Tz-En ChangTing-Jung HsiaoShu-Cheng LiangCheng-Ling TaiShih-Yuan LuYu-Jen YouChu-Jen ChenTE-CHENG SUYo-Lun YangChih-Yuan ChenJer-Ren Yang2025-05-152025-05-152025-06https://www.scopus.com/record/display.uri?eid=2-s2.0-86000561710&origin=resultslisthttps://scholars.lib.ntu.edu.tw/handle/123456789/729305Multi-stage solid solution treatments (SSTs) for two 7075 aluminum alloys, Al-5.98Zn-2.86Mg-1.61Cu, wt. % (denoted as 7075-LCu) and Al-5.91Zn-2.83Mg-1.98Cu, wt.% (denoted as 7075-HCu), have been investigated. Due to their different Cu contents, the two alloys possess different volume fractions of the primary second phases in the as-received plates. In this work, X-ray diffraction (XRD) was employed to identify the second phases in the alloys and differential scanning calorimetry (DSC) to characterize the melting points of the second phases. To verify the types of second phases in specimens after each stage of solution treatment with the ageing treatments, electron probe micro analysis (EPMA) was also employed. The main micro-scale sized second phases in two as-received alloys are the η phase and the S phase; their nominal compositions are Mg(Zn, Al, Cu)2 and Al2CuMg, respectively. Therefore, multi-stage SSTs were designed respectively for these two alloys. In these SSTs, the temperature of each stage was set slightly lower than the melting points of the corresponding second phases (η phase and S phase) in an attempt to achieve complete solid solutioning in the matrix without incipient melting. After the SSTs, aging processes with natural aging (NA) and artificial aging (AA) were applied. Under the newly designed multi-stage SSTs with (NA + AA), both the strength and elongation of these two alloys were significantly improved as compared to their counterparts treated by conventional single-stage SST with (NA + AA). The results showed that the elongation of 7075-LCu improved from 11.8 % (single-stage SST with NA + AA) to 15.0 % (multi-stage SST with NA + AA), a 27 % increase, and the corresponding ultimate strength improved from 597 MPa to 606 MPa. Similarly, the elongation of 7075-HCu improved from 12.6 % (single-stage SST with NA + AA) to 15.2 % (multi-stage SST with NA + AA), a 20 % improvement, and the corresponding ultimate strength improved from 596 MPa to 613 MPa. The effects of the elimination of second phases in the aluminum matrix without incipient melting on the properties of subsequent peak-aging treatments are discussed.Al-Zn-Mg-Cu alloyCu additionDifferential scanning calorimetry (DSC)Electron probe micro analysis (EPMA)Incipient meltingSecond phaseSolid solution treatmentTransmission electron microscopy (TEM)X-ray diffraction (XRD)[SDGs]SDG12journal article10.1016/j.msea.2025.148157