DEVELOPING RARE-EARTH PERMANENT-MAGNET ALLOYS FOR GAS ATOMIZATION

The development of inert gas atomization (IGA) as a primary production route for Nd-Fe-B type magnets has not been commercially successful d ue to a cooling rate which is much lower than the maximum achievable i n melt-spinning (MS). It is further complicated by the fact that powde r particles of a range of sizes are produced which solidify at differe nt rates and form significantly different microstructures. The role of the cooling rate is analysed in a general way by processing the same alloy composition by IGA and MS. MS allows a much broader but controll ed range of cooling rates to be studied than is possible in IGA. Gener al MS concepts of underquenching and overquenching are applied to IGA to indicate the state of the microstructure. Although the bulk of the IGA powder was formed in an underquenched condition, energy products a pproaching those obtainable in optimally quenched MS ribbons could be achieved in the finest size fraction (5-10 mu m) of powder. Changes in susceptibility shaw the general trend of improvement in hard magnetic property with decreasing scale of the microstructure. Quenchability d iagrams show that TIC additions to the base alloy increase the quencha bility and may allow future IGA alloys to be produced in an overquench ed condition.