DILATOMETRIC AND THERMOGRAVIMETRIC INVESTIGATION OF NDFEB ALLOY CONTAINING COBALT AND DYSPROSIUM

The temperature dependence of the change in volume and oxidizing prope rties of an industrial magnetically hard alloy containing 32 mass % Nd , 2.5 mass % Dy, 4 mass % Co, 1.2 mass % Al, and 1.2 mass % B (the rem ainder is Fe) is investigated. Dilatometric investigations show that r etrograde volume change occurs in the range 127-302 degrees C in this alloy and may lead to cracking in magnet blanks on heating or cooling. The variation in oxidation rate of the alloy with increase in tempera ture is shown by differential thermal analysis. The results explain th e irreversible reduction in magnetic properties on heating NdFeB-alloy magnets above 200 degrees C and permit the recommendation of addition s of alloying elements (Dy, Co, etc.).