What drives polymorphism in metal alloys?

[abstracted6]

I was wondering what the driving force behind polymorphism is. What causes polymorphic materials to change crystal structures at certain temperatures? Such as the iron phases alpha-ferrite, gamma-austenite, delta-ferrite, hexaferrum?

 

[pukb]

The driving force for poly morphism in metal alloys is difference in free energies of two phases at different temperatures