Gauge invariance of parametrized systems and path integral quantization

Gauge invariance of systems whose Hamilton-Jacobi equation is separable is improved by adding surface terms to the action fuctional. The general form of these terms is given for some complete solutions of the Hamilton-Jacobi equation. The procedure is applied to the relativistic particle and toy universes, which are quantized by imposing canonical gauge conditions in the path integral; in the case of empty models, we first quantize the parametrized system called ``ideal clock'', and then we examine the possibility of obtaining the amplitude for the minisuperspaces by matching them with the ideal clock. The relation existing between the geometrical properties of the constraint surface and the variables identifying the quantum states in the path integral is discussed.Comment: 23 page

Influencia del tiempo de revenido a 780oC sobre la resistencia al creep del acero ASTM A335 P91

Se investigó el efecto del tiempo de revenido a la temperatura de 780 °C sobre la resistencia al creep del aceroASTM A335 grado P91. La temperatura de revenido correspondió a la temperatura de revenido industrialy los tiempos evaluados se eligieron de tal manera de acumular, entre el revenido previo del material de suministro(40 minutos) y el tratamiento realizado en el laboratorio, tiempos de 3; 4; 5; 5,5; 5,7; 6 y 7 horas,por lo que en la práctica se aplicó un doble revenido. Posteriormente, las muestras fueron ensayadas a 600 ºCy 190 MPa hasta rotura. Los resultados muestran que el tiempo de revenido a 780 ºC, tiene un efecto muyimportante en la resistencia al creep del acero P91. Hasta 3 horas de revenido, el acero P91 mostró una buenaresistencia al creep con una velocidad mínima de creep de 7x10-9 s-1. Una marcada caída en la resistencia alcreep se observó para un tiempo de revenido de 5 horas (1.5x10-7 s-1), con una posterior recuperación a las5.5 horas (3x10-8 s-1). Este comportamiento al creep probablemente esté relacionado a la evolución de laspartículas MX durante el revenido. El tamaño promedio de las partículas de segunda fase en las probetas revenidasa 780º C con diferentes tiempos y sometidas a creep a 600º C, indicarían de manera indirecta un estadode disolución y re-precipitación de las partículas MX que ocurre durante el revenido. La rotura por creepocurre por la nucleación, crecimiento y coalescencia de cavidades en regiones próximas a los límites degrano de austenita previa, resultando en una fisura y propagación de la fisura hasta su rotura.Palabras clave: Acero 9% Cr, Comportamiento a creep, Resistencia a creep, Precipitación

Efecto del tratamiento térmico sobre la resistencia mecánica, corrosión intergranular y exfoliación de la aleación de aluminio 7075

En este estudio, los efectos de los tratamientos térmicos en el pico de envejecimiento (T6), sobreenvejecimiento(T73) y proceso de regresión y re-envejecimiento (RRA, regresión a 200 °C durante 10 min)sobre la microestructura, resistencia mecánica, corrosión por exfoliación y corrosión intergranular de aleaciónde aluminio 7075 ha sido investigado mediante microscopia óptica, microscopía electrónica de barrido,ensayos de tracción, ensayos ASTM G110, ensayos ASTM G34, curva de polarización y espectroscopia deimpedancia electroquímica (EIS).Los resultados de los ensayos de corrosión intergranular (ASTM G110) y los ensayos de corrosión por exfoliación(ASTM G34), aplicados a los tres tratamientos definen un orden de susceptibilidad a la corrosión dadapor: T6 T73 RRA. Las tendencias de la resistencia a la corrosión son confirmadas mediante curvas depolarización y ensayos EIS. El tratamiento térmico RRA mejora la resistencia a la corrosión intergranularexfoliaciónde la aleación de aluminio 7075 sin sacrificar su resistencia mecánica en comparación con eltratamiento térmico T6. El efecto del tratamiento térmico sobre la resistencia mecánica y la resistencia a lacorrosión se explica en función del estado de precipitación en la matriz y en los límites de grano.Palabras clave: Aleación de aluminio 7075, propiedades mecánicas, corrosión intergranular, corrosión exfoliación,espectroscopia de impedancia electroquímica

