Brittleness and fracture mechanism of sintered Dy-doped NdFeB magnets

Abstract

The brittleness and fracture behaviors of the sintered and the two-stage aged Dy-doped NdFeB magnets were studied by a unique method of acoustic emission testing and Vickers hardness indentation method in this paper. A detailed analysis on the crack propagation mechanism along the grain boundary of the main grain phase (Nd, Dy) 2 Fe 14 B was done. By comparing the acoustic emission energy count value (E n ) with the Vickers hardness indentation load (P), it is shown that there is a linear relationship between E n and P for both the sintered and the two-stage aged Dy-doped NdFeB magnets. According to the slope of E n versus P linear lines, it can be found that the two-stage aged Dy-doped NdFeB magnet is more brittle than the sintered one. It is due to that the Ndrich grain boundary phase of the two-stage aged Dy-doped NdFeB magnet is formed as thin film and uniformly distributes around the main grain phase, which plays a significant role in increasing the intrinsic coercive force of the magnet, but decreasing its interface binding strength. Therefore, the resistance of crack propagation along the grain boundary decreases and the brittleness increases.

Experimental Investigation on the Surface Formation Mechanism of NdFeB during Diamond Wire Sawing

“…Its hardness is lower than most hard and brittle crystalline materials such as monocrystalline silicon and SiC. As shown in Figure 12 a,b, NdFeB magnets are composed of thousands of micrometric-sized grains with a pomegranate-like microstructure, including the NdFeB phase, the B-rich phase, and the Nd-rich phase, which acts as a binding phase [ 33 , 34 , 35 ]. Due to the very soft and chemically active nature of the Nd-rich phase, the intergranular toughness is very weak.…”