Atom probe characterization of precipitation in an aged Cu–Ni–P alloy

A temporal evolution of clusters associated with age hardening behavior in a Cu–Ni–P alloy during ageing at 250 °C for up to 100 ks after solution treatment has been carried out. A three-dimensional atom probe (3DAP) analysis has showed that Ni–P clusters are present in the as-quenched condition, and that the cluster density increases as the ageing time increases. The clusters have a wide range of Ni/P ratios when they are relatively small, whereas larger clusters exhibit a narrow distribution of the Ni/P ratio, approaching a ratio of approximately two. These results would indicate that the clusters with various Ni/P ratios form at the early stage of precipitation and the ratio approaches a value identical to that of the equilibrium phase at 250 °C as the clusters enlarge during ageing.

Research highlights
► We characterize the clustering behavior in a Cu-Ni-P alloy during ageing at 250°C.
► The clusters have a wide range of Ni/P ratios when they are relatively small.
► Larger clusters exhibit a narrow distribution of the ratio.
► Hardness increases almost linearly with the logarithm of ageing time beyond 100s.
► We believe increasing density and size of the clusters leads to the age hardening.