Influence of tempering time at 780°C on the creep resistance of ASTM A335 P91 steel

<p></p><p>ABSTRACT It has been investigated the effect of tempering time at 780°C on the creep strength of ASTM A335 grade P91 steel. The tempering temperature corresponded to the industrial tempering temperature and the times evaluated were chosen in such a way to accumulate, between the tempering prior as received (40 min) and the treatment carried out in the laboratory, times of 3; 4; 5; 5,5; 5,7; 6 and 7 hours, so that in practice a double tempering was applied. Subsequently, the samples were creep tested at 600 ºC and 190 MPa up to rupture. The results show that a tempering time to 780 ºC has a very significant impact on the creep strength of the P91 steel. Up to 3 hours of tempering, the P91 steel retains its creep strength, with a minimum creep rate of 7x10-9 s-1. This creep strength falls off sharply to the 5 hours of tempering (1.5x10-7 s-1), and retrieved to the 5.5 hours (3 x10-8 s-1). This creep behavior is probably related to the evolution of the MX particles during tempering. The average size of the particles of the second phase in the samples tempered to 780 °C during different times and subjected to creep to 600 °C, would indirectly indicate a state of dissolution and reprecipitation of MX particles, which occurs during the tempering. Creep rupture occurs by the nucleation, growth and coalescence of cavities, in regions close to the prior austenite grain boundaries, resulting in a crack and propagation up to fracture.</p><p></p

Effect of heat treatment on the mechanical strength, intergranular and exfoliation corrosion of 7075 aluminum alloy

<p></p><p>ABSTRACT In this study, the effects of heat treatments in the peak aged (T6), overaged (T73) and retrogression and reaging (RRA, retrogression at 200 °C for 10 min) on the microstructure, mechanical strength, exfoliation corrosion and intergranular corrosion of aluminum alloy 7075, has been investigated by optical microscope, scanning electron microscopy, tensile tests, ASTM G110 tests, ASTM G34 tests, polarization curve and electrochemical impedance spectroscopy (EIS). The results of intergranular corrosion tests (ASTM G110) and exfoliation corrosion tests (ASTM G34), applied to the three treatments defines an order of susceptibility to corrosion given by: T6 > T73 > RRA. The trends of the corrosion resistance were further confirmed by polarization curve and EIS test. The RRA heat treatment improved the intergranular-exfoliation corrosion resistance of aluminum alloy 7075 without sacrificing the mechanical strength compared to T6 temper. The effect of heat treatment on mechanical strength and corrosion resistance is explained based on the state of precipitation in the matrix and grain boundaries.</p><p></p

Active galactic nuclei: what’s in a name?

Active Galactic Nuclei (AGN) are energetic astrophysical sources powered by accretion onto supermassive black holes in galaxies, and present unique observational signatures that cover the full electromagnetic spectrum over more than twenty orders of magnitude in frequency. The rich phenomenology of AGN has resulted in a large number of different "flavours" in the literature that now comprise a complex and confusing AGN "zoo". It is increasingly clear that these classifications are only partially related to intrinsic differences between AGN, and primarily reflect variations in a relatively small number of astrophysical parameters as well the method by which each class of AGN is selected. Taken together, observations in different electromagnetic bands as well as variations over time provide complementary windows on the physics of different sub-structures in the AGN. In this review, we present an overview of AGN multi-wavelength properties with the aim of painting their "big picture" through observations in each electromagnetic band from radio to gamma-rays as well as AGN variability. We address what we can learn from each observational method, the impact of selection effects, the physics behind the emission at each wavelength, and the potential for future studies. To conclude we use these observations to piece together the basic architecture of AGN, discuss our current understanding of unification models, and highlight some open questions that present opportunities for future observational and theoretical progress.Comment: Accepted for publication in Astronomy & Astrophysics Review, 56 pages, 25 figure