Powder metallurgy and liquid-phase sintering

Permanent magnets based on Nd2Fe14B are now being produced in commercially quite significant quantities following their discovery in 1984. 

The preparation of the alloys can proceed by two essentially different methods. Vacuum melting in an induction furnace is the more common method. First, Fe and B are melted in alumina crucibles under purified argon gas. Subsequently, the reaction vessel is degassed under vacuum and Nd metal is added to the melt after the latter has reached a temperature only slightly above the Fe-B liquidus temper- 

The second method involves the so-called calciothermic reduction of rare earth oxides by means of calcium in the presence of 3d metals under a protective atmosphere (Cech 1974, Herget and Domazer 1975, Sun Guangfei et al. 1985, Herget 1987, Zhou et al. 1987). This process is commonly referred to as the reduction-diffusion (R-D) process. Basically, it involves two steps. In the first step, performed at 900-1100 °C, the actual reduction of Nd203 takes place according to the reactions  

8Nd203 + 10Fe2O3 + 56Fe + 8B + 54Ca Nd16Fe76B8 + 54CaO, or 

To obtain the composition required it is necessary to apply an excess of Nd203 and Ca. The second step is a leaching step performed at room temperature in which the excess calcium and calciumoxide are removed from the reaction products by means of water, 

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