Engineering topics
Kramer, Matthew J
Publications and source records attributed to Kramer, Matthew J.
On-the-fly machine-learning for high-throughput experiments: search for rare-earth-free permanent magnets
Advanced materials characterization techniques with ever-growing data acquisition speed and storage capabilities represent a challenge in modern materials science, and new procedures to quickly assess and analyze the data are needed. Machine learning approaches are effective in reducing the complexity of data and rapidly homing in on the underlying trend in multi-dimensional data. Here, we show that by employing an algorithm called the mean shift theory to a large amount of diffraction data in high-throughput experimentation, one can streamline the process of delineating the structural evolution across compositional variations mapped on combinatorial libraries with minimal computational cost. Data collected at a synchrotron beamline are analyzed on the fly, and by integrating experimental data with the inorganic crystal structure database (ICSD), we can substantially enhance the accuracy in classifying the structural phases across ternary phase spaces. We have used this approach to identify a novel magnetic phase with enhanced magnetic anisotropy which is a candidate for rare-earth free permanent magnet.
Size-Induced Chemical and Magnetic Ordering in Individual Fe-Au Nanoparticles
Formation of chemically ordered compounds of Fe and Au is inhibited in bulk materials due to their limited mutual solubility. However, here we report the formation of chemically ordered L12-type Fe3Au and FeAu3 compounds in Fe–Au sub-10 nm nanoparticles, suggesting that they are equilibrium structures in size-constrained systems. The stability of these L12-ordered Fe3Au and FeAu3 compounds along with a previously discovered L10-ordered FeAu has been explained by a size-dependent equilibrium thermodynamic model. Furthermore, the spin ordering of these three compounds has been computed using ab initio first-principle calculations. All ordered compounds exhibit a substantial magnetization at room temperature. The Fe3Au had a high saturation magnetization of about 143.6 emu/g with a ferromagnetic spin structure. The FeAu3 nanoparticles displayed a low saturation magnetization of about 11 emu/g. This suggests a antiferromagnetic spin structure, with the net magnetization arising from uncompensated surface spins. First-principle calculations using the Vienna ab initio simulation package (VASP) indicate that ferromagnetic ordering is energetically most stable in Fe3Au, while antiferromagnetic order is predicted in FeAu and FeAu3, consistent with the experimental results.
Structural and Magnetic Evolution of Bimetallic MnAu Clusters Driven by Asymmetric Atomic Migration
The nanoscale structural, compositional, and magnetic properties are examined for annealed MnAu nanoclusters. The MnAu clusters order into the L10 structure, and monotonic size-dependences develop for the composition and lattice parameters, which are well reproduced by our density functional theory calculations. Simultaneously, Mn diffusion forms 5 Å nanoshells on larger clusters inducing significant magnetization in an otherwise antiferromagnetic system. The differing atomic mobilities yield new cluster nanostructures that can be employed generally to create novel physical properties.
Relating Dynamic Properties to Atomic Structure in Metallic Glasses
Atomic packing in metallic glasses is not completely random but displays various degrees of structural ordering. While it is believed that local structures profoundly affect the properties of glasses, a fundamental understanding of the structure–property relationship has been lacking. In this article, we provide a microscopic picture to uncover the intricate interplay between structural defects and dynamic properties of metallic glasses, from the perspective of computational modeling. Computational methodologies for such realistic modeling are introduced. Exploiting the concept of quasi-equivalent cluster packing, we quantify the structural ordering of a prototype metallic glass during its formation process, with a new focus on geometric measures of subatomic “voids.” Atomic sites connected with the voids are found to be crucial in terms of understanding the dynamic, including vibrational and atomic transport, properties. Normal mode analysis is performed to reveal the structural origin of the anomalous boson peak (BP) in the vibration spectrum of the glass, and its correlation with atomic packing cavities. Through transition-state search on the energy landscape of the system, such structural disorder is found to be a facilitating factor for atomic diffusion, with diffusion energy barriers and diffusion pathways significantly varying with the degree of structural relaxation/ordering. The implications of structural defects for the mechanical properties of metallic glasses are also discussed.
