Mechanical Properties of Small-Scale Sputtered Metallic Glasses

Abstract

Sputtered metallic glasses (MGs) represent a unique class of materials because their nonperiodic arrangements are far from equilibrium. This microstructure gives rise to their exceptional mechanical properties; for example, experiments and simulations on the deformation of small-scale sputtered Zr-based MGs demonstrate their exceptional compressive and tensile strengths in excess of 1 GPa and exceptional tensile ductility of ~150%. We report a new property that emerges in sputtered MGs: age-induced strengthening. We measured the compressive strengths of cylindrical pillars with diameters between 300 nm to 1.1 μm, which were carved from a 5 μm-thick sputtered Zr–Ni–Al thin film that was aged in a nitrogen environment for three years. Nanomechanical experiments revealed that the aged samples had a stiffness of 91 ± 4 GPa and a yield strength of 2.7 ± 0.2 GPa for all cylinder sizes, which represents a nearly 43% increase in yield strength and a 31% increase in the elastic modulus compared to equivalently sized as-sputtered samples. We also observed nano-sized induced failure suppression: samples with diameters below 600 nm deformed smoothly and noncatastrophically. Those with larger diameters deformed via a series of observable and detectable shear bands that propagated to the surfaces. Molecular dynamics (MD) simulations of uniaxial compression of chemically equivalent Zr–Ni–Al MG nanowires revealed that the underlying physics of enhanced strengths involves the evolution of local disorder that can be quantified in the number of fivefold atomic bonds. The average amount of fivefold bonding increased systematically with energetic relaxation and the maximum compressive stress. Dynamic mechanical analysis (DMA) revealed the presence of hydrides within the MG. Hydrogen diffusion into the host matrix resulted in an increase in the local volume such that more–mobile atoms (i.e., Ni and Al) can redistribute and relax into a more–energetically favorable configuration. Experiments and simulations in this work demonstrate that sputtered MGs strength by 43% when solely aged for three years, i.e., without any accompanying annealing or mechanical treatment, which originates from atomic-level microstructural relaxation in these materials. This provides a useful foundation for simple design of advanced materials whose mechanical properties can be predicted and prescribed a priori using physical principles of atomic-level relaxation.