Matthew J. Kramer

Development of suitable interatomic potentials for simulation of liquid and amorphous Cu―Zr alloys

We present a new semi-empirical potential suitable for molecular dynamics simulations of liquid and amorphous Cu–Zr alloys. To provide input data for developing the potential, new experimental measurements of the structure factors for amorphous Cu64.5Zr35.5 alloy were performed. In this work, we propose a new method to include diffraction data in the potential development procedure, which also includes fitting to first-principles and liquid density and enthalpy of mixing data. To refine the new potential, we used first-principles and liquid enthalpy of mixing data published earlier combined with the densities of liquid Cu64.5Zr35.5 measured over a range of temperatures. We show that the potential predicts a liquid-to-glass transition temperature that agrees reasonably well with experimental data. Finally, we compare the new potential with two previously developed semi-empirical potentials for Cu–Zr alloys and examine their comparative and contrasting descriptions of structure and properties for Cu64.5Zr35.5 liquids and glasses.

Using atomistic computer simulations to analyze x-ray diffraction data from metallic glasses

We propose a method of using atomistic computer simulations to obtain partial pair correlation functions from wide angle diffraction experiments with metallic liquids and their glasses. In this method, a model is first created using a semiempirical interatomic potential and then an additional atomic force is added to improve the agreement with experimental diffraction data. To illustrate this approach, the structure of an amorphous Cu64.5Zr35.5 alloy is highlighted, where we present the results for the semiempirical many-body potential and fitting to x-ray diffraction data. While only x-ray diffraction data were used in the present work, the method can be easily adapted to the case when there are also data from neutron diffraction or even in combination. Moreover, this method can be employed in the case of multicomponent systems when the data of several diffraction experiments can be combined.

Compressive creep behavior of Mo5Si3 with the addition of boron

Abstract Mo5Si3 shows high creep resistance but poor high temperature oxidation resistance. Previous work has shown that the oxidation rate of Mo5Si3 can be decreased with the addition of boron. By adding 1.3 wt % boron to a silicon deficient composition, a three phase microstructure composed of Mo5Si3 (T1), Mo5Si3, and a ternary Mo5(Si,B)3 (T2) phase was synthesized. The compressive creep rate of this composition was evaluated at 1240–1320 °C and 120–180 MPa. The average creep stress exponent and activation energy for the three phase material were found to be n = 4.3 and E a = 396 kJ mol . TEM analysis of the crept microstructure of the boron modified material reveals no evidence for dislocation activity in T1. Only basal slip was observed in the T2 phase while polygonal sub-grain structures were observed in Mo3Si.

Processing and mechanical properties of a molybdenum silicide with the composition Mo–12Si–8.5B (at.%)

Abstract Alloys with the nominal composition Mo–12Si–8.5B (at.%) were prepared by arc-melting or powder-metallurgical processing. Cast and annealed alloys consisted of approximately 38 vol.% α-Mo in a brittle matrix of 32 vol.% Mo3Si and 30 vol.% Mo5SiB2. Their flexure strengths were approximately 500 MPa at room temperature, and 400–500 MPa at 1200°C in air. The fracture toughness values determined from the three-point fracture of chevron-notched specimens were about 10 MPa m1/2 at room temperature and 20 MPa m1/2 at 1200°C in air. The relatively high room temperature toughness is consistent with the deformation of the α-Mo particles observed on fracture surfaces. Three-point flexure tests at 1200°C in air and a tensile test at 1520°C in nitrogen indicated a small amount of high temperature plasticity. Extrusion experiments to modify the microstructure of cast alloys were unsuccessful due to extensive cracking. However, using powder-metallurgical (PM) techniques, microstructures consisting of Mo3Si and Mo5SiB2 particles in a continuous α-Mo matrix were fabricated. The room temperature fracture toughnesss of the PM materials was on the order of 15 MPa m1/2.

Cerium: An Unlikely Replacement of Dysprosium in High Performance Nd–Fe–B Permanent Magnets

Replacement of Dy and substitution of Nd in NdFeB-based permanent magnets by Ce, the most abundant and lowest cost rare earth element, is important because Dy and Nd are costly and critical rare earth elements. The Ce, Co co-doped alloys have excellent high-temperature magnetic properties with an intrinsic coercivity being the highest known for T ≥ 453 K.

Fabrication of bulk nanocomposite magnets via severe plastic deformation and warm compaction

We demonstrate that a SmCo/FeCo based hard/soft nanocomposite material can be fabricated by distributing the soft magnetic α-Fe phase particles homogeneously in a hard magnetic SmCo phase through severe plastic deformation. The soft-phase particle size can be reduced from micrometers to smaller than 15 nm upon deformation. Up to 30% of the soft phase can be incorporated into the composites without coarsening. A warm compaction process of the plastically deformed powder particles then produces bulk nanocomposite magnets of fully dense nanocomposites with energy product up to 19.2 MGOe owing to effective interphase exchange coupling, which makes this type of nanocomposite magnets suitable for high energy-density applications at high temperatures.

Growth of large-grain R-Mg-Zn quasicrystals from the ternary melt (R = Y, Er, Ho, Dy and Tb)

Abstract The growth of large (up to 0.5 cm3). single-grain R-Mg-Zn quasicrystals (R = Y, Er, Ho, Dy and Tb) from a ternary melt is described in detail. The quasicrystals grown by this technique have a composition R8.7Mg34.6Zn56.8, are thermodynamically stable and have a dodecahedral morphology with clearly visible pentagonal facets. The quasicrystalline phase has been examined by X-ray diffaction, high-resolution transmission electron microscopy, elemental analysis and transport measurements. In addition, results are presented for a closely related ternary crystalline phase.

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.

Anomalous temperature-dependent transport in YbNi2B2C and its correlation to microstructural features

We address the nature of the ligandal disorder leading to local redistributions of Kondo temperatures, manifested as annealing-induced changes in the transport behavior of the heavy fermion system

One-Pot Synthesis of Urchin-like FePd–Fe3O4 and Their Conversion into Exchange-Coupled L10–FePd–Fe Nanocomposite Magnets

{\mathrm{YbNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}.