The interfacial energy of coherent twin boundaries in copper

In 1951, Fullman determined the interfacial energy of coherent twin boundaries in copper annealed at 950°c, by optical examination of the equilibrium dihedral angles developed at the intersection of surface traces of twin and grain boundary planes. The value 0·045 was obtained for the ratio γt/γG of the twin and grain boundary energies. From similar observations of thermally etched twin and grain boundary groove angles at a copper-lead vapour interface, the result 0·026 was deduced for γt/γG. Using the mean value 0·035 for the ratio γt/γG, and assuming γG to be approximately 600 erg cm.−2 in copper (Fisher and Dunn 1952), γt is about 21 erg cm−2. In recent years, a knowledge of the energy γt of coherent twin boundaries has assumed importance as a guide to the stacking fault energy and dislocation width, and Follman'a result has been the basis of various estimates of these quantities in copper (Seeger and Schoeok 1953) and other metals (Hirsoh and Thornton 1958). A more precise technique to measure γt/γG would therefore be very useful, and it is the purpose of this letter to report the improvement in precision which electron transmission microscopy can give, compared to the optical method used by Fullman.