Co-effect of microstructure and surface constraints on plastic deformation in micro- and mesoscaled forming process

The plastic deformation of materials is affected by external boundaries and microstructures, both of which can induce a serious phenomenon, viz., size effects in the microforming process. The interaction of these two kinds of size effects, however, is still not clear, thus significantly hinders the understanding and development of the microforming process. In this study, the co-effect of microstructure and surface constraints on the plastic deformation of material was investigated by using the barreling compression tests (BCT) of micro- and mesoscaled cylindrical pure copper specimens. Through comparing the barreling degree and the distribution of deformation bands of compressed specimens in different size scales, a size effect on plastic deformation caused by the interaction of microstructure and surface constraints was observed. To determine the mechanism of the observed size effect, an extended upper bound solution of barreling was firstly developed by employing the surface layer theory and validated by comparing the predicted average and scatter values of barreling magnitude with the experimental measurements in different size scales. Based on the validated solution, the energies dissipated by intergrain restrictions and surface constraints in the deformation of different scaled specimens were obtained and compared, and the mechanism of size effect on plastic deformation was determined as the change of dominating restrictions to grain rotation from intergrain restriction to surface constraints. Furthermore, the critical size scale point for the transformation of dominating restrictions as well as the dependence of the critical size scale point on the shape of the specimen and surface friction was obtained. The research thus provides an in-depth understanding of the plastic deformation in micro- and mesoscaled deformation and thus facilitates the forming of the desired geometry and shape and achieving of the tailored quality of the micro- and mesoscaled deformed parts.