As July and almost two week holiday I took faded away like a distant memory, I started to organize results obtained so far and thinking what to do next. Since there was quite a big discrepancy in observations regarding KoBo-ed zinc, a decision was made that new investigations should be conducted. Especially troubling was incoherency in grain sizes observed on different scans taken from different part of sample. New scans was taken along the radius (almost 1 mm) of the Zn wire in both transverse and longitudinal sections of sample. This was aimed at finding whether grain size varies along radius (as it might be expected after deformation that incorporates twisting of extrusion die).
No such tendency was observed. However I found out that microstructure consist of small, equiaxed grains (around 10 microns) embedded inside much bigger (above 150 microns) ones. This kind of microstructure is quite queer and definitely isn't typical for single phase material. Other interesting observed feature is aligning of some small grains into straight intergranular lines. Both of those observations need careful further analysis in order to determine their origin and underlying physical mechanism.