Precipitation strengthening mechanisms in Cu/Ni alloyed steel

The microstructural evolution and yield strength changes of medium-Mn steels with Cu， Ni， and Cu/Ni composite additions after aging at 500 ℃ are systematically compared using electron backscatter diffraction （EBSD）， transmission electron microscopy （TEM） with energy-dispersive spectroscopy （EDS）， and mechanical property tests. The precipitation strengthening mechanism is further revealed. The results show that aging at 500 ℃ increases the yield strength by approximately 125， 540 MPa， and 575 MPa for the Cu， Ni， and Cu/Ni alloyed steels， respectively， due to precipitation strengthening. The strengthening effect in the Ni and Cu/Ni alloyed steels is much greater than that in the Cu alloyed one， mainly because the number of precipitated nanoparticles in the former two steels is much greater than that in the Cu alloyed steel. Although Cu addition promotes the precipitation of NiAl phases， leading to larger size and higher number of Cu-NiAl composite precipitates in the Cu/Ni alloyed steel compared with the single NiAl precipitates in the Ni alloyed steel， the precipitation strengthening effect of the Cu-NiAl composite precipitates is not significantly higher than that of the single NiAl precipitates. This is because the latter， with its single precipitate sizes below the critical size， strengthen through the dislocation shearing mechanism， whereas the former， with its composite precipitates exceeding the critical size， strengthen via the dislocation Orowan mechanism， resulting in weakened precipitation strengthening. Consequently， the aging precipitation strengthening effect of Cu/Ni steel is not higher than that of the Ni steel.