Study of deformation and ductile fracture behaviors in micro-scale deformation using a combined surface layer and grain boundary strengthening model

Abstract

A constitutive model considering the composition of surface grain, grain boundary and grain interior and their contributions to the flow stress or strength of materials in micro-scale plastic deformation is developed and termed as a combined surface layer and grain boundary strengthening model in this research. To determine the composition of the three interior microstructural parts of materials, optical microscope and digital image processing technologies are employed. A series of micro-tensile experiments using the specimens with three different geometrical shapes and microstructural grain sizes are conducted for study of deformation and ductile fracture behaviors of material. The model is implemented in finite element analysis and validated via physical experiments. The relationship among fracture strain, grain size and stress triaxiality of the deforming material is thus established. It is found both fracture strain and stress triaxiality increase with the decrease of grain size, while the high stress triaxiality leads to small fracture strain for the given grain size. Through observation of the fractographs, it is revealed that the domination of shear fracture in the ‘cup-cone’ fracture increases with grain size. The research thus helps understand the ductile fracture in micro-scale deformation and facilitates deformation based working process determination and application.