Study of interrupted aging in Al-Cu-Mg alloys, A

Abstract

Recently, a novel interrupted aging treatment, T6I6, was reported to improve the strength and toughness of heat treatable Al alloys. It consisted of the following three stages, namely, 1) artificially underaging at a typical temperature (150-200C) and quenching to room temperature, 2) reheating to a lower aging temperature (60-100C) for times that result in hardening, and 3) reheating to the initial aging temperature again and holding until peak hardness is observed. The improvements were reported to be due to precipitate refinement and secondary precipitation during the low temperature age. However, the improvements in properties have not been consistently reproduced in the literature, and an explanation for the discrepancies remains unclear. Therefore, the focus of this study was to investigate the interrupted aging processes and the effects of each aging stage, as well as the effect of cold work, on precipitate development in commercial 2024 Al and a high-purity variant of the commercial 2014 alloy, Al-4.2Cu-0.4Mg (wt%). The aging responses, precipitation behavior, and microstructural changes were examined using Vickers microhardness, electrical resistivity, and electron microscopy. Small angle X-ray scattering (SAXS) was also used to characterize the precipitate size and volume fraction during the initial underaging and subsequent stages of interrupted aging. It was found that, although secondary hardening was observed during the low temperature aging, apparent softening upon re-heating was observed and the peak hardness showed little improvement. In addition, in 2024 alloys which were cold worked after SHT or after underaging, no further strengthening was observed during secondary aging at 65C. Likewise, subsequent aging to peak hardness showed no improvements. In order to understand these results, an attempt was made to examine the mechanism of hardening during the underaging and secondary hardening treatments using SAXS. Formation of 1 nm diameter solute clusters up to ~3 vol% was found to contribute primarily to the initial rapid hardening and secondary strengthening during interrupted aging. The subsequent softening was attributed to the reversion of these 1 nm clusters. Further analyses of the observed cluster hardening involved estimating the modulus and/or surface/chemical strengthening. The shear modulus of the clusters was estimated to be 2.9-3.3 GPa higher than that of 2024, and additional surface energy from cluster shearing was estimated to be 0.16-0.63 J/m2.