TMA study of phase evolution in Nd-Fe-B material during mechanical milling and annealing

Phase evolution of the matrix Nd/sub 2/Fe/sub 14/B magnetic phase in a pre-alloyed Nd/sub 15/Fe/sub 77/B/sub 8/ alloy lump during mechanical milling (MM) and subsequent annealing was investigated mainly by thermomagnetic analysis (TMA). TMA is also extended to an examination of the structural features of the Nd/sub 2/Fe/sub 14/B grains in mechanically milled and annealed material. The Hopkinson effect in the TMA of the milled and annealed material is also discussed in relation to the grain structure and magnetic properties. The Nd/sub 15/Fe/sub 77/B/sub 8/ alloy can be amorphized completely by a high-energy MM. On annealing of the amorphous material, fine (alpha)-Fe crystallizes first from the amorphous phase. These fine (alpha)-Fe crystallites react with the remaining amorphous phase afterwards, leading to crystallization of Nd/sub 2/Fe/sub 14/B with ultra fine grain structure. The milled and annealed Nd/sub 15/Fe/sub 77/B/sub 8/ alloy shows an unusual TMA curve with magnetization maximum just below T/sub c/ of the Nd/sub 2/Fe/sub 14/B phase. This is attributed to a Hopkinson effect associated with the ultra-fine grain structure of the Nd/sub 2/Fe/sub 14/B phase in the milled and annealed material.