R. T. Fingers

Creep deformation of a soft magnetic iron–cobalt alloy

The U.S. Air Force is in the process of developing magnetic bearings, as well as an aircraft integrated power unit and an internal starter/generator for main propulsion engines. These developments are the driving force for the new emphasis on the development of high saturation, low loss magnets capable of maintaining structural integrity in high stress and high temperature environments. It is this combination of desired material characteristics that is the motivation of this effort to measure, model, and predict the creep behavior of such advanced magnetic materials. Hiperco® Alloy 50HS, manufactured by Carpenter Technology Corporation, is one of the leading candidates for these applications. Material specimens were subjected to a battery of mechanical tests in order to study and characterize their behaviors. Tensile tests provided stress versus strain behaviors that clearly indicated: a yield point, a heterogeneous deformation described as Luders elongation, the Portevin–LeChatelier effect at elevated te...

Microstructure and magnetic properties of Fe–Co alloys

Fe–Co soft magnetic alloys exhibit high magnetic saturation, high yield strength, and moderate core loss. Use of such materials in cyclic high temperature high stress environments, such as generators and magnetic bearings, gives impetus to determining material properties. In particular, Hiperco® Alloy 50HS, provided by Carpenter Technology Corporation, has been a subject of our study. In order to fully understand the overall behavior of the alloy, both mechanical and magnetic properties must be investigated. Magnetic performance is a function of grain size, which varies with the annealing process. Fe–Co samples have been treated by various annealing recipes ranging in temperature from 1300 to 1350 °F and magnetic saturation along with hysteresis loop measurements made using a vibrating sample magnetometer. An etching and sample preparation process was developed and microstructural analyses were performed. The correlation between composition, heat treatment, microstructure, and magnetic properties of these...

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...

Effects of Zr, Nb, and Cu substitutions on magnetic properties of melt-spun and hot deformed bulk anisotropic nanocomposite SmCo type magnets

Structure and magnetic properties of both melt-spun and hot deformed bulk Sm–Co type nanocomposite magnets have been investigated with various metal additions, including Zr, Cu, and Nb. The Zr and Nb additions play important roles in constraining grain growth, resulting in an increase of coercivity Hc. The Cu addition significantly improves the squareness of BH loops as well as the energy product (BH)max. A typical hot deformed bulk anisotropic nanocomposite SmCo type magnet with Mr(hard)∕Mr(easy)∼0.4, Hc∼9kOe and (BH)max of 13.2MGOe was obtained.

Fully dense anisotropic nanocomposite Sm(Co,Fe,Zr,Cu,B)z (z=7.5–12) magnets

Fully dense anisotropic nanocomposite Sm(Co0.58Fe0.31Zr0.05Cu0.04B0.02)z (z=7.5–12) magnets have been synthesized via rapid hot pressing and hot deformation processes. The highest (BH)max∼10.6MGOe was observed for a magnet with z=10. X-ray diffraction and M-H measurements indicated that the easy magnetization direction of magnets prefers to be in the hot pressing direction. Transmission electron microscopy investigation confirmed that plastic deformation is an important route for forming magnetic anisotropy in the Sm–Co-type nanocomposite magnets. Some stripe and/or platelike patterns have been observed inside the nanograins (50–200nm), which may present as twins, and stacking faults. The (0001) twins have been observed in the 2:17R phase.Fully dense anisotropic nanocomposite Sm(Co0.58Fe0.31Zr0.05Cu0.04B0.02)z (z=7.5–12) magnets have been synthesized via rapid hot pressing and hot deformation processes. The highest (BH)max∼10.6MGOe was observed for a magnet with z=10. X-ray diffraction and M-H measurements indicated that the easy magnetization direction of magnets prefers to be in the hot pressing direction. Transmission electron microscopy investigation confirmed that plastic deformation is an important route for forming magnetic anisotropy in the Sm–Co-type nanocomposite magnets. Some stripe and/or platelike patterns have been observed inside the nanograins (50–200nm), which may present as twins, and stacking faults. The (0001) twins have been observed in the 2:17R phase.

