A study of grain size effect on mechanical behaviour, fracture mechanism, texture evolution and springback in microforming process

Size effects play a significant role in microforming process, and any dimensional change on microstructure or specimens can have a great impact on materials’ mechanical properties. In this research, the grain size effect on deformation behaviour, fracture and grain orientation of phosphor bronze foil are investigated in the form of the ratio of materials’ thickness (T) to average grain size (D) by micro tensile tests. The ratio is designed to be greater than, less than and equal to 1 respectively. The results show that the amount of plastic deformation decreases with the decrease of the ratio T/D, which indicates the grain size plays a significant role in the deformation process and grain deformation modes differ when the ratio changes. It is also found that their fractograph reflect different features in terms of micro-dimples and cleavage planes, further demonstrating that when T/D>1, the material has a tendency to experience ductile fracture, while the material would like to conduct brittle fracture when T/D<1. So the ratio T/D can be regards as the divide of ductile fracture and brittle fracture when it is approximately equal to 1. Additionally, for T/D<1, a new constitutive model is proposed based on the classic composite model. The model’s results are compared with the experimental ones and the efficiency of the developed models is verified.