Stabilized Nanocrystalline Alloys: The Intersection of Grain Boundary Segregation with Processing Science

Abstract

Processing science for nanocrystalline metals has largely focused on far-from-equilibrium methods that can generate many grain boundaries with excess defect energy. Conversely, the science of stabilizing nanocrystalline alloys has largely focused on the lowering of that excess defect energy through grain boundary segregation, bringing nanocrystalline structures closer to equilibrium. With increasing technological adoption of stabilized nanocrystalline alloys, there is a substantial need for research at the intersection of these two fields. This review lays out the basic thermodynamic issues of the two subfields and surveys the literature on the most common processing methods, including severe plastic deformation, ball milling, physical vapor deposition, and electrodeposition. We provide an overview of studies that have examined grain boundary segregation through each of these methods and identify general themes. We conclude that there is substantial scope for more systematic work at the intersection of these fields to understand how nonequilibrium processing affects grain boundary segregation. Expected final online publication date for the Annual Review of Materials Science, Volume 51 is August 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Towards achieving ultrahigh thermal stability of fine-grained alumina fibers via segregation-induced ordered superstructures

“…The dense continuous alumina fibers were obtained from green fibers by a two-step annealing process (methods for details). The smooth surface and almost completely dense cross-sectional morphology of continuous alumina fibers with an average diameter of 11.18±1.13 μm and a density of 3.95 g/cm 3 were shown in Fig. 1, a and b, whose grain size floated around 200 nm.…”

“…Many properties of engineered materials are closely related to grain boundary structure and morphology 1 . Grain boundary design plays a crucial role in materials development and has been an effective way to improve the stability of nanomaterials over the past decades [2][3][4][5][6][7] . Fine-crystal nanomaterials are known to have a strong tendency to coarsen at high temperatures 8,9 , which dramatically limits their applications in the high-temperature domain.…”

New trends in additive manufacturing of high-entropy alloys and alloy design by machine learning: from single-phase to multiphase systems

“…Therefore, PBF processes are usually used to fabricate the parts with complex structures, while DED processes are usually used to repair engineering and aerospace components. Moreover, DED processes also have significant feasibility in making functionally graded metallic materials [54] .…”

Microstructural Transitions during Powder Metallurgical Processing of Solute Stabilized Nanostructured Tungsten Alloys

“…In nanocrystalline metals, the deliberate introduction of solute that has an energetic preference to segregate to grain boundaries-often referred to as grain boundary doping (including deliberate alloying and impurity segregation)-has emerged as an effective means of stabilizing nanocrystalline materials against thermal [1][2][3][4][5], mechanical [6][7][8], and irradiation instabilities [9][10][11]. Because these materials contain a high-volume fraction of grain boundaries with excess free energy, they are not easily processed via traditional thermomechanical processing routes and instead rely on far-from-equilibrium processes such as physical vapor deposition (PVD), electrodeposition, high-energy ball milling, and severe plastic deformation (SPD) [12]. While all these techniques can produce the desired grain boundary-rich microstructures, deposition-based processes are limited in their scalability, while it is difficult to achieve simultaneous alloying and grain size refinement via SPD.…”