Rare-earth magnet, method of manufacturing rare-earth magnet, and rotator
A rare-earth magnet is an R-T-B-based rare-earth magnet containing a rare-earth element R, a transition metal element T, and boron B. The rare-earth magnet further contains Cu and Co, while having a Cu concentration distribution with a gradient along a direction from a surface of the rare-earth magnet to the inside thereof, Cu having a higher concentration on the surface side of the rare-earth magnet than on the inside thereof, and a Co concentration distribution with a gradient along a direction from the surface of the rare-earth magnet to the inside thereof, Co having a higher concentration on the surface side of the rare-earth magnet than on the inside thereof. The rare-earth magnet is excellent in corrosion resistance.
1. An R—Fe—B-based rare-earth magnet comprising a crystal particle group containing a rare-earth element R, the magnet comprising:
a center part; and
a surface part surrounding the center part and including crystal particles of the crystal particle group, an alloy containing R, Cu, Co, Al, and Fe being present in an R-rich phase included at a grain-boundary triple junction of the crystal particles, wherein:
the rare-earth magnet contains Ni;
the total of Cu, Co, and Al contents in the R-rich phase is at least 13 at %;
the alloy containing R, Cu, Co, Al, and Fe is present only in the surface part of the rare-earth magnet and is present only in a region of the magnet whose depth is 400 μm or less from an outer surface of the magnet;
in a direction perpendicular to the outer surface of the magnet, a thickness of the center part is greater than a thickness of the surface part;
the thickness of the surface part in the perpendicular direction is at least 100 μm and 400 μm or less;
a gradient of a concentration distribution based on unit weight of the rare-earth magnet of the Cu is 0.01 to 5 wt %/mm, a gradient of a concentration distribution based on unit weight of the rare-earth magnet of the Co is 0.01 to 5 wt %/mm, a gradient a concentration distribution based on unit weight of the rare-earth magnet of the Al is 0.01 to 5 wt %/mm, and a gradient of a concentration distribution based on unit weight of the rare-earth magnet of the Ni is 0.001 to 0.1 wt %/mm;
each of the gradients is in the direction perpendicular to the outer surface of the rare-earth magnet and directed from the outer surface to an inside of the magnet such that concentrations of the Cu, Co, Al, and Ni are highest at the outer surface; and
each value of the gradients is an average concentration gradient from a position where a depth from the outer surface of the rare-earth magnet is 20 μm to a position at ¼ of a thickness of the rare-earth magnet.
2. The rare-earth magnet according to claim 1 , wherein the total of Cu and Al contents in the crystal particles is 2 at % or less.
3. A rotator comprising the rare-earth magnet according to claim 2 .
4. The rare-earth magnet according to claim 1 , wherein the crystal particle group occupies at least 85 vol % of the whole rare-earth magnet.
5. A rotator comprising the rare-earth magnet according to claim 4 .
6. A rotator comprising the rare-earth magnet according to claim 1 .
7. The rare-earth magnet according to claim 1 , wherein the thickness of the surface part in the perpendicular direction is at least 200 μm and less than 400 μm.
8. The rare-earth magnet according to claim 1 , wherein an average value of content of Al in the R-rich phase at the grain boundary triple junction within the surface part is 6 to 8 at %.