Grain refinement and mechanical properties improvements in a high strength Cu–Ni–Si alloy during multidirectional forging

•Multidirectional forging is a passport to ultrafine grain structures.•Fiber structure formed after cold rolling.•Nano-grains appeared in the RTMDF-ed and CTMDF-ed samples.•The CTMDF-ed Cu-Ni-Si alloy shows high ultimate tensile strength. Microstructure evolutions and properties variations of a high-strength Cu–Ni–Si alloy during multidirectional forging have been systematically investigated using microstructure characterizations and properties measurements. Experiment results indicated that nano-scale grains formed in the block samples having been multidirectional forged (MDF) at room/cryogenic temperatures. With the increase of the MDF cycle, the sub-grain structure was further refined to nano-grains with tens of nanometers in diameter. The ultimate tensile strength of the cryogenic temperature multidirectional forged sample was 1057 MPa, the yield strength was 923 MPa, and the fracture elongation was 7.2%. Rather than the solid solution strengthening from solutes impeding the glide of dislocation motion and the second phase particle precipitation strengthening from the Orowan looping, the grain boundary strengthening from the Hall–Petch effects plays crucial roles in strengthening the samples having been cryogenic multidirectional forged. These findings indicated that multidirectional forging is a passport to an ultrafine-grained structure and high yield strength material for the nuclear industry.