Nanotwins Formation in Accumulative Roll-Bonded Brass

Accumulative roll-bonding (ARB) is a severe plastic deformation process that uses rolling to produce ultrafine grains in coarse grained metallic materials. In this study, ARB has been applied on 70/30 brass up to 6 cycles at ambient temperature and non-lubricated conditions to apply a true strain up to 4.8 Von Mises strain. Microstructures of ARBed brass samples were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicated that during ARB cycles, the grain size decreased from micron-size to nano-size and mechanical twins were widely observed throughout the microstructure after cycle 1. However, after cycle 3, the twinning activity became significantly limited and deformation occurred via shear bands formation. After cycle 6, the measured average grain size was about 50 nm and nanotwins were observed originating from grain boundaries and gain boundary junctions. With the reduction in the grain size down to nanometer, the pole mechanism was not the dominant mechanism of nanotwin formation and nanotwins were mainly produced via partial dislocation emission from grain boundaries and grain boundary junctions.