Study Of Mechanical Behaviors and Structures of Bulk Metallic Glasses with High-energy Synchrotron X-Ray Diffraction

This dissertation addresses two critical issues in the mechanical behaviors and structures of bulk-metallic glasses (BMGs): (1) the effect of composition, fabrication method, and pretreatment of plastic deformation on mechanical properties and structures of BMGs; (2) the mechanical response and structural evolution of BMGs in the elastic and plastic region.

(Cu₅₀Zr₅₀)₉₄Al₆ and (Cu₅₀Zr₅₀)₉₂Al₈ amorphous alloys were used to study the effect of composition on mechanical properties and structures of BMGs. The (Cu₅₀Zr₅₀)₉₄Al₆ alloy exhibits lower yield stress and Young’s modulus, higher Poisson’s ratio, worse thermal stability, and better plasticity than (Cu₅₀Zr₅₀)₉₂Al₈. Both the topological and chemical effects of Al addition account for the differences of mechanical and physical properties between them.

A Zr₅₅Ni₅Al₁₀Cu₃₀ glass-forming alloy with injection casting (the melting temperatures are 1,550 K and 1,250 K, respectively) and with suction casting was fabricated. The results indicate that despite their amorphous structures, the suction-casting samples exhibit a lower yield stress, lower Young’s modulus, and larger plastic strain than the injection-casting samples (the melting temperature is 1,550 K) due to more quenched-in free volumes in suction casting, which results from the higher cooling rate.

The inhomogeneous plastic deformation in Zr₅₀Cu₄₀Al₁₀ BMG samples was introduced by four-point-bend fatigue. There is almost no difference of the stress-strain behaviors between the deformed and undeformed samples. Elastostatic compression was used to introduce homogeneous deformation in Zr₇₀Cu₆Ni₁₆Al₈ BMG samples. The preloaded samples are softer with decreases of yield strength and Young’s moduli. Anisotropy was observed in the preloaded samples despite their small magnitudes, which even occurred at a relatively low temperature and applied stress level.

The structural evolution of Zr₇₀Cu₆Ni₁₆Al₈ BMG in the elastic region was analyzed with anisotropic pair density function. The analysis of the first shell of Zr₇₀Cu₆Ni₁₆Al₈ glass confirms the structural changes in the elastic region. The bond reorientation leads to direction dependent changes in the chemical short-range order. The structural evolution in the plastic region of Zr₇₀Cu₆Ni₁₆Al₈ BMG is investigated as well. The serrations were observed for both the stress-displacement and full width at half maximum-displacement curves. The excess free volume was measured, which increases with increasing the displacement.