Monica Kapoor

A procedure to create isoconcentration surfaces in low-chemical-partitioning, high-solute alloys.

A proximity histogram or proxigram is the prevailing technique of calculating 3D composition profiles of a second phase in atom probe tomography. The second phase in the reconstruction is delineated by creating an isoconcentration surface, i.e. the precipitate-matrix interface. The 3D composition profile is then calculated with respect to this user-defined isoconcentration surface. Hence, the selection of the correct isoconcentration surface is critical. In general, the preliminary selection of an isoconcentration value is guided by the visual observation of a chemically partitioned second phase. However, in low-chemical -partitioning systems, such a visual guide is absent. The lack of a priori composition information of the precipitate phase may further confound the issue. This paper presents a methodology of selecting an appropriate elemental species and subsequently obtaining an isoconcentration value to create an accurate isoconcentration surface that will act as the precipitate-matrix interface. We use the H-phase precipitate in the Ni-Ti-Hf shape memory alloy as our case study to illustrate the procedure.

Influence of Grain Boundary Character and Annealing Time on Segregation in Commercially Pure Nickel

Commercially pure nickel (Ni) was thermomechanically processed to promote an increase in Σ3 special grain boundaries. Engineering the character and chemistry of Σ3 grain boundaries in polycrystalline materials can help in improving physical, chemical, and mechanical properties leading to improved performance. Type-specific grain boundaries (special and random) were characterized using electron backscatter diffraction and the segregation behavior of elements such as Si, Al, C, O, P, Cr, Mg, Mn, B, and Fe, at the atomic level, was studied as a function of grain boundary character using atom probe tomography. These results showed that the random grain boundaries were enriched with impurities to include metal oxides, while Σ3 special grain boundaries showed little to no impurities at the grain boundaries. In addition, the influence of annealing time on the concentration of segregants on random grain boundaries was analyzed and showed clear evidence of increased concentration of segregants as annealing time was increased.

Investigation into Solute Stabilizing Effects in Nanocrystalline Materials: An Atom Probe Characterization Study

Over the past few years, there has been a concerted interest in understanding how solute segregation to grain boundaries stabilizes nanocrystalline materials against grain growth and stress effects [1-3]. To elucidate this behavior, the ability to quantitatively probe the chemistry of the grain boundary is essential. Atom probe tomography is ideally suited to achieve this level of atomic scale chemical analysis. This talk will address how atom probe is providing insights into solute segregation that leads to a variety of different nanocrystalline stabilization conditions.