R-Fe—B sintered magnet and making method

1. An R—Fe—B base sintered magnet of a composition consisting essentially of 12 to 17 at % of R which is at least two of yttrium and rare earth elements and essentially contains Nd and Pr, 0.1 to 3 at % of M 1 which is at least one element selected from the group consisting of Si, Al, Mn, Ni, Cu, Zn, Ga, Ge, Pd, Ag, Cd, In, Sn, Sb, Pt, Au, Hg, Pb, and Bi, 0.05 to 0.5 at % of M 2 which is at least one element selected from the group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W, 4.8+2×m to 5.9+2×m at % of B wherein m stands for atomic concentration of M 2 , up to 10 at % of Co, up to 0.5 at % of carbon, up to 1.5 at % of oxygen, up to 0.5 at % of nitrogen, and the balance of Fe, containing R 1.1 (Fe,(Co)) 14 B intermetallic compound as a main phase, and having a coercivity of at least 10 kOe at room temperature, wherein

the magnet contains a M 2 boride phase at a grain boundary triple junction, but not including R 1.1 Fe 4 B 4 compound phase, has a core/shell structure that the main phase is covered with HR-rich layer composed of (R,HR) 2 (Fe,(Co)) 14 B, wherein HR is at least one element selected from Tb, Dy and Ho, the thickness of HR-rich layer is in range of 0.01 to 1.0 μm, and moreover the outside of HR-rich layer is covered with grain boundary phases comprising an amorphous and/or sub-10 nm nanocrystalline (R,HR)—Fe(Co)-M 1 phase consisting essentially of 25 to 35 at % of (R,HR), with the proviso that R and HR are as defined above and HR is up to 30 at % of R+HR, 2 to 8 at % of M 1 , up to 8 at % of Co, and the balance of Fe, or the (R,HR)—Fe(Co)-M 1 phase and a crystalline phase or a sub-10 nm nanocrystalline and amorphous (R,HR)-M 1 phase having at least 50 at % of R, wherein a surface area coverage of the (R,HR)—Fe(Co)-M 1 phase on the main phase with HR-rich layer is at least 50%, and the width of the intergranular grain boundary phase is at least 10 nm and at least 50 nm on the average.

2. The sintered magnet of claim 1 wherein in the (R,HR)—Fe(Co)-M 1 phase, M 1 consists of 0.5 to 50 at % of Si and the balance of at least one element selected from the group consisting of Al, Mn, Ni, Cu, Zn, Ga, Ge, Pd, Ag, Cd, In, Sn, Sb, Pt, Au, Hg, Pb, and Bi.

3. The sintered magnet of claim 1 wherein in the (R,HR)—Fe(Co)-M 1 phase, M 1 consists of 1.0 to 80 at % of Ga and the balance of at least one element selected from the group consisting of Si, Al, Mn, Ni, Cu, Zn, Ge, Pd, Ag, Cd, In, Sn, Sb, Pt, Au, Hg, Pb, and Bi.

4. The sintered magnet of claim 1 wherein in the (R,HR)—Fe(Co)-M 1 phase, M 1 consists of 0.5 to 50 at % of Al and the balance of at least one element selected from the group consisting of Si, Mn, Ni, Cu, Zn, Ga, Ge, Pd, Ag, Cd, In, Sn, Sb, Pt, Au, Hg, Pb, and Bi.

5. The sintered magnet of claim 1 wherein a total content of Dy, Tb and Ho is up to 5.5 at %.

6. The sintered magnet of claim 5 wherein the total content of Dy, Tb and Ho is up to 2.5 at %.

7. A method for preparing the R—Fe—B base sintered magnet of claim 1 , comprising the steps of:

shaping an alloy powder into a green compact, the alloy powder being obtained by finely pulverizing an alloy consisting essentially of 12 to 17 at % of R which is at least two of yttrium and rare earth elements and essentially contains Nd and Pr, 0.1 to 3 at % of M 1 which is at least one element selected from the group consisting of Si, Al, Mn, Ni, Cu, Zn, Ga, Ge, Pd, Ag, Cd, In, Sn, Sb, Pt, Au, Hg, Pb, and Bi, 0.05 to 0.5 at % of M 2 which is at least one element selected from the group consisting of Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W, 4.8+2×m to 5.9+2×m at % of B wherein m stands for atomic concentration of M 2 , up to 10 at % of Co, and the balance of Fe,

sintering the green compact at a temperature of 1,000 to 1,150° C.,

cooling the sintered compact to room temperature,

machining the sintered compact into the shape near the desired end product shape,

placing a powder of HR-containing compounds or intermetallic compounds (HR stands for at least one element selected from Tb, Dy and Ho) on the surface of the sintered magnet,

heating the powder-coated magnet in vacuum at 700 to 1,100° C. for HR to permeate through the grain boundaries and to diffuse among the sintered magnet,

cooling the magnet body to a temperature of 400° C. or below at a rate of 5 to 100° C./min, and

aging treatment including exposing at a temperature in the range of 400 to 600° C. which temperature is lower than the peritectic temperature of (R,HR)—Fe(Co)-M 1 phase so as to form the (R,HR)—Fe(Co)-M 1 phase at a grain boundary, and cooling to a temperature of 200° C. or below.

8. The method of claim 7 wherein the alloy contains Dy, Tb and Ho in a total amount of up to 5.0 at %.

9. The method of claim 7 wherein the magnet contains up to 0.5 at % of HR which has been diffused into the magnet as a result of the grain boundary diffusion step.

10. The method of claim 7 wherein the magnet contains Dy, Tb and Ho in a total amount of up to 5.5 at %.