Structural transformation in Al-51 at% Zn eutectoid alloy observed in internal friction
Résumé
The Al-51 at% Zn alloy shows a total transformation of the eutectoid mixture at this concentration. It is therefore ideal for studying the evolution of the alpha and beta phases as a function of temperature. In the present work, we have clearly demonstrated a correlation between the evolution of the microstructure observed by scanning electron microscopy (SEM) and the results obtained by isothermal mechanical spectroscopy (IMS) [1]. IMS measurements on this alloy were carried out over a very wide frequency range (10⁻⁵–50 Hz) and the results obtained show that it exhibits different relaxation peaks as a function of temperature between ambient and 540 K. Upon heating, two peaks P1 and P2 appear below the eutectoid transition temperature (550 K). Both peaks are thermally- activated; P1 decreases and disappears with increasing measurement temperature while P2 appears and increases continuously until the eutectoid transition temperature. These peaks have been associated with thermally induced diffusion of atoms across the α-β interface; this could correspond, according to the literature, to a change in the shape of the lamellar interface between the α and β phases [1]. The SEM observations were performed after several quenches at different temperatures (385 K, 433 K and 493 K after a holding time of 2 h 30 min) chosen in the temperature range of the internal friction peaks evolution. The results obtained allowed us to associate the P1 peak to the lamellar structure whose destruction leads to the collapse of P1. On the other hand, the P2 peak is associated to the globules coalescence and as this coalescence increases the amplitude of this peak becomes more important. The analysis of our SEM observations has clearly highlighted the transformation mechanisms of this eutectoid mixture by specifying without ambiguity the passage from the lamellar structure to the globular structure and then the collapse of the latter.