Engineering topics

Lewis, L. H.

Publications and source records attributed to Lewis, L. H..

3 records

DOE OSTI2022

Effects of tensile loading during annealing of alnico melt spun ribbons

Conventional magnetic annealing (MA) of the permanent magnet alloy alnico involves application of an external magnetic field at temperatures within the spinodal decomposition range. This field biases the growth of the Fe-Co rich, ferromagnetic α 1 -phase in an energetically favorable 〈001〉 direction in alignment with the applied field within an Al-Ni rich, paramagnetic α 2 -phase. Utilizing a magnetic field to bias the α 1 -phase may limit alnico from reaching theoretical coercivity due to (1) the field having maximum biasing ability at temperatures near the Curie temperature where large α 1 -phase nanorods form and (2) connectivity of the α 1 -phase occurs unavoidably during MA. Both decrease the effective shape anisotropy of the α 1 -phase, thereby reducing coercivity. Herein, we explore tensile-loading as a biasing mechanism to control and optimize the final alnico nanostructure beyond that achieved by MA. Two samples of melt-spun alnico were heat-treated at 860 °C for 5 minutes: one sample was subjected to 10 MPa tensile stress for comparison with a stress-free control sample. Structural and magnetic characterization revealed that the stress-annealed ribbon sample possessed expected phase assemblages, but was distinguished by a ∼2× larger grain diameter and an elongated anisotropic α 1 -phase within grains that were oriented to a shear stress along 〈001〉 directions at an angle of ∼45° relative to the loading direction. Both types of annealing produced a similar increase in the coercivity and remanence, but a decrease in saturation magnetization.

Rinko, E. A.↗

DOE OSTI2021

Borderline first-order magnetic phase transition in AlFe 2 B 2

The thermal evolution of lattice parameters coupled with heat capacity data provide insight into tailorable magnetism-structure attributes in the orthorhombic compound AlFe 2 B 2 that was synthesized with and without small additions of gallium. Temperature-dependent X-ray powder diffraction experiments conducted through the magnetic phase transition reveal that the a- and b-parameters of both samples increase with increasing temperature while the c-parameter decreases. While a weak volumetric thermal expansion is noted over a range of temperatures well below and above the magnetic phase transition, anomalous behavior was observed within the phase transition region itself to reveal a magnetostructural phase transition with borderline first-order character in the Ga-modified sample but of more second-order character in the Ga-free sample. It is established that the nearest-neighbor Fe-Fe interatomic distance within the (ab)-plane plays a dominant role in influencing the magneto-functional response of these compounds. The magnetocaloric properties are discussed in the context of temperature-induced changes of the interatomic bonding that are influenced by the hypothesized presence of iron antisite defects in the AlFe 2 B 2 lattice.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2021

Towards additive manufacturing of magnetocaloric working materials

The feasibility of three-dimensional printing via the Directed Energy Deposition (DED) technique is demonstrated for the ferromagnetic AlFe 2 B 2 (aka 1-2-2) compound, which shows promise as a working material for room-temperature magnetocaloric devices. Like many intermetallic materials, the 1-2-2 compound is brittle at room temperature; not only is it difficult to synthesize from the melt, it is challenging to form it into useful geometries necessary for effective heat transfer in magnetocaloric devices. Using Laser Engineered Net Shaping (LENS TM ) synthesis from two powder precursors, AlB 2 and Fe, samples of simple (rod) and complex (honeycomb) architectures were successfully synthesized. Furthermore, the phase constitution and distribution, magnetic transition temperature, and saturation magnetization determined for the LENS TM -synthesized rod sample are all similar to those obtained for 1-2-2 materials made by conventional casting. Further, the measured magnetic entropy change, as determined from Maxwell's Relations applied to magnetic data, indicates that the magnetocaloric response of the annealed LENS TM -printed rod samples are comparable to those of the 1-2-2 compounds made by conventional techniques. These results confirm that complex magnetofunctional shapes, which are needed for efficient energy conversion in magnetocaloric devices, can be rapidly fabricated from the AlFe 2 B 2 compound.

36 MATERIALS SCIENCE↗