A Study of Dislocation Arrays at Spherical Indentations in LiF as a Function of Indentation Stress and Strain

Abstract

Abstract A systematic study has been made of the dislocation arrays produced by indenting (001) surfaces of LiF with steel and nylon spheres. The arrays are discussed in terms of position along a curve relating “indentation stress” to “indentation strain”, these parameters being determined by the geometry of the contact between indenter and specimen. Three stages of behaviour are distinguished: (i) elastic stage, to which the classical Hertzian contact theory is applicable; (ii) initial yield stage, to which standard yield criteria may be applied to predict the spatial location and critical resolved shear stress for initiation of dislocation motion; (iii) subsequent growth of plastic zone, in which the relative activity of the various slip systems determine the indentation behaviour. A mechanism for the initiation of dislocation flow below an indenting sphere has been proposed on theoretical grounds, and subsequently supported by experimental evidence; this mechanism differs from alternative proposals in that the dislocation sources operate within the interior of the crystal rather than at the crystal surface. An evaluation of the dislocation activation stress for LiF is thereby made, the value obtained being in reasonable agreement with results taken from conventional compression tests. The indentation stress‐strain behaviour is then discussed in the light of current thought on indentation theory, and some shortcomings in present ideas are pointed out.