Effect of Preparation on Glass Formation and Magnetic Properties of Nd-Fe-Co-Al-B Alloys
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Effect of Preparation on Glass Formation and Magnetic Properties of Nd-Fe-Co-Al-B Alloys G. Kumar1,2, J. Eckert1, L.Q. Xing1,3, A. Güth1, S. Roth1, W. Löser1, and S. Ram2 1 IFW Dresden, Institute of Metallic Materials, D-01171 Dresden, Germany. 2 Materials Science Centre, Indian Institute of Technology, Kharagpur, 721302, India. 3 Present address: Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD 21218, U.S.A. ABSTRACT The effect of alternative preparation methods (copper mold casting, melt spinning, and mechanical attrition) on amorphization and properties of Nd57Fe20Co5Al10B8 and Nd40Fe40Co5Al8B7 alloys has been investigated. For all differently prepared samples an amorphous phase is formed upon solidification or solid sate reaction. However, the samples prepared by different processing routes exhibit different transformation behavior in thermal analysis. The cast Nd57Fe20Co5Al10B8 rod exhibits crystallization at 790 K followed by melting at 810 K. Neither appreciable endothermic reaction due to a glass transition nor a supercooled liquid region have been observed. Mechanically alloyed powders and ball-milled prealloys reveal two exothermic DSC peaks in the range of 650-850 K. The J-H hysteresis loops of samples synthesized by different routes show that the unique atomic order responsible for hard magnetic properties can only be accessed at moderate cooling rate of the melt as realized in copper mold casting. Rapidly quenched ribbons, mechanically alloyed powders and ball-milled ingots do not show hard magnetic properties at room temperature. These results indicate that amorphous samples with different local atomic order can be prepared by different processing routes. INTRODUCTION Ferromagnetic amorphous alloys are very promising engineering materials due to their favorable magnetic and extraordinary mechanical properties [1,2]. These properties appear due to the absence of magnetocrystalline anisotropy and crystal defects in amorphous materials. The discovery of hard magnetic properties in amorphous Nd-Fe ribbons and Nd-Fe-Al cast cylinders has led to several current advances in the development of bulk ferromagnetic amorphous materials and related technologies [3,4]. A coercive force as high as 277 kAm-1 at room temperature has been reported for Nd60Fe30Al10 cast rods [4]. An increase of coercivity by a factor of 10 at liquid nitrogen temperature has also been reported [5]. The presence of hard magnetic properties in these amorphous alloys has been explained on the basis of the formation of small clusters with large random magnetic anisotropy. Moreover, previous investigations revealed that Nd-Fe-Co-Cu-Al and Nd-Fe-Co-Al-B amorphous alloys prepared by copper mold casting or melt spinning methods exhibit different properties [6,7]. This paper compares the different properties of Nd57Fe20Co5Al10B8 and Nd40Fe40Co5Al8B7 alloys prepared by three different methods, i.e., copper mold casting, melt spinning, and mechanical alloying (MA)/ball milling (BM). The small discrepancy in th
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