Studies on magnetic properties and microstructure of melt-spun nanocomposite R-8(Fe,Co,Nb)(86)B-6 (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 hav e been studied for the development of more economical and medium strength p ermanent magnets; The nanocomposite structure with a grain size around 15-3 0 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 phas e 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 coer civities are 3.3 kOe in Nd8Fe86B6 and 4.9 kOe in Pr8Fe86B6 samples. Nb subs titution significantly reduces the grain size of alpha-Fe and increases the coercivity. The highest coercivities obtained are 5.5 kOe in Nd-8(Fe0.97Nb 0.03)(86)B-6 and 9.3 kOe in Pr-8(Fe0.92Nb0.08)(86)B-6 samples. Co substitut ion 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 C o substituted samples leads to the formation of a more homogeneous and fine r 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 furth er found that Co substitution significantly increases the Curie temperature of the 2.: 14: 1 phase and improves the temperature dependence of the satu ration magnetization. (C) 1999 Elsevier Science B.V. All rights reserved.