Solid state grain alignment of permanent magnets in near-final shape
Magnet microstructure manipulation in the solid state by controlled application of a sufficient stress in a direction during high temperature annealing in a single-phase region of heat-treatable magnet alloys, e.g., alnico-type magnets is followed by magnetic annealing and draw annealing to improve coercivity and saturation magnetization properties. The solid-state process can be termed highly controlled abnormal grain growth (hereafter AGG) and will make aligned sintered anisotropic magnets that meet or exceed the magnetic properties of cast versions of the same alloy types.
1. A method for treating a preshaped magnet body to impart magnetic properties, the step of concurrently heating a preshaped magnet body having a solid state with essentially full density of at least 98% of theoretical and applying a uni-axial stress to the solid state magnet body by applying an external mechanical load thereto such that the uniaxial stress is transmitted through the solid state magnet body in a direction at a temperature where a single phase magnet alloy of the magnet body exists and for a time that results in preferential solid state, stress-biased grain growth that imparts a grain-aligned magnetic microstructure to the magnet body.
2. The method of claim 1 wherein the heating is followed by magnetic annealing.
3. The method of claim 2 wherein the magnetic annealing then is followed by draw annealing.
4. The method of claim 1 wherein the uni-axial stress is applied by external uni-axial mechanical loading of the magnet body during heating.
5. The method of claim 4 wherein the uni-axial stress is applied by a static dead weight disposed on the magnet body during heating.
6. The method of claim 1 wherein the applied stress is compressive stress.
7. The method of claim 1 wherein the stress is controlled so that substantially zero strain occurs that avoids plastic flow of the microstructure.
8. The method of claim 1 wherein grain growth occurs in a direction normal to the direction of applied stress when the magnet body has a cubic crystal structure.
9. The method of claim 1 wherein the grain-aligned microstructure is polycrystalline or a monocrystalline.
10. The method of claim 1 wherein the grain-aligned magnetic microstructure occurs by stress-biased grain growth and grain rotation toward a preference direction.
11. The method of claim 1 wherein the magnet body comprises an alloy comprising Al, Ni, and Co.
12. An anisotropic magnet made by the method of claim 1 .
13. An anisotropic magnet made by the method of claim 2 .
14. The method of claim 1 wherein the magnet body is a sintered body.