Abstract
Grain boundary structure‐property relationships influence bulk performance and, therefore, are an important criterion in materials design. Materials scientists can generate different grain ...boundary structures by changes in temperature, pressure, and chemical potential because interfaces attain their own equilibrium states, known as complexions. Complexions undergo first‐order transitions by changes in thermodynamic variables, which results in discontinuous changes in properties. Grain boundary complexion engineering is introduced in this paper as a method for controlling complexion transitions to improve material performance. This International Conference on Sintering 2017 lecture describes the tools for grain boundary complexion engineering: complexion equilibrium and time‐temperature‐transformation (
TTT
) diagrams. These tools can be implemented in processing design to tailor grain boundary properties, including grain boundary mobility. While impactful, these diagrams are often limited in scope because they are currently empirically derived. This article discusses how measurement techniques can be combined with data analytical methods to build mechanistically derived complexion equilibrium and
TTT
diagrams.
Grain boundaries can undergo phase-like transitions, called complexion transitions, in which their structure, composition, and properties change discontinuously as temperature, bulk composition, and ...other parameters are varied. Grain boundary complexion transitions can lead to rapid changes in the macroscopic properties of polycrystalline metals and ceramics and are responsible for a variety of materials phenomena as diverse as activated sintering and liquid-metal embrittlement. The property changes caused by grain boundary complexion transitions can be beneficial or detrimental. Grain boundary complexion engineering exploits beneficial complexion transitions to improve the processing, properties, and performance of materials. Here, we review the thermodynamic fundamentals of grain boundary complexion transitions, highlight the strongest experimental and computationalevidence for these transitions, clarify a number of important misconceptions, discuss the advantages of grain boundary complexion engineering, and summarize existing research challenges.
The solid‐state transformation behavior of cobalt dititanate (CoTi2O5) has been studied at different temperatures. The starting structure consisted of a duplex (1:1 molar) mixture of CoTiO3 and TiO2 ...grains. A seed layer of CoTi2O5 grains was incorporated at the upper surface of the green, precursor powder samples. Cobalt dititanate is a member of the pseudobrookite family (Cmcm, orthorhombic); it is an entropy‐stabilized compound, hence is stable only at elevated temperatures (T > 1140°C). It was observed that transformation initiated adjacent to the seed layer. First, a narrow layer of CoTi2O5 forms at the diphase boundaries, creating a 3‐D network that propagates at a constant velocity through the structure. The velocity of the reaction front follows an Arrhenius dependence on temperature. Both the rates of nucleation and growth increase with increasing temperature. At 1300°C, the velocity was estimated to be in excess of 10 mm/h. The progression of the reaction front was modelled based on a discontinuous template growth mechanism, with enhanced diffusion along the boundary between the product and reactant phases. The activation energy derived for boundary diffusion was 475 ± 69 kJ/mol. Behind the reaction front, the microstructure consists of a continuous matrix of CoTi2O5, with isolated remnant grains of CoTiO3 and TiO2. Further transformation occurs more slowly by solid‐state diffusion through the CoTi2O5 phase. Studies of the crystallographic orientation of the CoTi2O5 phase showed that it was pseudo‐ single crystal, with a preferred growth direction of 100. The system exhibits novel features, which have not been reported previously for ceramic systems.
Mechanical alloying was employed to produce a nanostructured Mo25Nb25Ta25W25 multi-principal element alloy (MPEA) with enhanced mechanical properties. Overall, a 400% increase in hardness was ...achieved, as compared to similar cast alloys, via mechanical alloying and optimized long-term annealing treatments. Furthermore, advanced characterization, including aberration-corrected scanning transmission electron microscopy, was conducted to elucidate processing-structure-property relationships in which it was determined that, although the introduction of impurities via mechanical alloying is common and thought to be deleterious, impurities can lead to an impressive enhancement of mechanical properties. More specifically, in this study, Fe and N impurities resulted in the formation of nanoscale, ceramic secondary phases. The observed strengthening was attributed, at least in part, to the ceramic impurity phases. Overall, we suggest that a deliberate doping strategy may be employed in the future to tailor MPEA chemistry and thereby achieve superior mechanical properties.
Abstract
It is often advantageous to display material properties relationships in the form of charts that highlight important correlations and thereby enhance our understanding of materials behavior ...and facilitate materials selection. Unfortunately, in many cases, these correlations are highly multidimensional in nature, and one typically employs low-dimensional cross-sections of the property space to convey some aspects of these relationships. To overcome some of these difficulties, in this work we employ methods of data analytics in conjunction with a visualization strategy, known as parallel coordinates, to represent better multidimensional materials data and to extract useful relationships among properties. We illustrate the utility of this approach by the construction and systematic analysis of multidimensional materials properties charts for metallic and ceramic systems. These charts simplify the description of high-dimensional geometry, enable dimensional reduction and the identification of significant property correlations and underline distinctions among different materials classes.
The role of reactive elements (RE) is an important topic in understanding the oxidation behavior of high‐temperature alloys. In this work, the influence of codoping alumina with two different RE ...elements (500 ppm Hf + 500 ppm La) was studied. The kinetics of oxygen grain‐boundary (GB) transport were studied at 1400°C using metallic nickel particles as markers. The results were compared with data obtained on the corresponding singly doped compositions; alumina‐500 ppm La, and alumina‐500 ppm Hf. The results showed that singly doping with La did not have any benefit compared to undoped alumina, whereas singly doping with Hf resulted in a slowing of transport by a factor of ~7. The behavior of the codoped sample was very similar to that of the singly doped Hf composition. For all the studied compositions, atomic scale characterization using high‐angle annular dark‐field scanning transmission electron microscopy and atom probe tomography (APT) revealed strong segregation of the dopant ions to the alumina grain boundaries. In the codoped sample, APT revealed evidence of oxygen excess and aluminum depletion at the GB core.
We introduce a phenomenological theory of dislocation motion appropriate for two-dimensional lattices. A coarse grained description is proposed that involves as primitive variables local lattice ...rotation and Burgers vector densities along distinguished slip systems of the lattice. We then use symmetry considerations to propose phenomenological equations for both defect energies and their dissipative motion. As a consequence, the model includes explicit dependencies on the local state of lattice orientation, and allows for differential defect mobilities along distinguished directions. Defect densities and lattice rotation need to be determined self-consistently and we show specific results for both square and hexagonal lattices. Within linear response, dissipative equations of motion for the defect densities are derived which contain defect mobilities that depend nonlocally on defect distribution.
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•An approach to characterize complexion transition kinetics is presented.•The fundamentals of bulk phase and complexion TTT diagrams are compared.•Avrami-type analysis quantifies the ...time dependence of complexion transitions.•Newly constructed complexion TTT diagrams enable engineered microstructures.•Future challenges and recommendations for complexion TTT diagrams are discussed.
Grain 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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