A survey of the effect of cryogenic treatment and natural ageing on structural changes and second-phase precipitation in Al–Zn–Mg–Cu alloy

• Published in: Journal of thermal analysis and calorimetry 2023-10, Vol.148 (20), p.10713-10725
• Main Authors: Altuntaş, Gözde, Özdemir, Abbas Tamer, Bostan, Bülent
• Format: Article
• Language: English
• Subjects: Activation energy; Aging; Aging (natural); Alloys; Aluminum base alloys; Analytical Chemistry; Calorimetry; Chemistry; Chemistry and Materials Science; Cryogenic effects; Cryogenic treatment; Crystal structure; Crystals; Differential scanning calorimetry; Dislocation density; Hardness; Inorganic Chemistry; Magnesium; Mathematical analysis; Measurement Science and Instrumentation; Nucleation; Physical Chemistry; Polymer Sciences; Precipitates; Specialty metals industry; Strain energy; Structure; Transformation temperature; X-ray spectroscopy; Zinc; Zinc compounds
• ISSN: 1388-6150; 1588-2926
• DOI: 10.1007/s10973-023-12414-8

In this work, the impacts of shallow cryogenic (−40 °C) and natural ageing treatments on the precipitation of the second phases in a 7xxx alloy were investigated. Using the differential scanning calorimetry (DSC) analysis technique, transformation temperatures of the phases both in standard natural aged and naturally aged after cryogenic treatment were determined. As a result of the analyses made, the activation energies of the precipitates formed in the material were calculated with the Kissinger, Takhor and Augis-Bennett equations and their effects on diffusion were considered. With DSC analysis, nucleation characteristics of the η ′ phase (MgZn2) were determined by the Avrami parameter from the Ozawa equation. Also, features and distributions of different types of precipitates were analysed by means of scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS) and X-ray diffraction (XRD) techniques, respectively. Particularly, during the cryogenic and natural ageing processes, dislocation densities, strain values of the planes, texture coefficients and so subsequent after effects on the crystal structure were assessed. It was anticipated that excess dislocations generated during sub-zero treatment preferably settled on suitable planes to subside the strain energy. Finally, it was noticed that compared to natural aged samples, application of the cryogenic treatment before ageing upsurged the hardness of the material on the average by15%. This was possibly because of the η′ precipitates nucleated mainly on dislocations. Graphical abstract