Transmission electron microscopy study of cross-slip and of Kear-Wilsdorf locks in L12ordered Ni3Fe

Abstract

Abstract In L12 ordered alloys the cross-slip modes of the superlattice dislocations are of special interest since a sessile configuration (Kear-Wilsdorf (KW) lock) can be formed by cross-slip from {111} glide planes to {010} planes where the screws are assumed to be locked. Single crystals of ordered Ni3Fe were deformed at various temperatures below the critical order-disorder temperature; contrary to most Ll2 ordered alloys Ni3Fe does not show an increase of the flow stress with increasing temperature. At room temperature the density of screws is very low, indicating that they are annihilated by cross-slip. The superlattice screw dislocations show a large dissociation (antiphase-boundary (APB) fault) on (111) cross-slip planes but none on (010) planes. In specimens deformed at 600 K the weak-beam method of TEM was applied to analyse the superlattice dislocations. The four-fold dissociation of the superlattice dislocations with a character near edge orientation is clearly resolved. The screws are either dissociated on a (111) plane, on a (111) plane or on a (010) plane; the last leads to the formation of a KW lock. The observed KW locks show a large dissociation on the (010) planes. Since the density of the KW locks is rather low they have no effect on the mechanical properties. The reason for their low density is discussed, and it seems that the cross-slip behaviour is a complex function of both the anisotropy of the APB energy and the core structure of the dislocations. It is assumed that the cross-slipped configurations that are observed in Ni3Fe are formed mainly by the help of localized internal stresses.