Behaviours of Metallic Materials in Initial Stage of Plastic Deformation

Investigation of the behaviour of plastic deformation in materials is very important in the fields of plastic working and strength evaluation of materials. Metallic materials are generally polycrystalline aggregate, and it is seldom possible to deduce the deformation behaviour of an aggregate directly from the behaviour of single crystals because of the interaction of each grain with the neighboring grains and grain boundaries. In the present paper, the concept of G.I. Taylor that the plastic strain in each grain of a polycrystal is the same as that in a bulk specimen was assumed, and the deformation behaviour in polycrystals was studied in relation to the fundamental equations for X-ray stress measurement The lattice strains of aluminum and low carbon steel measured by X-ray diffraction technique agreed with the analytical ones at a state of uniaxial tension in the initial stage of plastic deformation, indicating that the stress on each grain depends upon its orientation of crystal in plastic deformation. However, it is considered that the effect of anisotropy of work-hardening on each slip system may influence the measured values as the applied strain increases. Examination of the unloaded conditions after plastic deformation made it clear that the stress was unloaded uniformly in each grain. Therefore, it is considered that the uniform local stress model can be applied to both the loading and unloading processes in the case elastic deformation. Since the present experimental results can be explained by accepting this analysis, it is concluded that the behaviour of these materials is approximated by Taylor's model in the initial stage of plastic deformation.