54 research outputs found
On spinodal decomposition in alnico---a transmission electron microscopy and atom probe tomography study
Alnico is a prime example of a finely tuned nanostructure whose magnetic
properties are intimately connected to magnetic annealing (MA) during spinodal
transformation and subsequent lower temperature annealing (draw) cycles. Using
a combination of transmission electron microscopy and atom probe tomography, we
show how these critical processing steps affect the local composition and
nanostructure evolution with impact on magnetic properties. The nearly 2-fold
increase of intrinsic coercivity () during the draw cycle is not
adequately explained by chemical refinement of the spinodal phases. Instead,
increased Fe-Co phase () isolation, development of Cu-rich
spheres/rods/blades and additional rod precipitation that occurs
during the MA and draw, likely play a key role in enhancement.
Chemical ordering of the Al-Ni-phase () and formation of Ni-rich
() may also contribute. Unraveling of the subtle effect of these
nano-scaled features is crucial to understanding on how to improve shape
anisotropy in alnico magnets
Recommended from our members
Phase Separation in Lean-Grade Duplex Stainless Steel 2101
The use of duplex stainless steels (DSS) in nuclear power generation systems is limited by thermal instability that leads to embrittlement in the temperature range of 204Ā°C to 538Ā°C. New lean-grade alloys, such as 2101, offer the potential to mitigate these effects. Thermal embrittlement was quantified through impact toughness and hardness testing on samples of alloy 2101 after aging at 427Ā°C for various durations (1ā10,000 h). Additionally, atom probe tomography (APT) was utilized in order to observe the kinetics of Ī±āĪ±ā² separation and G-phase formation. Mechanical testing and APT data for two other DSS alloys, 2003 and 2205, were used as a reference to 2101. The results show that alloy 2101 exhibits superior performance compared to the standard-grade DSS alloy 2205 but inferior to the lean-grade alloy 2003 in mechanical testing. APT data demonstrate that the degree of Ī±āĪ±ā² separation found in alloy 2101 closely resembles that of 2205 and greatly exceeds 2003. Additionally, contrary to what was observed in 2003, 2101 demonstrated G-phase like precipitates after long aging times, although precipitates were not as abundant as was observed in 2205
Recommended from our members
An atom probe perspective on phase separation and precipitation in duplex stainless steels
Three-dimensional chemical imaging of FeāCr alloys showing Fe-rich (Ī±)/Cr-rich (Ī±') phase separation is reported using atom probe tomography techniques. The extent of phase separation, i.e., amplitude and wavelength, has been quantitatively assessed using the Langer-Bar-on-Miller, proximity histogram, and autocorrelation function methods for two separate FeāCr alloys, designated 2101 and 2205. Although the 2101 alloy possesses a larger wavelength and amplitude after annealing at 427 Ā°C for 100ā10 000 h, it exhibits a lower hardness than the 2205 alloy. In addition to this phase separation, ultra-fine NiāMnāSiāCu-rich G-phase precipitates form at the Ī±/Ī±' interfaces in both alloys. For the 2101 alloy, Cu clusters act to form a nucleus, around which a NiāMnāSi shell develops during the precipitation process. For the 2205 alloy, the Ni and Cu atoms enrich simultaneously and no coreāshell chemical distribution was found. This segregation phenomenon may arise from the exact Ni/Cu ratio inside the ferrite. After annealing for 10 000 h, the number density of the G-phase within the 2205 alloy was found to be roughly one order of magnitude higher than in the 2101 alloy. The G-phase precipitates have an additional deleterious effect on the thermal embrittlement, as evaluated by the AshbyāOrowan equation, which explains the discrepancy between the hardness and the rate of phase separation with respect to annealing time (Gladman T 1999 Mater. Sci. Tech. Ser. 15 30ā36).Keywords: atom probe tomography, thermal embrittlement, phase separation, Cu cluster, Fe-Cr alloy, G-phaseKeywords: atom probe tomography, thermal embrittlement, phase separation, Cu cluster, Fe-Cr alloy, G-phas
Strength can be controlled by edge dislocations in refractory high-entropy alloys
Energy efficiency is motivating the search for new high-temperature (high-T) metals. Some new body-centered-cubic (BCC) random multicomponent āhigh-entropy alloys (HEAs)ā based on refractory elements (Cr-Mo-Nb-Ta-V-W-Hf-Ti-Zr) possess exceptional strengths at high temperatures but the physical origins of this outstanding behavior are not known. Here we show, using integrated in-situ neutron-diffraction (ND), high-resolution transmission electron microscopy (HRTEM), and recent theory, that the high strength and strength retention of a NbTaTiV alloy and a high-strength/low-density CrMoNbV alloy are attributable to edge dislocations. This finding is surprising because plastic flows in BCC elemental metals and dilute alloys are generally controlled by screw dislocations. We use the insight and theory to perform a computationally-guided search over 10(7) BCC HEAs and identify over 10(6) possible ultra-strong high-T alloy compositions for future exploration
