Group theory analyses of transition structures related to the γ→ε transformation in Fe–Mn–Si based alloys

The fcc(γ)→hcp(ε) martensitic transformation in Fe–Mn–Si based shape memory alloys proceeds through a stacking fault mechanism. In the present paper, the symmetry characteristics of stacking faults are described by using the layer group theory, indicating that the intrinsic SF belongs to P6̄2m and the extrinsic fault to P321. The fault tetrahedron exhibits some sort of symmetry too. According to various repeating structures of the close-packed planes, transition states of the formed martensite are listed and exemplified. A thermodynamic method is proposed to calculate their chemical free energies and evaluate their stabilities which are in the order of 2H>4H>5H>6H>8H.