Mechanical Behaviors Of A TiZrNiCuBe Bulk Metallic Glass

In the present dissertation, the mechanical behaviors of Ti40Zr25Ni3Cu12Be20 bulk metallic glass (BMG) have been systematically investigated using unixial compression and tensile tests, as well as nanoindentation technique at different testing temperatures ranging from 123 K to 658 K.The dramatic effect of sample size on the plastic deformation capability of the glassy alloy tested was studied. Compared with the larger one, the smaller size glassy sample with the same chemical composition exhibits significantly pronounced plasticity, suggesting a"smaller is softer"trend of metallic glasses. The phenomenon is attributed to the fact that the smaller size alloy, which experienced a faster cooling rate during solidification, contains a larger amount of heats of relaxation and crystallization, favoring the preferential nucleation of shear bands and thus allowing enhanced plasticity upon compressive loading.The Ti-based BMG studied exhibits a significant enhancement in both compressive strength and plasticity at low temperatures. The ductilization of the BMG system can be evidently attributed to the formation of dense shear bands and the rotation mechanism of shear bands. The cryogenic surroundings can effectively slow down the mobility and diffusion of the atoms and consequently, suppress the nucleation and growth of nanocrystals during the deformation process, allowing the simultaneous improvement in the mechanical responses of the glassy alloy subjected to compressive loading far below the ambient temperature. The deformation-induced crystallization occurs during low temperature compression tests of the studied alloy while the crystallization in the low temperature deformed sample is not as profound as that in the room temperature deformed sample.The superplastic deformation behaviors of the studied alloy were examined. The alloy shows an extraordinary compressive superplastic formability in the supercooled liquid region, typically evidenced by a large compressive strain up to 0.83. The superplastic flow behaviors depend strongly on the testing temperatures and initial strain rates. At low temperatures or high strain rates, the viscosity measured was found to dramatically decrease with increasing strain rate (i.e. non-Newtonian flow) and a stress overshoot is detected in the first steps of strain. At low strain rates or high testing temperatures, the studied glass exhibits a Newtonian flow behavior. It is found that the relationship between the strength and the testing temperature of the metallic glasses obeys the following relation:σ= aE ( b ? T / Tg). For the testing temperature regime of 0～0.9Tg,σ/ E= 0.0066T/Tg+0.0206 whereas for the testing temperature regime of 0.9～1.1Tg,σ/ E= 0.1163T/Tg+0.1278.The fracture behaviors of the studied BMG subjected to uniaxial compression tests at high temperature have been investigated. At high temperatures, the compressive fracture surface of BMG becomes much rougher than that at room temperature. With increasing the testing temperature, a different pattern from the vein appeared on the fracture surface, which is viscous flow layer. This fracture feature is most likely due to the external heat fields and the adiabatic heating created by plastic flow. As the testing temperature increases, the fracture angle correspondingly increases as well. The normal stress-dependent yield criterion (i.e., the Mohr-Coulomb criterion) should be used to elucidate the controlling fracture mechanisms of metallic glass deformed under various testing temperatures.The mechanical properties of pre-deformed Ti-based metallic glass have been examined. With increasing pre-strain, the plasticity of the pre-deformed sample initially increases, and then decreases with further increasing pre-strain. The hardness of the pre-deformed samples exhibits the reverse trend. For the case of 10 % pre-strain, the pre-deformed sample exhibits the largest plastic strain prior to failure. At the same time, as the pre-strain increases, the quantity of shear bands observed on the outer surface and the density of vein patterns shown in macroscopical fracture surface also increases initially and then decreases. The thermal analysis results reveal that the 10 % pre-strained sample possesses the largest quantity of free volume. The saturation of free volume in metallic glassy sample during pre-deformation can be successfully interpreted based on the free volume theory proposed by Spaepen.The time-dependent plastic deformation properties of Ti40Zr25Ni3Cu12Be20 bulk and ribbon metallic glass were investigated using nanoindentation technique at room temperature. The stress exponent n, defined asε? =Aσn, has been derived from the load-displacement curve and is used as a measure of the creep resistance. It was found that the measured stress exponent increases rapidly with increasing indentation size, exhibiting a positive size effect. The size effect in n obtained from the bulk sample is more pronounced than that obtained from the ribbon sample.The reverse plasticity of Ti-based bulk metallic glasses was investigated using cyclic nanoindentation tests. The deviation between the unloading and reloading curves gradually diminishes with the increasing number of cycles for the Ti40Zr25Ni3Cu12Be20 metallic glass. Smaller loading/unloading rate leads to a larger deviation between the unloading and reloading curves for the Ti40Zr25Ni3Cu12Be20 metallic glass. Metallic glasses with high Tg show smaller deviation between the unloading and reloading curves.