The influence of the difference of orientation of two crystals on the mechanical effect of their boundary

Abstract

Abstract The influence of the crystal boundaries on the mechanical properties of metals has long been recognized, and various theories have been advanced regarding the structure of the boundary region between adjacent crystals. Since in general the lattices of two neighbouring crystals growing from independently formed nuclei will not register, there must be some modification of the lattices where they join. The two principal theories as to the nature of this modification are (a) that a structureless or amorphous layer of a thickness of at least some tens of atoms occupies the space between crystals, and (b) that the atoms arranged on a transitional lattice joining one crystals to another. There are two ways of distinguishing between these theories. In the first place, the amorphous theory regards the atoms of the boundary layer as being disposed at random, whereas the transitional theory considers each atoms as having a calculable position; and secondly, the structure and properties of the amorphous layer should be independent of the relative orientations of the axes of the bounding crystals, while a transitional lattice should vary in its structure and properties according to the angle through which the transition is made. That the second of these criteria can be applied has already been indicated by the author (Chalmers 1937). A number of specimens of tin cylindrical form, some consisting of a few crystals with longitudinal boundaries, were subjected to tensile tests, and the results showed that the different behaviour of different specimens could only be accounted for by the hypothesis that the effect of a crystal boundary depends on the difference of orientation of the crystals.