High-throughput design of high-performance lightweight high-entropy alloys
Developing affordable and light high-temperature materials alternative to Ni-base superalloys has significantly increased the efforts in designing advanced ferritic superalloys. However, currently developed ferritic superalloys still exhibit low high-temperature strengths, which limits their usage. Here we use a CALPHAD-based high-throughput computational method to design light, strong, and low-cost high-entropy alloys for elevated-temperature applications. Through the high-throughput screening, precipitation-strengthened lightweight high-entropy alloys are discovered from thousands of initial compositions, which exhibit enhanced strengths compared to other counterparts at room and elevated temperatures. The experimental and theoretical understanding of both successful and failed cases in their strengthening mechanisms and order-disorder transitions further improves the accuracy of the thermodynamic database of the discovered alloy system. This study shows that integrating high-throughput screening, multiscale modeling, and experimental validation proves to be efficient and useful in accelerating the discovery of advanced precipitation-strengthened structural materials tuned by the high-entropy alloy concept
Detecting Cage Crossing and Filling Clusters of Magnesium and Carbon Atoms in Zeolite SSZ-13 with Atom Probe Tomography
The conversion of methanol to valuable hydrocarbon molecules is of great commercial interest, as the process serves as a sustainable alternative for the production of, for instance, the base chemicals for plastics. The reaction is catalyzed by zeolite materials. By the introduction of magnesium as a cationic metal, the properties of the zeolite, and thereby the catalytic performance, are changed. With atom probe tomography (APT), nanoscale relations within zeolite materials can be revealed: i.e., crucial information for a fundamental mechanistic understanding. We show that magnesium forms clusters within the cages of zeolite SSZ-13, while the framework elements are homogeneously distributed. These clusters of just a few nanometers were analyzed and visualized in 3-D. Magnesium atoms seem to initially be directed to the aluminum sites, after which they aggregate and fill one or two cages in the zeolite SSZ-13 structure. The presence of magnesium in zeolite SSZ-13 increases the lifetime as well as the propylene selectivity. By using operando UV-vis spectroscopy and X-ray diffraction techniques, we are able to show that these findings are related to the suppression of aromatic intermediate products, while maintaining the formation of polyaromatic compounds. Further nanoscale analysis of the spent catalysts showed indications of magnesium redistribution after catalysis. Unlike zeolite H-SSZ-13, for which only a homogeneous distribution of carbon was found, carbon can be either homogeneously or heterogeneously distributed within zeolite Mg-SSZ-13 crystals as the magnesium decreases the coking rate. Carbon clusters were isolated, visualized, and analyzed and were assumed to be polyaromatic compounds. Small one-cage-filling polyaromatic compounds were identified; furthermore, large-cage-crossing aromatic molecules were found by isolating large coke clusters, demonstrating the unique coking mechanism in zeolite SSZ-13. Short-length-scale evidence for the formation of polyaromatic compounds at acid sites is discovered, as clear nanoscale relations between aluminum and carbon atoms exist
Elucidating the Structure and Composition of Individual Bimetallic Nanoparticles in Supported Catalysts by Atom Probe Tomography
