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

Alnico 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. Two methods of reducing γ+σ were explored using gas-atomized material. Rapid solutionization at 1250°C followed by water quenching can eliminate the γ+σ network caused by SPS. Low temperature magnetic annealing can suppress γ growth in later high temperature heat treatments. Reducing γ using either of the two approaches increases saturation magnetization and remanence by 10-30% of commercial values. By further optimizing the heat treatment process, gas-atomized alnico 8H material should exceed the maximum energy product for all alnico grades