Interfacial energy and structure in F.C.C. metals and alloys

Abstract

Abstract It is shown by transmission electron microscopy that a rather obvious distinction exists for the interfacial free energies of high-angle grain boundaries, low-angle grain boundaries, and twin boundaries in f.c.c. metals and alloys. The characteristics of these differences are analysed, and a balance of interfacial free energy argument is used to calculate relative interfacial free energy ratios. The average low-angle grain boundary free energy in nickel and 304 stainless steel is found to be 383 and 342 ergs/cm2 respectively, corresponding to average misorientations of 1·9° and 1·7°. The coherent twin boundary free energy in nickel and stainless steel is found to be 41 and 18 ergs/cm2 respectively, while the ratio of coherent twin boundary free energy to non-coherent twin boundary free energy is measured as 0·070, 0·090 and 0·100 in nickel, 304 stainless steel and Cu-5 at. % Al respectively. Dislocation core energies are measured for nickel and 304 stainless steel to be 1·2 and 1·3 ev per atomic length of edge dislocation, representing approximately 6% of the corresponding dislocation self-energies. Low-angle grain boundaries of the general tilt variety possess a predominantly edge dislocation structure, while high-angle grain boundaries in f.c.c. metals and alloys possess a variable structure consisting of infrequent dislocations, steps, ledges, and a high proportion of coincident boundary-plane sections. The interfacial free energy of high-angle grain boundaries can vary from three to six times the average low-angle grain boundary free energy over a range of misorientations 10° < Θ < 90°. Grain boundaries having energies between the mean values of low-angle and high-angle grain boundaries, and misorientations 2° < Θ < 8°, possess a complex, transitional structure. Grain boundaries in f.c.c. metals and alloys act as sources of imperfections; specifically dislocations are emitted from steps or ledges in the grain boundaries which in most instances are coincident with the {111} planes. A high compatibility of interfacial structure and energetics is observed, and a simple phenomeno-logical model is proposed for the observations of grain boundary structure which is essentially consistent with the experimental findings. This model envisages grain boundary formation as the result of atomic slip on {111} planes, or the analogous shuffling of other coincidence planes to produce dislocation-ledge structures.