Abstract

VALERA MANEIRO, María del V; LUIGGI AGREDA, Ney J; MACLELLAN B, Luis E  and  PRIN, José L. Study of precipitation in an AA7075 rolled alloy. Rev. Fac. Ing. UCV [online]. 2013, vol.28, n.1, pp.115-121. ISSN 0798-4065.

The effect of the deformation applied by cold rolling on the kinetics of phase transformation in a commercial AA7075 alloy has 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 oversaturation and a high internal energy. Samples under condition (a) when heated between 25 and 600° undergo a process of atomic redistribution leading to the re-arrangement of the solute atoms (Zn and Mg mainly), which gives rise to the precipitation of Guinier-Preston zones (GP) at temperatures below 170º, followed by the precipitation of the η′ phase in the vicinity of 250° while the ηphase precipitates at temperatures below 350º, the last two MgZn₂ rich. The microstructure in condition (b) when heated shows a different evolution regarding the disappearance of characteristic peaks in the homogenized microstructure, especially at low temperatures, accelerating the transition of η to ηphase when increasing the rolling rate. The transformations detected by DSC are displaced when the heating rate Φ is increased, confirming that the kinetics of phase transformation is controlled by diffusion, even in the severely deformed samples. This allows us the use of the iso-conversion method to evaluate the activation energy Q of the most prominent phase, in this case ηthe phase. The variation of Q with the deformation applied allows the conclusion that a direct interaction between the recrystallization and precipitation processes occurs, where for a critical thickness reduction (~ 50%) the precipitation of the ηphase is promoted. The microstructural state for both conditions, at different stages of the study, has been observed by transmission electron microscopy (TEM) confirming the occurrence of a dislocation reorganization process.

Keywords : AA7075; Cold Rolling; Recovery; Recrystallization; Precipitation.

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