Effect of niobium addition on the NdFeB alloy and magnet

Abstract

The effect of niobium addition on the microstructure and phase compositions of an Nd 2(Fe 0.98Nb 0.02) 14B alloy and an Nd 16Fe 76.3Nb 0.3B 7.4 magnet have been investigated. The microstructure of the as-cast alloy contains four phases, the Nd 2Fe 14B matrix phase, an NbFeB ternary phase, Nd-rich regions and free iron. After homogenisation the alloy contains only the first two phases and very small amounts of Nd-rich phase. Quantitative electron probe microanalysis of the NbFeB phase revealed a composition of approximately Nb 26Fe 32B 42. This ternary phase was found in the as-cast alloy, the homogenised alloy and also, to a lesser extent, in the magnet. The shape of this phase in the as-cast and the sintered magnet was hexagonal whereas, in the homogenised alloy, it was also observed in an acicular form. In the as-cast alloy the ternary phase was found to nucleate within the free iron phase and then grows at the expense of the surrounding free iron. This provides strong evidence for the role of Nb in the removal of free iron from the microstructure. The results indicate that, by controlling the solidification behaviour of the alloy, free iron may be prevented from nucleating or at least reduced to a very thin film near the surface of the ingot which can be removed by machining to produce NdFeB material without free iron.