Sequential nucleation of phases in a 17-4PH steel: Microstructural characterisation and mechanical properties

A major concern for 17-4PH steels operating at elevated temperatures is embrittlement due to Fe-rich (alpha) and Cr-enriched (alpha') phase separation, along with precipitation of other detrimental phases. In this study the sequence of microstructural changes at the atomic scale in a 17-4PH steel is characterized by atom probe tomography (APT) at two different ageing temperatures, 480 degrees C and 590 degrees C. In the earliest stages of heat treatment at both temperatures, APT reveals that dislocations and matrix defects are highlighted by the segregation of NbN/CrN ionic species, providing heterogeneous nucleation sites for subsequent Cu-rich precipitates (CRPs) and Nb-rich precipitates, respectively. At the lower temperature, Cr-rich a'-phase also nucleates and ultimately a Mn, Ni, and Si-rich (MNS) phase was observed to form. The evolution in number density and fraction of CRPs and Cr-rich alpha'-phase, the latter of which was not observed at the higher temperature, has been quantified and their respective contributions to the overall precipitation hardening of the material has been estimated. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. This is an open access article under the CC BY license.