Age-hardening effect and formation of nanoscale composite precipitates in a NiAlMnCu-containing steel

A NiAlMnCu-containing steel during ageing at 500 °C up to 100 h exhibits an interesting age-hardening effect. Careful atom probe tomography investigations confirm that small clusters, enriched with Ni, Al, Mn and Cu, formed in the steel after a short ageing of 0.25 h. Composite precipitates, consisting of a NiAl-rich and a Cu-rich component side by side, formed after 4 h ageing, a phenomenon which corresponded to a peak of 445 VHN in hardness. A decrease in hardness of the steel occurred after ageing for 100 h, due to the coarsening of the composite precipitates. Quantitative chemical composition measurements reveal that early-stage formation of composite precipitates involves the decomposition of metastable solute clusters, and their growth is associated with the further partitioning of solutes from the ferrite matrix into the composite precipitates.