Compositing, Crystallizing And Behaving Of Bulk Metallic Glasses

Bulk metallic glasses (BMGs) with excellent mechanical and physical properties open a new research field for structural and functional materials. However, the catastrophic brittle fracture at room temperature has restricted the extensive application of BMGs. The brittleness of BMGs at room temperature is one of the key problems to be resolved in the near future.Previous researches showed that the formation of plastic solid solution phase in micrometer scale in the BMG matrix during casting is very effective to improve the plasticity of BMGs at room temperature. It is normally deemed that the in situ formed second phase in micrometer scale should be plastic to improve the plasticity of the BMG-based composites. Based on the understanding of brittleness of BMGs at room temperture, a new route to improve plasticity is proposed in this dissertation, i.e. the second phase in the composite is effective to improve the plasticity of BMGs as long as it can block the propagation of single shear band and induce the formation of multiple shear bands, in despite of the plasticity of the second phase. Following this idea, in situ formed intermetallic/BMG composite was designed and fabricated. This composite showed both high fracture strength and remarkable plasticity at room temperature (plastic strain 4.4%). Further study showed that the second phase with different sizes can affect the plasticity of BMG-based composites with different mechanism: if the second phase is in nanometer range, it can induce the aboundant nucleation of shear bands and improve the plasticity of the composite; if the second phase is in micrometer range and larger than the shear band spacing, it can block the propagation of single shear band and induce the multiplication of shear bands effectively, resulting in an improved plasticity; if the second phase is larger than nanometer range, but smaller than shear band spacing, it does not affect the plasticity obviously.The thermal stability and crystallization behavior of BMGs is a well concerned basic problem. In this dissertation, the high density electric pulse was found to be effective to induce the crystallization and alter the type of crystalline phases in BMGs. ForⅥalloy, the crystallization started in the supercooled liquid region (400-450℃) during the isothermal annealing. However, obvious crystallization reaction was found at a temperature well below the glass transition temperature (338℃) after annealing under an electric pulse with a density of 1410A/mm2. After crystallization under high density electric pulse, the crystalline phases included NiTi2 and an unknown metastable crystalline phase, besides Be2Zr and Zr2Cu phases which precipitated after isothermal annealing. Prolonging the holding time of the electric pulse annealing, the full wave at half maximum (FWHM) of the halo peak for amorphous phase became narrow, indicating an enhanced short range order. During the continuous heating, the high pressure up to 5Gpa supressed the crystallization ofⅥalloy. The crystallization under 5Gpa started at about 410-430℃, which is much higher than the value of onset temperature for crystzllization (393℃) found in DSC at a same heating rate. During isothermal annealing, the application of a magnetic field of 12T increased the volume fraction of crystallized phases in Fe-based BMGs, while the direction of magnetic field did not affect the volume fraction distinctly.Up to now, because of the lack of effective method to control magnetic orientation, a complicated and costly work flow including ingot crushing and powder preparation-magnetic orientation and molding-mintering or bonding-magnetization and inspection is necessary to fabricate bulk permanent magnets. However, the magnets fabricated with sinitering or bonding methods are not fully compact, resulting in some disadvantages such as low corrosive resistance. In this dissertation, magnetic annealing of Fe-based BMGs was proposed to attempt to induce the orientation of magnetic phases during the nucleation and growth of these crystalline phases. It might be a new approach to fabricate fully density anisotropic permanent magnets from the BMG solids or melts directly, avoiding the powder preparation process. (Fe71B21Nd8)96Nb4 BMG samples were annealed at 700℃under a magnetic field with a density of 12T. After excluding the shape anisotropy, it was found that the inherent coercive and maxium energy product (BH)max for samples annealed with a parallel magnetic field were much higher than those with a perpendicular field, while the values for samples annealed without a field ranged between them. These results demonstrated that the application of high density magnetic filed was effective to induce anisotropic magnetic properties during the crystallization of BMGs. Magnetic annealing of Fe-based BMGs precursor is feasible to fabricate bulk NdFeB permanent magnets with a short workflow, indicating great scientific significance and application potential.