William A. Soffa

Reducing thermal conductivity of binary alloys below the alloy limit via chemical ordering

Substitutional solid solutions that exist in both ordered and disordered states will exhibit markedly different physical properties depending on their exact crystallographic configuration. Many random substitutional solid solutions (alloys) will display a tendency to order given the appropriate kinetic and thermodynamic conditions. Such order-disorder transitions will result in major crystallographic reconfigurations, where the atomic basis, symmetry, and periodicity of the alloy change dramatically. Consequently, the dominant scattering mechanism in ordered alloys will be different than that in disordered alloys. In this study, we present a hypothesis that ordered alloys can exhibit lower thermal conductivities than their disordered counterparts at elevated temperatures. To validate this hypothesis, we investigate the phononic transport properties of disordered and ordered AB Lennard-Jones alloys via non-equilibrium molecular dynamics and harmonic lattice dynamics calculations. It is shown that the thermal conductivity of an ordered alloy is the same as the thermal conductivity of the disordered alloy at ≈0.6T(melt) and lower than that of the disordered alloy above 0.8T(melt).

Influence of crystallographic orientation and anisotropy on Kapitza conductance via classical molecular dynamics simulations

We investigate the influence of crystallographic orientation and anisotropy on local phonon density of states, phonon transmissivity, and Kapitza conductance at interfaces between Lennard-Jones solids via classical molecular dynamics simulations. In agreement with prior works, we find that the Kapitza conductance at an interface between two face-centered cubic materials is independent of crystallographic orientation. On the other hand, at an interface between a face-centered cubic material and a tetragonal material, the Kapitza conductance is strongly dependent on the relative orientation of the tetragonal material, albeit this dependence is subject to the overlap in vibrational spectra of the cubic and tetragonal materials. Furthermore, we show that interactions between acoustic phonons in the cubic material and optical phonons in the tetragonal material can lead to the interface exhibiting greater “thermal anisotropy” as compared to that of the constituent materials. Finally, it is noted that the relati...

Comparative material issues for fast reliable switching in STT-RAMs

With its fast write and read, small cell size, non-volatility and excellent endurance, Spin Transfer Torque-RAM STT-RAM) has a high potential of dominating the embedded and standalone memory world in the near future. In this paper, the suitability of different classes of magnetic materials constituting the STT-RAM free layer is reviewed for faster switching and thermal stability. We identify the following material classes for faster switching in the thermally stable free-layer of a STT-RAM: (a) In-plane materials with high HK and low MS. While the high HK deters the magnetization during the easy to hard axis switching, it helps with switching past the equator, making the switching speeds for high and low HK materials comparable. However, high HK materials benefit from higher thermal stability. (b) Perpendicular materials with low damping have the same switching speed as in-plane materials but greater switching probability because of a lower critical current. The demagnetization field helps the free layer to start switching to the hard axis, but hinders it from switching further to the easy axis beyond the equator. (c) Anti-ferromagnetically capped partially-perpendicular materials. Capping with a Va layer decreases the demagnetization field, which promotes faster switching.

Interplay of ordering and spinodal decomposition in the formation of ordered precipitates in binary fcc alloys: Role of second nearest-neighbor interactions

In this paper, the possible interaction of ordering and phase separation tendencies in the formation of an ordered precipitate phase (A3B/L12) within a binary supersaturated fcc solid solution is investigated using computational thermodynamics based on a generalized Bragg–Williams model incorporating first and second nearest-neighbor interactions. The formulation synthesizes and expands upon previous works and incorporates a strong pedagogical approach to elucidate the essential elements of the problem. The diffusional pathways governing microstructural development are predicted to be more complex, allowing for a multiplicity of decomposition mechanisms when second nearest-neighbor interactions are incorporated into the solution energetics, even in this mean field approximation. These higher order interactions markedly influence phase equilibria and phase stability. Ordering and clustering tendencies are not mutually exclusive but can occur synergistically, e.g. a conditional spinodal decomposition is predicted contingent on prior ordering of initially non-stoichiometric, disordered solid solutions. The role of second nearest-neighbor interactions on thermodynamic stability is discussed explicitly and compared to the classic treatments limited to first nearest-neighbor interactions only.

Diffusional Phase Transformations in the Solid State

Abstract This chapter deals with diffusional phase transformations. We first define a phase and move on to discuss various ways of classifying phase transformations. We then discuss in detail the energetics (thermodynamics) and kinetics of diffusional phase transformations. Transformations discussed include: precipitation, atomic ordering, spinodal decomposition, massive and cellular transformations in the light of the sections on energetics and kinetics. A final section of the role of symmetry in developing microstructure concludes the chapter.

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).

Lengthscale effects on exchange coupling in Co-Pt L10 + L12 nanochessboards

The Co-Pt nanochessboard is a quasi-periodic, nanocomposite tiling of L10 and L12 magnetic phases that offers a novel structure for the investigation of exchange coupling, relevant to permanent magnet applications. Periodicity of the tiling is controlled by the rate of cooling through the eutectoid isotherm, resulting in control over the L10-L12 exchange coupling. First order reversal curve analysis reveals a transition from partial coupling to nearly complete exchange-coupling in a Co40.2Pt59.8 nanochessboard structured alloy as the periodicity is reduced below the critical correlation length. Micromagnetic simulations give insights into how exchange coupling manifests in the tiling, and its impact on microscopic magnetization reversal mechanisms.

Re-Examination of A1 → L10 Ordering: Generalized Bragg-Williams Model with Elastic Relaxation

The tetragonal lattice relaxation has been included in the thermodynamics of the fcc→L10 ordering to produce a first-order character of the transition within the mean field description of the binary solution energetics. In view of growing interest in such systems e.g. Fe-Pd and Co-Pt alloys, which display a wide range of applications relevant to current and futuristic technologies, the fcc→L10 two-phase field is re-examined utilizing a generalized Bragg-Williams approach including first and second nearest neighbor interactions. The thermodynamic behavior is examined in the limit of T→0K and discussed in terms of the implications of the Third Law of Thermodynamics.

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.

Strain induced microstructural and ordering behaviors of epitaxial Fe38.5Pd61.5 films grown by pulsed laser deposition

Epitaxial films of Fe38.5Pd61.5 at the L10-L12 eutectoid composition have been grown on MgO (001) oriented substrates by pulsed laser deposition. The effect of deposition temperature on the magnetic, microstructural, and crystallographic natures of these films are discussed. The films in this study exhibit atomic ordering with increasing deposition temperature, transitioning from the disordered face centered cubic (FCC) phase to an L12 ordered phase, which is tetragonally distorted due to epitaxial strain. This distortion leads to a perturbation in the Fe occupancy of the Pd superlattice sites at nonstoichiometric compositions. Additionally, Fe38.5Pd61.5 films grown at 550 °C have been found in an unique two-phase microstructure of prismatic, Fe60Pd40 disordered FCC secondary phases with 10–100 nm facets oriented along the ⟨110⟩ substrate directions, embedded within a nearly stoichiometric ordered L12-Fe27Pd73 matrix. These secondary phase precipitates exhibit single domain magnetic axis rotation, while t...