Interferometric studies on the directional hardness of some single metal crystal

Abstract

A directional effect has been observed when indentation hardness tests were made with a Double-Cone indenter on single crystals of Tin and Bismuth, The distortion around the impressions made during, the teats was studied by multiple beam interferometric techniques. The surface contours shown by the fringes revealed deformation of an unsymmetrical nature, with "piling-up" and "sinking-in occurring in directions mutually at right angles. The fringes also revealed slip traces, of step heights of the order 200 A.u. or less, crossing the hill formations. As the direction of indentation on the surface was changed, the observed interference patterns varied continuously, the amount of deformation passing from a maximum to a minimum and returning to the original form when the indenter had rotated through 180°. Because of the presence of the slip traces an attempt was made to explain the directional effects observed, in terms of the plastic deformation properties of the crystals and the direction of the forces exerted on the crystal during indentation. As the Indenter was rotated with respect to the crystal, it was found that the value of the shear stress t component on the activated slip planes varied in a similar manner as the measured lengths of the indentations. It is suggested that the hardness variation is a measure of the change in yield stress in the crystal, as the, deformation takes place under strainless conditions.