Effect Of Multiple Cold-drawn On The Microstructure And Mechanical Properties Of Co_68.15Fe_4.35Si(12.25)B_15.25 Microwires

In this dissertation, the effect of multiple cold-drawn on the mechanical properties and microstructure of melt-extraction Co68.15Fe4.35Si12.25B15.25 microwires was systematically investigated mainly by employing the DMA, Istron 5848 , scanning election microscope (SEM), transmission election microscope (TEM).The result shows that the fracture strength increases with radius reduction, reaches a maximum around 30% radius reduction, and then decreases with furthure increase of radius reduction. Plastic-deformation ability also increases with radius reduction, reaches a maximum around radius reduction is 30%, and then decreases with furthure increase of radius reduction. Uniaxial cyclicloading experiments on dynamic mechanical analysis (DMA) shows that in amorphous wires deformation can induce"word-harding"effect. Nano-indentation experiments shows that the hardness increases after cold-drawn predeformation.The cold-drawn removes the concaved track on the as-quenched amorphous wires. After the surface flaws were removed by wire drawing, the average fracture strength increased. The cold-drawn predeformation increases the homogeneity of the wire shape. As the same time, after drawning, numerous"deformation marks"i.e. shear bands appear on the surface of the amorphous wires. The generation of deformation marks and the intersection of these marks can retard the expand of main shear band, increase its fracture strength.As the drawing process proceeds, the inhomogeneous elemental distribution has been eliminated, and the difference of micro-mechanical properties between the surface and center is diminished. This indicating that"weakest links"became fewer in the samples and the applied stress can be distributed more homogeneously with fewer stress concentration sites, which can also result in higher fracture strength.DSC measurements were carried out for different amorphous wires. DSC curves among these wires have two typical features: exothermic enthalpy increases in the intitial deformation process, then disappear; endothermic enthalpy decreases with increase radius reduction. This indicates the localized ordering dicreases at first, and then increases with furthure drawing process.Transmission Electron Microscopy was used to investigate the microstructure evolution during different drawing process. The microscopic examination showed presence of a fine dispersion of nanocrystals in cold-drawn amophous wires, and the dimension of the nanoparticles increases with furthure increase of radius reduction. Auto-Correlation Function was applied to characterize localized ordering of the as-quenched and cold-drawn amorphous wires, indicating that the localized ordering of the amorphous increases with furthure increase of radius reduction.The strain-rate sensitivity m increases with radius reduction. The activation energy of shear transformation zones can be calculated based on the strain-rate sensitivity m. The activation energy of shear transformation zones decreases with icrease of radius reduction, which reflcets the STZ in amorphous wires can be activated more easily after drawning and the rearrangement of local atoms bacomes easily. So the plastic deformation capability of amorphous wires becomes better.