Early Stages of Cu Precipitation in 15-5 PH Maraging Steel Revisited - Part I: Experimental Analysis

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 15Kmin(-1) of as-quenched 15-5 PH maraging steel in a differential scanning calorimeter (DSC), two exothermal reactions are observed, one at approximately 300 degrees C and the other one around 500 degrees C. 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.