Why ferrite nucleates first in bainitic phase transformation???

This answer has lots of details in it hidden in the form of equations and lots of experimental concepts which is hard to explain by writing all here. So, I will explain to you in a short but most insightful way.

Now, you know that when temperature decreases during transformation as we go below A1 we get phases ferrite and cementite which we call pearlite. You might have also gone through this in phase transformations kinetics that nucleation and growth rates increase as we decrease temperature(increase undercooling) then reaches a maximum value and start decreasing.

So, try to think this here. When we are decreasing temperature and following certain conditions as bainite formation is permissible, but due to low-temperature diffusion decreases exponentially and so the nucleation and growth rate. The cementite phase which is first metastable and second have a high C percentage to form is hard to diffuse properly at that time because lots of energy will be needed to spend but in return, we won’t be able to reduce that in the output after forming the cementite.

But the ferrite which needs much less C to form will also need lesser energy spent but on output, we can get a relatively more reduction in free energy. So energetically ferrite is more preferable to nucleate first. You need to know that it’s ferrite is however a supersaturated solution.

Also, it contains lots of C which will either diffuse further at partitions or some at ferrite grains itself as in case of the lower bainite. Also, the important thing to note is that the next ferrite which will form after that will follow the displacive mechanism as iron and other solute elements do not have proper diffusivity for diffusion-control growth. Hence after nucleation, the growth happens as shear displacive.

you can ask me further for any doubts.