Influence of quench rate and hydrogen absorption on the magnetic properties of melt-spun Nd15Fe77B8

The effects of wheel speed and hydrogen absorption on the magnetic properties of melt-spun Nd₁₅Fe₇₇B₈ have been studied using Mössbauer spectroscopy, magnetization measurements, differential scanning calorimetry and thermopiezic analysis (TPA).

Samples prepared at wheel speeds up to 30 m s⁻¹ consist mainly of microcrystalline tetragonal Nd₂Fe₁₄B. At speeds greater than 50 m s⁻¹ the samples are composed of an amorphous phase, which orders magnetically at 435 K and crystallizes at 890 K. A mixture of crystalline and amorphous phases is obtained at intermediate wheel speeds.

TPA measurements show that hydrogen absorption proceeds via as many as three stages; the primary absorption of microcrystalline Nd₂Fe₁₄B is at 680 K, whereas that of the ingot is at 530 K and that of the amorphous alloy is at 620 K. The Curie temperature and hyperfine field in amorphous Nd₁₅Fe₇₇B₈ (430 K and 27 T respectively) are considerably less than in crystalline Nd₂Fe₁₄B. Hydrogenation raises the hyperfine field but leaves the Curie temperature almost unchanged.