Fig. 7 Grain size (a) and misorientation distributions (b) of BM, AS-HAZ and WNZ samples.
The characteristics of precipitates in BM, AS-HAZ and WNZ were investigated by TEM and HRTEM viewed along with the <110>Al projection. The results are shown in Fig. 8, Fig. 9 and Fig. 10, respectively. In Fig. 8a, some sub-structures remained in BM after solid solution treatment. Many coherent Al3(Sc,Zr) particles with typical Ashby-Brown strain characteristics can be observed in grains, however, the grain boundary phase and precipitate-free zone (PFZ) were inconspicuous. At higher magnification, many smaller short rod-shaped precipitates could be distinguished clearly in the grains (Fig. 8b). According to the HRTEM images and corresponding inverse fast Fourier transform (FFT) images, one kind of these rod-shaped precipitates precipitated along (11-1)Al and remained semi-coherent relationship with matrix (framed by rectangle 1), were well accepted as 𝜂′ phases [25, 26]. Another kind of smaller precipitates consisted of several atomic layers in thickness and width, and showed fully coherent with the matrix (framed by rectangle 2 and 3), were known as GPII zones which often found in Al-Zn-Mg-(Cu) alloys [27, 28]. TEM and HRTEM investigations confirmed that GPII zones and 𝜂′ phases were the dominant precipitates in BM.