Technical Papers and Notes - Institute of Metals Division - Size Effects in the Deforrmation of Aluminum

In order to observe size effects associated with a grain boundary, aluminum single crystals and bicrystals of various cross-sectional dimensions were deformed in tension. Typical stress-strain curves are presented in conjunction with measurements of the effects of changes in crystal dimensions. Some effects of the growth rate used in preparation of crystals are also described. It is indicated that the macroscopic constraints at a grain boundary are responsible for the most prominent grain-boundary effect, the production of multiple slip. Two measurements of the width of the region of double slip are in agreement. It is concluded that the yield stress of aluminum is affected markedly by the inherent oxide coating. THE general problem to be examined is that of dimensional effects in the plastic deformation of metal crystals. These effects will be termed "size effects" and are here defined as follows: A size effect occurs in a series of crystals if their plastic-deformation properties are a function of crystal dimensions. Clearly then size effects depend on the nature of the boundary conditions imposed on the crystal. Crystal boundaries can be of three types: interphase, inter-crystalline, and free surface. Such deformation effects as are caused by distribution of phases, grain size, lineage structure, and single crystal external geometry are specific types of size effects. This discussion will be confined primarily to grain-boundary and single-crystal size effects, though in practice most "single crystals" not only have a number of small angle boundaries but also have one or more surface phases, so that the boundary conditions appropriate to a free surface do not necessarily apply. Hence, to this extent interphase size effects will be considered. The question of size effects associated with grain boundaries (called here intercrystalline size effects)