TRANSFORMACIONES DE FASES EN ACEROS DE BAJO CARBONO / Phase Transformations In Low Carbon Steel

Mediante la aplicación de las técnicas de Calorimetría Diferencial de Barrido (DSC), Resistividad Eléctrica (ρ) y Microscopía Electrónica de Barrido (MEB), se realizó un estudio de la transformación de fases en dos tipos de aceros microaleados de bajo contenido en carbono. La cinética de transformación no isotérmica fue estudiada en estado normalizado y en muestras sometidas a un tratamiento de austenización a 1100 ºC durante 30, 60 y 120 minutos, seguida de un temple en agua a 20 ºC. Las curvas de DSC para los aceros normalizados y para los tratados y templados muestran comportamientos diferentes evidenciando su sensibilidad al cambio microestructural. En ambos casos se reportan tres transformaciones endotérmicas y tres exotérmicas, correspondiente las exotérmicas a la precipitación de fases ricas en carbono en diferentes proporciones respecto a la matriz de hierro, mientras que la primera endotérmica se asocia con la disolución de cementita en el caso normalizado y de martensita en el caso templado, y las otras dos muy puntuales las cuales se asocian con la transición de ferrita magnética a ferrita amagnética y a la transformación alotrópica α → g, respectivamente. La determinación de la entalpía relacionada con los picos puntuales de transformación, permitió calcular la fracción transformada asociada y evaluar su respectiva energía de activación. La resistividad en función del tiempo de austenización, presentó un comportamiento descendente hasta un plateau, indicativo de la fracción de átomos de carbono retenida durante la austenización y por ende asociadas con la fracción de martensita formada. La técnica de microscopía electrónica de barrido, evidenció para muestras en estado normalizado una microestructura ferrítico-perlítica y las templadas una microestructura martensítica.ABSTRACTBy applying the techniques of Differential Scanning Calorimetry (DSC), Electrical resistivity (ρ) and scanning electron microscopy (SEM) was carried out a study of phase transformation in two types of microalloyed low carbon steels. The non isothermal transformation kinetics was studied on normalized samples and on austenitized samples at 1100 ºC for 30, 60 and 120 minutes followed by quenching in water at 20 ºC. DSC curves for normalized and quenched steels show different behaviors demonstrating its sensitivity to microstructural change. In both cases was reported three endothermic and three exothermic transformations, corresponding the exothermic peaks to the precipitation of carbon-rich phases in different proportions with respect to the iron matrix, while the first endothermic peak is associated to the dissolution of cementite in the normalized samples and to the martensite in the quenched samples, and the two very specific peaks was associated with the transition from ferrite magnetic to ferrite non-magnetic and α → g, allotropic transformation, respectively. The good definition of the endothermic peaks permitted to calculate the fraction transformed and evaluates their respective activation energy. The resistivity as a function of austenitizing time present a downward trend to a plateau, indicative of the fraction of carbon atoms trapped in solid solution and thus associated with the fraction of martensite formed. Micrographs obtained by Scanning electron microscopy showed a ferritic-pearlitic microstructure in the normalized samples and a martensitic microstructure on quenching samples.Keywords: Microalloyed Steel, Phase Transformations, DSC, Electrical Resistivity, SEM

Isochronal study of Al-Mg, Al-Mn, and Al-Mn-Mg alloys using electrical resistivity and thermoelectric power

We have studied the phase transformation kinetics occurring in aluminum alloys containing Mn, Mg, and Mn-Mn-Mg by means of electrical resistivity (rho) and thermoelectric power (deltaS). The alloy samples were annealed isochronally at temperatures ranging from ambient temperature to 615 °C. Both rho and deltaS allowed the separation of several stages of transformation associated to either the precipitation or dissolution of phases that occur during the annealing process. The alloys containing Mn show a strong deltaS growth or a marked r drop between 450 °C and 550 °C, linked to the precipitation of the Mn-rich Al6(Mn,FE) equilibrium phase. While the Mg in aluminum generates a series of maxima and minima of both rho and deltaS associated to the pre-established precipitation sequence: GP Zones <FONT FACE=Symbol>® b</FONT>' phase <FONT FACE=Symbol>® b</FONT> phase, a combination of effects ensues in the Mn- and Mg-containing alloys, the Mg effect being enhanced at temperatures below 350 °C and that of the Mn striking the same behavior above such temperature. Our study ascertains that the Mg speeds up the precipitation and lowers the activation energy of the Al6(Mn,Fe) phase, the latter having been evaluated by the multiple temperature method

Experimental Study of the Interaction between Recrystallization and Precipitation Processes of an AA8011 Commercial Alloy

Phase changes in a commercial AA8011 alloy from different initial microstructure conditions were studied using thermoelectric power (ΔS), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM) techniques with the purpose of obtaining evidence of the interaction between recovery-precipitation and recrystallization-precipitation processes occurring during nonisothermal heating at different rates. Thermoelectric power and its thermal derivative reflect this evidence by a displacement of the characteristic precipitation peaks, the recovery and recrystallization contributions remaining masked by the strong incidence of the iron precipitation on that property, while DSC measurements detect the emergence of new peaks not observed on thermograms of homogenized samples. An exhaustive study of these peaks permits direct differentiation between precipitation and recovery-recrystallization contributions. TEM confirms the interaction between both processes by means of local observations

ESTUDIO DE LA PRECIPITACIÓN EN UNA ALEACIÓN AA7075 DEFORMADA / Study of Precipitation in an AA7075 Rolled Alloy

