A certain amount of time is necessary for the formation of different crystalline phases during cooling of the melt. The more thermodynamically stable this phase is, the less time is necessary for its crystallization. With very rapid cooling, there is not enough time for the formation of all the crystalline phases and solid material remains amorphous (metallic glasses form from metal melts). Air cooling is not the fastest way of cooling, but it is fast enough that not all possible crystalline phases can form. On the other hand, with extremely slow cooling in the oven, practically everything that can crystallize from this melt crystallizes.

Thank you for your answer. The solution temperature was far bellow melting point of the alloy. What can be the source of the eutectic type 2? Incipient melting or solid recrystalization? Vadim Verlotski

as Mohammad Najmi asked Vadim Verlotski , allow me to add some more.

Diffusion was working well below melting point, right? Are you sure the structured formed is eutectic, and not eutectoid? Elongated and slightly columnar grained structure requires slow to moderate cooling to form, even be it intermetallic precipitates (eutectic type 2). You can also note tendency of hasty new-phase formation along grain boundary at air-cooling case as well.

Thank you dear Sumit Bhowmick for your good answer.

It's accepted that diffusion is a major participant below melting point.

Also, because there in no evidence of incipient melting during solution heat treatment (as indicated in air-cooled sample), it is obvious that solid diffusion caused type 2 microstructure.

Have you ever seen such a binary microstructure in Ni-based superalloys?

I have no experience of physcially working with Ni-based superalloys, but given the enormous number of alloying elements, such microstructures are not unusal, I guess. You can run some SEM-EDS analysis on these two types of eutectics/eutectoids, (TEM would have been even better), and possibly find some hints about constituents.