Plastic deformation of silver chloride I. Internal stresses and the glide mechanism

Abstract

The experiments of Obreimow & Schubnikoff (1927) on the birefringence produced by the plastic deformation of single crystals of rock salt have been extended to a polycrystallirie material. Rolled sheets of silver chloride have been recrystallized and then deformed plastically in various ways—by simple extension and by bending, for example. The sheets are transparent and very ductile and, since silver chloride is cubic in structure, the birefringence patterns observed under the microscope provide a picture of the distribution of the internal stresses uncomplicated by natural double refractions. It is suggested that results obtained with this optical method are applicable to metals. Silver chloride appears to deform by glide, and when the glide packets are observed on edge the glide plane and glide direction to the crystal structure has been studied by making observations upon these bands and upon the glide lines formed on the surfaces of bars of square cross-section consisting effectively of chains of single crystals. The orientations of the fifteen sets of glide bands examined in this way were all consistent with glide movements in a <110> direction; the glide plane, however, was not always a crystallographic plane of low indices. In the six cases in which the measurement was possible, it lay within 9° of the plane in the.<110> zone on which the maximum shear stress, resolved in the <110> direction, acted. It is concluded that silver chloride deforms by ‘pencil glide’, the mechanism postulated by Taylor & Elam in 1926 to explain the plastic behaviour of a-iron. The transmission of pencil glide across grain boundaries is discussed. The residual stresses observed by the optical method in polycrystalline sheets may be divided into three groups: (1) A system of stresses set up between the glide zones of each grain and alternating with a period equal to the spacing of the glide zones. A detailed analysis of these is given in the second paper (part II). (2) Alternating stresses produced when a system of glide zones meets a grain boundary. (3) ‘Heyn stresses’ produced by the nonuniformity of plastic deformation from grain to grain.