David E. Laughlin

Amorphous and nanocrystalline materials for applications as soft magnets

Abstract This review seeks to summarize the recent developments in the synthesis, structural characterization, properties, and applications in the fields of amorphous, bulk amorphous, and nanocrystalline soft magnetic materials. Conventional physical metallurgical approaches to improving soft ferromagnetic properties have relied on the optimization of chemical and microstructural features. Within the last decade, the development and rapid increase in research of nanocrystalline materials has shown that through proper modifications, revolutionary contributions can be made to better materials’ properties. A wide range of materials’ properties are examined in this review, including: kinetics and thermodynamics, structure, microstructure, and intrinsic and extrinsic magnetic properties.

Nano-scale materials development for future magnetic applications

Developments in the field of magnetic materials which show promise for future applications are reviewed. In particular recent work in nanocrystalline materials is reviewed, with either soft or hard behavior as well as advances in the magnetic materials used for magnetic recording. The role of microstructure on the extrinsic magnetic properties of the materials is stressed and it is emphasized how careful control of the microstructure has played an important role in their improvement. Important microstructural features such as grain size, grain shape and crystallographic texture all are major contributors to the properties of the materials. In addition, the critical role that new instrumentation has played in the better understanding of the nano-phase magnetic materials is demonstrated.

Structure and magnetic properties of (Fe0.5Co0.5)88Zr7B4Cu1 nanocrystalline alloys

The development of Fe73.5Si13.5B9Nb3Cu1 (FINEMET) by Yoshizawa et al. and Fe88Zr7B4Cu1 (NANOPERM) by Inoue et al. have shown that nanocrystalline microstructures can play an important role in the production of materials with outstanding soft magnetic properties. The FINEMET and NANOPERM materials rely on nanocrystalline α-Fe3Si and α-Fe, respectively, for their soft magnetic properties. The magnetic properties of a new class of nanocrystalline magnets are described herein. These alloys with a composition of (Fe,Co)–M–B–Cu (where M=Zr and Hf) are based on the α- and α′-FeCo phases, have been named HITPERM magnets, and offer large magnetic inductions to elevated temperatures. This report focuses on thermomagnetic properties, alternating current (ac) magnetic response, and unambiguous evidence of α′-FeCo as the nanocrystalline ferromagnetic phase, as supported by synchrotron x-ray diffraction. Synchrotron data have distinguished between the HITPERM alloy, with nanocrystallites having a B2 structure from the ...

Effects of Ag underlayers on the microstructure and magnetic properties of epitaxial FePt thin films

In this work Ag underlayers, with a slightly larger unit cell than FePt, were found not only to induce epitaxial growth of the FePt films but also to reduce the FePt ordering temperature. Without using the Ag underlayer, the FePt film deposited onto the Si substrate was fcc disordered. By the use of the Ag underlayer, it was observed that the FePt unit cells were expanded in the film plane. This has caused the shrinkage of the FePt unit cells along the film normal direction and resulted in the in situ ordering of the FePt thin film at reduced temperatures. The microstructural and magnetic properties of the FePt/Ag films at varied substrate temperature and FePt thickness were studied to investigate the L10 FePt ordering.

Precipitation hardening in aluminum alloy 6022

Although the precipitation process in Al-Mg-Si alloys has been extensively studied, the understanding of the hardening process is still incomplete, since any change in composition, processing and aging practices, etc., could affect the precipitation hardening behavior. In this paper, hardness measurements, differential scanning calorimetry and transmission electron microscopy have been utilized to study the precipitation hardening behavior in aluminum alloy 6022.

Spinodal decomposition in age hardening copper-titanium alloys

Abstract The early stages of spinodal decomposition in age hardening Cu-Ti alloys have been studied by electron microscopy. The alloys (1.55, 3.08 and 5.17 w/o Ti) decomposed on the quench from solutionizing temperatures into Ti enriched and Ti lean regions. Superlattice reflections, at 1 5 {420} m positions as well as 1 2 {210} m reflections were observed in the diffraction patterns of the as quenched 5.17 w/o Ti alloy. The alloys continued to decompose when aged at elevated temperatures. A sequence of microstructures was used to show that continuous phase separation, and hence spinodal decomposition, was the mechanism of decomposition. The metastable two phase structure which formed from the spinodal process was aligned and periodic from the start of the process. The Ti enriched phase was ordered, with the Dla(Ni4Mo; 14/m) structure. Reversion experiments were performed to determine the position of the coherent metastable solvus. When aging treatments were performed near this solvus. heterogeneous nucleation of the metastable phase was observed.

