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

Phase evolution of the matrix Nd₂Fe₁₄B magnetic phase in a pre-alloyed Nd₁₅Fe₇₇B₈ 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 Nd₂Fe₁₄B 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 Nd₁₅Fe₇₇B₈ alloy can be amorphized completely by a high-energy MM. On annealing of the amorphous material, fine α-Fe crystallizes first from the amorphous phase. These fine α-Fe crystallites react with the remaining amorphous phase afterwards, leading to crystallization of Nd₂Fe₁₄B with ultra fine grain structure. The milled and annealed Nd₁₅Fe₇₇B₈ alloy shows an unusual TMA curve with magnetization maximum just below T_c of the Nd₂Fe₁₄B phase. This is attributed to a Hopkinson effect associated with the ultra-fine grain structure of the Nd₂Fe₁₄B phase in the milled and annealed material.