Nd(FeCo)Al alloys

In this work, we are investigating the Nd₆₀Fe_xCo_30-xAl₁₀ (0 < x < 30) alloys prepared by mold casting and melt spinning. One of the stricking feature observed in these alloys is the appearance of glass transition in Co-rich compositions. The subsitituion of Co by Fe decreases the primary crystallization temperature and obscures  the glass transition. This is the underlying reason for the absence of glass transition in Fe-rich compostition, which contradicts the previous arguments about the occurence of glass transition abobe melting temperature. We have also found that all the as-cast samples exhibit two-phase microstructure comprising of Nd-rich and Fe-rich regions. As illustrated in Fig 3, the dark sphears are enriched with Fe and light matrix is rather Nd-rich. Furthermore, the TEM investigations have revealed that the Fe-rich sphears contain metastable A1 or µ-phase (depending on composition under investigation) and Nd-crystallites. This kind of separation is possibly driven by the positive (+1 kJ/mol)  heat of mixing between Nd and Fe. The melt is single phase above liquidus temperature but separates into two regions with different compositions on lowering the temperature below liquidus. Such a liquid-liquid phases separation is a consequence of energy minimization. This is the region why as-cast binary NdFe (near eutectic) and Nd-30Fe(Co)-10Al alloys always exhibit Nd-rich and Fe-rich regions.

The µ (A1) is found to be hard magnetic with a Curie temperature of about 240 °C. The tentative composition of the A1 phase was found to lie in the range of NdFe to NdFe₄. The partial (up to 5 at%) substitution of Fe by Al in the A1 phase  leads to the formation of µ-phase without affecting the magnetic properties. However, further increase in Al content causes a phase transformation from µ to d (tetragonal). The A1 phase is metastable in character and transforms inot Nd₅Fe₁₇ (hexagonal) while µ transforms into d  upon short annealing at 600 °C. In conclusion hitherto, we have elucidated that, that the A1 and µ phases are responsible for the large coercivities (0.4 T) in as-cast Nd-Fe (hypoeutectic) and Nd₆₀Fe_xCo_30-xAl₁₀ (0 < x < 30) alloys (Fig. 4), respectively. We have studied the coercivity  as a function of temperature (10 K < T < 400 K) for as-cast and melt-spun Nd₆₀Fe_xCo_30-xAl₁₀ (0 < x < 30) alloys. The strong domain wall pinning (Gaunt et al.) governs the coercivity mechanism in these alloys. The Nd-crystallites and amorphous matrix acting as pinning centres impede the domain walls of ferromagnetic A1(µ) phase resulting in a large room temperature coercivity.

Fig. 3 : SEM micrographs of mold-cast Nd60FexCo30-xAl10  rods (3 mm diameter).	Fig. 4 : The M-H loops of mold-cast Nd60FexCo30-xAl10  rods at room temperature.

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