| dc.description.abstract |
In some Zr-based bulk metallic glass composites (BMGCs), the ZrCu B2 phase can be precipitated from the matrix. When the ZrCu B2 phase subjected to the stress from the shear banding, it will absorb the energy of shear band and transform into ZrCu B19′ phase, and so as to improve the plasticity of Zr-based BMG. However, the particle size and distribution of ZrCu B2 phase in Zr48Cu47.5Al4Co0.5 BMG cannot be well controlled in general casting. Large agglomerated and inhomogeneous distributed ZrCu B2 phase were usually found in the Zr48Cu47.5Al4Co0.5 BMG samples. Therefore, the concept of inoculation in conventional solidification process is applied in this study. The Ta particles (size of 5–30 µm) with 0 ~ 1.0 vol.% were added into Zr48Cu47.5Al4Co0.5 BMG matrix as the inoculant. By using the ultrahigh melting point of tantalum and immiscible with Zr-base substrate, the Ta particles can be uniformly dispersed in the Zr-based alloy melt as the nucleation sites for precipitating ZrCu B2 phase, and form a homogeneously distributed ZrCu B2 phase in the matrix of Zr48Cu47.5Al4Co0.5 BMG. Then, the different cooling rates of solidification process are further used to control the particle size of ZrCu B2 phase.
Based on the results of XRD analysis, Zr48Cu47.5Al4Co0.5 alloy rods with 0 ~ 0.75 vol.% Ta particle additions made by the copper mold at the temperature of -30°C present amorphous structure co-existing with ZrCu B2 phase. However, when the temperature of copper mold increases to higher than -20 °C, the sample with 0.75 vol.% Ta particle additions will be totally crystallized. After adding Ta particles, the precipitates of ZrCu B2 phase in the Zr48Cu47.5Al4Co0.5 alloy matrix exhibit more even distribution and round shape. But when decrease the cooling rate of solidification, the large amount of ZrCu B2 precipitates will agglomerate and form a large particle. According to the results of DSC analysis, with increasing the amount of Ta particles and decreasing the cooling rate of solidification, the enthalpy value of crystallization exothermic peak decreases, which means that the volume fraction of amorphous matrix decreased and the precipitate of ZrCu B2 phase increased.
The results of compression test reveal that the sample of Zr48Cu47.5Al4Co0.5 added with 0.75 vol.% Ta particle performs the highest mechanical properties, 1750 MPa yield stress, 1890 MPa fracture stress, and 14 % plastic strain. This is 6.5 % improvement of plastic strain in comparison with its base alloy. | en_US |