Ruthenium (Ru) suppresses the precipitation of deleterious topologically close-packed (TCP) phases in high refractory content single-crystal Ni-base superalloys. The effectiveness of Ru as a TCP suppressant appears to be the net effect of its limited solubility in the TCP phase, a lower density of structural growth ledges for atomic attachment at the TCP/matrix interface, and destabilization of the c¢ phase at elevated temperatures. These characteristics combine to limit the growth rates of precipitates and decrease the driving force for TCP precipitation, which has the secondary effect of reducing the TCP nucleation rate. The reduction in c¢ volume fraction upon the addition of Ru is particularly potent due to the sensitivity of the rhenium (Re) content in the c matrix to changes in the c¢ volume fraction.

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The elemental partitioning characteristics of advanced single-crystal Ni-base superalloys containing additions of both Pt and Ru have been investigated using atom probe tomography. Detailed microanalysis revealed Ru additions partitioned preferentially to the disordered g matrix, whereas Pt additions tended to partition to the ordered intermetallic g9 precipitates. The stability of the g9 precipitates at elevated temperatures coupled with the subtle changes in elemental partitioning associated with the additions of Cr and Ru were found to significantly influence the bare metal cyclic oxidation behavior of the experimental single-crystal alloys. Although the presence of Pt was observed to greatly enhance oxidation characteristics, these beneficial gains were impacted by Cr and Ru additions as they both increased the tendency for oxide spallation during cyclic thermal exposures.

As nickel-base single crystals are being implemented in gas turbine engines operating at temperatures in excess of 1600K, maintaining structural properties at these elevated temperatures has become an increasingly important problem. Typically, elevated levels refractory alloying elements are added to these single crystal alloys to enhance the degree of solid solution strengthening within the microstructure. The presence of Re, W, Mo, Ta in Ni-base superalloys strongly influences parameters, such as the stacking fault energies, lattice misfit, and shear modulus of the crystalline lattices, that govern the creep response of the material. However, this is a challenge when dealing with high refractory alloys that are susceptible to the precipitation of deleterious topologically-close-packed (TCP) phases. Recent investigations have demonstrated Ru additions to be beneficial with respect to hindering the formation of these TCP phases and consequently improving the creep properties of these advanced single crystal alloys. Substantial effort has been dedicated towards understanding the fundamental effects and the mechanisms by which Ru additions alter the structure of the γ and γ' phases. Much of this knowledge has been effectively applied to the development of a new class of single crystal Ni-base superalloys with improved strength and a temperature capability significantly higher than those of existing "second" or "third" generation alloys. Nevertheless, optimization of the alloy chemistries remains notoriously difficult as various component design requirements related to the solidification characteristics, overall blade density, coating compatibility and cost must also be carefully considered when developing these materials. This investigation addresses some of the issues associated with the development of Ru-bearing, high refractory content Ni-base superalloys and describes how interrelationships between the properties, processing, microstructure and chemistry of these alloys can be balanced to yield a practical "fourth" generation single crystal superalloy.

Food SterilizationThe aim of food sterilization is to produce a sterile product of the highest possible quality. The classical method is canning, in which the product is sealed and then processed until it has reached the correct level of sterility. This may result in a lowquality product; the kinetics of sterilization and quality loss reactions are such that a better product is obtained by heating to higher temperatures for shorter times. This is best done in a continuous process with three sections: (i) a heating section in which product is first heated to the required temperature; (ii) a holding section in which it is held at temperature long enough to ensure sterility; and (iii) a cooling section prior to packaging. The holding section is generally a horizontal or slightly inclined tube.Continuous processes are capable of producing a higherquality product than canning. It is possible to process liquids, using forced convective heat transfer, at much higher heating and cooling rates, on the order of l"C/s, than is possible in processes which rely on thermal conduction or natural convection in the can. A number of commercial food sterilization processes involve the transport and heating of solid-liquid food mixtures of high solids fractions (Holdsworth, 1993). These flows can consist of up to 50-60% solids, particle diameters up to 25 mm in diameter, and carrier fluids which are generally non-Newtonian. Fluid velocities are restricted by the requirement to cause as little damage as possible to the mixture. It is difficult to process solid-liquid mixtures rapidly as solids heating is thermal-conduction-controlled; however, new volumetric heating technologies, such as microwave (Ayappa et al., 1991) and electrical (ohmic) heating (Biss et a]., 1989; Parrott, 1992) allow solids to be heated at the same rates as liquids.Volumetric processes potentially remove heat-transfer limitations on processing particles during the heating stage. Despite this, Zhang and Fryer (1993) demonstrate that significant quality loss can still accumulate during cooling. It is thus vital to study the whole process. To design processes and confirm the sterility of the final product, the temperatures of solidliquid mixtures must be known. Little information is available on the heating rates of the particles and the liquid and of the flow patterns of the food mixture; the design of plant is thus largely empirical. Food will have a range of residence times in any flow situation, and so the process must be designed so that the fastest moving piece of fluid must be sterilized while the slowest moving part is not overcooked.The APV Baker ohmic heater forms the basis for the electrical heating system modeled here. As described by Parrott (1992), the heater consists of a vertical or near-vertical tube up to 10 m in length and 75 mm in diameter, containing a series of up to seven electrode housings, each containing a single cantilever electrode across the tube. Up to 3 ton/h of flood flow upward through the tube past the electrodes; current density ...