Mechanisms for suppressing discontinuous precipitation and improving mechanical properties of NiAl-strengthened steels through nanoscale Cu partitioning

Control of discontinuous and continuous precipitation is crucial for tailoring the microstructure and mechanical properties of NiAl-strengthened steels. Through a combination of atom probe tomography, transmission electron microscopy, electron backscatter diffraction, first-principles calculations, and mechanical tests, we demonstrate that Cu is effective in not only promoting the nano-scale continuous NiAl precipitation but also in suppressing the coarse-scale discontinuous NiAl precipitation at grain boundaries, which results in the development of new NiAl-strengthened steels with a high yield strength (1400 MPa) and good ductility (10%). Our analyses indicate that the mechanisms for suppressing discontinuous NiAl precipitation are twofold. The main one is the acceleration of continuous NiAl precipitation through Cu partitioning, which swiftly reduces the matrix supersaturation, thereby decreasing the chemical driving force for the growth of discontinuous precipitates. The other is the reduction of grain boundary energy through Cu segregation, which is likely to decrease the nucleation rate of discontinuous precipitates. Consequently, Cu increases the number density of continuous NiAl nanoparticles by more than fivefold, which leads to a twofold enhancement in the strengthening and an improvement in the over-aging resistance of NiAl-strengthened steels. The effects of Cu on the precipitation strengthening mechanisms were quantitatively evaluated. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.