K.F. Russell

Review of atom probe FIB-based specimen preparation methods.

Several FIB-based methods that have been developed to fabricate needle-shaped atom probe specimens from a variety of specimen geometries, and site-specific regions are reviewed. These methods have enabled electronic device structures to be characterized. The atom probe may be used to quantify the level and range of gallium implantation and has demonstrated that the use of low accelerating voltages during the final stages of milling can dramatically reduce the extent of gallium implantation.

Atom Probe Tomography of Nanoscale Particles in ODS Ferritic Alloys

An atom probe tomography characterization of the microstructure of as-processed and crept mechanically-alloyed, oxide-dispersion-strengthened (MA/ODS) ferritic alloys has been performed. The significant enrichments of Cr, W, Ti, Y, O, C and B in the vicinity of dislocations and the presence of ultrastable 4-nm-diameter Ti-, Y- and O-enriched particles appears to be responsible for their improved high temperature mechanical properties.

Atom probe characterization of the microstructure of nuclear pressure vessel surveillance materials after neutron irradiation and after annealing treatments

Abstract Microstructural changes due to neutron irradiation of weld and forging materials were characterized using the atom probe field ion microscope (APFIM). Neutron-induced clusters containing Cu, P, Ni, Mn and Si were detected in the high copper weld (0.24 at.% Cu) after irradiation to fluences of 6.6 × 1022 and 3.47 × 1023 n m−2; only phosphorus atmospheres were observed in the low copper forging material (0.02 at.% Cu) irradiated to an intermediate fluence of 1.5 × 1023 n m−2. These results are in agreement with previous studies and with their respective measured transition temperature shifts. In addition, APFIM experiments were carried out on the high fluence weld material after two post-irradiation annealing treatments. The first annealing treatment of 168 h at 454°C is similar to the proposed condition for in situ pressure vessel annealing and the second, 29 h at 610°C, is similar to the final stress relief heat treatment employed in vessel fabrication. Annealing at 454°C led to coarsening of the copper-enriched precipitates and a 92% recovery of the radiation-induced transition temperature shift. Essentially complete rehomogenization of the solutes was obtained in the simulated stress relief treatment at 610°C.

Embrittlement of low copper VVER 440 surveillance samples neutron-irradiated to high fluences

Abstract An atom probe tomography microstructural characterization of low copper (0.06 at.% Cu) surveillance samples from a VVER 440 reactor has revealed manganese and silicon segregation to dislocations and other ultrafine features in neutron-irradiated base and weld materials (fluences 1×10 25 m −2 and 5×10 24 m −2 , E>0.5 MeV, respectively). The results indicate that there is an additional mechanism of embrittlement during neutron irradiation that manifests itself at high fluences.

Low temperature copper solubilities in Fe-Cu-Ni

An atom probe characterization of the copper concentration in the matrix of a model Fe–1.1at.%Cu–1.4% Ni alloy has been performed after isothermal ageing for various extended times at low temperatures. The matrix copper concentrations from material annealed at 500, 550 and 600°C are approximately 10% lower than the equilibrium values predicted from the Thermocalc™ program with the Kaufman database. Isothermal annealing for 4000 h at either 400 or 300°C was not sufficient to attain the equilibrium copper concentration. The kinetics of copper precipitation was investigated with the Starink-Zahra model derived for nucleation and growth reactions and a good fit with the atom probe data was found for an activation energy of 250 kJ mol?1.

Improvement in the ductility of molybdenum alloys due to grain boundary segregation

Abstract An improvement in the tensile ductility from the traditional 3–20% in 6.35-mm-thick molybdenum weldments has been achieved through the addition of Zr, Al, C and B at the ppm level. Atom probe tomography has revealed segregation of Zr, B and C to and depletion of O at the grain boundaries in the base metal and the heat affected zone.

APFIM studies of the phase transformations in thermally aged ferritic FeCuNi alloys: comparison with aging under neutron irradiation

Abstract An atom probe field ion microscopy characterization has been performed on a ternary Fe1.28wt%Cu1.43wt%Ni model alloy. Two sets of heat treatments at 400 and 500°C were selected to follow the decomposition of the solid solution. The atom probe results suggest that the precipitation of copper from the body-centered cubic matrix involves classical nucleation, growth and coarsening processes. Nickel does not appear to be involved in these processes. The nickel and iron detected in the small particles in the early stage is rejected from the particles during subsequent growth and coarsening. The comparison of these data with those observed in neutron-irradiated pressure vessel steels and in Fe0.28wt%Cu0.7wt%Ni model steel indicates that thermal aging of ternary FeCuNi model alloys is not an appropriate process to elucidate the phase transformations in neutron-irradiated pressure vessel steels, and that the high nickel level detected in the clusters in the irradiated pressure vessel steels may be due to a “radiation-induced” rather than a “radiation-enhanced” precipitation process.

Influence of long-term thermal aging on the microstructural evolution of nuclear reactor pressure vessel materials : an atom probe study

Abstract Atom probe field ion microscopy (APFIM) investigations of the microstructure of unaged (as-fabricated) and long-term thermally-aged (~100 000 h at 280°C) surveillance materials from commercial reactor pressure vessel steels were performed. This combination of materials and conditions permitted the investigation of potential thermal aging effects. This microstructural study focused on the quantification of the compositions of the matrix and carbides. The APFIM results indicate that there was no significant microstructural evolution after a long-term thermal exposure in weld, plate and forging materials. The matrix depletion of copper that was observed in weld materials was consistent with the copper concentration in the matrix after the stress relief heat treatment. The composition of cementite carbides aged for 100 000 h were compared to the Thermocalc™ prediction. The APFIM comparisons of materials under these conditions are consistent with the measured change in mechanical properties such as the Charpy transition temperature.

Effect of neutron-irradiation on the spinodal decomposition of Fe-32% Cr model alloy

Abstract A comparison has been performed between the extent of decomposition found in an Fe-32% Cr alloy that was both neutron-irradiated at 290°C and thermally aged for the equivalent time at the same temperature. In addition, these results are compared to data obtained from a series of specimens isothermally aged at 470 and 500°C. These atom probe results indicate that neutron irradiation has a significant effect on both the kinetics of decomposition and the morphology of the chromium-enriched α′ phase. The results are consistent with the neutron irradiation significantly changing the location of the phase field in the phase diagram.

Characterization of phosphorus segregation in neutron-irradiated Russian pressure vessel steel weld

An atom probe field ion microscopy characterization of three Russian pressure vessel steels has been performed. Field ion micrographs of several lath boundaries have indicated that they are decorated with a semicontinuous film of discrete brightly-imaging precipitates that were identified as molybdenum carbonitrides. In addition, extremely high phosphorus levels were measured at the lath boundaries. The phosphorus was found to be confined to an extremely narrow region indicative of monolayer type segregation. The phosphorus coverage determined from the atom probe results of the unirradiated materials agree with predictions based on McLeans equilibrium model of grain boundary segregation. The boundary phosphorus coverage of a neutron-irradiated weld material was significantly higher than in the unirradiated material. Ultrafine darkly-imaging copper- and phosphorus-enriched precipitates were also observed in the matrix of the neutron-irradiated material.