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Even though Cu precipitation during isothermal aging of binary Fe-Cu alloys and of Cu-alloyed maraging steel has been carefully studied in the past, no detailed investigations of the early stages of Cu clustering upon continuous aging have been carried out so far. During continuous aging with a heating rate around 15 K min À1 of as-quenched 15-5 PH maraging steel in a differential scanning calorimeter (DSC), two exothermal reactions are observed, one at approximately 300 8C and the other one around 500 8C. These DSC signals are attributed to the nucleation and growth of Fe-rich Cu clusters in the lower temperature range, followed by Cu-enrichment of these clusters and seamless transformation into bcc-Cu precipitates at higher temperatures. Part I of this investigation focuses on the experimental characterization of the mechanisms governing these reactions in a comprehensive manner, where the authors first present the results of this analysis based on dilatometry, differential scanning calorimetry, hardness testing, and 3D atom probe tomography. The latter method is capable of detecting early clusters, which are not yet accessible by transmission electron microscopy. In the companion paper, part II, thermo-kinetic simulations are presented, which support the interpretation of the observed reactions and substantially aid in understanding the underlying mechanisms.

Cu precipitation in steel has been investigated numerous times. Still, a consistent simulation of the nucleation, growth and coarsening kinetics of Cu precipitates is lacking. Major reason for this is the fact that Cu precipitation involves complex physical interactions and mechanisms, which go beyond the classical precipitation models based on evaporation and absorption of precipitate-forming monomers (atoms). In the present work, we attempt a comprehensive modeling approach, incorporating coalescence results from Monte Carlo simulation, prediction of the nucleus composition based on the minimum energy barrier concept, diffusion enhancement from quenched-in vacancies, dislocation pipe diffusion, as well as the transformation sequence of Cu-precipitates from bcc-9R-fcc. Our simulations of number density, radius and phase fraction coincide well with experimental values. The results are consistent over a large temperature range, which is demonstrated in a TTP-plot.