Melt Extraction Of Amorphous Metallic Microwires And The Influence Of Cold Drawing On Its Performances

The basic principle of melt extraction technique (MET) is to use a high-speedwheel to extract a thin liquid layer from the molten alloy and then rapidly solidify tobe wires under its viscosity, surface tension and gravity. Several nonequilibrium evenamorphous metallic, ceramic wires and ribbons can be fabricated by MET, andexhibit excellent magnetic, electric and mechanical properties due to their highcooling rate and geometric symmetry. In the present thesis, the fabrication processand characterization of melt-extracted amorphous metallic microwires wasinvestigated and the correlation between process parameters and microstructure wasrelated. Uniaxial tensile deformation and fracture reliability analysis ofmelt-extracted amorphous microwires was performed. The effect of cold drawing onthe structural evolution, mechanical and magnetic properties was investigated aswell.It was found that the variation of wire diameters vs. wheel velocity passesthrough a maximum and then decreases with the increase of wheel velocity. UniformCoFe-based amorphous microwires with larger GMI value of360%was fabricatedusing Cu wheel with speed of30m/s.The roundness of the wires was influencedmainly by the molten alloy feed rate. Circular wires with roundness exceed98%wasfabricated when the feed rate is lower than90μm. While viscosity and surfacetension of the molten varied considerably by temperature and determining the wireformation process. Thermal pretreatment of the extracted wheel was identified tofacilitate the wettability between the molten and wheel, thus improved the geometryof the wire.A high-resolution CCD video camera recorder was used to monitor the changingof the surface shape of molten alloy contacting the wheel tip under differentconditions. Liquid metal spread along the wheel rotary direction and a sharpenfrontier was formed under the shear forcce applied to the puddle. The thickness δ andthe length of the extracted boundary layer Lcincreased with increasing wheel velocity.The formation region of Rayleigh waves correlated with the fiber diameter, wheelradius, wettability between the molten layer and extracted wheel. Heat transfer duringextraction process can be devided to three regions: in the puddle, move along thewheel and in the air. It was found that the mechanism of the wire formation duringmelt extraction was controlled by the process parameters. Momentum transferdetermined the thickness of the extracted layer in the low wheel speed region; whilein the low wheel speed region, heat transfer turned out to be a dominant factor.Micro-sized CoFe-based amorphous microwires exhibit high tensile fracture strength and nonlinear tensile behavior while decrease with the increase of thediameter due to the decrease of amorphous nature and the formation of concavedtrack and cavity, which will cause the stress concentration during tensile process. AFeSiB amorphous microwire shows a pronounced tensile plasticity of0.6%after alinear and nonlinear deformation during tensile test. The submicro-sized dimplesformed on the fracture surface reveal the plastic fracture behavior in the range ofmicro-or nano scale for these amorphous microwires. The plasticity origin of thesemelt-extracted FeSiB amorphous microwires can be elucidated by the formation,growth and coalescence of nanovoids in the plastic zone during crack propagation.The strength and plasticity of CoFe-based amorphous microwires shows adependence of strain rate upon dynamic loading. The tensile strength decreases withthe increase of strain rate while plastic property shows an opposite effect. Criticalshear offset formed during tensile process represent the stable shear deformationability and can be used as an important standard to measure the level of plasticity ofamorphous alloy. The higher Weibull modulus and threshold stress of CoFe-basedamorphous microwires compared with other metallic microwires indicats its excellentfracture reliability and failure safety.The CoFe-based amorphous microwires can be successfully cold-drawn with upto75%cross-sectional area reduction. All of the drawing wires exhibit smoothsurfaces without any visible scratches, while the grooves and fluctuations in theas-quenched wire can be seen occasionally on the surface. Cold-drawing can alsoeffectively reduce the composition segregation and improve macroscale chemistryhomogenization in the wires. HRTEM results confirm the presence of nano-sizedcrystallites precipitated in the amorphous matrix during drawing and growth rapidlywith the increase of drawing degree. The tensile ductility and tensile strengthincreased gradually with cross-sectional area reduction ratio until51%, and decreasedwith further deformation. The microwire with51%drawn exhibits the highest tensileductility of1.64%and tensile strength of4250MPa. Interestingly, the GMI effectalso attains the maximum value of160%at10MHz when drawn to51%(30%largerthan that of the as-cast wires), before decreasing with further cold deformation. Theanisotropy field Hkundergoes a rapid increase from1Oe to5Oe at10MHz after thefirst drawing step before a relatively small increase of2Oe with further drawing.Afterwards the anisotropy field levels off at7Oe. The generation of radial andunaxial residual stress can alter the mechanical and magnetic property of cold-drawnmicrowires. The compressive residual stress can close the crack tip and decrease thecrack propagation rate, leading to an improved mechanical property. The temperatureincrease during the drawing process cannot target the change of microstructure, whilethe generation of residual stress during drawing decreases the energy barrier fornanocrystallites nucleation and promote the phase transformation. Nano-sized crystallites can also alter the number and distribution of free volume generated duringwire fabrication process and will dissipate the fracture energy in front of the crack tip,block its quick extend, leading to the improved mechanical property. The complexdependence of GMI characteristics on cold drawing has been elucidated by a fullconsideration of residual stress and nanocrystalline structure, as well as geometricaldefects. The axial residual tensile stress and circumferential compressive stressinduced by the cold-drawing process together increase the volume of the outer shelland hence the circumferential permeability, giving rise to an improved GMI ratio.The existence of large-size nanocrystals after51%drawing causes an increase ofmagnetocrystalline anisotropy and magnetic hardness, resulting in a deterioration ofthe soft magnetic property and hence the reduction of the GMI ratio.