Effects of heat treatment and processing modifications on microstructure in alnico 8H permanent magnet alloys for high temperature applications

Abstract

2014ii In addition, I would like to thank my friends, family, and loved ones for their endless encouragement and love during my college career. ABSTRACTAlnico is a commercially available series of permanent magnet alloys, which form a nanoscale bcc-intermetallic spinodally decomposed structure. The major mechanism of coercivity in these alloys is shape anisotropy produced by the spinodal morphology. A series of heat treatments, with and without external magnetic field, is crucial to developing optimal magnetic properties. Alnico 8H has a coercivity of 1900 Oe, the highest of all commercial alnico grades, and was used to investigate heat treatment effects on microstructure and magnetic properties.Conventional alnico 8H permanent magnets are manufactured by casting or sintered blended elemental powder techniques. Oxides, especially in sintered varieties, can make up as much as 4% of the magnet, reducing total magnetization. Detrimental grain boundary γ phase, and a newly observed σ phase are present in finished commercial magnets, reducing coercivity. Intragranular γ was also found in sintered varieties. Growth of γ occurred as much as 200°C below its reported thermodynamically stable temperature, as shown in recent phase diagrams.Pre-alloyed alnico 8H (without minor Si, S, or Nb additions) was gas atomized into fine single-phase spherical powders to investigate alternative consolidation methods and processing routes. Hot isostatic pressing at 1250°C produces a fully dense compact with small amounts of γ phase and very a low oxide content. Heat-treating in a similar manner to commercial alnico yields a maximum energy product slightly exceeding standard commercial values. Spark plasma sintering (SPS) at lower temperatures generates compacts 80-92% dense, with a large intercellular network of γ and σ phases.

Algorithms for design optimization of chemistry of hard magnetic alloys using experimental data

“…and it was observed in a few samples [21]. Gamma phase must be avoided, as it is detrimental for magnetic properties.…”

“…The following text will provide the reader with a brief idea regarding the role of various alloying elements and its effect on H c and B r [21]:…”

“…(BH) max is dependent on both Br and Hc and it is proportional to Hc at low Hc. For example, a recent study on nanostructured magnetic material suggests that it is possible to achieve a very high magnetic energy product for fine wires of the order of 10 nm [21].…”

Processing of Alnico Magnets by Additive Manufacturing

“…Selected material pieces left over from the cylinder machining process were also subjected to the same SOQ process to verify any microstructural changes occurring. SOQ is a common and necessary practice in alnico magnet fabrication in order to "reset" the nanostructure to a single phase (B2) before developing the α 1 + α 2 nanostructure in a spinodal transformation on cooling or heating [3,11,[26][27][28]. The cylinders were cleaned with ethanol and vacuum sealed in a quartz tube (45 torr).…”

Imanes Permanentes y su Producción por Pulvimetalurgia

“…Afterwards, mixing is required to ensure homogeneity within the material. Pressing to the desired shape can be done using a range of techniques, although isostatic pressing, either cold or hot, is preferred (Dillon, 2014;Tang et al, 2015).…”