Size dependent mechanical behavior of Zr-Ni thin metallic glass films

Bulk metallic glasses are known for their superior mechanical properties with respect to crystalline counterparts owing to a yield strength close to the theoretical limit and a large elastic deformation (~2%). Nevertheless, the shear band instability that occurs during plastic deformation of BMGs leads to a macroscopic brittle behavior, thus significantly undercutting their potential applications. Therefore, research is moving toward nano-scale metallic glasses in order to investigate size effects aiming to mitigate the catastrophic effects of instabilities on the material behavior. Nevertheless, although there are evidences about a change of mechanical behavior when small length scales are involved, the specimen manipulation is extremely delicate and contrasting results are reported about the deformation mechanisms and the trend of the main mechanical properties when reducing the specimen size. Open questions are represented by the origin of size effects and by the critical size below which they are activated. In this PhD thesis, the mechanical properties of thin Zr65Ni35 metallic glass films - deposited by DC magnetron sputtering - have been investigated for thickness ranging from 200 up to 900 nm. The mechanical behavior was studied for films deposited on Si substrate and for freestanding films as well. Zr65Ni35 films exhibit the same atomic structure as indicated by the absence of shift of diffraction peaks and by the constant values of the mass density, elastic properties, and activation volume. However, the cracking mechanisms of the film on the substrate are thickness dependent, resulting from a thickness confinement effect on the development of the plastic zone. The mechanical properties of freestanding films were investigated using an original technique of micro-tension controlled by internal stresses. Homogeneous plastic strains (up to 10 %) combined with very high stresses (up to 3500 MPa) were attained. The specimen cross-sectional area was the key parameter affecting the probability to get percolation of defects involved in the plastic deformation as confirmed by the constant value of the estimated activation volume analyzing stress relaxation phenomena. The effect of the composition on the mechanical properties has been investigated as well and, in this case, the changes in mechanical behavior were preferentially attributed to modifications of the metallic glass atomic structure. Specifically, we show that crystallization occurs for Zr-rich specimens, while the increment of the Ni content leads to an increase of the mass density, elastic constants, hardness, and activation volume indicating the formation of Zr-Ni bonds and an enhanced entropic contribution.