Precipitation Behavior of AA2618

Abstract

The precipitation behavior of AA2618 was studied by a multitude of characterization techniques: microhardness testing, lattice parameter measurement through X-ray diffraction (XRD), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and atom probe field ion microscopy (APFIM). The matrix lattice parameter increased during the first 20 hours of natural aging, due to the formation of Cu clusters and decreased over the next 24 hours, due to the formation of Mg-rich clusters. Prior natural aging weakened subsequent artificial aging hardening at 180°C, 200°C, and 230°C, due to the cluster reversion that delayed the precipitation of strengthening phases. The matrix lattice parameter exhibited erratic changes during artificial aging that corresponded to the formation and partial dissolution of Guinier-Preston-Bagaryatsky (GPB) zones, the transformation of GPB zones to GPB2 zones, and the precipitation of S¢. The structural changes during the artificial aging of AA2618 occur in this sequence: supersaturated solid solution fi clusters + GPB fi GPB + GPB2 fi GPB2 + S¢ fi S¢+ S fi S.

Characterisation of precipitation upon cooling of an AA2618 Al–Cu–Mg alloy

“…After a plateau during ca. 2 h measured by Lu et al [3], the hardness increases in the second stage to reach its peak value after ca. 9 h at 200 C where equilibrium S (Al 2 CuMg) and metastable S' precipitates are found.…”

effects of heat treatment on the microstructure and mechanical properties of the AA2618 DC cast plate /

“…However, Si additions retard the age hardening at room temperature due to preferential interaction of vacancies with Si atoms [15]. Precipitation of other phases, such as O-AhCu, is also possible in alloys with higher Si contents in which Mg2Si forms in greater amounts, thereby consuming more Mg from the matrix [16].…”

“…The early stages of natural ageing in laboratory Al-Cu-(Mg) alloys and in AA2024 were investigated using positron annihilation and X-ray absorption spectroscopy by Klobes et al [39] who also concluded that the natural aging hardening process was associated with the growth of co-clusters and no S" phase formed during natural aging in the investigated Al-Cu and Al-Cu-Mg alloys. Lu et al [16] found that prior natural aging of 2618 alloy weakened the hardening in the subsequent artificial aging at 180, 200, and 230 °C, but had no such effect at 150 °C.…”

Misfit-induced changes of lattice parameters in two-phase systems: coherent/incoherent precipitates in a matrix

“…Countless observations of diffraction-line broadening due to microstrain have been reported; in fact, analysis of diffractionline broadening has become a standard method of microstructural characterization (Mittemeijer & Welzel, 2012). However, little experimental work has been devoted to the change of the (average) macroscopic lattice parameter by misfit strain Mittemeijer & van Gent, 1984;Zhou et al, 1991;Mü ller et al, 1997;Timmermans & Froyen, 1999;Chechenin et al, 2002;Bruno et al, 2003;Lu et al, 2007). Against the above background, this manuscript presents a review of misfit-induced (average, macroscopic) lattice-parameter changes, as predicted and as observed.…”