Kovove Mater. 60 2022 209–222 DOI: 10.31577/km.2022.4.209 209 Metallurgical characterization of natural aging effects on pre-deformed Al 7075/T651 alloy during retrogression and re-aging heat treatment G¨ozdeAltunta¸ s*, B¨ ulent Bostan Gazi University, Faculty of Technology, Department of Metallurgical and Materials Engineering, Ankara, Turkey Received 20 January 2022, received in revised form 18 April 2022, accepted 12 May 2022 Abstract In this study, a new heat treatment route was designed, and the effects of pre-deformation on the retrogression and re-aging (RRA) and 17500 h natural aging (NA) heat treatment were investigated. The specimens that were pre-deformed during the RRA heat treatment and the subsequent NA specimens were compared. As a result of the heat treatments, the phase volume ratios in each specimen were calculated and compared with two different methods. Firstly, the distributions of the phases and the geometrical changes of the precipitates were interpreted by scanning electron microscopy (SEM). Elemental ratios in the precipitates were obtained by energy-dispersive X-ray spectrometry (EDS). Particle size, dislocation density, lattice strain, and texture coefficient values were calculated by the X-ray diffraction method (XRD), and their effects on crystallography were examined in detail. As a result of the differential scan- ning calorimetry (DSC) analysis, it was observed that the transformation temperatures of the phases changed with the heat treatment route. As a result of all analyses, the connec- tions between the hardness values were interpreted. The results showed that the hardness values, dislocation densities, and lattice strain of the pre-deformed RRA + NA specimens were increased compared to the pre-deformed RRA specimens. K e y w o r d s: natural aging, retrogression and re-aging heat treatment, dislocation density, texture coefficient, 7075 aluminum alloys 1. Introduction 7000 series (Al-Zn-Mg-Cu) aluminum alloys have high hardness and strength and can be hardened by precipitation [1, 2]. Due to their low densities, they have been frequently used in the defense industry, space, and aviation in recent years and can compete with steel at the same time [3]. The aging process of these alloys generally continues with natural aging after solution heat treatment followed by quenching. However, the long duration of natural aging makes artificial aging heat treatment more frequently used. Natural aging has a more profound effect on mi- crostructural properties than artificial aging. There- fore, it is necessary to fully understand the precipitates during natural aging [4–6]. The order of precipitation in 7000 series alloys was accepted as follows [7, 8]: *Corresponding author: e-mail address: gozdealtuntas@gazi.edu.tr Solid solution/GP zones/metastable η ′ /stable η- MgZn 2 . There are two types of GP zones. GPI zones are created over a wide temperature range from room tem- perature to 140–150 ◦ C. It is often seen when natural aging is done. It is consistent with the internal order- ing of Zn and Al/Mg on the matrix lattice. GP II zones are formed by aging at temperatures above 70 ◦ C af- ter quenching at temperatures above 450 ◦ C [9]. Some studies suggest that the GPI zones formed during ag- ing at a low temperature dissolve at 120 ◦ C and form GP II zones that can transform to η ′ to gain strength [10]. Others suggest that the GPI zones are the core of the η ′ phase [11, 12]. It is thought that the metastable η ′ phase is the main strengthening phase and provides the hardening of the aged 7000 series alloys [13]. The η phase consists of MgZn 2 precipitates, which tend