Fault energies in ordered and disordered Ni3Fe

Abstract

The weak-beam method of electron microscopy has been used to study the dissociation and the fault energies in disordered and ordered Ni₃Fe. For the ordered structure this is of particular interest since in this case the plastic behaviour is determined by the dissociation mode of the superlattice dislocations. Single crystals of Ni₃Fe were quenched to suppress the formation of the long-range ordered phase and strained at room temperature. From the dissociation of the Shockley partials the stacking fault energy of the disordered f.c.c. phase γ = 95 ± 14 mJ m ⁻² was deduced. A section of the disordered and deformed crystal was subsequently fully ordered (Ll₂ superlattice structure) and additionally deformed. The complex stacking fault energy γ_csF = 123 ± 13 mJ m ⁻² was deduced from dissociated unit dislocations which were sessile since they had been introduced before ordering. The deformation of the ordered crystal proceeds by the formation of superlattice dislocations. These dislocations show a four-fold dissociation and measurements of the APB energy yielded a value of γ_APB= 133 ± 8 mJ m⁻², which is in good agreement with recently calculated values. The dissociation can be used to explain the observed dislocation structure.