Microstructures And Magnetic Properties Of Alnico And RE-Fe-B Based Alloys

Permanent magnetic materials,including neodymium-iron-boron?Nd-Fe-B?,samarium-cobalt?Sm-Co?,hard ferrites and Alnico,play a significant role in enhancing the efficiency of electric devices in electric power generators,conditioning,conversion,transportation,medical instruments,magnetic recording media and so on.Rare earth?RE?based permanent magnets?Nd-Fe-B,Sm-Co?have obvious advantages in magnetic properties on the non-rare-earth permanent magnets?hard ferrites and Alnico?.The excessive use of RE-based permanent magnets,however,has resulted in depletion of RE resources and has caused detrimental consequences such as climate changes.Due to the high demand of RE-based permanent magnets,the prices of critical RE-elements neodymium?Nd?,praseodymium?Pr?,dysprosium?Dy?and terbium?Tb?have hugely fluctuated leading to a colossal pressure on the related industries.Therefore,it is,necessary to be smart and serious about developing permanent magnetic materials that consume less RE or no RE resources for various applications.Alnico alloys are non-RE permanent magnets containing major constituents Al,Ni,Co and Fe,plus minor constituents such as Cu and Ti.Because of the high Curie temperature?T_C?and temperature stability,these alloys have enormous potentials for replacing the Nd-based permanent magnets for high-temperature applications above 450 K.It is distinguished from scientific,technological and economic point of view to investigate the prospects of RE free and RE permanent magnetic materials.In this dissertation,Alnico,RE-Fe-B and nanocomposite magnets were fabricated by various methods including casting followed by various heat treatments,rapid solidification and spark plasma sintering?SPS?.Efforts have been made to establish a relationship between the fabrication and heat treatment conditions,the evolution of microstructures and magnetic properties of the investigated alloys.The respective mechanisms of elemental alloying and metallurgy,microstructural evolution,magnetic hardening and coercivity have been explored in detail.Initially in Chapter 1,fundamentals of magnetism and permanent magnets,the existing problems and developing trends of permanent magnets have been described and discussed systematically,following experimental procedures and methodology in Chapter 2.In the first major research part?Chapters 3 and 4?,Alnico alloys were fabricated and investigated systematically.In Chapter 3,cast Alnico alloys were fabricated and different heat treatments were applied to study the spinodal decomposition process.It was found out that the high temperature?⁸35 ^oC?treatment under magnetic field is the most effective treatment for the designated composition.The best magnetic properties in the alloys were obtained by thermomagnetic treatment for five to ten minutes followed by two low temperature tempering at⁶50 ^oC and⁵40 ^oC.An obvious improvement in the intrinsic coercivity(H_cj)was observed which indicated that uphill diffusion at a small scale took place during tempering,though no obvious structural changes were noted.Continuous cooling in magnetic field resulted in moderate magnetic properties.Treatment without magnetic field installed isotropic properties in the alloys which were lower than the properties of anisotropic magnets treated under magnetic field.To obtain refine spinodal nanostructure,Ta element was added into the alloys after optimizing the treatment conditions.Ta refined the spatial dimension of the ferromagnetic rod-like structure,enhanced the magnetic properties and Curie temperature of Alnico alloys.Reducing the spatial dimension of Fe-Co rich?₁ phase is considered to be the most promising route for improving the magnetic properties of Alnico alloys.It was found,during the study of cast Alnico alloys in Chapter 3,that the spatial dimension of?₁ phase could not be reduced below a certain limit by conventional processing methods or by adding refractory elements.Furthermore,the dimensions of various phases are very critical for the development of composite alloys.Therefore,in Chapter 4,Alnico ribbons were fabricated by designing a simplified heat treatment.By modifying the heat treatment conditions,very narrow?⁵ nm?and long?¹00 nm??₁ rods embedded in weakly magnetic matrix were obtained which resulted in an aspect ratio of²0,which is double of the highest aspect ratio obtained in Alnico cast alloys.The temperature stability of the ribbons were investigated from sub-ambient temperature?10 K?upto the working temperature of the Alnico ribbons?800 K?and it was found that Alnico ribbons,like Alnico cast alloys,possess unprecedented temperature stability.In the major second research part?Chapters 5?,different elements were added into Nd-Fe-B and Ce-Fe-B alloys to study the microstructure evolution and magnetic property.Firstly,the effects of refractory element Ta on the magnetic properties,phase transition temperature,and microstructure of Nd-Fe-B alloys were investigated.Ta showed promising grain refinement characteristics in these alloys,which was further exploited in the fabrication of composite magnets in Chapter 6.Next,the synergetic effects of Alnico elements on the microstructures,magnetic properties,thermal stabilities,phase transition temperatures and intergranular exchange interactions of Nd-Fe-B and Ce-Fe-B alloys were studied.The site preferences of Alnico constituent elements were investigated by Chromatic aberration corrected scanning transmission electron microscopy,and it was found that the thin grain boundary phase in RE-Fe-B ribbons was ferromagnetic and hence it could not decouple the RE-Fe-B nano-grains.As discussed above,Alnico alloys have high T_C and high temperature stability,but low H_cj and magnetic energy density,?BH?_max.On the other hand,RE-Fe-B alloys have high H_cj,high?BH?_max,but low T_C and poor temperature stability.It would be of great scientific,technological and economical interest if these two alloys could be combined together in some proportion to make composite magnets with good comprehensive properties.In the major third research part?Chapter 6?,nanocrystalline composite magnets were fabricated by melt-spinning and spark plasma sintering?SPS?techniques.Nd-Fe-B/Alnico magnetic nanocomposite alloys were fabricated and core-shell structure with 2:14:1 phase as the core and Alnico rich phase as the shell was obtained by optimizing the composition and processing conditions.The?BH?_max was enhanced upto 150 kJ/m³ in nanocomposite magnets.The T_C was enhanced significantly which is attributed to the field penetration of Alnico phase into the hard2:14:1 phase,in addition to the substitution of Fe by Co and Ni.No kink was observed in the demagnetization curves,which suggested the presence of exchange interactions among the phases of the nanocomposite alloys.By using Ce-Fe-B and Alnico powders as starting materials,SPS magnets were fabricated by formerly optimized conditions.It was observed that two phase structure coexist in the alloys interacting magnetostatically.The Alnico phase constituted?₁ and?₁ phases,while the Ce-Fe-B phase typically constituted Ce₂Fe₁₄B phase and CeFe₂ phase.The composition of Alnico was tuned to have a lower treatment temperature;however,heat treatment conditions for the two phase structure still need to be optimized,because it was observed that the optimized SPS processing conditions for Ce-Fe-B alloys were not very effective for the decomposition of Alnico phases.Finally in Chapter 7,the results are summarized and the prospects of future research work are presented.