Structural Origins for Enhanced Thermal Stability and Glass-Forming Ability of Co-B Metallic Glasses with Y and Nb Addition

doi:10.1007/s40195-022-01506-3

Abstract

Abstract:

The effects of Y and Nb addition on thermal stability, glass-forming ability (GFA), and magnetic softness of Co₇₅B₂₅ metallic glass (MG) were comprehensively investigated. The experimental results indicated that the thermal stability, GFA, and magnetic softness of the studied MGs increase in the order Co₇₅B₂₅ < Co₇₃Nb₂B₂₅ < Co_71.5Y_3.5B₂₅ < Co_69.5Y_3.5Nb₂B₂₅. The structural origins of the improved properties were revealed by ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) calculations. Results showed that the B-centered prism units are the primary structure-forming units of the four MGs, connect through vertex-, edge-, and face-shared (VS, ES, and FS) atoms, and Co-centered units tend to connect with Co/B-centered units via the intercross-shared (IS) atoms. The addition of Y and Nb not only plays the role of connecting atoms but also enhances both bond strengths and the fractions of icosahedral-like units in increasing order Co₇₅B₂₅ < Co₇₃Nb₂B₂₅ < Co_71.5Y_3.5B₂₅ < Co_69.5Y_3.5Nb₂B₂₅, which is conducive to the enhancement of the structural stability, atomic packing density, and viscosity, thereby improving thermal stability and GFA. In addition, the improvement of structural stability and homogeneity leads to enhanced magnetic softness.

Key words: Co-based metallic glasses, Thermal stability, Glass-forming ability, Soft magnetic property, Ab initio molecular dynamics simulations, Local atomic structure

Shuang Ma, Junyu Zhang, Xudong Wang, Rie Y. Umetsu, Li Jiang, Wei Zhang, Man Yao. Structural Origins for Enhanced Thermal Stability and Glass-Forming Ability of Co-B Metallic Glasses with Y and Nb Addition[J]. Acta Metallurgica Sinica (English Letters), 2023, 36(6): 962-972.

Figures/Tables 9

Fig. 1 a DSC curves and b hysteresis loops of the four melt-spun MGs

Table 1 Thermal parameters, tc/dc, and magnetic parameters of the MGs

MGs	T_g (K)	T_x (K)	ΔT_x (K)	t_c/d_c (mm)	B_s (T)	H_c (A/m)
Co₇₅B₂₅	-	714	-	0.03	0.98	14.1
Co₇₃Nb₂B₂₅	-	803	-	0.05	0.83	4.6
Co_71.5Y_3.5B₂₅	822	855	33	0.24	0.73	1.5
Co_69.5Y_3.5Nb₂B₂₅	840	883	43	1.00	0.57	1.2

Fig. 2 a Total PDFs of the four MGs, and the partial PDFs for b Co75B25, c Co73Nb2B25, d Co71.5Y3.5B25, e Co69.5Y3.5Nb2B25 MGs at 300 K

Fig. 3 Fractions of major VPs centered by a Co and b B atoms of the four MGs at 300 K. The typical motifs of VP are also shown, and the fractions of icosahedral-like (ico-like) and trigonal prism (tri-prism) VPs are listed in the inserted table

Fig. 4 Matrices of spatial correlation index Cij for the dominant VPs centered by Co, Y, Nb, and B atoms in a Co75B25, b Co73Nb2B25, c Co71.5Y3.5B25, d Co69.5Y3.5Nb2B25 MGs at 300 K. The colors represent the strong (red) and weak (blue) probability for the VPs to correlate with each other

Fig. 5 a Three-dimensional configuration of the simulation box. b-d Extended VPs linked by vertex-, edge-, face-, and intercross-shared (VS, ES, FS, and IS) atoms, showing the MRO structures among the four studied MGs

Fig. 6 Density of states (DOS) for a Co75B25, b Co73Nb2B25, c Co71.5Y3.5B25, d Co69.5Y3.5Nb2B25 MGs. e The sum of total DOS for spin-up and spin-down channels of the four MGs at the Fermi level

Fig. 7 Time dependence of MSD for all of the MGs at different temperatures

Fig. 8 a XRD pattern of Co69.5Y3.5Nb2B25 MG isothermally annealed for 10 min at different temperatures. The result of the melt-spun ribbon is shown for comparison. b Unit cell structure of (Co, Nb, Y)23B6 phase from different visual views. c Sites correspond to VPs in (Co, Nb, Y)23B6 phase

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