| The preparation technologies, microstructures and properties of as-cast in situ Zr-Based bulk metallic glass (BMG) composites with ductile crystalline precipitates were studied in this dissertation. Especially, the spheroidization mechanism of dentritic crystal, spheroidization process for primary dentrites β phase, preparation technologies of BMG matrix composite with spheroidal crystal as well as the interaction model between micrometer-sized ductile sphroical crystalline inclusions and the shear band, also and formation mechanism of the multiple shear band during deformation of cast in-situ BMG composites with ductile precipitates were studied in detail. The essential research work for the dissertation was focused on the attention of BMG composites in five aspects: (1) Two types of as-cast in situ BMG matrix composite with dentritic β-Zr(Ti,Nb), labeled as W and M, were prepared by the methods of melt water quenching and low pressure casting /chilling in water-cooled copper mould for homogeneous Zr56.2Ti13.8No5.0Cu6.9Ni5.6Be12.5 melt. It is shown that the volume fractions of developed dentritic β phase in composite W and fine equiaxed β-phase grain in the composite M were determined to be about 30% and 20% respectively. Compression test results at room temperature revealed yield strengths of 1200MPa and 1450MPa as well as plastic strain of 10.2% and 7% respectively. The differences in morphology, size and volume fraction of β phase precipitates between both composites were primarily attributed to the different cooling rate during their preparation process. In turn, the different microstructural characteristics exhibited varieties of their mechanical properties. A relatively large amount of coarse β phase was favorable to increasing plastic strain at room temperature, while the yield strength trended to decrease with β volume faction increase. (2) Considering significant effect of the morphology of crystal on the properties of the material, the microstructure and preparation method for BMG composite with in situ formed ductile crystalline inclusions was designed and a Zr56.2Ti13.8No5.0Cu6.9Ni5.6Be12.5 BMG composite labeled as S1 and with in situ formed spheroidal crystal was successfully synthesized. The average diameter and volume fraction of the spheroidal phase approximately equal to 18μm and 30% respectively. The yield strength, ultimate strength and plastic strain of S1 composite reached to 1325MPa, 1800MPa and 12.0% respectively. Compared to the composite with dentritic crystal under both same chemical composition and volume fraction, the above-mentioned yield strength and fracture plastic strain of...
|
PDF Full Text Request