The properties of materials change, sometimes catastrophically, as alloying elements and impurities accumulate preferentially at grain boundaries. Studies of bicrystals show that regular atomic ...patterns often arise as a result of this solute segregation at high-symmetry boundaries, but it is not known whether superstructures exist at general grain boundaries in polycrystals. In bismuth-doped polycrystalline nickel, we found that ordered, segregation-induced grain boundary superstructures occur at randomly selected general grain boundaries, and that these reconstructions are driven by the orientation of the terminating grain surfaces rather than by lattice matching between grains. This discovery shows that adsorbate-induced superstructures are not limited to special grain boundaries but may exist at a variety of general grain boundaries, and hence they can affect the performance of polycrystalline engineering alloys.
Grain boundary complexions Cantwell, Patrick R.; Tang, Ming; Dillon, Shen J. ...
Acta materialia,
January 2014, 2014, 2014-01-00, Letnik:
62
Journal Article
Recenzirano
Grain boundaries exhibit phase-like behavior in which their structure, chemistry and properties may change discontinuously at critical values of thermodynamic parameters such as temperature, pressure ...and chemical potential. Therefore, grain boundaries (and other interfaces such as surfaces and heterophase boundaries) can be treated as thermodynamically stable interfacial states. To differentiate these interfacial states from bulk phases, the term “complexion” has been introduced. A variety of terminology has been used to describe complexions and complexion transitions. In many cases, several terms exist that describe essentially the same phenomenon. We give an overview of complexion-related terminology, suggest a preferred nomenclature and discuss a classification framework that can be used to categorize complexions and complexion transitions. The field of grain boundary complexions has evolved rapidly in the past decade due to advances in experimental equipment – in particular, aberration-corrected transmission electron microscopy – and progress in computational simulation methods. Grain boundary complexion transitions are the root cause of a wide variety of materials phenomena – such as abnormal grain growth, grain boundary embrittlement and activated sintering – that have defied mechanistic explanation for years. In this overview, we review the history and theory of grain boundary complexion transitions, their role in materials processing and their effect on materials properties.
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.
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.
Bulk phase transformation kinetics were not well understood before Davenport and Bain developed time–temperature-transformation (TTT) diagrams for steel alloys in the 1930s. These powerful diagrams ...revolutionized the heat treatment of steel and other alloys. Grain boundaries and internal interfaces are now known to behave in a phase-like manner, referred to as ‘complexions’, and their transitions can be represented on TTT diagrams. We present experimental grain boundary complexion TTT diagrams for polycrystalline Al2O3 and Y2O3. Grain boundary mobility discontinuities in Y2O3 occur at different temperatures for different annealing times, an unusual pattern that becomes understandable when viewed on a complexion TTT diagram. Similarly, the anisotropy of Al2O3 complexion kinetics can be visualized with these diagrams. Complexion TTT diagrams are a graphical tool to control interface-related phenomena such as diffusion, creep, oxidation, and microstructure evolution. They could explain why two-step sintering produces dense nanocrystalline ceramics and offer insight into other processes as well.
Display omitted
Cross-sectional micrograph of spark plasma sintered tungsten with multimodal grain size distribution: (a) using focused ion beam (b) Bright field TEM image.
Display omitted
► High applied external ...pressure during SPS led to high density of the samples. ► The consolidated samples by SPS had a multimodal size distribution. ► Ultrafine grains were present within the samples sintered at low temperatures. ► High Vickers hardness was obtained compared to commercial tungsten. ► The consolidated samples were proved to be pure by chemical analysis.
Preparation of fine grained, hard and ductile pure tungsten for future fusion reactor applications was tested using the bottom-up approach via powder consolidation by spark plasma sintering (SPS) at different temperature (1300–1800
°C) and pressure (90–266
MPa) conditions. Pure tungsten powders with an average particle size of about 1
μm were sintered to high density (about 94%) with almost no grain growth at a temperature below 1400
°C and an applied pressure up to 266
MPa. These samples had a multi-modal grain size distribution (resembling the size distribution of the initial powder) and a very high Vickers hardness (up to 530
kg/mm
2). Above 1500
°C fast grain growth occurred and resulted in a drop in hardness. XRD on the surface of bulk samples showed a small amount of tungsten oxides; however, XPS and EDS indicated that these oxides were only surface contaminants and suggested a high purity for the bulk samples. The results demonstrate that SPS can lead to ultrafine and nanocrystalline tungsten if used to consolidate pure nano tungsten powders.
Anti-thermal behavior of materials Cantwell, Patrick R.; Holm, Elizabeth A.; Harmer, Martin P. ...
Scripta materialia,
07/2015, Letnik:
103
Journal Article
Recenzirano
Thermally activated processes such as diffusivity, grain growth, oxidation, and catalysis are often modeled using the Arrhenius equation, in which the steady-state process rate increases with ...increasing temperature, yielding a positive activation energy. However, in some systems, the process rate is constant or decreases with increasing temperature. Mechanistic explanations for many types of anti-thermal behavior are lacking. By learning how to control anti-thermal behavior, major advances are possible in fields ranging from catalysis to nanocrystalline alloys to high efficiency engines.
Display omitted
•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.
PDF
