Deformation behavior of crystalline/amorphous Al-Si nanocomposites with nanolaminate or nanofibrous microstructures

Deformation mechanisms in sputter-deposited crystalline Al/amorphous Si nanocomposites with nanolaminate or nanofibrous morphology are characterized by nanoindentation, micropillar compression testing and transmission electron microscopy (TEM). The nanofibrous composite having crystalline Al nanofibers with ∼40–50 nm in length and 15–20 nm in diameter embedded in amorphous Si exhibits strain hardening to a maximum flow stress of 2.9 GPa and no shear band (SB) formation in compression up to plastic strain exceeding 24%. On the other hand, nanolaminate composite that is composed of 80 nm crystalline Al layers and 20 nm amorphous Si layers exhibits catastrophic SBs starting at plastic strains in the range of 5–10%. Cross-sectional TEM of the deformed samples reveals a high density of stacking faults and twin boundaries in Al nanofibers and no microshear bands in the nanofibrous composite, suggesting plastic deformation in amorphous Si phase and crystalline Al nanofibers. Molecular dynamics simulations revealed that the plastic deformation in amorphous Si phase in the cosputtered films could be favored by the decrease in flow strength of amorphous Si with increasing Al solute concentration trapped in Si.

Figure 1

Microstructure of focused ion beam (FIB)-prepared cross-sectional lamellae of the multilayer film. (a) Transmission electron microscopy (TEM) bright-field image. (b) Selected area electron diffraction (SAED) pattern of the highlighted portion. (c) Elemental scanning TEM (STEM) mapping showing alternating layers of Al and Si. (d) High-resolution STEM (HRSTEM) BF image highlighting the crystalline Al and amorphous Si layer. (e) Fast Fourier transform (FFT) pattern of the crystalline Al layer from the marked region A. (f) FFT pattern of the amorphous Si layer from the marked region B.

Figure 2

Microstructures of the nanofibrous Al-Si film. (a) Low-magnification transmission electron microscopy (TEM) bright-field (BF) image. (b) Selected area diffraction (SAD) pattern from region A. (c) SAD pattern from region B. (d) Enlarged scanning TEM (STEM) BF near the surface showing the nanofibers of Al. (e) Elemental STEM mapping showing the distribution of Al and Si. (f) High-resolution STEM BF image of the film near the substrate showing the nanograins of Al embedded in amorphous Si matrix. The inset shows the magnified version of one Al grain showing different plane direction in [011] zone axis. (g) Schematic representation of the growth of the Al fiber in Si matrix.

Figure 3

(a) Transmission electron microscopy (TEM) bright-field image of the cross-section directly under the indent in Al80/Si20 multilayer film having the inset showing the typical indentation impression. (b) High-magnification image of the indentation mark focusing a few layers of Al and Si. (c) and (d) represent the scanning TEM (STEM) elemental mapping of Al and Si layers, respectively. (e) Plot showing the variation of true plastic strain as a function of layer number from (b).

Figure 4

(a) High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) image showing the fracture of the amorphous Si layers within the crystalline Al layers underneath the nanoindents. (b) High-resolution image from the dotted circle of (a). (c) Fast Fourier transform (FFT) of the region marked as A in (b). (d) High-resolution HAADF-STEM image showing the interface of the Al and fractured Si layers below the indent.

Figure 5

Representative deformation morphology of indented cosputtered film. (a) Low-magnification bright-field transmission electron microscopy (TEM) image of a cross-section directly under the indent. (b) Enlarged image of the indent showing deformation in the Al nanograins. (c) High-resolution STEM (HRSTEM) BF image showing the twin boundary in the (111) planes of Al. (d) Fast Fourier transform (FFT) pattern from the (111) plane of Al showing the twin structure.

Figure 6

(a) Engineering stress–engineering plastic strain curves of the multilayer and fibrous film. (b) Plots depicting the variation of strain hardening rate with true plastic strain.

Figure 7

(a) Scanning electron microscopy (SEM) micrograph of the compressed pillar of the Al80/Si20 multilayer film prepared by focused ion beam (FIB) method. Inset shows the representative SEM image of the pillar before deformation. (b) Transmission electron microscopy (TEM) bright-field image showing the shear bands along the diagonal of the pillar.

Figure 8

(a) Scanning electron microscopy (SEM) micrograph of the compressed pillar prepared from the fibrous film having the inset showing the pillar before compression. Transmission electron microscopy (TEM) micrographs of the compressed pillar. (b) Low-magnification bright-field (BF) image. (c) Enlarged BF image. (d) High-resolution STEM (HRSTEM) BF image showing one plastically deformed nanofiber of Al. (e) HRSTEM image from the highlighted dotted square in (d). Inset images of (e) shows the fast Fourier transform (FFT) patterns from the dotted red rectangular markers of the locations A and B.

Figure 9

Calculated ${\sigma }_{\mathrm{flow}}$ from Eq. (3) for the crystalline Al layer.

Figure 10

Schematic representation of the deformation mechanism in the multilayer below the indenter tip.

Figure 11

Schematic representation of the deformation mechanism in film with fibrous morphology.