7 research outputs found
Isochronal study of Al-Mg, Al-Mn, and Al-Mn-Mg alloys using electrical resistivity and thermoelectric power
No full textWe 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
No full textPhase 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 PRECIPITACIĂ“N EN UNA ALEACIĂ“N AA7075 DEFORMADA / Study of Precipitation in an AA7075 Rolled Alloy
No full textEl efecto de la deformación, por laminado en frío, sobre la cinética de transformación de fases en una aleación comercialAA7075 ha sido estudiado por Calorimetría Diferencial de Barrido (DSC). Diferentes condiciones microestructurales departida han sido utilizadas: (a) homogeneizada y templada, la cual se caracteriza por una elevada sobresaturación de solutoy una baja densidad de tensiones internas y (b) deformada hasta un 85% de reducción de espesor luego del tratamientoaplicado en (a), caracterizada por una elevada sobresaturación de soluto y una alta concentración de tensiones internas. Lasmuestras bajo la condición (a) al ser calentadas entre 25 y 600ºC sufren un proceso de redistribución atómica, que conllevaa que los átomos de soluto (principalmente Zn y Mg) se reacomoden para dar paso a la precipitación de zonas Guinier–Preston (GP), a temperaturas por debajo de 170ºC, seguido de la precipitación de la fase η′ en la vecindad de 250ºC y quea temperaturas por debajo de los 350ºC precipite la fase η, las dos últimas ricas en MgZn2. La microestructura en condición(b) al ser calentada muestra flujogramas diferentes en cuanto a la desaparición de picos característicos de la microestructurahomogeneizada, principalmente a bajas temperaturas, acelerando la transición de la fase η’ a la fase η cuando se aumentala deformación aplicada. Las transformaciones detectadas por DSC se desplazan cuando la razón de calentamiento Φes aumentada, confirmando que la cinética de fases es controlada por el mecanismo de difusión, incluso en muestrasseveramente deformadas. Esto nos permite usar el método de iso–conversión para evaluar la energía de activación Q dela fase más prominente, en este caso la fase η. La variación de Q con la deformación aplicada demuestra que ocurre unainteracción directa entre los procesos de recristalización y de precipitación, donde existe un valor crítico de reducción deespesor (∼50%) a partir del cual se facilita la precipitación de la fase de equilibrio η. El estado microestructural para ambascondiciones, en diferentes etapas del estudio, es observado por Microscopía Electrónica de Transmisión, confirmando unareorganización de las dislocaciones introducidas al aplicar diferentes grados de laminación.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°C undergo a process of atomicredistribution leading to the re–arrangement of the solute atoms (Zn and Mg mainly), which gives rise to the precipitationof Guinier-Preston zones (GP) at temperatures below 170ºC, followed by the precipitation of the η′ phase in the vicinity of250°C while the η phase precipitates at temperatures below 350ºC, 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 η’ to η phase when increasing the rollingrate. The transformations detected by DSC are displaced when the heating rate Φ 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 η 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 (∼ 50%) the precipitation of the η 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
Isochronal study of Al-Mg, Al-Mn, and Al-Mn-Mg alloys using electrical resistivity and thermoelectric power
No full textCharacterization by thermoelectric power of a commercial aluminum-iron-silicon alloy (8011) during isothermal precipitation
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