Understanding and controlling the structure and composition of nanoparticles in supported metal catalysts are crucial to improve chemical processes. For this, atom probe tomography (APT) is a unique tool, as it allows for spatially resolved three-dimensional chemical imaging of materials with sub-nanometer resolution. However, thus far APT has not been applied for mesoporous oxide-supported metal catalyst materials, due to the size and number of pores resulting in sample fracture during experiments. To overcome these issues, we developed a high-pressure resin impregnation strategy and showcased the applicability to high-porous supported Pd-Ni-based catalyst materials, which are active in CO2 hydrogenation. Within the reconstructed volume of 3 Ć 105 nm3, we identified over 400 Pd-Ni clusters, with compositions ranging from 0 to 16 atom % Pd and a size distribution of 2.6 Ā± 1.6 nm. These results illustrate that APT is capable of quantitatively assessing the size, composition, and metal distribution for a large number of nanoparticles at the sub-nm scale in industrial catalysts. Furthermore, we showcase that metal segregation occurred predominately between nanoparticles, shedding light on the mechanism of metal segregation. We envision that the presented methodology expands the capabilities of APT to investigate porous functional nanomaterials, including but not limited to solid catalysts
Nanoscale Chemical Imaging of Zeolites Using Atom Probe Tomography
Understanding structureācompositionāproperty relationships in zeoliteābased materials is critical to engineering improved solid catalysts. However, this can be difficult to realize as even single zeolite crystals can exhibit heterogeneities spanning several orders of magnitude, with consequences for, for example, reactivity, diffusion as well as stability. Great progress has been made in characterizing these porous solids using tomographic techniques, though each method has an ultimate spatial resolution limitation. Atom probe tomography (APT) is the only technique so far capable of producing 3D compositional reconstructions with subānanometerāscale resolution, and has only recently been applied to zeoliteābased catalysts. Herein, we discuss the use of APT to study zeolites, including the critical aspects of sample preparation, data collection, assignment of mass spectral peaks including the predominant CO peak, the limitations of spatial resolution for the recovery of crystallographic information, and proper data analysis. All sections are illustrated with examples from recent literature, as well as previously unpublished data and analyses to demonstrate practical strategies to overcome potential pitfalls in applying APT to zeolites, thereby highlighting new insights gained from the APT method
Lattice Parameter Evolution during the Ī²-to-Ī± and Ī²-to-Ļ Transformations of Iron- and Aluminum-Modified Ti-11Cr(at.%)
Ī²-titanium (Ī²-Ti) alloys are useful in diverse industries because their mechanical properties can be tuned by transforming the metastable Ī² phase into other metastable and stable phases. Relationships between lattice parameter and Ī²-Ti alloy concentrations have been explored, but the lattice parameter evolution during Ī²-phase transformations is not well understood. In this work, the Ī²-Ti alloys, Ti-11Cr, Ti-11Cr-0.85Fe, Ti-11Cr-5.3Al, and Ti-11Cr-0.85Fe-5.3Al (all in at.%), underwent a 400 Ā°C aging treatment for up to 12 h to induce the Ī²-to-Ļ and Ī²-to-Ī± phase transformations. Phase identification and lattice parameters were measured in situ using high-temperature X-ray diffraction. Phase compositions were measured ex situ using atom probe tomography. During the phase transformations, Cr and Fe diffused from the Ļ and Ī± phases into the Ī² matrix, and the Ī²-phase lattice parameter exhibited a corresponding decrease. The decrease in Ī²-phase lattice parameter affected the Ī±- and Ļ-phase lattice parameters. The Ī± phase in the Fe-free alloys exhibited Ī±-phase c/a ratios close to those of pure Ti. A larger Ī²-phase composition change in Ti-11Cr resulted in larger Ļ-phase lattice parameter changes than that for Ti-11Cr-0.85Fe. This work illuminates the complex relationship between diffusion, composition, and structure for these diffusive/displacive transformations
Nanoscale tomography reveals the deactivation of automotive copper-exchanged zeolite catalysts
Copper-exchanged zeolite chabazite (Cu-SSZ-13) was recently commercialized for the selective catalytic reduction of NOX with ammonia in vehicle emissions as it exhibits superior reaction performance and stability compared to all other catalysts, notably Cu-ZSM-5. Herein, the 3D distributions of Cu as well as framework elements (Al, O, Si) in both fresh and aged Cu-SSZ-13 and Cu-ZSM-5 are determined with nanometer resolution using atom probe tomography (APT), and correlated with catalytic activity and other characterizations. Both fresh catalysts contain a heterogeneous Cu distribution, which is only identified due to the single atom sensitivity of APT. After the industry standard 135,000 mile simulation, Cu-SSZ- 13 shows Cu and Al clustering, whereas Cu-ZSM-5 is characterized by severe Cu and Al aggregation into a copper aluminate phase (CuAl2O4 spinel). The application of APT as a sensitive and local characterization method provides identification of nanometer scale het- erogeneities that lead to catalytic activity and material deactivation
- ā¦