L1 0 structure formation in slow-cooled Fe-Au nanoclusters
An ordered L1 0 structure has been formed in near-stoichiometric Fe-Au alloy nanoparticles. The L1 0 structure with a = 0.367 nm and c = 0.360 nm was observed in nanoclusters with diameters below 10 nm after slow cooling from 600 °C. The stable L1 0 structure formed from a parent fcc solid solution phase observed in the as-formed clusters. The fcc phase has a lattice parameter of 0.417 nm, significantly expanded compared to both Au and γ-Fe. The saturation magnetization and coercivity of both fcc and L1 0 structures were much lower than expected considering Fe dilution effects suggesting competing ferromagnetic and anti-ferromagnetic ordering.
Exchange-coupled nanoscale SmCo/NdFeB hybrid magnets
Nanoscalehybridmagnets containing SmCo5 and Nd2Fe14B hard magnetic phases have been produced via a novel “in-one-pot” processing route. The grain size of the processed bulk composite materials is controlled below 20 nm. The refinement of the nanoscale morphology leads to effective inter-phase exchange coupling that results in single-phase like magnetic properties. Energy product of 14 MGOe was obtained in the isotropic nanocomposite magnets at room temperature. At elevated temperatures, the hybridmagnets have greatly improved thermal stability compared to the Nd2Fe14B single-phase counterpart and have substantially increased magnetization and energy products compared to the single-phase SmCo5 counterpart.
Effect of Nb and W substitutions on the stability of the A15 Mo3Si phase
A combination of theoretical and experimental studies on the stability of Mo3Si with Nb and W substitutions has been presented in this paper. The first principles calculations suggest that the formation enthalpy of the A15 phase favors Nb substitution up to 37.5 atom %. In case of W substitution, the formation enthalpies increase monotonically. Experimental studies on the other hand, showed that the A15 phase is destabilized when approximately 27.5 atom % Nb is added while 7.5–10 atom % W substitution was sufficient to destabilize A15 phase. It was further determined that upon destabilization of the A15 structure, Nb partitions preferentially to the T1 phase, whereas W partitions preferentially to a solid solution with Mo.
Icosahedral short-range order in amorphous Cu80Si20 by ab initio molecular dynamics simulation study
Short-range order in liquid and amorphous structures of Cu80Si20 is studied by ab initio molecular dynamics simulations. We performed the simulations at 1140 and 300 K respectively to investigate the local structure change from liquid to amorphous. The result of structure factor in comparison with experimental data indicates that our simulation of amorphous Cu80Si20 is reliable. By using the bond-angle distribution function, Honeycutt–Andersen index, Voronoi tessellation method, and the atomistic cluster alignment method, the icosahedral short-range order in the system is revealed. Strong Cu–Si interaction was also observed.
Reactive gas atomization processing for Fe-based ODS alloys
Gas atomization reaction synthesis was employed as a simplified method for processing oxide dispersion forming precursor Fe-based powders (e.g., Fe–Cr–Y–Hf). During this process a reactive atomization gas (i.e., Ar–O2) was used to oxidize the powder surfaces during primary break-up and rapid solidification of the molten alloy. This resulted in envelopment of the powders by an ultra-thin (t < 50 nm) metastable Cr-enriched oxide shell that was used as a vehicle to transport oxygen into the consolidated microstructure. Subsequent elevated temperature heat treatment promoted thermodynamically driven oxygen exchange reactions between trapped films of Cr-enriched oxide and internal (Y, Hf)-enriched intermetallic precipitates, resulting in highly stable nano-metric mixed oxide dispersoids (i.e., Y–Hf–O) that were identified with X-ray diffraction. Transmission electron microscopy and atom probe tomography results also revealed that the size and distribution of the dispersoids were found to depend strongly on the original rapidly solidified microstructure. To exploit this, several oxide dispersion strengthened microstructures were engineered from different powder particle size ranges, illustrating microstructural control as a function of particle solidification rate. Additionally, preliminary thermal–mechanical processing was used to develop a fine scale dislocation substructure for ultimate strengthening of the alloy.