Bulk amorphous and partially crystallized alloys in Nd-Fe-(Al, B) system with hard magnetic properties prepared by arc melting

Bulk Nd₆₀Fe₃₀Al_10-xB_x (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 (T_x) of 802 K, eutectic temperature (T_e) of 925 K, and reduced crystallization temperature (T_x/T_e) 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 (ΔH_mix) and mismatch entropy (S_σ) corresponding the empirical criteria for the achievement of high AFA. Furthermore, amorphous-forming composition regions (AFCRs) were calculated by giving a limitation for formation of an amorphous phase to ΔH_mix and S_σ 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 Nd₆₀Fe₃₀Al_10-xB_x alloys is interpreted from the result that Nd₆₀Fe₃₀B₁₀ is located near the edge of the AFCR in Nd-Fe-B system.