Research On Magnetic Properties And Crystallization Behavior For Fe-(Co)-Nd-B-Nb Bulk Amorphous Alloys

The amorphous alloys have excellent mechanical properties, physical propertiesand corrosion-resistance properties in comparison with crystallization alloys.However, these alloys are usually prepared with high cooling rate which leads to alow dimension. In recent years, the preparation of multi-component bulk amorphousalloys have excited and been one of the hottest topics in the materials sciencecommunity. Ferromagnetic materials have the widest application among thefunctional materials. Therefore, Fe-based bulk amorphous alloys have attractedmuch attention because of their good soft magnetic properties. Some Fe-based bulkalloys with hard magnetic properties can be obtained by a devitrification annealingprocess. The fully Fe-based dense magnets prepared by a devitrification annealingprocess have obvious advantages, such as high density, casting in precise dimensionseasily and low cost, which leads a wide application future. However, the glassforming ability （GFA） of these alloys is not very good and the value of the intrinsiccoercive force （iHc） obtained after crystallization is not very impressive. Moreover,the relationship between the microstructure and the magnetic behavior is not clearduring the crystallization process. In this work, the glass forming ability, magneticproperties and crystallization behavior of Fe–（Co）–Nd–B–Nb bulk amorphous alloyshave been investigated. A variety of Fe-based bulk amorphous alloys with good GFAhave been prepared. Furthermore, bulk permanent magnets with highiHchave beenobtained by a devitrification annealing process. Some of the main experimentalresults are listed as follows:The proper addition of Nb could improve GFA effectively inFe_68-xNd₇B₉25)Nb_x（x=0⁶） alloys. A Fe-based bulk amorphous alloy with good GFAand soft magnetic property has been obtained when Nb content is4at%. The valueof saturation magnetization （Ms） is95.8Am2/kg. Moreover, it is found that Msdecreases with increasing Nd content. The alloys show good GFA and soft magnetic properties when Nd content is5¹0at%.For Fe_71-xNd_xB₂₅Nb₄（x=5,7,10） bulk amorphous alloys, the crystallizationprocess is sensitive to Nd content. The Fe₆₆Nd₇B₂₅Nb₄（x=5） bulk amorphous alloyhas three stages for the crystallization process. After annealing treatment, the alloyconsists of soft magnetic phases, such as α-Fe phase, Fe3B phase andnon-ferromagnetic phase. The hard magnetic Nd2Fe14B phase is not been foundresulting in lowiHc. The Fe₆₄Nd₇B₂₅Nb₄（x=7） bulk amorphous alloy has two stagesduring the crystallization process. The alloy contains soft magnetic phases, such asα-Fe phase, Fe3B phase, non-ferromagnetic phase and hard magnetic Nd2Fe14Bphase. The increasingiHcresults from the presence of hard magnetic Nd2Fe14B phase.With further increasing Nd addition up to10at%, Fe₆₁Nd₇B₂₅Nb₄（x=10） bulkamorphous alloy shows one stage crystallization process. The alloy consists ofnon-ferromagnetic phase and hard magnetic Nd2Fe14B phase. HighiHcis obtainedafter annealing treatment. Hence, after annealing, Fe_71-xNd_xB₂₅Nb₄（x=5,7,10） bulkamorphous alloys have the changing trend of presence of magnetic phases withincreasing Nd content, such as, soft magnetic phaseâ†'soft magnetic phase+hardmagnetic phaseâ†'hard magnetic phase.It is worth noting that the Fe₆₁Nd₇B₂₅Nb₄bulk alloy annealed at943K for20min shows the highestiHcwith the value of1191.1kA/m. The value of remanence（Br） and maximum energy product （（BH）max） is0.42T and31.72kJ/m3, respectively.The alloy consists of the Nd2Fe14B phase, the NdFe4B4phase and a small amount ofNbFeB phase. For the as-cast alloy, a small amount of NbFeB phase with the grainsize of500nm is observed by transmission electron microscopy （TEM）. Theprecipitation of the NbFeB phase in the as-cast alloy cannot prohibit the crystallinegrowth, resulting in large grain size of NdFe4B4phase and Nd2Fe14B phase.The Fe_64-xCo_xNd₇B₂₅Nb₄（x=10,20,30,40） shows good GFA when Fe ispartially substituted by Co. Compared with the Fe₆₄Nd₇B₂₅Nb₄alloy, the values of Brand （BH）maxof Fe_64-xCo_xNd₇B₂₅Nb₄（x=10,20,30,40） alloys decrease, while the value ofiHcincreases with Co addition. When Co content is20at%,iHcof the alloyhas the value of1164.3kA/m after annealing at1003K for20min.The effect of cooling rate on the magnetic properties and crystallizationbehavior has been studied in Fe₆₄Nd₇B₂₅Nb₄and Fe₆₁Nd₇B₂₅Nb₄alloys. It is foundthat the value of Msof bulk sample is higher than that of ribbon sample forFe₆₄Nd₇B₂₅Nb₄alloy, while the value of Msof bulk sample is lower than that ofribbon sample for Fe₆₁Nd₇B₂₅Nb₄alloy. After annealing treatment, the hardmagnetic properties of bulk samples are better than that of ribbon samples for bothalloys. It is noting that the metastable Fe23B6phase is observed in both bulkFe₆₄Nd₇B₂₅Nb₄sample and ribbon sample during the crystallization process.However, after annealing at higher temperature, Fe23B6phase disappears for the bulksample, while it still remains for the ribbon sample. It might the reason that Nbatoms tend to replace Fe atom at8c sites for ribbon sample, so that the structure ofFe23B6phase is more stable.