Gerard M. Ludtka

The effect of high magnetic field on phase stability in Fe-Ni

Identically prepared samples of Fe0.85Ni0.15 were annealed either in the ambient magnetic field or in a field of 29 T. Room temperature x-ray powder diffraction measurements that were performed after magnetic annealing showed that the ratio of the volume of the γ to α phase is decreased in the field-annealed sample by a factor of 2. First-principles calculations of the magnetic structure in the presence of a magnetic field are used to compute the resulting change in free energy. Analysis in terms of the phase diagram calculated with and without a magnetic field is in substantial agreement with the measurements.Identically prepared samples of Fe0.85Ni0.15 were annealed either in the ambient magnetic field or in a field of 29 T. Room temperature x-ray powder diffraction measurements that were performed after magnetic annealing showed that the ratio of the volume of the γ to α phase is decreased in the field-annealed sample by a factor of 2. First-principles calculations of the magnetic structure in the presence of a magnetic field are used to compute the resulting change in free energy. Analysis in terms of the phase diagram calculated with and without a magnetic field is in substantial agreement with the measurements.

An investigation of superplasticity in a thermomechanically processed U-6nb (α + γ2) alloy

A uranium-2 molybdenum (U-2Mo) alloy was shown to exhibit superplastic behavior over the β + γ two-phase field temperature regime and over a limited temperature span in the α + γ field. At Oak Ridge, two distinct processes were developed that evolved microstructures conducive to superplasticity. These microstructures were shown to exhibit superplasticity (elongations >500 pct) over a broad range of strain rates, from 2.5 × 10-4 to 1 × 10-2 s-1. A maximum value of 700 pct elongation was reached at 695 °C and a true constant strain rate of 2.5 × 10-3 s-1. This study details the processing sequences, microstructures, strain-rate sensitivity, and maximum elongation data generated to characterize the superplastic U-2Mo alloy. In addition, the fracture and cavitation analyses conducted on constant strain-rate tensile test specimens are discussed.

Exchange Coupling Nanophase Fe-Pd Ferromagnets Through Solid State Transformation

This study continues previous work on off-stoichiometric Fe-Pd alloys using a combined reaction strategy during thermomechanical processing [1,2]. Severe plastic deformation of the initial disordered fcc gamma phase (γ) of compostion Fe-35at.%Pd, followed by heat treatment in the two phase field produces a nano-composite ferromagnet comprised of soft alpha phase/ferrite (α) in a high-anisotropy L10 FePd matrix. The length scale and morphology of the transformation products have been characterized using x-ray diffraction, and scanning electron microscopy. The transformed microstructures exhibit strong texture retention similar to the stoichiometric alloy suggesting a massive ordering mode. The alloy has shown a proclivity to exchange couple at a length scale not in agreement with proposed theories of exchange coupling [3,4]. The magnetic properties were measured using standard vibrating sample magnetometry (VSM). This research has been supported by the National Science Foundation (NSF-DMR).

Electron Microscopy Study of Hypostoichiometric Fe-Pd Nanocomposites Resulting from Combined Reactions Thermomechanical Processing

Hypostoichiometric Fe-Pd binary alloys (35-45 at% Pd) were severely deformed (>90%) and subsequently aged to induce concomitant recrystallization, precipitation, and ordering. This thermomechanical processing strategy was articulated by Hornbogen [1] over thirty years ago. The resulting exchange-coupled ferromagnets contain ferrite precipitates and a complex metastable two-phase lamellar transformation product comprised of ordered L10 and a metastable FCC phase. The later duplex microconstituent is suggested to form in conjunction with a so-called pseudospinodal reaction [2] involving emerging cubic and tetragonal phases, whereby phase separation and ordering result from continuous changes in composition and a reduction in symmetry, cubic to tetragonal. The deformation texture of the parent austenite is substantially retained in the transformation product, resulting in anisotropy of the magnetic properties as determined by magnetometry (VSM). This paper presents electron microscopy results elucidating the crystallography and morphology of the phase mixtures including HREM. Magnetic field annealing is also included as a branch of our thermomechanical processing strategy, and we discuss the influence of the external fields on recrystallization, precipitation, and ordering.

Texture Evolution In Fe-1%Si as a Function of High Magnetic Field

The effect of a 1.5T, 15T and 30T magnetic field on texture evolution in Fe-1%Si was investigated by annealing samples for 1 hour at 787°C, (27° above the Curie temperature, Tc = 760°C). The intensity of the Goss texture component increased with increasing field strength accompanied by a drastic increase in grain size.

Investigation and Analytical Description of Acoustic Production by Magneto-Acoustic Mixing Technology

Magneto-Acoustic Mixing Technology is a novel manufacturing method that combines two magnetic fields to produce high-intensity sonication for liquid-state materials processing. This method may be adapted to the manufacture of various materials that benefit from a combination of high temperature, magnetic fields, and acoustic energy. In this work, acoustic generation mechanisms are described in detail and found to be dependent on the skin depth of the induction currents. Analytical models of acoustic pressure are derived, based on two mutually exclusive vibration mechanisms, crucible and melt vibration. Additionally, grain size evidence of acoustic pressure distribution is presented as preliminary model validation.

Microstructural Modification of a Cast Iron by Magnetic Field Processing

The current study deals with the microstructural modification of a nodular cast iron during solidification under the influence of high magnetic fields (up to 18 tesla).

Magneto-acoustic Interfacial Reaction-Based Nanoparticle Synthesis: A Direct Path to Manufacturing Metal Matrix Nanocomposites

This work investigates a novel nanoparticle fabrication methodology: combined reaction and acoustic cavitation abrasion of a solid in contact with a liquid. Magneto-Acoustic Mixing Technology is used to produce nanometer- to micron-sized particles by chemical and acoustic mechanisms between diamond particles and a stainless steel surface in the presence of a metallic liquid (Mg), where it is found that particle–surface interactions and cavitation generate particles more efficiently together than independently, producing unique chemistries. The individual and combined influence of sonic power and chemical reaction on particle size, volume fraction, chemistry, magnetic properties, and applicability to metal matrix nanocomposite fabrication are studied.

Phase Transformation in a High Flux Magnetic Field

A pure Fe and a binary iron carbon alloy were examined using neutron diffraction in a high flux magnetic field to characterize the influence of the field on the equilibrium allotropic phase transformation. Previous literature indicated the transformation of a common steel alloy from ferrite to austenite increased by 3°C per tesla applied. This work found that this common alloy was not typical for all commercial steels, however the change was found to be consistent across hypoeutectoid carbon levels. A change in the normal hysteresis between heating and cooling was measured, as the magnetic field favored the ferromagnetic phase. The measured temperature change was related to the corresponding change in Gibbs free energy and its reduction of the stable critical nucleus size, which predicts finer grain sizes and lath spacings when steel is subjected to a field during a phase transition. This allows the Hall-Petch relationship to explain the enhanced physical properties seen using this technique.