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The melt-spun SmFe12Bx (x=0,0.50,0.75,1.00,1.25 and 1.50) ribbons were prepared at 40 m.s 1,and their structure and magnetic properties were studied by powder X-ray diffraction (XRD),vibrating sample magnetometer (VSM) and transmission electron microscopy (TEM).XRD results indicate that SmFe12Bx alloys with 0.50 ≤ x ≤ 1.00 are composed of single-phase TbCu7-type structure.Moreover,it is found that the boron addition can inhibit the emergence of soft magnetic phase α-Fe and result in the increase in the axial ratio c/a.After annealing at 650 ℃ for 0.5 h,the metastable phase TbCu7 initially decomposes into the stable phase Sm2Fe14B (Nd2Fe14B-type) and α-Fe.The value of magnetic moment per Fe atom increases slightly from 1.75μB for boron-free sample to 1.80μB for the x =0.75 sample and then decreases again.In addition,the best magnetic properties of maximum energy product [(BH)max] of 14.56 kJ.m-3,coercivity (Hcj) of 172.6 kA.m-1 and remanence (Br) of 0.45T are obtained for the SmFe12B1.00 alloy.Based on transmission electron microscopy (TEM) results,the average size of grains is around 197 nm for B-free sample and decreases to 95 nm for x =1.00 sample,indicating that the addition of boron can refine grains.