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Mechanical strengthening by grain refinement is a method whereby a material's strength and hardness can be increased by decreasing the average crystallite grain size. The empirical Hall-Petch relationship mathematically describes grain boundary strengthening and provides guidance for a straightforward way to produce stronger materials. While the phenomenon has been widely explored in nanocrystalline metals, the difficulty associated with fabricating high-quality dense nanocrystalline ceramics has left unanswered the question of the validity and extent of the relationship in ceramics. Prior studies suggest the occurrence of an inverse Hall-Petch response in ceramics with grain sizes 100 nm. This paper demonstrates a novel integrated approach, comprised of nanopowder processing and high-pressure, low-temperature sintering to fabricate bulk, fully dense and high-purity nanocrystalline ceramics with unprecedentedly small nanometersized grains. Using magnesium aluminate spinel as an archetypal hard ceramic, the hardness of this transparent ceramic armor is shown to rigorously follow the Hall-Petch relationship down to grain sizes of 28 nm. Consequentially, the nanocrystalline spinel ceramics are shown to exhibit a 50% increase in hardness over a corresponding order of magnitude reduction in grain size without a decline in density or fracture resistance. Additionally, the produced nanocrystalline ceramics have an optical transparency near theoretical. Reaching an exceptional hardness of 20.2 GPa at 28 nm, the behavior shows no evidence supporting an inverse Hall-Petch effect., Published in Acta Materialia v69 p9-16, 2014. Sponsored in part by Office of Naval Research.

The group at Lehigh University has implemented the successful application of Seeded Polycrystal Conversion (SPC) to ferroelectric materials. Initial work at Lehigh established the feasibility of using the SPC process to grow single crystals of the relaxor-based ferroelectric PbMg(1 /3)Nb(2/3))O(3-) 5mol%PbTiO3 (PMN-3 5PT) from seed crystals of the same composition. However, much work was needed to refine this process. Maintaining single crystal growth until full conversion of the polycrystal can only be achieved with a fundamental understanding of the factors that influence the migrating boundary. The present work addressed, in particular, the roles of liquid phase content and chemistry, matrix grain size, and porosity, as previous results have indicated these to be key variables. Some strides have been made in understanding the roles of each of the above key variables. In fact, over the past year, the Lehigh group has begun to model the effect of matrix grain size on single crystal growth. By improving this model, a better understanding of why single crystal growth slows can be achieved by pinpointing the mechanism of growth. Additionally, great progress has been made in microscopic characterization of samples as Lehigh has acquired a dual-beam focused ion beam (FIB). This instrument allows for the TEM preparation of samples across specific boundary interfaces in the PMN- PT system, previously unattainable with traditional sample preparation techniques. This will help gain a better understanding of the liquid phase chemistry and its role in the SPC process.