Study on microstructure and properties of hot pressed/hot deformed Nd2(FeCo)14B/α-(FeCo)magnetic materials
Time:05/10/2011
Study on microstructure and properties of hot pressed/hot deformed Nd2(FeCo)14B/α-(FeCo)magnetic materials
Nanocomposite rare earth magnetic material has potential excellent magnetic properties,its theoretical energy product is about twice of traditional sintered NdFeB.It may become a new generation of permanent magnetic materials for its low rare earth content and low price.
In this dissertation,the Nd2Fe14B/α-Fe-type nanocomposite magnets have been prepared by melt spinning and subsequent die-upsetting.X-ray diffraction(XRD),differential scanning calorimeter(DSC),transmission electron microscope(TEM),vibrating sample magnetometer (VSM)and composite property measurement system(PPMS)have been employed.The effect of Co on crystallization process has been analyzed.The effect of Co and hot deformation temperature on microstructure and magnetic properties has been investigated.Intergranular interaction was analyzed usingδM(H)curves and irreversible susceptibility curves.Nd9Fe85.5-xCoxBB5.5(x=0,1,3,5)ribbons（v=35m/s）were composed mainly of amorphous phase,α-Fe/α-(FeCo)phase and Nd2Fe14B/Nd2(FeCo)14B phase.Besides,a metastable phase, Nd3(FeCo)62B14B were found in Nd9Fe85.5-xCoxBB5.5 (x=5)ribbons.Because Co element enter into the soft and hard phase,the Curie temperature of soft and hard phase of Nd9Fe85.5-xCoxB5.5B(x=5) increased from 770℃and 370℃to 850℃and 395℃compared to the Co-free ones.Hot pressed Nd9Fe85.5-xCoxBB5.5(x=0,1,3,5)were not dense due to the lack of Re-rich phase.When the Co content increased from 1at%to 5at%,the intergranular exchange coupling got weaker and the coercivity and maximum energy product increased slightly.
The grain sizes of hot deformed Nd9Fe85.5-xCoxBB5.5(x=0,1,3,5)decreased significantly induced by Co substitution.When x=1,the grain sizes of soft and hard phase reduced from 61nm and 168 nm to 24nm and 50nm,when x=5,however,because of the dicomposition of Nd3-(FeCo)62B14B metastable phase in deforming-temperature，the grain size of two phase both increased slightly.Co substitution produced more Nd2(FeCo)14B which possessing lower magnetic anisotropy constant and it would weak the coercivity of the magnets.By analyzing theδM(H) curve we found that,the coercivty of Co-free ally was controlled by long-range magnetostatic interaction and the coercivity of Co-contain ally was controlled by short-range exchange coupling.
The grain size increased with the substitution of 5at%Co and the role of short-range exchange coupling were weakened,so the coercivity was increased.The closely relation between intergranular interaction and microstructure reveals that microstructure plays a more important role on the coercivity compared with alloy composition.Deformation temperature increased from 1143K to 1203K,the grain size of Nd2(FeCo)14B of hot deformed Nd9Fe80.5Co5BB5.5 increased from 70nm to 90nm.Because of the effect of microstructures,the remanence,coercivity and energy product decreased from 0.67T,218kA/m and 40.9kJ/m to 0.56T,158kA/m and 25.9 kJ/m respectively.By the analysis of 3 3δM(H)curves and irreversible susceptibility curves,we found that the coercivity of magnets deformed at 1143K was controlled by intergranular exchange coupling,while the coercivity of magnets deformed at 1203K was controlled by long-range magnetic coupling interaction,and this may be the reason for higher coercivity of former.
Weak texture appeared in hot deformed Nd9Fe85.5-xCoxBB5.5 (x=0,1,3,5)and the texture strength was closely related with deformation temperature and Co content.The texture was stronger in magnets deformed at 1203K than in magnets deformed at 1143K.When Co content increased from 1at%to 5at%,texture got stronger.
