John C. Horwath

Absence of Long-Range Chemical Ordering in Equimolar FeCoCrNi

Equimolar FeCoCrNi alloys have been the topic of recent research as “high-entropy alloys,” where the name is derived from the high configurational entropy of mixing for a random solid solution. Despite their name, no systematic study of ordering in this alloy system has been performed to date. Here, we present results from anomalous x-ray scattering and neutron scattering on quenched and annealed samples. An alloy of FeNi3 was prepared in the same manner to act as a control. Evidence of long-range chemical ordering is clearly observed in the annealed FeNi3 sample from both experimental techniques. The FeCoCrNi sample given the same heat treatment lacks long-range chemical order.

Magnetic and Vibrational Properties of High-Entropy Alloys

The magnetic properties of high-entropy alloys based on equimolar FeCoCrNi were investigated using vibrating sample magnetometry to determine their usefulness in high-temperature magnetic applications. Nuclear resonant inelastic x-ray scattering measurements were performed to evaluate the vibrational entropy of the 57Fe atoms and to infer chemical order. The configurational and vibrational entropy of alloying are discussed as they apply to these high-entropy alloys.

Thermomagnetic analysis of FeCoCrxNi alloys: Magnetic entropy of high-entropy alloys

The equimolar alloy FeCoCrNi, a high-entropy alloy, forms in the face-centered-cubic crystal structure and has a ferromagnetic Curie temperature of 130 K. In this study, we explore the effects of Cr concentration, cold-rolling, and subsequent heat treatments on the magnetic properties of FeCoCrxNi alloys. Cr reductions result in an increase of the Curie temperature, and may be used to tune the TC over a very large temperature range. The magnetic entropy change for a change in applied field of 2T is ΔSm = −0.35 J/(kg K) for cold-rolled FeCoCrNi. Cold-rolling results in a broadening of ΔSm, where subsequent heat treatment at 1073 K sharpens the magnetic entropy curve. In all of the alloys, we find that upon heating (after cold-rolling) there is a re-entrant magnetic moment near 730 K. This feature is much less pronounced in the as-cast samples (without cold-rolling) and in the Cr-rich samples, and is no longer observed after annealing at 1073 K. Possible origins of this behavior are discussed.

An evaluation of diagnostic techniques relevant to arcing fault current interrupters for direct current power systems in future aircraft

Arc fault circuit interrupters (AFCI) are a relatively new development in the area of circuit protection. Typical circuit breakers are designed to detect an overload current in the protected circuit and trip open within a specified period of time. The AFCI is a device that detects and interrupts an arcing fault when it occurs. An arc fault is a dangerous situation with very high temperatures in the arc. These high temperatures are sufficient to ignite wire insulation and other combustibles in the vicinity of the arc. Standard circuit breakers and ground fault circuit breakers generally will not trip in the event of an arc fault since there is sufficiently high impedance in the circuit to limit current below the trip level of its characteristic curve. Numerous arc faults that have been observed on aging aircraft, due to failing insulation, have spurred an interest in AFCIs for both commercial and military aircraft. Such AFCIs assess the AC current waveform to distinguish that an arc is occurring in the circuit. When this occurs, the AFCI will open to mitigate damage to the wiring system. In the AC case, the opening device will generally take advantage of the natural current zero in the AC wave form to clear the circuit. Protective devices for DC systems cannot exploit this characteristic. In addition, the charge transport properties (conductivity) of the DC arc are affected by the gas density (i.e., altitude) surrounding the arc. This paper presents experimental results intended to help evaluate the potential applicability of the typical diagnostics used in AC AFCIs as a valid means of detecting a series DC arc. This is particularly relevant to future aircraft power systems employing 270 Volt DC distribution. Also, these DC arcs have been characterized at low atmospheric pressures to simulate high altitude operation

Persistent current in coils made out of second generation high temperature superconductor wire.

We report the results of an experimental study of a persistent coil made out of YBa2Cu3O7−δ coated conductors. The magnitude of the persistent current and the rate of decay were investigated. Two distinct modes of relaxation are evident—one is flux creep and the other, which is much faster, is of less obvious origin. Our conclusion is that the persistent current in such a coil can be large enough and decay slowly enough so that coated conductors can be used to make persistent coils for variety of applications.

