The enhancement of oxygen ionic conductivity by over two orders of magnitude in an electroceramic oxide is explicitly shown to result from nanoscale enrichment of a grain boundary layer or complexion with high solute concentration. A series of CaCeO polycrystalline oxides with fluorite structure and varying nominal Ca solute concentration elucidates how local grain boundary composition, rather than structural grain boundary character, primarily regulates ionic conductivity. A correlation between high grain boundary solute concentration above ∼40 mol%, and four orders of magnitude increase in grain boundary conductivity is explicitly shown. A correlated experimental approach provides unique insights into fundamental grain boundary science, and highlights how novel aspects of nanoscale grain boundary design may be employed to control ion transport properties in electroceramics.

As heat treated, Rene' 88 DT was found to contain some 42.5 % y ' , with both the cooling (24.5%) and aging (18.0%) forms exhibiting a very low positive (0.05%) mismatch to the matrix. Small amounts of gram boundary MsB2 TiNblCJ, and traces of MsBs,were also present. The rate of coohng from the solutioning temperature could be related to the resultant cooling y ' diameter by a regression equation, as could the aging conditions. The rate of cooling, as well as the grain size, were found to effect the gram boundary sermtions that were formed on cooling from the supersolvus annealing temperature. Both of these features, and thus the cooling rate, mav effect mechanical oronerties. On nrolonaed hiah tempemture' exposure, the y ' phases 1 grow, and more M&z precioitates above 650°C (1200°F 1. followed bv MT&L above 705 '"C (i300°F), and an inbagranul~ p phase at ?6O"C (1400°F). Simple regression equations were fitted to the kinetics of growth of both the cooling and aging y ' by the use of a Larsen-Miller parameter to normalize the-time and temperature of exposure. This abowed the mediction of the growth of the Y ' Dhases and the possible iden&ication of the average service tempe&ure that a part experienced. The growth of y ' during high temperature exposure, rather then any precipitation of deleterious phases, was found to limit the service range of this structurally stable alloy. Indeed, the formation of M&e and n start only after the aging y' had comnletelv re-solutioned. Re& 8SDT is an extremely stable alloy for prolonged 650°C (1200°F) service. Its sunerior orooerties reflect a hieh Y' content and a y "whose low positive mismatch allows a highly coherent and finer y' precipitation. These advantages are optimized by a super solvus annealing practice that eliminates large, sub-solvus y' phases, which promote crack nucleation in fatigue, and interfere with the formation of serrated grain boundaries.

Commercially pure α-Ti was serial sectioned using a Xe plasma focused ion beam (PFIB) scanning electron microscope and orientation maps were obtained on the parallel layers by electron backscatter diffraction. The orientations and shapes of 13900 grains and 92100 grain faces were characterized. The mean number of faces per grain was 14.2. The grain boundaries were classified according to the three misorientation parameters and two grain boundary orientation parameters. There were more grain boundaries with 180°-twist and 180°-tilt character than expected in a random distribution. Furthermore, grain boundary planes with prismatic orientations were more common than those with basal orientations. The grain boundary with the greatest relative area had a 28°/[0001] misorientation and (314 � 0) and �72 � 5 � 0� grain boundary planes. Compared to earlier instruments with Ga-ion sources, the milling speed of the PFIB makes it possible to collect ten times more data in a comparable time.

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a b s t r a c tGrain boundaries and other interfaces can undergo complexion transitions from one thermodynamic state to another, resulting in discontinuous changes in interface properties such as diffusivity, mobility, and cohesive strength. The kinetics of such complexion transitions has been largely overlooked until recently. Just as with bulk phase transformations, complexion transition kinetics can be represented on time-temperature-transformation (TTT) diagrams. An experimental complexion TTT diagram is presented here for polycrystalline Eu-doped spinel annealed at 1400-1800°C. This material developed a microstructure with a bimodal grain size distribution, indicating that a complexion transition occurs within this temperature range. The time and temperature dependence of this complexion transition was analyzed and used to produce a grain-boundary complexion TTT diagram for this system. Complexion TTT diagrams have the potential to be remarkably useful tools for manipulating the properties of internal interfaces in polycrystalline metals and ceramics. The development of experimental complexion TTT diagrams is likely to have an important impact on the field of grain-boundary engineering, and hence the development of these experimental diagrams should be an intense area of focus in the coming years.

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and a gamma matrix high in Cr, Co and MO.

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