Research On The Magnetic Properties, Temperature Stability And Impact Toughness Of Sintered Nd-Fe-B Magnets

It plays a very important role for sintered Nd-Fe-B magnets in our daily lives because of their excellent magnetic properties. Especially for their applications in the motor, it requires high temperature stability, high maximum energy product and low cost for sintered Nd-Fe-B magnets. So, in order to develop new applications and reduce the cost, it is very important and practical to improve the composition and control the microstructure and sintered conduct of sintered Nd-Fe-B magnets. Sintered Nd-Fe-B permanent magnetic materials are very brittle. It is very difficult to machining because of their poor mechanical properties, which increases the cost of sintered Nd-Fe-B magnets and severely limits their applications.We studied the effect of additive elements and heat treatment on the magnetic properties, temperature stability and impact toughness of sintered magnets in this paper. Magnetic hysteresis loop tracer is used to measure the magnetic properties of sintered magnets, and SEM is used to investigate the microstructure of sintered magnets. We analyzed the reasons that affect impact toughness of sintered Nd-Fe-B magnets by investigating the microstructure and fracture micrograph of sintered magnets. Based on fracture mechanics and magnetization reversal mechanism, the relationships among the magnetic properties, impact toughness and microstructure of sintered Nd-Fe-B magnets were discussed. At last, we obtained the ultra-high intrinsic coercivity Nd-Fe-B magnets with excellent impact toughness. The main results are listed as follows:(1) Addition of Nb can increase the intrinsic coercivity of sintered Nd-Fe-B magnets, improve the squareness and temperature stability of sintered magnets, and has little effect on the remanence and maximum energy product of sintered magnets. The magnetic properties of sintered Nd-Fe-B magnets changes regularly with the content of Nb. Addition of Nb can drastically improve the impact toughness of sintered magnets. However, when the content of Co addition is 5 at%, the effect of Nb addition on the impact toughness of sintered Nd-Fe-B magnets is not very obviously. But the trend of variation on impact toughness of sintered magnets is the same with sintered Nd-Fe-B magnets without Co addition.(2) Addition of heavy rare earth (Dy, Tb) can effectively increase the intrinsic coercivity of sintered Nd-Fe-B magnets, but decrease the remanence and maximum energy product of sintered magnets. Addition of Tb is first combined with the Nd-rich phase, and then enters into the main phase (NdaFe14B). Addition of heavy rare earth (Dy, Tb) can improve the impact toughness of sintered Nd-Fe-B magnets. Especially, the impact toughness of sintered magnets can be greatly increased by a large amount of Dy addition, and the impact toughness of sintered magnets is 53% higher than sintered Nd-Fe-B magnets without Dy addition.(3) The effect of annealing treatment on the intrinsic coercivity of sintered Nd-Fe-B magnets is different with the content of B. When the content of B is 6 at% and 8 at%, the intrinsic coercivity of sintered magnets can be greatly improved by annealing treatment. When the content of B is 7 at%, annealing treatment has little effect on the intrinsic coercivity of sintered magnets. The intrinsic coercivity of sintered Nd-Fe-B magnets with Nb addition can be greatly improved by annealing treatment, and annealing treatment has little effect on maximum energy product of sintered magnets. As for sintered Nd-Fe-B magnets with ultra-high intrinsic coercivity, a small amount of Nb addition can improve the magnetic properties of sintered magnets. When the content of Nb addition is 1.0 at%, the maximum of intrinsic coercivity is attained, which is 671.9 kA/m at 513 K. As for sintered Nd-Fe-B magnets with different B content, the trend of variation on impact toughness of magnets is different by annealing treatment. However, as for sintered Nd-Fe-B magnets with different Nb content, the trend of variation on impact toughness of magnets is same by annealing treatment. Annealing treatment can increase the impact toughness of sintered magnets, and the range that the impact toughness of sintered magnets is increased is almost same.(4) Addition of Co decreases the intrinsic coercivity and the temperature coefficient of remanence for sintered Nd-Fe-B magnets, and increases the temperature coefficient of coercivity for sintered Nd-Fe-B magnets. The variation of the microstructure is the main reason that the temperature coefficient of coercivity for sintered Nd-Fe-B magnets increases with the Co content. Addition of Co can reduce the loss of total magnetic flux, the loss of reversible magnetic flux, the temperature coefficient of reversible magnetic flux and the loss of irreversible magnetic flux, and improve the temperature stability of sintered Nd-Fe-B magnets. When the content of Co addition is 7.5 at%, the optimum value of temperature stability for sintered Nd-Fe-B magnets is obtained. The impact toughness of sintered Nd-Fe-B magnets with the addition of Co first decreases, reaches a minimum, and then starts to increase. When the content of Co addition is 7.5 at%, the impact toughness of sintered magnets is worst.(5) The temperature stability of ultra-high intrinsic coercivity Nd-Fe-B magnets which were produced by advanced commercial manufacture technology was investigated, and compared the impact toughness of sintered magnets with common Sm2Co17-type magnets and the ultra-high intrinsic coercivity Nd-Fe-B magnets that were produced by traditional manufacture technology. The results show that the temperature coefficients of remanence and coercivity are very small, and the impact toughness of sintered magnets is higher than common Sm2Co17-type magnets, but obviously lower than the ultra-high intrinsic coercivity Nd-Fe-B magnets that were produced by traditional manufacture technology.