Crystal structure, microstructure and magnetic properties of rapidly solidified samarium-cobalt based alloys

The SmCo-based permanent magnets are drawing much more attention since the early 1970\u27s for their highly attractive features such as, high energy product (15 MGOe–30 MGOe), reliable coercive force, best temperature characteristics and excellent corrosion and oxidation resistance. These interesting and highly demanding features have made Sm-Co the ideal material in dynamic applications such as generators and motors. The melt-spun ribbons of Sm-Co-based magnetic materials produced by rapid solidification exhibited higher anisotropy, improved microstructures and better magnetic properties (Mr ∼ 8.5 kG, Hc ∼ 4.1 kOe and (BH)max ∼ 18.2 MGOe and a high remanence ratio of 0.9). This research reports the structure and magnetic properties of rapidly solidified SmCo permanent magnets of simple binary alloy systems modified with Nb and C additions. Melt spinning at 40 m/s resulted in the formation of the metastable TbCu7-type structure in all instances regardless of alloying additions. While the unalloyed Sm12Co88 alloy displayed a coercivity of 0.5 kOe, alloying additions resulted in a systematic and profound increase in coercivity, with maximum values exceeding 37 kOe. TEM revealed the presence of fcc Co, formed as a result of the non-equilibrium processing. The alloying additions had a profound influence on the scale of the microstructure, reducing the SmCo7 grains from the micron-scale to the 100 nm range and the scale of the Co from 80 nm to 10 nm. The nanoscale Co soft magnetic phase enables exchange coupling to the hard magnetic phase, resulting in high remanence ratios (∼0.7). For a range of compositions, from SmCo5.67 to SmCo8, the coercivity was highest in Sm-rich compositions (17.5 kOe) and decreased to ∼3 kOe as Sm content decreased. At higher wheel speed during melt-spinning, the higher chances of formation of Co precipitate and the reduced size of Co precipitates helped to improve the remanence. At higher wheel speed and at higher concentration of alloying additions the magnetization process was dominated by pinning mechanism. During the order-disorder phase transformations, the as-solidified alloys in the TbCu7-type structure exhibited coercivsty as high as 7.85 kOe, which increased to greater than 9 kOe by heat-treatment. The magnetization processes were also strongly influenced by the structural state during order-disorder phase transformations, initially it was totally controlled by nucleation followed by the domain wall pinning