Atom Probe Tomography Investigation of Nanoscale Ordered Precipitates in an Experimental Steel

Summary form only given. The precipitation phenomena in a secondary hardening experimental steel alloyed with Ni and Al have been investigated by atom probe tomography (APT). The overall precipitation behavior during tempering has been sequenced as follows: a constant heating rate of 20degC/min was applied to small rods, and they were subsequently quenched from the desired temperature in order to monitor the development of the microstructure, which consists of a fine dispersion of secondary hardening complex carbides (Mo, Cr, V carbides) and intermetallic B2 (CsCl) ordered NiAl particles. The early stages of precipitation of ordered precipitates was investigated by following the behavior of homo- and hetero-atomic Ni, Al pairs by plotting the pair correlation function as derived from the 3D data-sets. For longer aging times, it appears that complex carbides precipitate together with an intermetallic NiAl phase. However, the composition of both phases was found to vary during tempering; specifically, a significant amount of iron is still present in the core of the ordered phase even in the overaged conditions. Additionally, ordering in NiAl intermetallic precipitates was monitored by probing along a superstructure [200] direction. A large displacement shift of the Ni sublattice with respect to the Al-sublattice in the direction perpendicular to the analysis was observed. This effect has been simulated and can be related to trajectory aberrations during the evaporation sequence