Development And Formation Mechanism Of High Bs Fe-based Nanocrystalline Alloys

High B_s nanocrystalline aloys?HBNAs?are structurally characterized by nano-sized?-Fe grains in amorphous matrix and functionally characterized by high B_s above1.80 T with unparalleled magnetic softness.These aloys characterized by high Fe content?above 82 at.%?without large atoms exhibit attractive magnetic properties and low materials cost,which are ideal candidates for next-generation electronic materials or being used as core materials in transformers,motors,sensors and electric vehicles.To obtain high performance HBNAs is of great significance to the wide applications of this kind of soft magnetic materials.However,HBNAs always suffer from the harsh requirement of ribbon production and annealing processes due to the low amorphous forming ability?AFA?and poor thermal stability.For the HBNAs,the effects of constituent elements on the crystalization behaviors and soft magnetic properties,as well as the formation mechanism of nanocrystals are not clear yet.In addition,the structure of the as-quenched ribbons is very sensitive to the ribbon production process,which affects the crystalization process greatly and makes the formation of nanocrystals in HBNAs more complicated.In this dissertation,these issues were systematically studied.According to the statistical investigations of HBNAs,the compositions of ferromagnetic elements?Fe,Co?,amorphous forming elements?Si,B,P,C?and nucleation motivating elements?Cu?were determined.From the perspective of atomic radius mismatch,mixing entropy and binary phase diagrams,the effects of constituent elements on AFA and nanostructure refinement were analyzed systematicaly.The guiding principles for the compositional design of HBNAs were then proposed and the Fe_84.75Si₂B₉P₃C_0.5Cu_0.75 aloy were successfuly developed.The magnetic properties of the designed aloy after annealing under different conditions were characterized and the precipitating phases during the annealing process were identified.It was found that the optimal annealing window for the HBNAs is the optimal nanocrystallization stage,during which uniform nanostructure with fine grains and excelent magnetic properties can be obtained.The designed Fe_84.75Si₂B₉P₃C_0.5Cu_0.75 aloy after optimal annealing exhibit both high B_s above 1.83 T,?_e above 1.2×10⁴,low H_c of⁴.5 A/m and P_50/1.0=0.17 W/kg,P_400/1.0=0.56 W/kg,P_1000/1.0=0.82 W/kg,as well as excelent frequency and temperature properties,proving that the guiding principles for the compositional design of HBNAs are reasonable,which can be used to design high performance HBNAs.Based on the proposed compositional design rules,the Fe₈₃(Si₂B₁₁P₃C₁)_1-x_/17Cux?x=0-1.25 at.%?,Fe83+x?Si2B11P3C1??16.25-x?/17Cu0.75?x=-3-3 at.%?and Fe83Cu1Si2?B14-x_-yPxCy?al oy systems were designed to investigated the effects of nucleation motivating elements,ferromagnetic elements and metaloid elements on the crystalization behaviors and magnetic properties.It was found that the appropriate addition of Cu/Fe will promote the nucleation of?-Fe grains,resulting in the formation of uniform nanostructure with fine grains and high crystalinity.While the excessive addition of Cu/Fe will lead to the formation of primary crystals in the as-quenched ribbons,resulting in the formation of coarse?-Fe grains,which greatly deteriorate the magnetic softness.As for the introduction of different kinds of metaloid elements,such as addition of P,C elements,which can promote the formation of?-Fe grains and enhanced the thermal stability of the nanostructure.The correlation between the compositions,processes,structure and magnetic properties were hence established.The key to obtain fine nanostructure and excelent magnetic softness in HBNAs is to obtain as-quenched precursors with high number density?-Fe nuclei,to ensure a synchronous and competitive grain growth during the subsequent nanocrystallization process.These results are of great significance to the optimization of compositions and processes for the HBNAs.The magnetic properties of HBNAs are determined by their nanostructure,the investigation of the formation mechanism of nanocrystals in HBNAs contributes to a controlled nanostructure and magnetic properties.The structure evolution process of Fe_84.75Si₂B₉P₃C_0.5Cu_0.75 aloy was characterized in detail and found that uniform nanostructure with fine grains can be retained for a long time at a low annealing temperature.The formation of intermetalic phases will lead to the formation of non-uniform nanostructure with coarse grains.Based on the structure evolutions and thermal analyses,a“dual phase co-growth”model was developed to understand the kinetics of nano-grain growth,which agrees quite well with the experimental results.The nanostructure and elemental distribution show that the nanostructured aloys exhibit a core-shell like structure.The Fe-rich?-Fe grains are surrounded by the amorphous interphase,which is rich in metaloid elements.In addition,the model parameters calculated from the experimental results are very close to the fitting results,proving that the proposed kinetic model is effective in predicting the grains growth during the annealing process.The nanostructure stability of HBNAs is ultimately governed by the thermal stability of the intergranular amorphous matrix.The formation of intermetalic phases in the intergranular amorphous matrix will destruct the shielding layer,resulting in the coarsening of?-Fe grains by grain coalescence.By controlling the annealing process to form nanosized?-Fe grains in amorphous matrix,will give a synergetic increase of B_s and?_e as well as hardness,which outperforms the variety of soft magnetic materials.For the HBNAs with low AFA,the formation of primary crystals is noticeable in the as-quenched ribbons,which affects the subsequent annealing process greatly.By adjusting the wheel speed during the ribbon production process,fuly amorphous as-quenched ribbons and the ribbons with primary crystals were obtained,and the cross-section structure of the as-quenched ribbons was then characterized.It was found that the as-quenched ribbons exhibit a gradient structure that number density and size of the primary?-Fe grains decrease from the free to wheel side.As a result,the wheel side usualy exhibits a fuly amorphous structure.The comparative thermal analyses showed that the growth of the primary crystals can readily take place,resulting in a non-uniform crystalization process as large primary crystals grow faster than small ones.To investigate the effects of the primary crystals on the nanocrystallization process,the nanostructure of the free side and wheel side of the ribbons after annealing were further characterized in detail.Interestingly,after annealing,the free side with a high number density and small sized primary crystals exhibit a more uniform nanostructure with fine?-Fe grains than the fuly amorphous wheel side,which can be attributed to the synchronous and competitive growth of the primary crystals.Thus,the presence of small primary crystals with a high number density is conducive to obtain fine nanostructure,which in turn enhances the soft magnetic properties and improve thermal stability of HBNAs.