Martin P. Harmer

Processing and microstructure development in Al2O3-SiC ‘nanocomposites’

Abstract Composites consisting of Al 2 O 3 + 5 vol.% 0·15 μm SiC particles were prepared by pressureless sintering. The optimum conditions for achieving dense and uniform microstructures by conventional ceramic processing are given in detail. The SiC particles were found to strongly inhibit grain growth of the Al 2 O 3 matrix. Densification was also significantly retarded by these ultra-fine particles, and possible explanations for this behavior are discussed.

The role of a bilayer interfacial phase on liquid metal embrittlement.

The formation of single-layer complexes between different metals is a cause of liquid metal embrittlement. Intrinsically ductile metals are prone to catastrophic failure when exposed to certain liquid metals, but the atomic-level mechanism for this effect is not fully understood. We characterized a model system, a nickel sample infused with bismuth atoms, by using aberration-corrected scanning transmission electron microscopy and observed a bilayer interfacial phase that is the underlying cause of embrittlement. This finding provides a new perspective for understanding the atomic-scale embrittlement mechanism and for developing strategies to control the practically important liquid metal embrittlement and the more general grain boundary embrittlement phenomena in alloys. This study further demonstrates that adsorption can induce a coupled grain boundary structural and chemical phase transition that causes drastic changes in properties.

Role of segregating dopants on the improved creep resistance of aluminum oxide

Recent studies have demonstrated that p.p.m. levels of rare-earth dopant ions (e.g. Y, La, Nd) wield a beneficial and highly potent influence on the creep properties of alumina. In addition, codoping with ions of disparate sizes (Nd, Zr) resulted in even further enhancement of the creep behavior. In all cases, the dopant ions were found to strongly segregate to grain boundaries. Creep rates were not influenced by the presence of second phase precipitates, verifying that the creep improvement is a solid solution effect. In an attempt to clarify the exact mechanism(s) that controls creep behavior of the doped aluminas, various advanced characterization techniques have been applied including: secondary ion mass spectrometry, scanning transmission electron microscopy, orientation image microscopy, and extended X-ray absorption fine structure as well as atomistic computer simulation and studies of the creep kinetics. Although no definitive mechanism has been established, a logical explanation is that outsize ions segregate to more energetically favorable grain boundary sites, and improve creep resistance by blocking a few critical diffusive pathways. This mechanism is sufficiently general that it may be applicable to other ceramic systems.

Control of microchemical ordering in relaxor ferroelectrics and related compounds

Abstract A review is given of some of the ways to control microchemical domains (due to ordering on the B-sites) in relaxor ferroelectrics and related compounds. Depending on the system, ordering can he controlled by heat treatment, chemical composition and stoichiometry. Specific examples are given of both thermally- and chemically-induced ordering in relaxor ferroelectric and perovskite-related materials.

The Phase Behavior of Interfaces

Grain boundaries between crystals, which can control materials properties, can interconvert between well-defined equilibrium structures. Common metal or ceramic objects are made up of many bonded micrometer-scale single crystals, or grains. Their interfaces, called grain boundaries, play a decisive role in determining the properties and processing of almost all engineering materials. Because of their structural and chemical complexities, the description of grain boundaries has lacked a satisfactory conceptual framework, and grain boundaries tend to be viewed as disordered regions with kinetically trapped structures. Recent work has shown that grain boundaries can be described as interface-stabilized phases (also called interphases or complexions) that are chemically and structurally distinct from any bulk phases (1–4). On page 206 of this issue, Baram et al. (5) extend our understanding of grain boundaries by examining interface structures that form between a gold particle in contact with metal oxide surfaces. They show that these nanoscale interface structures are equilibrium phases that obey thermodynamic rules analogous to those for bulk phases.

