Microstructure Evolution in a Cast and Homogenized Nd-Fe-B-Cu Alloy: Experimental Study and Thermodynamic Calculations

A commercial grade Nd_15.5Fe₇₈B₅Cu_1.5 alloy was prepared through the melting-casting route. The as-cast (AC) alloy was characterized by X-ray diffraction (XRD) and scanning electron microscopy techniques. XRD and energy-dispersive spectroscopy analyses revealed the presence of Nd₂Fe₁₄B, Nd₂Fe₁₇, Nd-rich phase (Nd/NdCu eutectic), and α-Fe phases in the AC specimen. Retainment of α-Fe in the AC microstructure makes a core-shell type microstructure in this condition. Thermodynamic predictions based on the non-equilibrium (Scheil-Gulliver) solidification calculations validate the retainment of both α-Fe and Nd₂Fe₁₇ phases in the AC microstructure. A homogenization treatment has been conducted at 1373 K for 2 h, which effectively results in a substantial reduction in the phase fraction of remnant α-Fe.