Atomic‐Scale Characterization of Aluminum‐Based Multishell Nanoparticles Created by Solid‐State Synthesis

Abstract

Many metallic alloys contain nanoparticles created by solidstate precipitation and, to date, these nanoparticles show simple chemical structures (either homogenous or core-shell). Creating more-complex nanoparticles in metallic alloys (e.g., particles with a core surrounded by multiple concentric shells) would be interesting for two reasons. First, this would represent a proof of concept of a generic, solid-state approach based on a simple heat treatment, applicable to a large variety of metallic and nonmetallic systems with particular characteristics (i.e., a matrix with solubility for solute elements varying with temperature whose diffusion kinetics are mismatched). [1,2] Second, in the field of structural metallic alloys, such complex, tailored nanoparticles can better fulfill the often contradictory requirements for particle stiffness, lattice-parameter mismatch, and shearability, which control the elastic interactions of nanoparticles with dislocations (and hence the alloy's strength) and their coarsening resistance via intrinsic diffusivities and interfacial free energies (and hence the alloy's aging resistance). [3][4][5][6] While core/multishell nanoparticles have been synthesized in liquid suspensions, [7][8][9][10][11] they have never been created within a solid matrix to the best of our knowledge.Core/single-shell particles form in aluminum alloys upon heat treatment; for example, Al 3 (Sc, rare earth (RE)) nanoparticles with RE-rich cores and Sc-rich shells [2,12] and Al 3 (Li, Sc) particles with Al 3 (Sc, Li) cores and Al 3 Li shells. [3,13] In this Communication, we present a general methodology for synthesizing nanoparticles with a core and two concentric shells, which we demonstrate for a quaternary AlÀLiÀScÀRE alloy by combining the methods used to create single-shell nanoparticles in ternary AlÀScÀRE, [2,12] and AlÀLiÀSc alloys. [3,13] Here, core/double-shell nanoparticles were obtained by applying the following heat treatment to an Al-6.3Li-0.069Sc-0.018Yb alloy: a) homogenization at 6408C followed by quenching in an iced brine, b) aging at 3258C for 8 h, followed by quenching down to 1708C, and c) aging for one week at this temperature and quenching in ice brine.Scanning electron microscopy (SEM) observations after the 6408C homogenization step detected primary particles (%1-5 mm in size) at grain boundaries containing Sc, Yb, and Li. As a result, the a-Al matrix had a somewhat depleted composition of Al-5.5Li-0.055Sc-0.008Yb as measured by atom-probe tomography (APT) performed immediately after homogenization. The first 3258C aging step avoids the formation of the stable AlLi d-phase (with the B 32 structure [14] ), while providing a large thermodynamic driving force for homogenously distributed precipitates of core/shell Al 3 X (L 12 ) nanoparticles, where X can be Li, Sc, and/or Yb, as all three binary L 12 trialuminides exist. [4,15,16] The second 1708C aging step was performed close to the metastable d'-Al 3 Li solvus curve [17] to minimize the thermodynamic driving force for homogeneous ...

Atom probe tomography—A cornerstone in materials characterization

“…However, thanks to the new capabilities of the APT technique, they have been observed and chemically analyzed. Core-shell nanostructures have been observed by Monachon et al, 28 attributed to the excellent spatial resolution that can be obtained for complex topologies. Krug et al have reported additional APT analysis of core-shell precipitates for aluminum-scandium-based systems.…”

Structure and growth of core–shell nanoprecipitates in Al–Er–Sc–Zr–V–Si high-temperature alloys

“…This high microhardness is achieved via a two-stage aging process: first aging at 350 ⁰C permits the high-diffusivity elements (Er and Sc) to form the inner part of the precipitate core; subsequent aging at 400 C allows the slower diffusing elements (Zr and V) to form shells enveloping the Er-and Sc-enriched core. While atomprobe tomography (APT) has been recently used to study these nanoprecipitates [10,12,13], to the best of our knowledge, there are not detailed studies using high-resolution transmission electron microscopy (HRTEM) for imaging these nanoprecipitates, thereby, revealing their core-shell structure.…”

Atomically resolved calcium phosphate coating on a gold substrate

“…It offers 3-D imaging and chemical composition measurements at the atomic scale (0.1-0.3 nm spatial resolution in depth, 0.3-0.5 nm laterally). [32][33][34] This capability has led to the study of a wide range of materials on the sub-nanometer to nanometer length scale for a broad range of physical problems; for example, studies of alloys and superalloys, 35,36 nanowires, 37 nanoparticles, 38 transistors, 39 powders, 40 and even metallic thin films and electrodeposits. [41][42][43][44] APT has also been utilized for studying biomineralization.…”