Development of Mo Containing Fe-Cr-Co Permanent Magnets by Modified Single Step Thermomagnetic Treatment

Abstract

This paper is focused on a new approach, which was aimed to ease up the development of Fe-Cr-Co-Mo permanent magnets. Tri-arc melting technique under inert atmosphere of argon was used for the development of Fe-Cr-Co-Mo alloy. Solution treatment was done at a temperature of 1250 °C for 5 h followed by water quenching and then samples were subjected to thermomagnetic treatment (TMT) at 630 °C at predetermined cooling rates. The influence of TMT and cooling rates on the final magnetic properties of the alloy were investigated. Microstructural, X-ray diffraction, and magnetic characterization were carried out. It was observed that cooling rates and TMT temperature both affect the magnetic properties of material. Spinodal structure was observed at 50 k magnification of scanning electron microscope: rod like α 1 phase (FeCo-rich) is embedded in the matrix of α 2 -phase (Cr-rich). The average length of the α 1 rod like particles is 150 ± 30 nm and the diameter is 30 ± 10 nm. Resultant magnetic properties are as 900 Oe (H c ), 10.4 kG (B r ), and 4.3 MGOe (BH max ) with a two-step aging process and 810 Oe (H c ), 10.6 kG (B r ), and 3.6 MGOe (BH max ) with continuous cooling up to 540 °C. The current method provides a quick and low cost manufacturing route for the Fe-Cr-Co-Mo permanent magnets with magnetic properties comparable with that of AlNiCo with the added advantage of having high ductility.