Oxide Dispersion Strengthened

APT characterization of ultrastable particles and solute segregation to dislocations in MA/ODS ferritic alloys

Background:

Mechanical-alloying (MA) of powders enables new classes of alloys to be developed which contain immiscible or low solubility phases. The mechanical milling also produces alloys that are far from the equilibrium state. This combination enables new types of solute interactions to be studied and exploited. Mechanical-alloying of fine pre-alloyed metal and Y₂O₃ powders has been shown to produce oxide dispersion-strengthened (ODS) ferritic alloys with dramatically improved high temperature mechanical properties. This material has been characterized with a variety of techniques including atom probe tomography and electron microscopy in order to establish the microstructural features responsible for the improve mechanical properties.

Accomplishment:

Atom probe tomography revealed that the improved high temperature mechanical properties of a MA/ODS alloy [Fe-12.3 wt% Cr-3% W-0.39% Ti-0.25% Y₂O₃] are correlated with the presence of ultrastable 4-nm-diameter Ti-, Y- and O-enriched particles. The Ti-, Y- and O-enriched particles were stable during creep experiments for at least 4,000 h at 850°C and 14,500 h at 800°C. Electron microscopy revealed that that the MA/ODS alloy had undergone partial recrystallization after annealing in vacuum for 1 h at 1300 ºC. Atom probe tomography revealed that the Ti-, Y- and O-enriched particles were still present after annealing for 10 h at 1300 ºC and that only a small amount of coarsening had occurred. In addition, significant enrichments of Cr, W, Ti, Y, O, C and B were measured in the vicinity of dislocations, which indicates that both interstitial and substitutional atoms can form Cottrell atmospheres. Despite the solute enrichment at the dislocation, no evidence of preferential coarsening of the Ti-, Y- and O-enriched particles near dislocations was observed. Atom probe tomography also revealed that the oxygen content of the ferrite was significantly higher than the equilibrium oxygen level. In addition, the oxygen atoms were frequently associated with the titanium, chromium and tungsten atoms indicating that these atoms were interacting in the matrix to possibly influence solute diffusion and the coarsening of the particles. This research paves a new way to understand the interaction of interstitial and substitutional solute atoms in alloys far from their equilibrium conditions.

SHaRE program collaborative research by M. K. Miller, D. T. Hoelzer, S. S. Babu, D. J. Larson, E. A. Kenik, P. J. Maziasz and K. F. Russell (ORNL)