Study On Brittleness Characterization And Fracture Mechanism Of Dy-doped Sintered NdFeB Permanent Magnetic Alloy

Sintered NdFeB permanent magnetic alloys are widely used in the modern industry and electronic technology because of their advantages,such as very high magnetic energy product,coercive force and high energy density.Especially in recent years with the development of new energy vehicles,the high-performance sintered NdFeB permanent magnetic alloys gradually form an international research focus.But sintered NdFeB magnets are typical brittle materials,their brittleness,which is an important mechanical property,has a direct effect on their quality and useful life.However,the most studies on the properties of sintered NdFeB magnets are centered on how to improve their magnetic properties rather than the mechanical properties.So it is very important to study effective brittleness testing and fracture mechanism of the sintered NdFeB magnets.The brittleness testing on five kinds of the Dy-doped sintered and two-stage aged NdFeB magnets by using acoustic emission testing with Vickers hardness indentation method were studied in this paper.At the same time,the low-temperature and high-temperature aging process were optimized respectively,and the influence on intrinsic coercivity Hcj and the microstructure of the aged Dy-doped NdFeB magnets were studied.The brittleness of the sintered and the optimized aged Dy-doped NdFeB magnets were quantitatively detected and the effect of aging process on the brittleness and the fracture along the main grain phase(Nd,Dy)2Fe14B of the Dy-doped NdFeB magnets were also analyzed.The research results showed that the acoustic emission testing with Vickers hardness indentation method was an effective brittle testing for the NdFeB magnets.There were always linear relationships between the acoustic emission energy count value,En,with the Vickers hardness indentation load,P;and between the indentation total crack length,L,with P.The slope,K,of the En-P relationship line was more correct than the slope,S,of the L-P one.Therefore the value K could be used as an important brittleness index of the Dy-doped sintered NdFeB permanent magnets.The experimental results of the aging processes optimization showed that the ageing temperature had great effect on the intrinsic coercive force,Hcj,but little effect on remanence,Br,morphology of the main grain phase(Nd,Dy)2Fe14B and grain size of the Dy-doped NdFeB magnets.The optimized ageing process for the N35TH model of the Dy-doped NdFeB magnets produced by Yantai Shougang magnetic materials Co Ltd..was 900℃×2h + 470℃×2h.The grain boundaries in the N35TH model of the Dy-doped sintered NdFeB magnet aged by the optimized ageing process became clear and smooth and the grain boundary Nd-rich phase changed into thin layer distributed continuously along the interfaces between the main grain phase(Nd,Dy)2Fe14B.Consequently,the static magnetic coupling and the domain wall pinning effect between the main phase grains were weakened and the intrinsic coercivity was greatly improved.But the interface binding strength between the main grain phase(Nd,Dy)2Fe14B was decreased due to the thin layer morphology of the grain boundary Nd-rich phase.Therefore the resistance of cracks propagation along the grain boundary decreased and intergranular fracture was easier to occur.That’s the brittleness of the aged Dy-doped sintered NdFeB magnet was higher than that of the sintered one.