Startling C Behavior in Mo-Free Steel Atom Probe Tomography Volumes

Atom probe tomography (APT) is now routinely used to study solute atom segregation at crystalline defects in different materials. The present study reports unexpected observations concerning carbon (C) segregation at dislocations in APT volumes analyzed from two different industrial steel grades. APT analyses reveal that C segregation at dislocations could only be observed with Mo co-segregation. Indeed, transmission electron microscopy (TEM) observations on APT tips and correlative TEM-APT analysis show that despite dislocations being present in the samples prior to APT analyses, C segregation was not observed in the absence of Mo segregation. Statistics on the distribution of C composition in the different APT volumes from Mo-free steels show important discrepancies, with 35% of the volumes exhibiting C content in solid solution five times higher than expected. It is concluded that APT measurements of both C segregation at dislocations and C content in solution in iron may be incorrect due to the possibility of dislocations leaving the APT samples when subjected to a high electric field before or during field evaporation.