||Since the first discovery of amorphous alloys in 1960, researchers have explored many unique mechanical, magnetic, and optical characteristics of such materials for potential applications. Up to now, well-developed processes, such as rapid quenching, sputtering, evaporation, pulse laser deposition, etc, have been applied for different applications in micro-electro-mechanical systems (MEMS). Due to the lack of ordered structure, amorphous alloys can bear a high stress in the elastic region. Their plastic deformation stability is also of interest and has been widely studied. The shear-band characteristic, a kind of inhomogeneous deformation mechanism, dominates the deformation after yielding at room temperature. While a shear band nucleate, its propagation usually cannot be arrested or stopped. In other words, the occurrence of matured shear bands needs to be prevented. There are two major approaches in this aspect. The first is to increase the material yield strength so as to delay the shear band nucleation. Another is to incorporate intrinsic or extrinsic particles so as to absorb the kinetic energy of shear bands in the amorphous matrix.|
In this study, we utilize three strategies to control the propagation of shear bands in thin film metallic glasses (TFMGs): sub-Tg annealing, the addition of strong element in solute form, and the introduction of strong nanocrystalline layers. For sub-Tg annealing, the base alloy system is Zr69Cu31, with a base film hardness of 5.1 GPa measured by nanoindentation. After annealing, the hardness exhibits ~30% increase. Without the occurrence of the phase transformation, as confirmed by X-ray diffraction, the possible reaction during sub-Tg annealing is attributed to structural relaxation, not crystallization. The full width at half maximum of the X-ray peak exhibits a decreasing trend in the using X-ray and transmission electron microscopy diffraction, meaning the excess free volumes forming during vapor-to-solid deposition process would be annihilated by localized atomic re-arrangement. Moreover, the formation of medium-ordering-range clusters was confirmed utilizing high-resolution transmission electronic microscopy. The denser amorphous structure leads to the increment of hardness.
For the addition of Ta in Zr55Cu31Ti14, sputtering provides a wide glass forming range with solubility of Ta approaching ~75 at%. With increasing Ta content, the elastic modulus and hardness increase slowly. A steep rise occurs at ~50 at% of Ta. Up to 75 at% of Ta, the elastic modulus and hardness approaches 140 GPa and 10.0 GPa, respectively (100% increment). Up to now, Ta-rich TFMGs exhibit the highest elastic modulus and hardness among all amorphous alloys fabricated using vapor deposition techniques. The irregular increase is attributed to the formation of Ta-Ta bonding. A large quantity of Ta bonds would lead to the formation of Ta-rich nanoclusters, drastically decreasing the strain rate while shear band propagates under nanoindentation and microcompression tests. The introduction of nanocrystalline Ta layers can not only effectively enhance the yield strength but also serve as the absorber for the kinetic energy of shear bands, revealing ductility in the microcompression test.