El efecto de la deformaci&oacute;n, por laminado en fr&iacute;o, sobre la cin&eacute;tica de transformaci&oacute;n de fases en una aleaci&oacute;n comercialAA7075 ha sido estudiado por Calorimetr&iacute;a Diferencial de Barrido (DSC). Diferentes condiciones microestructurales departida han sido utilizadas: (a) homogeneizada y templada, la cual se caracteriza por una elevada sobresaturaci&oacute;n de solutoy una baja densidad de tensiones internas y (b) deformada hasta un 85% de reducci&oacute;n de espesor luego del tratamientoaplicado en (a), caracterizada por una elevada sobresaturaci&oacute;n de soluto y una alta concentraci&oacute;n de tensiones internas. Lasmuestras bajo la condici&oacute;n (a) al ser calentadas entre 25 y 600&ordm;C sufren un proceso de redistribuci&oacute;n at&oacute;mica, que conllevaa que los &aacute;tomos de soluto (principalmente Zn y Mg) se reacomoden para dar paso a la precipitaci&oacute;n de zonas Guinier&ndash;Preston (GP), a temperaturas por debajo de 170&ordm;C, seguido de la precipitaci&oacute;n de la fase &eta;&prime; en la vecindad de 250&ordm;C y quea temperaturas por debajo de los 350&ordm;C precipite la fase &eta;, las dos &uacute;ltimas ricas en MgZn2. La microestructura en condici&oacute;n(b) al ser calentada muestra flujogramas diferentes en cuanto a la desaparici&oacute;n de picos caracter&iacute;sticos de la microestructurahomogeneizada, principalmente a bajas temperaturas, acelerando la transici&oacute;n de la fase &eta;&rsquo; a la fase &eta; cuando se aumentala deformaci&oacute;n aplicada. Las transformaciones detectadas por DSC se desplazan cuando la raz&oacute;n de calentamiento &Phi;es aumentada, confirmando que la cin&eacute;tica de fases es controlada por el mecanismo de difusi&oacute;n, incluso en muestrasseveramente deformadas. Esto nos permite usar el m&eacute;todo de iso&ndash;conversi&oacute;n para evaluar la energ&iacute;a de activaci&oacute;n Q dela fase m&aacute;s prominente, en este caso la fase &eta;. La variaci&oacute;n de Q con la deformaci&oacute;n aplicada demuestra que ocurre unainteracci&oacute;n directa entre los procesos de recristalizaci&oacute;n y de precipitaci&oacute;n, donde existe un valor cr&iacute;tico de reducci&oacute;n deespesor (&sim;50%) a partir del cual se facilita la precipitaci&oacute;n de la fase de equilibrio &eta;. El estado microestructural para ambascondiciones, en diferentes etapas del estudio, es observado por Microscop&iacute;a Electr&oacute;nica de Transmisi&oacute;n, confirmando unareorganizaci&oacute;n de las dislocaciones introducidas al aplicar diferentes grados de laminaci&oacute;n.ABSTRACTThe effect of the deformation applied by cold rolling on the kinetics of phase transformation in a commercial AA7075 alloyhas been studied by Differential Scanning Calorimetry (DSC). Different microstructural conditions were investigated:(a) homogenized and quenched, which is characterized by a high solute oversaturation and low internal energy and (b)deformed up to 85% thickness reduction after the treatment applied in (a), characterized by a high solute oversaturationand a high internal energy. Samples under condition (a) when heated between 25 and 600&deg;C undergo a process of atomicredistribution leading to the re&ndash;arrangement of the solute atoms (Zn and Mg mainly), which gives rise to the precipitationof Guinier-Preston zones (GP) at temperatures below 170&ordm;C, followed by the precipitation of the &eta;&prime; phase in the vicinity of250&deg;C while the &eta; phase precipitates at temperatures below 350&ordm;C, the last two MgZn2 rich. The microstructure in condition(b) when heated shows a different evolution regarding the disappearance of characteristic peaks in the homogenizedmicrostructure, especially at low temperatures, accelerating the transition of &eta;&rsquo; to &eta; phase when increasing the rollingrate. The transformations detected by DSC are displaced when the heating rate &Phi; is increased, confirming that the kineticsof phase transformation is controlled by diffusion, even in the severely deformed samples. This allows us the use of theiso-conversion method to evaluate the activation energy Q of the most prominent phase, in this case &eta; the phase. The variation of Q with the deformation applied allows the conclusion that a direct interaction between the recrystallizationand precipitation processes occurs, where for a critical thickness reduction (&sim; 50%) the precipitation of the &eta; phase ispromoted. The microstructural state for both conditions, at different stages of the study, has been observed by transmissionelectron microscopy (TEM) confirming the occurrence of a dislocation reorganization process.Keywords: AA7075, Cold Rolling, Recovery, Recrystallization, Precipitation

Calculo de Parámetros Electrónicos Para el Grafito Bernal

<abstract language="eng">Using ab-initio calculation, we have computed different electronic parameters associated with the transport coefficients of graphite Bernal. Software CRYSTAL¹ was used with a Pople standard STO- 21G* basis set. Different hamiltonians were tested, choosing a restricted Hartree-Fock one, because it generated the best qualitative results. All graphites studied present a valence bandwidth near 0.60 a.u., in agreement with the literature. In hexagonal Bernal graphite the splitting of the valence P band was accentuated, and E F = -0.00140 a.u. The surface Fermi is located around the HKH edge of the first Brillouin zone. In general such surfaces present a central electron orbit of maximal size in the plane, which diminish in size when the z component of the <IMG SRC="http:/img/fbpe/mr/v5n2/11049s6.gif">vector moves in KH direction. Integrating on the surface Fermi calculates the time relaxation parallel and the time relaxation perpendicular and finally we evaluate the parallel and perpendicular resistivity