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 carbide Ni6W6C has been identified in electroplated and sputtered nanocrystalline Ni–23at.% W annealed at 700°C. This carbide is unexpected and forms due to carbon contamination, which is ...difficult to avoid in practice. Carbon has a low solubility in the Ni(W) solid solution and may segregate strongly to grain boundaries while W anti-segregation may occur during carbide precipitation. Carbon contamination may therefore impact the thermal stability of nanocrystalline Ni–W in ways not previously considered.
Coulomb correlations can manifest in exotic properties in solids, but how these properties can be accessed and ultimately manipulated in real time is not well understood. The insulator-to-metal phase ...transition in vanadium dioxide (VO₂) is a canonical example of such correlations. Here, few-femtosecond extreme UV transient absorption spectroscopy (FXTAS) at the vanadium M
2,3 edge is used to track the insulator-to-metal phase transition in VO₂. This technique allows observation of the bulk material in real time, follows the photoexcitation process in both the insulating and metallic phases, probes the subsequent relaxation in the metallic phase, and measures the phase-transition dynamics in the insulating phase. An understanding of the VO₂ absorption spectrum in the extreme UV is developed using atomic cluster model calculations, revealing V3+/d² character of the vanadium center. We find that the insulator-to-metal phase transition occurs on a timescale of 26 ± 6 fs and leaves the system in a long-lived excited state of the metallic phase, driven by a change in orbital occupation. Potential interpretations based on electronic screening effects and lattice dynamics are discussed. A Mott–Hubbard-type mechanism is favored, as the observed timescales and d² nature of the vanadium metal centers are inconsistent with a Peierls driving force. The findings provide a combined experimental and theoretical roadmap for using time-resolved extreme UV spectroscopy to investigate nonequilibrium dynamics in strongly correlated materials.
Nanocrystalline Ni–W alloys are reported in the literature to be stabilized against high temperature grain growth by W-segregation at the grain boundaries. However, alternative thermal stability ...mechanisms have been insufficiently investigated, especially in the presence of impurities. This study explored the influence of oxygen impurities on the thermal stability and mechanical properties of electrodeposited Ni-23at%W with aberration-corrected scanning transmission electron microscopy (STEM) and nanoindentation hardness testing. The primary finding of this study was that nanoscale oxides were of sufficient size and volume fraction to inhibit grain growth. The oxide particles were predominantly located on grain boundaries and triple points, which strongly suggests that a particle drag mechanism was active during annealing. In addition, W-segregation was observed at the oxide/Ni(W) interfaces rather than the presumed Ni(W) grain boundaries, further supporting the argument that alternative mechanisms are responsible for thermal stability in these alloys. Lastly, alloys with nanoscale oxides exhibited a higher hardness compared to similar alloys without oxides, suggesting that the particles are widely advantageous. Overall, this work demonstrates that impurity oxide particles can limit grain growth, and alternative mechanisms may be responsible for Ni–W thermal stability.
The design of commercial‐grade specialty alumina for improved behavior relies on better understanding the complex interplay between high concentrations of intentional dopants and unintentional ...impurities, which are unavoidably introduced during industrial‐scale processing. In particular, dopants and impurities govern grain boundary structure/composition, grain growth, and bulk material properties. While the effects of single dopants in ultra‐high‐purity ceramics are well understood, the same concepts have not been adequately extended to commercial materials containing numerous impurities, especially when utilizing the latest atomic‐resolution characterization techniques to evaluate grain boundary structure and composition. Therefore, this work investigated the effects of varying co‐doping levels of MgO, CaO, and SiO2 on grain boundary structure and composition in commercial‐grade alumina by applying aberration‐corrected scanning transmission electron microscopy. First, it was observed that grain growth behavior, specifically grain morphology, in doped specialty aluminas was anomalous to ultra‐high‐purity alumina; in the composition range that was investigated in this study, Ca‐doped specialty alumina exhibited equiaxed grains, whereas Si‐doped specialty alumina exhibited elongated grains. Afterward, it was determined that elemental ratios of bulk doping concentrations differed from grain boundary compositions. Grain boundary compositions, as well as grain boundary structure, were therefore determined to be the dominant metrics to predict grain growth behavior. Overall, co‐doping effects on grain boundary structure and composition are the key parameters to consider to control commercial‐grade ceramics and improve material reliability.
Eu‐doped MgAl2O4 has been used to evaluate the kinetics of equilibrium grain boundary transformations, otherwise known as complexion transitions, by monitoring abnormal grain growth induced the ...nucleation of highly mobile complexions. The assumption in prior works was that abnormal grain growth can be charted using time‐temperature‐transformation (TTT) diagrams to reflect the complexion transition nucleation and growth kinetics. A model depending on doping concentration, grain size, and abnormal area fraction has also been recently developed to estimate excess grain boundary coverage, and thereby predict complexion types depending on microstructural descriptors. While useful, the grain boundary excess model has not been validated using atomic‐resolution characterization. This work aimed to directly validate the grain boundary excess model and complexion TTT diagrams by applying aberration‐corrected electron microscopy to characterize grain boundary structures and compositions (i.e., complexions) in Eu‐doped MgAl2O4. Eu doping concentrations of 100 ppm and 500 ppm were produced and bulk samples were annealed at 1600°C for different times. Forty‐five distinct grain boundaries were characterized. Overall, at least five grain boundary complexion types were identified and the grain boundary excess model was validated. Interpretability of the grain boundary excess model and correlations between grain boundary structures and compositions are discussed.
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.
Inconel 625, whereas designed as a single-phase solid-solution-strengthened alloy, is prone to formation of various precipitate phases under processing or service conditions. When this alloy is ...fabricated with additive manufacturing, the enhanced segregation of alloying elements to grain boundaries, interdendritic regions, or dislocation cores can influence microstructural evolution and modify local precipitation pathways. In this study, an Inconel 625 made with laser powder-bed fusion is characterized with comprehensive electron microscopy techniques combined with thermodynamic calculations, with emphasis on element segregation, precipitate formation, and their relation to the columnar sub-grains associated with additive manufacturing. Enrichment of both major (Nb, Mo) and minor (Si, N) solutes at dislocation cell walls is observed in the as-built samples, whereas stress-relief heat treatments promote formation of two types of globular precipitates,
i.e.,
(Nb, Mo, Si, N)-rich M
6
X and (Nb, N)-rich MX. Absence of the detrimental, needle- or plate-shaped
δ
-Ni
3
(Nb, Mo) phase leads to improved mechanical properties and is attributed to the higher concentrations of Si and N at cell walls, although the overall composition remains within the standard range for powder-bed fused alloys. These new insights into the critical role of minor alloying elements provide a potent guide for the design and processing of additively manufactured metallic materials.
Boron carbide is an excellent armor material due to its light weight and ultrahigh hardness. However, high‐rate mechanical behavior can be degraded by stress‐induced amorphization. In this paper, we ...review the progressive advances in the understanding of amorphization in three successive generations of boron carbide: stoichiometric (undoped), B‐rich, and B/Si codoped boron carbides. For each generation of boron carbide, the crystal structure and microstructure are first discussed. Then, we outline the experimental observations of amorphization made by Raman spectroscopy and transmission electron microscopy. The susceptibility of amorphization in each generation of boron carbide will be compared and the fundamental mechanisms that explain the reduction in amorphization for B‐rich and B/Si codoped boron carbides elucidated. Comments on future research directions to further broaden and deepen the understanding of stress‐induced amorphization of boron carbide are also provided.
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