Effect of Heat Treatment Time on Dy-Cu Alloy Diffusion Process in Dy-Containing Commercial Nd-Fe-B Sintered Magnets

doi:10.1007/s40195-017-0660-x

Abstract

Abstract:

In this paper, the grain boundary diffusion process (GBDP) using a Dy₇₀Cu₃₀ (at.%) alloy as the diffusion source was performed in a commercial sintered Nd-Fe-B magnet, and the effect of heat treatment time on the microstructure and magnetic properties of the magnet was investigated in detail. For the processed magnets heat-treated at 860 °C, as heat treatment time increased, the coercivity and the depth of (Nd,Dy)₂Fe₁₄B core-shell structure increased first and then decreased. However, when the heat treatment time was more than 2 h, the diffusion path of Dy from the Dy-rich shell phase into the Nd₂Fe₁₄B grains was revealed, and a nearly homogeneous (Nd,Dy)₂Fe₁₄B phase was formed, which brought on the decrease in both the depth of visible core-shell structure and the coercivity of Nd-Fe-B magnet.

Key words: Sintered Nd-Fe-B magnets, Grain boundary diffusion, Heat treatment time, Coercivity, Micromagnetic simulation

H. Y. Liu, G. Wang, Y. Hong, D. C. Zeng. Effect of Heat Treatment Time on Dy-Cu Alloy Diffusion Process in Dy-Containing Commercial Nd-Fe-B Sintered Magnets[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(5): 496-502.

Figures/Tables 7

Fig. 1 a XRD pattern of the Dy70Cu30 alloy ribbons; b DSC curve of the Dy70Cu30 alloy ribbons upon heating from room temperature to 850 °C at 10 °C/min

Fig. 2 a Coercivity values at 27 °C of the reference magnets exposed only to heat treatment without Dy-Cu diffusion, b Coercivity values at 27 °C of the GBDPed magnets under different heat treatment states, c demagnetization curves at 27 °C for the original magnet, diffusion-processed magnet using Dy70Cu30 alloy and the magnet only heat-treated at 860 °C/2 h + 500 °C/1 h without Dy-Cu diffusion

Table 1 Magnetic properties at 20 and 120 °C, temperature coefficient of remanence (α) and coercivity (β) in the temperature range from 20 to 120 °C for the Nd-Fe-B magnets before and after Dy-Cu diffusion treatment

Magnetic properties	Original magnet		Diffusion-processed magnet
Magnetic properties	20 °C	120 °C	20 °C	120 °C
H _c (kA/m)	1636	715	1937	940
J _r (T)	1.38	1.23	1.222	1.093
(BH)_max (MGOe)	44.4	30.6	34.4	26.09
α (%/°C)	- 0.110		- 0.106
β (%/°C)	- 0.563		- 0.515

Fig. 3 SEM images of a the original magnet and b only heat-treated magnet at 860 °C/2 h + 500 °C/1 h without Dy-Cu diffusion

Fig. 4 SEM images of the diffusion-processed magnets at 860 °C for 1-4 h, in which the distances from the right edge of each image to the magnet surfaces are 325 μm (1 h), 420 μm (2 h), 170 μm (3 h), 85 μm (4 h), respectively

Fig. 5 Average Dy content after GBDP at 860 °C for different time

Fig. 6 Calculated demagnetization curves for GBDPed samples at 860 °C for 2 and 4 h, respectively, based on the micromagnetic simulation model inserted

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