Bulk Amorphous and Partially Crystallized Alloys in Nd-Fe-(Al, B) System with Hard Magnetic Properties Prepared by Arc Melting

Abstract

Bulk Nd60Fe30Al10−x Bx (x = 0, 2.5, 5, 7.5 and 10 at%) alloys with hard magnetic properties were prepared by an arc melting method. The bulk alloys have weights of 10 and 100 mg with button shapes of 1 and 6 mm in thickness, respectively. The X-ray diffraction and TEM observation data reveal that the Nd60Fe30Al10 bulk alloy exhibits an amorphous phase while the other alloys consist of partial crystalline and amorphous phases. The bulk Nd60Fe30Al10 amorphous alloy of 1 mm in thickness exhibits crystallization temperature (Tx) of 802 K, eutectic temperature (Te) of 925 K, and reduced crystallization temperature (Tx/Te) of 0.87. The alloy exhibits hard magnetic properties at 298 K, i.e., saturation magnetization of 0.13 T, remanance of 0.09 T, and intrinsic coercive filed of 275 kA/m under an applied field of 1242 kA/m. These thermal and magnetic properties are nearly the same as those for the corresponding bulk amorphous cylinders of 1 to 12 mm in diameter prepared by copper mold casting. The replacement of Al with B in Nd‐Fe‐Al amorphous alloy causes a decrease in amorphous-forming ability (AFA), although their hard magnetic properties remain almost unchanged. The reason for the decrease in AFA was analyzed using mixing enthalpy (∆Hmix) and mismatch entropy (Sσ ) corresponding the empirical criteria for the achievement of high AFA. Furthermore, amorphousforming composition regions (AFCRs) were calculated by giving a limitation for formation of an amorphous phase to ∆Hmix and Sσ values, which arises from the statistics for 6500 amorphous alloys listed in a database. The calculated AFCR of the Nd‐Fe‐Al and Nd‐Fe‐B alloy systems agrees with the experimental data reported previously. The decrease in AFA by the replacement of Al with B in the Nd60Fe30Al10−x Bx alloys is interpreted from the result that Nd 60Fe30B10 is located near the edge of the AFCR in Nd‐Fe‐B system.