Mechanical properties and microstructures of Zr-Cu-Al-Ni bulk metallic glasses
Student thesis: Doctoral Thesis
The study aims at understanding the mechanical behaviors in a group of Zr-based bulk metallic glasses (BMGs). The first investigation was conducted on understanding the serrated fluctuation in compressive stress-strain curves, which was one of the most important characteristics in deformation of BMGs. By detailed examination on morphology of compressed BMGs, some characteristics in morphologies were indentified and analyzed. Shear bands, which were believed to play a key role in deformation in BMGs, were produced by severe deformation and investigated using high resolution transmission microscopy. Some attempts were also made to indentify differences between shear bands and matrix. Dependence of properties on compositions was also examined to understand the mechanism of deformation in these BMGs. A linear relationship between loading drops and displacements in serrations was found, which was related to testing machines. Careful inspections on morphologies of deformed samples show that shear bands propagate or developed in a serrated way, in which granular materials were sheared. By analogy with granular materials, the shear stress initiating shear sliding in shear bands was calculated to be the sum of forces induced by internal friction and dilatation, and follow a form of Mohr-Coulomb criterion. The liquid-like fracture indicated the possibility of friction welding during sliding on shear planes. The cold jointing of two samples during compression confirm the friction welding, and fracture of Quasicrystal-metallic glass composites after compression shows partial liquid-like characters, which implies that the liquid characters are probably related to glass transition instead of melting. Due to the liquid-like layer in shear bands during deformation, the interaction between liquid layer and solid matrix was considered during compression. Due to geometric confinement, samples with small aspect ratio can be highly compressed without breaking. By severe deformation of Zr65Cu15Al10Ni10 bulk metallic glasses, profuse shear bands were produced. Based on observations on these shear bands with high resolution transmission electron microscopy, the process of shear banding was discussed. Shear-induced branching and ordering were observed in shear bands subjected to high stress. By analogy with thermal relaxation, localized deformation was analyzed and multiple spring-piston systems were proposed for compression process of metallic glasses. Ex situ annealing were conducted at a temperature above glass transition for different durations (crystallization onset and up to 35% respectively). No visible crystallization was found within shear band or highly deformed regions around shear band in both cases, while large crystal phase appeared in the matrix. The disruption of short-range order in shear bands likely delays the crystallization and requires larger activation energy for crystallization, thus shear band showed a lower nucleation rate or higher crystallization temperature in conventional annealing. However, ion irradiation of Ar induces preferential crystallization of shear bands, which may be due to larger free volume in shear bands. Two phases were produced by the devitrification of shear bands: Zr52(CuNi)45.5Al2.5 face center cubic phase with lattice constant a=0.56 nm and Zr88(CuNi)8Al4 hexagonal phase with lattice constant a=0.227 nm and c=0.32 nm. Line scanning of compositions across these crystal belts implied that Zr-rich hexagonal phase located in shear bands and cubic phase may locate at the interface between shear bands and matrix. High deformation also induced macroscopic hardening (in true stress-strain curves) and microscopic softening (in Hardness). The former owes to induced cell-like structure, while the latter is attributed to profuse shear bands and ordering within them induced by deformation. Thermal and thermal-mechanical properties of ZrCuAlNi bulk metallic glasses were investigated and results showed that there are at least three relaxation processes existing: glass transition, Johari-Goldstein secondary relaxation and a third relaxation (γ relaxation). The transition temperature of γ relaxation is close to the temperature for homogenous deformation and the apparent activation energy for γ relaxation is near 1 eV, which is close to activation energy for diffusion of individual atom, implying the function of γ relaxation in deformation. Annealing (above γ transition but below secondary transition) led to increment of γ transition up to annealing temperature and did not change the electronic structures, which imply the topological nature. Measurements of electronic structure by Xrays photoelectron spectroscopy supported that high percentage of nearly free electrons was responsible for plasticity in metals.
- Mechanical properties, Metallic glasses, Bulk solids