In the displacive mechanism the change in crystal structure also alters the macroscopic shape of the sample when the latter is not constrained. The shape deformation during constrained transformation is accommodated by a combination of elastic and plastic strains in the surrounding matrix. It is the diffusion of atoms that leads to the new crystal structure during a reconstructive transformation. It might be argued that the orientation relationships between the forms of iron are obvious from these crystallographic similarities. Extreme pressure changes that, with the core electron bands widening, overlapping and hence participating more effectively in the bonding process leading to changes in the relative stabilities of the allotropes of iron. The iron-carbon phase diagram is not strictly applicable given the large curvature of the iron nanoparticles, which would tend to favour the formation of the most stable phase.