The formation of metastable Pr 2 Fe 23 B 3 phase and its effect on magnetic properties of melt-spun Pr 9 Fe 91 x B x (x = 4-12) have been investigated. Thermogravimetric analysis (TGA) and X-ray diffraction (XRD) studies show that the samples consist of Pr 2 Fe 14 B and -Fe phases at low B content, and Pr 2 Fe 14 B, -Fe and Fe 3 B at high B content. The metastable Pr 2 Fe 23 B 3 phase appears in the intermediate B content (8-12 at%). This metastable Pr 2 Fe 23 B 3 phase results in a poor

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... ess in the demagnetization curve and is detrimental to the hard magnetic properties. Transmission electron microscopy (TEM) studies reveal larger average grain sizes when the B content is at 8.5 at% or above. It is further found that this metastable phase can be transformed into Pr 2 Fe 14 B, -Fe, and Fe 3 B by annealing the sample at temperature of 750 C or above, leading to a more square demagnetization curve. However, B r of the nanocomposite decreases slightly due to the excessive grain growth during high-temperature annealing. Index Terms-Metastable phase, nanocomposite magnet, Pr 2 Fe 23 B 3 , rapidly quenching.