Studies on magnetic properties and microstructure of melt-spun nanocomposite R8(Fe,Co,Nb)86B6 (R = Nd, Pr) magnets

Fe-rich R-8(Fe, Co, Nb)(86)B-6 (R = Nd, Pr) alloys consisting of a mixture of exchange-coupled magnetically hard 2 : 14 : 1 and soft Fe(Co) phases have been studied for the development of more economical and medium strength permanent magnets; The nanocomposite structure with a grain size around 15-30 nm is obtained by crystallizing the as-made amorphous melt-spun ribbons. The coercivity is found to depend mainly on the grain Size of the soft phase which is very sensitive to the sample composition. The average grain size is about 30 nm in R8Fe86B6, but the microstructure is not homogeneous and there are several large alpha-Fe grains with size up to 50-100 nm. The coercivities are 3.3 kOe in Nd8Fe86B6 and 4.9 kOe in Pr8Fe86B6 samples. Nb substitution significantly reduces the grain size of alpha-Fe and increases the coercivity. The highest coercivities obtained are 5.5 kOe in Nd-8(Fe0.97Nb0.03)(86)B-6 and 9.3 kOe in Pr-8(Fe0.92Nb0.08)(86)B-6 samples. Co substitution for Fe increases the grain size of both the 2 : 14 : 1 phase and alpha-Fe and dramatically decreases the coercivity. Increasing the B content in Co substituted samples leads to the formation of a more homogeneous and finer microstructure and thus to a partial recovery of the coercivity from 2.3 kOe in Nd-8((Fe0.5Co0.5)(0.97)Nb-0.03)(86)B-6 to 4.3 kOe in Nd-8((Fe0.5Co0.5)(0.97)Nb-0.03)(82)B-10 and from 2.1 kOe in Pr-8((Fe0.5Co0.5)(0.94)Nb-0.06)(86)B-6 to 6.5 kOe in Pr-8((Fe0.5Co0.5)(0.94)Nb-0.06)(82)B-10. It is further found that Co substitution significantly increases the Curie temperature of the 2.: 14: 1 phase and improves the temperature dependence of the saturation magnetization. (C) 1999 Elsevier Science B.V. All rights reserved.