Stability of nanocrystalline metals: The role of grain-boundary chemistry and structure

Abstract

Nanocrystalline metals are transitioning from laboratory curiosities to engineering materials, in large part due to advances in improving their stability, making their exceptional properties more predictable and accessible. Nanoscale grains typically have a very strong innate tendency to coarsen, but the grain-boundary structure can be designed and tuned to lower its excess energy, reducing both the driving force for coarsening and the grain-boundary mobility. This article reviews two major strategies for achieving low-energy grain boundaries in nanocrystalline structures. First, grain-boundary alloying is discussed, including grainboundary segregation and its energetic competition with the formation of second phases; with sufficient grain-boundary segregation tendency it is possible to stabilize nanostructures to high temperatures. Second, methods of achieving low-energy crystallographic grainboundary structures are discussed, including the formation of nanotwinned structures and relaxing grain boundaries into low-energy structures through their interactions with partial dislocations. Both of these strategies have led to effective and implementable stable nanocrystalline materials, and point to many directions for future advancements.

100 years after Griffith: From brittle bulk fracture to failure in 2D materials

“…In the concept of grain-boundary complexion engineering, 25 this can be attempted on different interface length scales, for example, using ductile phases in a nanocomposite approach, 26 , 27 by extent amorphous interphases, 28 or local interface segregation. 29 , 30 While all of them hold promise and have been demonstrated to result in significant improvements of material toughness, the latter seems the most elegant approach with respect to general and scalable applicability, sustainable use, and recyclability of materials, as only minor fractions of alloying or rather doping elements are necessary to segregate or decorate the grain boundaries. Selection of the respective grain-boundary doping elements should be guided on the one hand by ab initio calculations, looking at the effect of the respective elements on strengthening or weakening interface cohesion, 31 and on the other hand by thermodynamic considerations of the segregation tendency of the employed dopant, 32 as the strength of segregation defines the efficiency of dopant use in modifying the interface properties.…”

A nanodispersion-in-nanograins strategy for ultra-strong, ductile and stable metal nanocomposites

“…Here we report a “nanodispersion-in-nanograins” strategy to achieve enhanced mechanical properties and thermal stability along with enhanced electrical conductivity in nanocrystalline metals. Different from the commonly adopted approaches such as hierarchical microstructural design and GB engineering 16 , we achieve a uniform intragranular dispersion of a high density of carbon nanoparticles. Finely dispersed intragranular nanoparticles not only amplify the strengthening effect of nanograins but also activate multiple hardening mechanisms via dislocation-nanoparticle interactions, thereby rendering unique combinations of high strength, work hardening, and tensile ductility in nanocrystalline metals.…”

Achieving strong and stable nanocrystalline Al alloys through compositional design

“…Microstructural instability stimulates extensive research endeavors of studying the constituent and alloying effects to preserve nanograins at elevated temperatures. Thermodynamic and kinetic strategies have been devised to mitigate GB migration and grain coarsening [14]. Specifically, thermodynamic approach exerts GB energy relaxation by decorating GBs with selective solute atoms and the GB segregation-induced thermal stability has been realized in NC Al-Pb [15], Co-P [16], Cu-Ta [17], Fe-Zr [18], Ni-P [19], Ni-W [20], Ni-Mo [21], and W-Ti [22] alloys.…”