THE FORMATION OF POLYTWINNED STRUCTURES IN FE-PT AND FE-PD ALLOYS

1. 1. The polytwinned structures in the FePt and FePd alloys derived from a nucleation and growth process in which the coherent ordered regions form aligned particle arrangements under the influence of the elastic strain energy. The impingement and coalescence of the ordered particles to form twin related structural domains give rise to a high density of APB's within the twin plates. 2. 2. The nuclei form as disks along the {110} planes of the cubic matrix and these nuclei may merge with an order parameter or c/a ratio less than the equilibrium value but as the strain energy of the system relaxes the c/a ratio approaches equilibrium. 3. 3. The polytwinned structure undergo a coarsening process under the mutual influence of the strain and surface energies analogous to "discontinuous coarsening" of lamellar two-phase aggregates resulting from cellular phase separation. © 1991.