Predicting the quench sensitivity of Al–Zn–Mg–Cu alloys: A model for linear cooling and strengthening

Abstract

In this work the quench sensitivity of Al-Zn-Mg-Cu alloys is studied through continuous cooling at constant rates of a range of alloys using differential scanning calorimetry (DSC), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and hardness testing. The DSC, TEM and SEM data show that the cooling reactions are dominated by a high temperature reaction (typically ~450 °C down to ~350 °C) due mostly to S-Al2CuMg phase formation, a medium temperature reaction (~350 °C down to ~250 °C) due predominantly to -Mg(Al,Cu,Zn)2 phase formation and a lower temperature reaction (~250 °C down to ~150 °C) due to a Zn-Cu rich thin plate phase. A new, physically-based model is constructed to predict rates of all reactions, enthalpy changes and resulting yield strength in the artificially aged condition. The model incorporates a recently derived model for diffusion-controlled reactions based on the extended volume fraction concept as well as recent findings from first principles modelling of enthalpies of the relevant phases. The model shows a near perfect correspondence with data on all 6 alloys studied extensively by cooling DSC and hardness testing, and 2 allows prediction of the influence of the 3 major elements and 3 dispersoid forming elements on quench sensitivity. Graphical abstract Highlights

Modeling of GP(I) zone formation during quench in an industrial AA7449 75 mm thick plate

“…Recently, Starink et al proposed a model for linear cooling and strengthening in Al-Zn-Mg-Cu alloys taking into account the formation of three phases during cooling. Their modeling approach considers the formation of the S (Al2CuMg) phase at high temperature, the η phase at medium temperature and a Zn\ \Cu rich platelet phase at lower temperature between 250°C and 150°C [18].…”

Study on quenching sensitivity of 7097 aluminum alloy

“…Moreover, it was shown that the Al 3 Zr dispersoids and grain boundary acted as preferential nucleation sites for the coarse equilibrium η phase ( figure 3). Especially, a small amount of high aspect ratio 'Plate-like' Y phase (it primarily contained with Al, Cu, Zn and Mg elements, and the crystallography was similar to T 1 (Al 2 CuLi) with hexagonal symmetry, lattice parameter a=0.429 nm, c=1.385 nm [27][28][29]) was showed on the dislocations and the dark 'Bean-like' Al 3 Zr dispersoids after 230°C/30 s isothermal treatment ( figure 7). Figure 8 shows the x-ray diffraction (XRD) pattern of the AA7097 after isothermal holding for different times at 320°C.…”

“…The results were consistent with the previous microstructure observations (figures 3 and 6). Besides, the quench-induced Y phase of 7xxx series aluminum alloys was discovered in recent years [27][28][29], which was preferentially precipitated on dislocations [29]. The Y phase was presented at the quenching cooling rate within 3.2 k s −1 and less than 300 k s −1 of AA7085 and AA7150, respectively [4].…”

Prediction and Experimental of Yield Strengths of As-Quenched 7050 Aluminum Alloy Thick Plates after Continuous Quench Cooling

“…2 of 11 Thus far, modeling the as-quenched effect is a potentially complex task because the effect results from the precipitation of multiple phases during quenching. Starink et al proposed a model for the quench-induced hardening effect after linear cooling in Al-Zn-Mg-Cu alloys, which accounted for the formation of three phases during cooling, including the formation of the S (Al 2 CuMg) phase at high temperatures, the η phase at medium temperatures, and a Zn/Cu-rich platelet phase at lower temperatures between 250 • C and 150 • C [8]. However, this model was designed for a long-term age hardening curve and is questionable for the quench-induced hardening effect in short quenching times.…”