Brittle metallic glass deforms plastically at room temperature in glassy multilayers

Bulk metallic glasses are emerging as a new class of materials that can have applications ranging from structural materials to materials for future nanotechnology. However, catastrophic mechanical failure is a serious issue hindering the use of these materials in engineering applications. Here we introduce an approach to understanding and solving the problem of brittleness of metallic glasses. We have shown that even a very brittle metallic glass (La based) can be forced to deform plastically at room temperature if it is made in the form of multilayers involving other metallic glasses, i.e., a two-phase glass. The mechanically soft glassy layer (La based) having a lower critical shear stress acts as a nucleation or an initiation site for shear bands and the mechanically hard glassy layer (Zr based) acts as an obstacle to the propagation of shear bands. This process results in the multiplication of shear bands. Since the shear bands are associated with a local rise in temperature, a large number of shear bands can raise the overall temperature of the soft layer and eventually can drive it to the supercooled liquid state, where deformation of metallic glass is very large and homogeneous. The results reported here not only clarify the mechanism of large plastic deformation in two-phase glassy alloys but also suggest the possibility of a different kind of two-phase bulk glassy alloys exhibiting large plastic deformation at room temperature.

Figure 1

(Color online) X-ray diffraction curves for La-based, Zr-based glassy thin films, and their multilayered structure. The insets show the cross-sectional transmission electron microscope image and the selected area electron-diffraction (SAED) pattern of the multilayered structure.

Figure 2

(Color online) (a) Cross-sectional high-resolution TEM image of multilayered film along with SAED patterns, (b) HAADF-STEM obtained from the interface of Zr- and La-based glassy layers, and (c) TEM image and the corresponding elemental mapping for (d) Zr and (e) La. This figure shows that the interface between the Zr- and the La-based glassy layers is atomically sharp.

Figure 3

(Color online) Scanning probe microscope images of the impressions made on (a) La-based, (b) Zr-based, and (c) multilayered glassy thin films. (d) The line profiles obtained in Figs. 3a, 3b, 3c, showing pileup of material around the indents.

Figure 4

(Color online) Load displacement curves obtained at different loading rates for (a) La-based, (b) Zr- based, and (c) multilayered glassy thin film. For clarity, these curves are shifted along the $x$-axis.

Figure 5

Typical SEM image of a pillar fabricated by FIB technique on Zr- and La-based multilayered glassy film. The side view of the fabricated pillar is featureless and it is hard to see any contrast in SEM image originating from Zr- and La-based glassy thin-film layers. Similar featureless surface topography was observed for the pillar fabricated from monolithic Zr- and La-based glassy thin films.

Figure 6

SEM images of (a) La-based, (b) Zr-based, and (c) multilayered glassy thin-film ($1.5\mu \text{m}$ diameter) pillars deformed after the microcompression tests performed under identical conditions using a Berkovich-type indenter.

Figure 7

SEM image of deformed pillars of diameter $\sim 2\mu \text{m}$. (a) La-based, (b) Zr- based, and (c) multilayer. Figures show large deformation in case of multilayered glassy thin film.

Figure 8

SEM image of deformed pillars of diameters (a) 1.5 and (b) $3.0\mu \text{m}$ fabricated from La-based glassy film. In case of (a) loading of the indenter is in the center of the pillar and it is the magnified image of Fig. 6a, whereas for (b) the loading was off centered. In both the cases pronounced cracking can be noticed.

Figure 9

SEM images of shear bands formed after scratching on (a) La-based, (b) Zr-based, and (c) multilayered glassy thin films. Multiple shear bands can be noticed in case of the multilayered glassy film.

Figure 10

(a) SEM image of the micropillar after off-axis microcompression testing, sometimes flowed material is observed on the sides of the pillar made from multilayered glassy thin films and (b) SEM image of broken pillar fabricated from multilayered glassy thin film after improper loading in microcompression test. The features observed on the side of the pillar are very different compared to the monolithic glass, where shear bands similar to bulk were observed.