Design Of Amorphous Alloys And Crystallization Behavior Of Amorphous Alloys In A High Magnetic Field

Amorphous alloys have unique physical and chemical properties due to their disorder structures. However, to our knowledge, effective theory is still absent in design of bulk amorphous alloys. Based on structure heredity existing between amorphous alloys and primary devitrification phases, the cluster model has been constructed from primary devitrification phases. In this paper, the effects of Al and Ga additions on the glass forming ability of Cu-based amorphous alloys were studied using Discrete-variational method (DVM) based on the density functional theory (DFT). It provides the guidance to design amorphous alloys.High magnetic field is often applied to prepare nanocomposite magnets, which is useful to control the precipitation of soft and hard magnetic phases. This might greatly enhance magnetic properties. In order to search optimum parameters of annealing process, it is necessary to know the crystallization behavior of Nd-Fe-B base amorphous alloys in a high magnetic field. Thus, the crystallization behavior of Nd-FeCo-B amorphous alloys was studied in high magnetic field in this paper. In addition, the effect of high magnetic field on microstructure and magnetic properties also was investigated. At last, the transition processes of crystallized phases in Nd-Fe-B alloys were discussed in the view of electronic structure. The conclusions are as follows:1. The effect of Al and Ga addition on glass-forming ability (GFA) of Cu-based alloys was investigated using the discrete variational method based on a Cu₆Zr₅ cluster model. The results show that GFA of amorphous alloys and stability of cluster increase by introducing small content of Al and Ga atoms, but reduce by more addition.2. Crystallization behavior of Nd-Fe-B base amorphous alloys in a high magnetic field and effect of high magnetic field on microstructure and magnetic properties were investigated using DTA, XRD, TEM and VSM. Crystallization process of amorphous Nd₅Fe₇0Co₈B₁7 alloy can be summarized as: Amorphous→α-Fe/Fe₃B/Nd₂Fe₂₃B₃→α-Fe/Nd_1.1Fe₄B₄. While that of amorphous Nd₉Fe₇8Co₈B₅ alloy is: Amorphous→α-Fe/TbCu₇ type phase→α-Fe/ Nd₂Fe₁₄B. High magnetic field promoted the crystallization in amorphous Nd-Fe-Co-B alloy and accelerated the transition processes from α-Fe/Fe₃B/Nd₂Fe₂₃B₃ to α-Fe/Nd_1.1Fe₄B₄ and α-Fe/TbCu₇ type phase to α-Fe/ Nd₂Fe₁₄B. Meanwhile, the external high magnetic field reduced the grain size of hard and soft phases and induced a uniform distribution of grains. The improvement in magnetic properties after magnetic crystallization is considered to be the resultof enhanced exchange coupling due to a reduction of grain size.3. Structure relations of crystallized phases of Nd-Fe-B amorphous alloys were analyzed with cluster structure. It was found that cluster models in which Nd atom located in centre in Nd2Fe2sB3 phase, TbCu₇ type phase and Nd₂Fe₁₄B phase were similar. The stabilities of precipitation phases were studied comparing with electronic structure. It showed that the stabilities of clusters of Nd2Fe23Ba phase and TbCu7 type phase were low, but high in Nd1.1Fe4B4 and Nd2Fei4B phase. It indicates that the crystallization processes of amorphous alloy are transformations from metastable structure to stable structure.