Substantial tensile ductility in sputtered Zr-Ni-Al nano-sized metallic glass

Abstract

In addition to the tensile experiments discussed in detail, we also examined the mechanical properties of the as-sputtered Zr-Ni-Al metallic glass via uniaxial compression experiments on cylindrical nanocompression specimens. Specimen were fabricated by FIB milling of the same ~4.5 µm thick sputtered film analyzed by TEM in Figure 2. The specimens were fabricated with diameters ranging from 225 nm to 1560 nm, and corresponding heights such that the aspect ratio was ~1:3 so as to avoid buckling during compression. The specimens were compressed at a strain rate of 1×10 -3 s -1 using the Hysitron PI-85 nanomechanical testing instrument in-situ in the Versa 3D scanning electron microscope. The resultant compression data is displayed in Figure A.1. These specimens exhibited two very distinct regimes of mechanical behavior depending on their size: ductile behavior and homogeneous flow was observed for "small" specimens with initial diameters ≤ 555 nm, while shear banding and localized failure was observed for "large" specimens with initial diameters ≥ 890 nm. These differences in mechanical response are apparent from the stress-strain curves, which show uniform plastic loading for the "small" specimens ( Figure A.1 (a)), and many strain bursts from the catastrophic shear banding events for the "large" specimens ( Figure A.1 (b)).The post-compression images of the specimens also illustrate the differences in mechanical response with the "small" specimens exhibiting homogeneous deformation near the top of the specimens ( Figure A.1 (c), (d)) and the "large" specimens exhibiting clear shear bands ( Figure A.1 (e), (f)). Overall, the compression data indicates a transition from homogeneous-like flow to shear banding at a specimen diameter between 555 and 890 nm.

3D nano-architected metallic glass: Size effect suppresses catastrophic failure

“…Previous studies have found that ~100 nm diameter metallic glass pillars can reach true tensile strains of ~25% prior to failure [8,9], and our previous work demonstrated that sputtered Zr-NiAl metallic glass nanopillars can reach true strains of ~150% for sample widths up to ~150 nm [11]. These studies show that reducing the characteristic dimension of metallic glass to the nanoscale can alleviate metallic glasses' Achilles' heel of brittle failure, thereby enabling the use of metallic glasses without catastrophic failure.…”

“…Amorphous Zr-Ni-Al was sputtered to coat these polymer scaffolds under the same conditions utilized in our previous study [11], namely by sputtering an alloyed Zr 56 Ni 22 Al 22 target (ACI Alloys, Inc.) with a base pressure less than 1 × 10 -6 Torr using a DC power supply at 100 W with resultant voltage of 320-390 V, under 3 mTorr argon in a magnetron sputter deposition system (ATC Orion sputtering system, AJA International, Inc.). The sputter deposition time was varied at times of 15,30,45,60,120, and 240 minutes in order to vary the thickness of the metallic glass coatings on the polymer scaffolds.…”

“…These nanolattices can be made arbitrarily large when experimental practicalities are neglected. We chose to work with sputter-deposited Zr-NiAl as the thin film metallic glass "coating" for the nanolattices based on this material's substantial tensile ductility at dimensions up to ~150 nm [11]. Nanolattices, or architected structural metamaterials, exhibit hierarchical ordering ranging from nanometer length scales in wall thickness to micron length scales in defining unit cells and beyond millimeter scales in the overall macroscale architecture, with many nano-architectures produced by using direct-laser-writing two-photon lithography [14][15][16][17].…”

“…The combination of metallic glasses' enhanced plasticity at small scales [5][6][7][8][9][10][11] with their other desirable properties (including high elasticity, an M A N U S C R I P T…”

“…The average bulk yield strength of 1.26 GPa was determined from the nano-tensile experiments in our previous study [11]. The elastic modulus of this as-sputtered metallic glass was determined to be 130 GPa, measaured by nanoindentation into a ~1 µm-thick film using the Oliver-Pharr method [44].…”