The different nanoprecipitates formed in a 6061 aluminum alloy during aging at 453 K (180 °C), with or without 168 hours of pre-natural aging (NA), and the age-hardening response of the alloy were investigated by atom probe tomography (APT) and hardness testing. A hardness plateau developed between 2 and 8 hours in both the artificial aging (AA) and artificial aging with pre-natural aging (NAAA) samples. The hardness of NAAA samples was lower than that of AA samples when artificially aged for the same time. A 168-hour NA led to the formation of solute atom clusters in the matrix. The NA accelerated the precipitation kinetics of the following AA. The solute atom clusters gave the highest hardness increment per unit volume fraction. The β″ precipitates were dominant in the samples at the hardness plateau. The average normalized Mg:Si ratios of the solute atom clusters and GP zones were near 1. The average Mg:Si ratio of β″ precipitates increased from 1.3 to 1.5 upon aging for 2 hours. The microstructural evolution of samples with or without NA and its influence on the strengthening effects are discussed based on the experimental results.

•A transition phase was observed at the coherent interfaces of M23C6 and matrix.•The transition phase has hexagonal structure, and is coherent with matrix and M23C6.•The M23C6 transforms from the matrix directly at the incoherent phase interface.The growth mechanism of grain boundary M23C6 carbides in nickel base Alloy 690 after aging at 715 °C was investigated by high resolution transmission electron microscopy. The grain boundary carbides have coherent orientation relationship with only one side of the matrix. The incoherent phase interface between M23C6 and matrix was curved, and did not lie on any specific crystal plane. The M23C6 carbide transforms from the matrix phase directly at the incoherent interface. The flat coherent phase interface generally lies on low index crystal planes, such as {011} and {111} planes. The M23C6 carbide transforms from a transition phase found at curved coherent phase interface. The transition phase has a complex hexagonal crystal structure, and has coherent orientation relationship with matrix and M23C6: {111}matrix//{0001}transition//{111}carbide, <112¯>matrix//<21¯10>transition//<112¯>carbide. The crystal lattice constants of transition phase are ctransition=3×amatrix and atransition=6/2×amatrix. Based on the experimental results, the growth mechanism of M23C6 and the formation mechanism of transition phase are discussed.

The grain boundary segregation in Alloy 690 was investigated by atom probe tomography. B, C and Si segregated at the grain boundary. The high concentration regions for each segregation element form a set of straight arrays that are parallel to each other in the grain boundary plane. The concentration fluctuation has a periodicity of about 7 nm in the grain boundary plane. Before the Cr23C6 nucleation at grain boundaries, the C–Cr co-segregate on one side of the grain boundaries while not the exact grain boundary core regions have been detected. The reasons why grain boundary carbides have coherent orientation relationship only with one side of nearby grain which grain boundary is located at high index crystal plane were discussed.Highlights► Grain boundary segregation in Alloy 690 was investigated by atom probe tomography. ► B, C and Si segregate at the grain boundary. ► Concentration of segregated atoms periodicity fluctuated in the grain boundary plane. ► C and Cr co-segregate on one side of the grain boundary before carbide nucleation.