The Formation of Twin Metallic Crystals

Abstract

IN the discussion in NATURE (Jan. 22, p. 120, and Mar. 12, p. 392), Mr. McKeehan has taken exception to statements of Carpenter and Tamura in a paper on the above subject on the grounds that the method of formation of twins depicted by them brings atom centres too close together. Twinning by reflection about a plane is considered, and the discussion hinges on the precise location of this plane with reference to the planes of atoms. Geometrically, a twin crystal of this type consists of two individuals united symmetrically about a plane, which is not one of systematic symmetry but is a possible crystal face (Tutton, “Crystallography and Practical Crystal Measurement,” 2nd ed. vol. 1, p. 500, where it is also stated that the plane of twinning is “usually one with low indices and indeed very often a primary face”). In view of the improbably small distance of approach of atoms required by Carpenter and Tamura's hypotheses, it appeared to be worth while examining the effect of adding to the above geometrical law of twinning the physical conditions (1) that the reflection plane can only be one such that the operation of twinning does not bring atom centres closer to one another than the closest distance of approach of atoms in either component of the twin, and (2) that the components of the twin have in common at least one plane of atoms. Briefly, these conditions imply minimum stress and maximum continuity of structure.