Evolution of Microstructure and Microchemistry in the High Temperature Sm(Co,Fe,Cu,Zr)_z Magnets

摘要

The evolution of microstructure and microchemistry of sintered and cast Sm(Co,Cu,FeZr)_z magnets were investigated under different aging temperatures, aging times and cooling rates. The magnetic properties were measured at each state and correlated with the microstructure and microchemistry determined by transmission electron microscopy and nanoprobe EDAX, respectively. Results shwo that the 1:5 precipitates are formed earlier than the lamella phase in both magnets. The precipitates are observed even after 5 min aging at 830 deg C. With longer aging the precipitates coalesce and start forming the cellular structure. In both magnets, unfirom cellular and lamellar structures are formed after 2 hours of aging at 800-850 deg C. However, the coercivity remains very low and develops only after slow cooling below 600 deg C. Lower aging temperatures lead to a smaller cell size, thinner boundaries, and to lower lamella density. Nanoprobe studies show a small Cu gradient across the 1:5/2:17 boundaries even at the early stages of aging. The gradient is enhanced significantly upon further heat treatment to the final optimum state. Increasing of Cu and Zr content in the magnet assists in the rapid formation of the right microstructure and microchemistry leading to high coercivity.