Linking stress-driven microstructural evolution in nanocrystalline aluminium with grain boundary doping of oxygen

Abstract

The large fraction of material residing at grain boundaries in nanocrystalline metals and alloys is responsible for their ultrahigh strength, but also undesirable microstructural instability under thermal and mechanical loads. However, the underlying mechanism of stress-driven microstructural evolution is still poorly understood and precludes rational alloy design. Here we combine quantitative in situ electron microscopy with three-dimensional atom-probe tomography to directly link the mechanics and kinetics of grain boundary migration in nanocrystalline Al films with the excess of O atoms at the boundaries. Site-specific nanoindentation leads to grain growth that is retarded by impurities, and enables quantification of the critical stress for the onset of grain boundary migration. Our results show that a critical excess of impurities is required to stabilize interfaces in nanocrystalline materials against mechanical driving forces, providing new insights to guide control of deformation mechanisms and tailoring of mechanical properties apart from grain size alone.

Correlating Atom Probe Crystallographic Measurements with Transmission Kikuchi Diffraction Data

“…This has enabled the emergence of atom probe crystallographic studies (Vurpillot et al, 2001; Moody et al, 2009; Gault et al, 2012 a ), which facilitate calibration of the reconstruction and advanced reconstruction approaches such as lattice rectification (Vurpillot et al, 2003; Moody et al, 2014; Breen et al, 2015), as well as the opportunity to fully define the crystallographic nature of individual grain boundaries and compare this directly to interfacial chemistry (Araullo-Peters et al, 2012; Yao, et al, 2013). It is possible to conduct chemico-textural orientation mapping at resolutions otherwise not available (Yen et al, 2015; He et al, 2016). However, there remain significant limitations and challenges with this avenue of APT analysis.…”

Building on Gleiter: The Foundations and Future of Deformation Processing of Nanocrystalline Metals

“…Upon the journey to ambient or elevated homologous temperatures, thermal energy and/or deformation [64][65][66] can perturb the delicate metastable nanocrystalline state, which often brings decreases in strength via grain growth to a larger, more thermodynamically preferable size. [67][68][69][70][71][72][73][74] In the absence of extrinsic stabilization, the grains grow and strength properties are lost. In Figure 5, Weissmu¨ller [75] presents a schematic that illustrates two alloy states proposed by Gleiter, [7] but includes a new principle where solutes with large enthalpies of segregation have energetically minimized the specific grain boundary energy through segregation to the grain boundary.…”

“…[126] Subsequently, he and co-workers used in-situ electron microscopy and 3D atom probe to explore the mechanics and kinetics of the oxygen stabilization, and quantify the excess boundary solute clusters needed for stabilization. [127] On the powder metallurgy side of things, cryomilling studies on the effects of O and N in Cu showed that in the as-cryomilled condition, the NC (~45 nm grain size) ''alloy'' had a composition including 0.512 pct O, 0.26 pct N, 0.027 pct C, 0.064 pct Fe, < 0.0005 pct Al, and 0.0094 pct Cr (all in wt pct). [128] The O and N contents in the starting powder were 0.45 and 0.004 wt pct, respectively, indicating the pick-up of both elements during cryomilling.…”

“…In this regard, the emerging high-resolution microscopy (e.g., high-angle annular dark-field scanning transmission microscopy (HAADF-STEM), transmission Kikuchi diffraction (TKD) analysis), and microanalysis techniques (e.g., electron energy loss spectroscopy (EELS), atom probe tomography (APT)) are critical. [64,69,[142][143][144][145] When correlated with synchrotron/ beamline studies, these methods may provide the best pathway to new discoveries. More so, while the majority of studies have focused on pure metals, [146][147][148] the role of impurity solutes is much less explored.…”

Stress-assisted grain growth in nanocrystalline metals: Grain boundary mediated mechanisms and stabilization through alloying

“…The notion of stabilizing nanocrystalline structures through grain boundary doping has been substantiated for a range of binary alloy systems at temperatures approaching 30% of their melting point [47][48][49]. Stabilization against mechanical grain growth has also been accomplished in nanocrystalline aluminum directly through the addition of grain boundary solute atoms [50] and indirectly through the presence of impurities [51] that effectively pin the boundaries against sliding and/or migration under stress. In the absence of stress-assisted grain growth, a transition to competing deformation mechanisms can be expected that will depend on the nanocrystalline grain size as well as extrinsic factors such as temperature and strain rate.…”