HAADF-TEM Tomography of the precipitation state in an Al-Zn-Mg alloy

Nano-tomography in Transmission Electron Microscopy (TEM) has been recently and successfully applied to the characterization of various precipitates embedded in a matrix (see for example [1,2]). In this context we have undertaken a study of the precipitation state of MgZn2-based particles in the Al-Mg-Zn system. The material is a medium-strength commercial alloy extensively used for automotive applications, where weldability is an important feature. A previous characterization by means of TEM, Small-Angle X-ray Scattering (SAXS) and Atom Probe Tomography (APT) [3] has shown that, in the so-called T7 tempering state (6 hrs. at 170°C): (i) the stable η-MgZn2 phase (hexagonal P63/mmc structure) mainly precipitates under the form of spheres, while minor variants, called η′, adopt a platelet shape; (ii) the total precipitated volume fraction, as measured by SAXS, is 2.54 ± 0.3 %; (iii) the precipitates mean radius is 3.6 nm with a standard deviation of 1.0 nm. According to the above size and volume fraction, TEM tomography appears as an interesting alternative tool to SAXS, with which it can be delicate to discern the η and η′ particles, and to APT, for which the density of precipitates makes any statistical approach hazardous.