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.