Chapter 49 Geometry of dislocation glide in Ll2γ′-phase: TEM observations

Abstract

This chapter discusses the geometry of dislocation glide in L1 2 γ´-phase. It emphasizes the geometry of dislocation glide in L1 2 alloys on three scales: (1) the shape of the spreading dislocations on a scale of microns or more, (2) the gross dissociation into APB- or SISF-separated superpartials on a scale of nanometers, and (3) the fine dissociation into partials of the parent f.c.c, lattice on a scale of angstroms. The chapter presents transmission electron microscopy (TEM) observations of dislocations and other related defects that characterize these two contrasting slip modes in γ´ and, on this basis, describes the controlling dislocation mechanisms. When the temperature exceeds about 800°C, the slip direction changes to (100), the shortest lattice translation vector in L1 2 ; the origin for this transition is described in the chapter. It was using a transmission electron microscope that Marcinkowski, Brown, and Fisher obtained the first direct experimental evidence for an important hypothesis, proposed almost half a century ago by Koehler and Seitz, about dislocations in superlattice structures: the dislocations would propagate more readily if they existed in groups coupled by antiphase boundaries (APB).