L–10 ordering and microstructure of FePt thin films with Cu, Ag, and Au additive

The influence of Cu, Ag, and Au additives on the L10 ordering, texture, and grain size of FePt thin films has been examined. Lattice parameter data indicated that Au and Ag additives tended to segregate from FePt, but Cu alloyed with FePt. FePt films with Au or Ag additive showed 1–2 kOe higher coercivity values compared to a pure FePt film after annealing at 450 °C and above for 10 min. The addition of at least 20 vol. % Cu to FePt boosted average coercivity values and increased (001)/(002) x-ray peak intensity ratios, suggesting an accelerated L10 ordering process for annealing temperatures exceeding 350 °C. Decreasing the film thickness promoted (001) film texture in FePt+20% Cu films, but higher annealing temperatures were required to achieve large coercivity. Au and Ag limited the average grain size compared to a pure FePt film. Cu additive increased the average grain size and film roughness.

Decomposition and ordering processes involving thermodynamically first-order order → disorder transformations

Abstract Clustering and ordering processes often occur concomitantly or synergistically during the decomposition of supersaturated solid solutions. However, the occurrence of spinodal decomposition in conjunction with an order → disorder transformation thermodynamically of first order under equilibrium conditions is generally not well understood. In this paper a simple graphical approach using free energy-composition diagrams and loci of thermodynamic instability delineating regions of continuous transformation is used to develop a framework for understanding the interplay between clustering and ordering occurring in metallic solid solutions where the ordered precipitate stems from a phase diagram configuration in which the ordered and disordered states are connected by a first-order phase transition. The approach is applied to the NiAl, NiTi and AlLi binary alloy systems in which the interaction of clustering and ordering appears to play an important role.

Polyol Process Synthesis of Monodispersed FePt Nanoparticles

Monodispersed FePt nanoparticles are synthesized by reduction of iron(II) acetylacetonate and platinum(II) acetylacetonate with 1,2-hexadecanediol as the reducing reagent in the polyol process. As-prepared FePt nanoparticles are chemically disordered with fcc phase. Transmission electron microscopy (TEM) images show a self-assembled particle array with an average particle size of 3 nm and a standard deviation about 10%. The transformation from chemically disordered fcc to chemically ordered L10 phase is achieved by annealing at 650 degrees C for 30 min in Ar atmosphere where the oxygen level is less than 1 ppm. Magnetic hysteresis measurements show a coercivity of 9.0 kOe at 293K, and 16.7 kOe at 5 K for the annealed FePt nanoparticles.

High-strength age hardening copper-titanium alloys: redivivus

In this review the decomposition of supersaturated Cu–Ti solid solutions and subsequent microstructural evolution are discussed in terms of modern views of precipitation from solid solution. This update is motivated by an anticipated emergence of these alloys as technologically significant high-strength, high-conductivity, precipitation hardened alloys over the next decade replacing conventional Cu–Be alloys in numerous applications. The decomposition of Cu–Ti alloys is shown to involve a complex interplay between clustering and ordering effects including a synergy between LRO and SRO similar to those observed in Ni4Mo-type concentrated solutions. New perspectives regarding metastable and stable phase equilibria in the system are discussed including the polymorphic nature of the Cu4Ti precipitate phase. The role of non-classical nucleation and spinodal decomposition in the initial breakdown of the supersaturated state is addressed within the context of a generalized nucleation theory. The fine-scale coherent/semicoherent two-phase mixtures which emerge coarsen according to a LSW coarsening law with an activation energy for the diffusion of Ti in Cu of approximately 50 kcal mol � 1 in excellent agreement with reported values from diffusion studies in the Cu–Ti system. Overaging in Cu–Ti age hardening alloys is associated with the emergence of a coarse lamellar microconstituent which nucleates at the grain boundaries of the parent matrix phase and the growth of these cells consumes the metastable, fine-scale coherent/semicoherent phase mixtures leading to a rapid degradation of mechanical properties. The activation energy for the growth of the cellular microconstituent is less than half that for bulk diffusion indicating interfacial/boundary diffusion control. It is suggested that to enhance and optimize the mechanical properties of the precipitation hardened Cu–Ti alloys, alloying and thermomechanical processing strategies should focus on controlling the nucleation and growth of the cellular or ‘‘discontinuous’’ precipitation reaction. # 2003 Elsevier Ltd. All rights reserved.