Synthesis And Properties Of Soft Magnetic Fe-based FeCoNiPCB Bulk Metallic Glasses

Fe-based bulk metallic glasses (BMGs) have been drawing considerable attention due to their high strength, excellent soft magnetic properties, good corrosion resistance, and low material cost. However, the poor plasticity at room temperature makes them difficult to process by conventional methods. Thermoplastic processing has been suggested to be an effective way for fabricating micro-/nano-parts and electromechanical devices on some soft magnetic Fe-based BMGs in view of the viscous flow workability in their supercooled liquid region. Nevertheless, the developed Fe-based BMGs do not simultaneously possess low glass transition temperature (Tg), wide supercooled liquid region (△Tx) and good glass-forming ability (GFA), which are unsuitable for thermoplastic forming. Therefore, this work aims at developing the ferromagnetic Fe-based BMGs with low Tg, large △Tx and high GFA. The effects of Co, Ni and Mo addition on the thermal stability, GFA, thermoplastic formability, mechanical and magnetic properties of a soft magnetic Fe75P10C10B5 (in at.%) metallic glass have been investigated.The thesis consists of following parts mainly:1. The addition of Co and Ni effectively enhances the thermal properties and GFA of the Fe-Co-Ni-P-C-B alloys. These glassy alloys simultaneously possess low Tg of 643-713 K, wide △Tx of 50-60 K and large dc of exceeding 1.5 mm in the composition range of 30<Fe <50,0<Co<45, and 0<Ni<35. The Fe40Co20Ni15P10C10B5 metallic glass exhibits the largest △Tx and dc of 60 K and 2.5 mm, respectively.2. The mechanisms of good thermal stability of supercooled liquid and high GFA for the Fe-Co-Ni-P-C-B alloys were clarified. A Fe3(P, B) phase was primarily precipitated during annealing for Fe75P10C10B5 metallic glass, whereas the (Fe, Co, Ni)3(P,B) and (Fe, Co, Ni)3C phases simultaneously precipitated for Fe40Ni15Co20P10C10B5 alloy. The simultaneous precipitation of two phases restrains the crystallization process, which results in the enhancement of stability of the supercooled liquid against crystallization.3. The developed Fe-based Fe-Co-Ni-P-C-B BMGs show good soft magnetic and mechanical properties, i.e., high Is of 0.65-1.38 T, low Hc of 2.8-5.4 A/m, high yield strength (σc,y) of 2846-3340 MPa, and Young’s modulus (E) of 155-180 GPa.4. The developed Fe-based Fe-Co-Ni-P-C-B BMGs exhibit low viscosity (η) of～108 Pa·s in their spuercooled liquid region. The micro-/nano-line pattern is successfully achieved by imprinting of the developed metallic glasses, indicating their excellent thermoplastic formability.5. The thermal stability of supercooled liquid and GFA of Fe-Co-Ni-P-C-B are remarkably enhanced by addition of Mo. The Fe40Co20Ni11Mo4P10C10B5 metallic glass possess the largest △Tx of 81 K and dc of 4.0 mm. The Is, Hc,σc,y, E and η of the BMGs are 0.74-1.13 T,2.8-3.8 A/m,2920-3150 MPa and ～107 Pa·s, respectively.