantization of a free Dirac field in 2+1-dimensional
backgrounds which are conformally ultrastatic, with a time-dependent conformal
factor. As it is typical for field theories, there is an infinite ambiguity in
the Fock representation of the canonical anticommutation relations. Different
choices may lead to unitarily inequivalent theories that describe different
physics. To remove this ambiguity one usually requires that the vacuum be
invariant under the unitary transformations that implement the symmetries of
the equations of motion. However, in non-stationary backgrounds, where time
translation is not a symmetry transformation, the requirement of vacuum
invariance is in general not enough to fix completely the Fock representation.
We show that this problem is overcome in the considered scenario by demanding,
in addition, a unitarily implementable quantum dynamics. The combined
imposition of these conditions selects a unique family of equivalent Fock
representations. Moreover, one also obtains an essentially unique splitting of
the time variation of the Dirac field into an explicit dependence on the
background scale factor and a quantum evolution of the corresponding creation
and annihilation operators.Comment: 24 pages. Document replaced to match published versio
El acuerdo para la mejora del crecimiento y el empleo en la perspectiva económica
El análisis económico del último gran pacto social en materia de mercado de trabajo, observado desde la perspectiva su incardinación en el contexto de otros acuerdos y reformas precedentes, asà como de las reformas que aún pueden considerarse pendientes, es el objetivo principal de este trabajo. En ese sentido, quizás el primer dato a destacar es el del papel de la concertación social, en términos de estabilidad polÃtica y social, en cuanto que torna en elemento preciso para afianzar una dinámica de crecimiento continuado de la formación bruta de capital, vÃa atracción de inversiones externas o vÃa estimulo al ahorro interno. Desde un punto de vista objetivo o material, la temporalidad —la lucha contra el exceso peso de la misma—, que ha sido la transformación más importante en las relaciones laborales en los últimos 20 años del siglo pasado y que caracterizó la evolución del mercado de trabajo español, se constituye en el eje vertebrador de la reforma, más que la propia creación de empleo. Pero también cabe resaltar, pese a que no se trata de una reforma integral, que cabe esperar que efectos del nuevo marco regulatorio se extiendan a su vez sobre la dinámica de la productividad, en la medida en que el encarecimiento relativo en el uso del trabajo temporal lleve a una búsqueda de recortes de los cotes laborales a través de mejoras en el rendimiento del trabajo. De obtenerse tales resultados, cabrÃa hablar de un éxito importante en la aplicación del acuerdo. Como punto de conclusión se ha de señalar en todo caso que todavÃa es pronto para poder hacer valoraciones sobre las consecuencias que la puesta en marcha del acuerdo para la mejora del crecimiento y el empleo. Hacer previsiones sobre su evolución futura es deslizarse por un terreno resbaladizo. El fuerte crecimiento del empleo que se ha puesto de manifiesto en la última década ha estado enmarcado en un entorno general de estabilidad económica de reducción de inflación y de tipos de interés en relación a la puesta en marcha del euro como moneda única. Las circunstancias del entorno ha cambiado en los últimos años y el fenómeno más relevante que se está produciendo en la economÃa y sociedad española de la última década es el creciente fenómeno inmigratorio. Los efectos de la incorporación de estos trabajadores a las nuevas condiciones del mercado laboral repercutirán en los resultados del acuerdo que aquà hemos recogido
Uniqueness of the Fock quantization of scalar fields in a Bianchi I cosmology with unitary dynamics
The Fock quantization of free scalar fields is subject to an infinite
ambiguity when it comes to choosing a set of annihilation and creation
operators, choice that is equivalent to the determination of a vacuum state. In
highly symmetric situations, this ambiguity can be removed by asking vacuum
invariance under the symmetries of the system. Similarly, in stationary
backgrounds, one can demand time-translation invariance plus positivity of the
energy. However, in more general situations, additional criteria are needed.
For the case of free (test) fields minimally coupled to a homogeneous and
isotropic cosmology, it has been proven that the ambiguity is resolved by
introducing the criterion of unitary implementability of the quantum dynamics,
as an endomorphism in Fock space. This condition determines a specific
separation of the time dependence of the field, so that this splits into a very
precise background dependence and a genuine quantum evolution. Furthermore,
together with the condition of vacuum invariance under the spatial Killing
symmetries, unitarity of the dynamics selects a unique Fock representation for
the canonical commutation relations, up to unitary equivalence. In this work,
we generalize these results to anisotropic spacetimes with shear, which are
therefore not conformally symmetric, by considering the case of a free scalar
field in a Bianchi I cosmology.Comment: 23 pages. Updated to match published versio
Desaceleración económica y crisis. El mercado de trabajo en el primer trimestre de 2008
En el primer trimestre de 2008 la economÃa española parece haber entrado ya claramente en una nueva fase del ciclo económico. Las previsiones de crecimiento económico se han reducido drásticamente respecto a las de trimestres anteriores. La desaceleración económica se traduce en el comportamiento de las principales variables laborales. El desempleo crece en España y en AndalucÃa y el crecimiento de la ocupación se ha detenido. La crisis es especialmente grave en el caso del sector de la construcción. La tasa de paro en AndalucÃa ha alcanzado el 14,83%.Las perspectivas para los próximos trimestres no son muy optimistas
Studying the Hall-Petch effect regarding sub-micrometer steel (0.6% C)
Este trabajo describe la obtención y caracterización mecánica
de una aleación de acero 0,6% C con estructura de
tamaño de grano inferior a 1 μm. El proceso para la obtención
de piezas masivas se inicia sometiendo el polvo a
severa deformación plástica en un molino planetario de
bolas y a continuación se realiza la consolidación a alta
presión y temperaturas entre 350 y 500 °C. El estudio de
la evolución del tamaño de grano muestra que los consolidados
sin tratamiento térmico posterior conservan su
estructura en el rango nanométrico. En muestras con tratamiento
térmico se observa un crecimiento controlado
debido a los numerosos puntos de nucleación y la presencia
de precipitados de cementita. Los resultados de dureza
y tamaño de grano obtenidos cumplen la relación de Hall
-Petch. Finalmente se analiza la influencia de las técnicas
de obtención y de caracterización mecánica empleadas
en este trabajo frente a diferentes fuentes bibliográficas.Postprint (published version
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