Compositional control of ferroelectric fatigue in perovskite ferroelectric ceramics and thin films

The mechanism and control of ferroelectric polarization fatigue (loss of polarization with cycling) in donor‐ and acceptor‐doped BaTiO3 ceramics and Pb(Zr1−xTix)O3 (PZT) thin films has been investigated. Experimental results clearly demonstrate that fatigue behavior is related to the defects within the materials. Donor‐doped BaTiO3 ceramics showed significantly improved fatigue characteristics when compared with acceptor‐doped materials. A similar but reduced effect has been observed in donor‐doped PZT thin films. The electric‐field‐assisted migration of charged species within ferroelectric materials may be responsible for the degradation/fatigue behavior. Results support the expectation that oxygen vacancies play an important role in fatigue that occurs as a result of polarization switching.

Structural features of Y-saturated and supersaturated grain boundaries in alumina

Abstract Grain boundary segregation of Y in α -Al 2 O 3 and evolution of the structural environment around the Y atoms have been investigated using high resolution STEM and EXAFS. The stages of incorporation of Y atoms by α -Al 2 O 3 grain boundaries, on average, are characterized by three composition regimes: (I) dilute to saturated; (II) supersaturated [where the degree of supersaturation is determined by the nucleation barrier for Y 3 Al 5 O 12 (YAG)]; and (III) equilibrium with YAG precipitates. The average Y grain boundary concentration in equilibrium with YAG precipitates has been determined to be ∼1/4 equivalent monolayer, and the maximum supersaturation concentration has been determined to be ∼1/2 equivalent monolayer. EXAFS revealed that accompanying the supersaturation of grain boundaries with Y is an increasing Y–O nearest neighbor coordination number and, simultaneously, a significantly increased degree of ordering of Y with respect to Al ions beyond nearest neighbor O. This Y–Al distance is the same as that for Y absorbed on the free surface of α -Al 2 O 3 , and the same as that expected for the Y–Al distance when Y substitutes for Al with the Y–O distance relaxed to that in Y 2 O 3 . This compositional and structural information has led to a clearer picture of how the grain boundary segregated Y concentration influences grain boundary structure. For dilute Y concentrations, Y ions preferentially fill sites in the grain boundary core which have well defined order only within the nearest neighbor shell of oxygens. As the Y concentration increases, Y begins to occupy near-boundary sites, forming two near-boundary layers, each adjacent to a grain surface. The near-boundary layer has nearest neighbor ordering extending at least to nearest neighbor cations. Nucleation of the YAG phase leads to the depletion of Y from these partially ordered layers.

Solute segregation to grain boundaries in MgO‐doped alumina

The spatial distribution of Mg solute in polycrystalline, single‐phase Al2O3 has been obtained, using a secondary ion imaging technique. Segregation of Mg to the Al2O3 grain boundaries is clearly observed—strong evidence that the principal role of MgO is to reduce the grain boundary mobility via a solute‐drag mechanism. In addition, Mg (dopant) and Ca (impurity) are shown to cosegregate, shedding further light on magnesium’s ability to stabilize microstructural evolution in Al2O3.

Sintering kinetics for a model final-stage microstructure: A study of AI2O3

Abstract Model experiments have been conducted on a series of alumina samples in which the microstructures have been tailored to conform to the classical configurations depicted in the models of final-stage sintering. Simultaneous measurements of sintered density, grain size, pore number density and pore size distribution were made as a function of sintering time at constant temperature (18500C). The data conformed to a model of grain-boundary-diffusion-controlled densification. Atom flux equations for grain-boundary diffusion transport and lattice diffusion transport were deduced from the data. The number of pores per unit volume was identified as the most critical factor influencing densification kinetics.

Heteroepitaxial growth of bulk single-crystalPb(Mg 1/3 Nb 2/3 )O 3 –32 mol% PbTiO 3 from (111) SrTiO 3

SrTiO3 was investigated as an alternate seed material to grow Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) ferroelectric single crystals by seeded polycrystal conversion. Fully dense polycrystalline samples of PMN–32 mol% PT doped with 3 vol% excess PbO were top-seeded with (111) SrTiO3 substrates. Annealing for 10 h at 1150 °C resulted in growth of PMN–32PT single crystals with sizes on the order of several millimeters. Orientation imaging microscopy confirmed that the grown crystal exhibited the same crystallographic orientation as that of the SrTiO3 seed. Elemental distributions analyzed using energy dispersive spectroscopy indicated that interdiffusi on of the relevant elements was negligible.