Effect of high-temperature aging on electrical properties of Hiperco® 27, Hiperco® 50, and Hiperco® 50 HS alloys

Some more electric aircraft concepts require soft magnetic FeCo materials to be stable at temperatures as high as 773 K for long periods of time. At this high operating temperature, aging related processes may occur that result in changes in material properties. The material supplier typically specifies only room-temperature properties, and only limited reports are available on properties at elevated temperatures. The change in properties as a function of time at 773 K will give information on the lifetime of the material to assist designers when selecting materials for high-temperature applications. We have conducted a study on the effects of long-term aging on the magnetic, mechanical, and electrical properties of Hiperco® 27, Hiperco® 50, and Hiperco® 50 HS FeCo soft magnetic alloys. Samples of each material were aged in argon for 100, 1000, 2000, and 5000 h at 773 K. Here, we report the changes in electrical resistivity after aging. Of the three alloys, high-temperature aging has the greatest effect o...

Fe–Co–V alloy with improved magnetic properties and high-temperature creep resistance

Advanced power systems require soft magnetic materials with a combination of high saturation magnetization and high creep resistance. When the Fe–Co–V alloy laminate is used in a rotor at high temperatures (500–600 °C) coupled with very high rpm, significant creep occurs, which destroys the device integrity. Since grain boundary slide is predominantly responsible for creep deformation in the Fe–Co–V alloy at temperature higher than ∼430 °C, the approach in this study was to reduce the volume fraction of the grain boundaries by making a Fe–Co–V alloy with very large grains. Very large grains, up to mm range, were readily obtained after small cold deformation of ∼3% followed by a normal recrystallization anneal. Fe–Co–V alloy with large grains displays lower coercivity and higher permeability than the commercial Fe–Co–V alloy. Even though its yield strength at 600 °C is lower than the commercial Fe–Co–V, the creep strains of the Fe–Co–V alloy with large grains are only 1/10–1/2 of that for the commercial al...

Magnetic properties of FeCo laminates subjected to axial loading

While manufacturing the stator/rotor assemblies of the aircraft power components, increased power losses are typically pronounced due to an imposed axial load onto the stack of magnetic laminates to increase the rigidity of the stack. An axial loader to enable core loss measurements to be made while the laminate stack is subjected to an axial load has been designed. The apparatus and multilayered slotted interface plates, allowing for the toroidal windings and simultaneous load application were used to evaluate the effect of compression up to 27.5 MPa on magnetic properties of commercially available Fe–Co based Hiperco® 50, Hiperco® 50 HS, and Hiperco® 27 alloys. For each composition, we tested two sets of samples: (1) completely insulated and (2) uninsulated. We report the increased losses due to an axial load and discuss the origin of these increased losses in terms of constant size anisotropy for completely insulated laminates.

Coercivity of bulk anisotropic nanocomposite Sm(CoFeTi)8–10 magnets

The present study investigates the effects of nonmagnetic metal additions on Sm(CobalFe0.2Ti0.05MxB0.01)z (x=0–0.04 and z=8–10) based melt-spun alloys. M=Nb, Ga, Cu, and Al were selected to form anisotropic nanocomposite bulk magnets via hot pressing followed by a hot deformation. The effects of Nb, Ga, Cu, and Al substitutions on intrinsic coercivity Hci and possible mechanisms behind the improvement in Hci, such as phase formations, anisotropy field HA, and microstructure were investigated. Experimental results indicated an over 20%–50% enhancement in Hci after hot deformation (HD) with Nb, Ga, and Cu additions for magnets with z=8 and z=10. Especially with the Nb addition, the Hci improved from 8.7 to 12 kOe for the z=8 magnet, and from 5 to 11 kOe for the z=10 magnet. An unusual enhancement in Hci, from 9 kOe after hot pressing (HP) at 700 °C to 11 kOe after HD at 850 °C, was observed in the Nb-doped magnet with z=10. Our analysis on possible mechanisms behind the improvement in Hci indicated that Ga ...