High frequency breakdown characteristics of various electrode geometries in air

In airborne and spacecraft electronic applications, minimum weight and volume constraints have always influenced electrical component and cabling designs. Insulation system specifications must include the total operating environment, voltage excursions, current capacity, mechanical integrity and aging. This is true for both power source equipment as well as special purpose electronics such as radar or high power transmitters, where bare or insulated conductors are often used for high frequency signal transmission, as well as power transmission. A brief review of relevant data is presented to exemplify the effects of high frequency content on breakdown voltages in air, for specific electrode configurations. At high frequencies, the spacing between electrodes or conductors becomes a deterministic parameter for the breakdown phenomenon known as multipactor. When the mean free path of an electron exceeds the order of electrode spacing or relative dimension of the high frequency system, multipactor breakdown may occur, usually at pressures less than 10/sup -2/ torr. Multipactor is prevalent within or around components of the high frequency system such as the waveguide, antenna, cabling, connectors and switches. Information that may be useful to designers of components and interconnections includes high frequency initiation phenomena and multipactor breakdown characteristics. Data concerning low pressure corona initiation on antennae is also presented.

Effects of chemical composition and B2 order on phonons in bcc Fe–Co alloys

The phonon density of states (DOS) gives insight into interatomic forces and provides the vibrational entropy, making it a key thermodynamic function for understanding alloy phase transformations. Nuclear resonant inelastic x-ray scattering and inelastic neutron scattering were used to measure the chemical dependence of the DOS of bcc Fe–Co alloys. For the equiatomic alloy, the A2→B2 (chemically disordered→chemically ordered) phase transformation caused measurable changes in the phonon spectrum. The measured change in vibrational entropy upon ordering was −0.02±0.02 k_B/atom, suggesting that vibrational entropy results in a reduction in the order–disorder transition temperature by 60±60 K. The Connolly–Williams cluster inversion method was used to obtain interaction DOS (IDOS) curves that show how point and pair variables altered the phonon DOS of disordered bcc Fe–Co alloys. These IDOS curves accurately captured the change in the phonon DOS and vibrational entropy of the B2 ordering transition.

High strength bulk Fe–Co alloys produced by powder metallurgy

Fe–Co alloys are extensively used in lamination form, but there are certain power generation applications that require Fe–Co rotors in bulk form. Experiencing only a dc magnetic field, these rotors can be as large as 0.5m in diameter, depending on the size of the generator. The forging of such large pieces of Fe–Co has proven to be difficult. The present study investigates powder metallurgy processing of a gas atomized FeCoNbV alloy through hot isostatic pressing (HIP) for manufacturing large size rotors with improved mechanical strength. Gas atomized FeCoNbV alloy powders with and without ball milling were hot isostatic pressed at temperatures between 675 and 850°C at a fixed pressure of 193MPa for up to 6h. Ball milling prior to HIP improved the yield strength. A further improvement in yield strength and in ductility was obtained after a disordering heat treatment at 730°C followed by a rapid quench to room temperature. The optimum HIP and annealing conditions resulted in samples with yield strengths of...

Effect of surfactant molecular weight on particle morphology of SmCo5 prepared by high energy ball milling

Surfactant-assisted high energy ball milling (HEBM) is a widely used technique for producing nanostructured magnetic materials with oleic acid (OA) being the most commonly utilized surfactant reported in literature to date. No conclusive explanation has been presented for the wide use of OA and only a few studies have deviated from its use. OA has a boiling point of 360 °C which presents issues for complete removal of the surfactant after the HEBM process. Exposing the nanostructured materials to the high temperatures required for surfactant removal is known to result in grain growth and oxidation. In other studies, select surfactant systems, such as octanoic acid or oleylamine, have been used, however, a systematic study examining the dependence of surfactant selection on overall particle (flake) morphology has yet to be performed. In this study, we have qualitatively and quantitatively examined the effects of surfactant selection on the morphology and magnetic properties of SmCo5 utilizing surfactants with lower boiling points that are structurally similar to OA. Our results demonstrate that there was little change in the morphological and magnetic properties for the different surfactants. The implication is that lower boiling point surfactants may be used for HEBM, which require less severe conditions for surfactant removal after milling thereby preserving the integrity of the powders.

Effect of milling time on magnetic properties and structures of bulk Sm-Co/α-(Fe, Co) nanocomposite magnets

Bulk Sm-Co/α-(Fe,Co) nanocomposite magnets were fabricated by hot pressing composite powders prepared by high-energy ball milling of magnetically hard SmCo5 powder and magnetically soft Fe powder. The bulk magnets had a nanocomposite structure consisting of Sm-Co matrix (1:5 H and 1:7 H phases) and α-(Fe,Co) phases. The Fe-Co particles were distributed uniformly in the Sm-Co matrix. The milling time strongly affects the structures and the magnetic properties of the bulk magnets. Increasing milling time led to a decrease of the amount of 1:5 H phase, an increase in the phase fraction of the 1:7 H phase, and a decrease in the amount of soft phase, which resulted in an increase in magnetization and a decrease in coercivity. Scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS) analyses revealed that inter-diffusion took place between the Sm-Co matrix and Fe particles during the processing.