Precipitation of a new platelet phase during the quenching of an Al-Zn-Mg-Cu alloy

Abstract

A previously undescribed high aspect ratio strengthening platelet phase, herein named the Y-phase, has been identified in a commercial Al-Zn-Mg-Cu alloy. Differential scanning calorimetry indicates that this phase only precipitates at temperature and cooling rate of about 150–250 °C and 0.05–300 K/s, respectively. This precipitate is shown to be responsible for a noticeable improvement in mechanical properties. Aberration corrected scanning transmission electron microscopy demonstrates the minimal thickness (~1.4 nm) precipitate plates are isostructural to those of the T1 (Al2CuLi) phase observed in Al-Cu-Li alloys. Low voltage chemical analysis by energy dispersive X-ray spectroscopy and electron energy loss spectroscopy gives evidence of the spatial partitioning of the Al, Cu and Zn within the Y-phase, as well as demonstrating the incorporation of a small amount of Mg.

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

“…a maximum of scattering intensity vs. q is present) means that the distance between clusters is of the same order as their size. Clusters with a blocky structure [29], GP(I) zones and the Y-phase [16] have been evidenced in Al-Zn-Mg-Cu alloys after fast cooling. Yet, phases with a high aspect-ratio such as GP(II) zones [17], the Y-phase or T 1 (Al 2 CuLi) phase lead to a highly non-isotropic scattering pattern [39].…”

“…In a more recent study, it was shown that this low temperature reaction originated from the precipitation of the platelet shaped Y-phase. The Y-phase features a high aspect ratio and a similar structure as the T 1 (Al 2 CuLi) phase, which is observed in Al-Cu-Li alloys [16]. Schloth et al evidenced by in-situ SAXS measurements the formation of nanosized precipitates in the AA7449 alloy during cooling close to industrial practice, which harden the material during quench [17].…”

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].…”

On the Characterization of a Hitherto Unreported Icosahedral Quasicrystal Phase in Additively Manufactured Aluminum Alloy AA7075

“…This reveals that the complexity of physical metallurgy -even for well-studied systems -continues to present itself and is abetted by advanced manufacturing and characterisation techniques. In a similar vein, a new low temperature phase in the age hardening sequence of 7xxx series Al-alloys was only recently revealed using advanced synchrotron characterisation [18], suggesting just how little is known about the engineering of inorganic materials as the next industrial revolution emerges. The images were collected upon tilting the same particle, exhibiting strong diffraction contrast (red arrow), in all three zone axes.…”