SUPERLATTICE DISLOCATIONS AND ANTIPHASE BOUNDARIES IN FE3SI

TEM investigations of D0(3)-ordered Fe3Si reveal dislocations belongin g to the slip system [100]{001}. The glissile superlattice dislocation s consist of two superlattice partial dislocations with Burgers vector b=a(0)/2[100](100) which are linked together by an antiphase (APE(NNN )) which changes the next nearest neighbour. The dissociation width is of the order of 100 nm. The TEM observations are analysed by means of thermodynamics. The state of order of the undisturbed crystal and the free energy of the APE using the concept of thermodynamic relaxation of an antiphase boundary are calculated. The behaviour of the APE in t he vicinity of a phase transition of the first and second kind is stud ied. It is shown that the APE can be seen as the B2 nucleus during pha se transition D0(3)-->B2 below the Curie temperature.