Shear banding mechanism in compressed nanocrystalline ceramic nanopillars

Localization of deformation in shear bands is widely observed preceding intense damage and fracture in ductile or granular materials. However, mechanisms about how shear bands form are generally not known. Here, we show that nanocrystalline ceramic (NCC) nanopillars of ZrN fracture by shear banding under compression. The microstructure evolution of entire shear banding process was in situ monitored by electron imaging and diffraction. Results show that the nanopillars deform through a series of granular activities due to intermittent nucleation and propagation of dislocations. The stress drops associated with the dislocation activities are relatively small since dislocation avalanches are restricted in NCCs because of the effect of nanograin size. Localized and cooperative granular activities, as well as nanocracks, are discovered in the regions where shear bands form. A model about shear band and nanocrack formation is proposed. These findings thus demonstrate for the first time of spatial and temporal nature of intermittent granular activities, leading to shear band formation in NCCs.

Figure 1

In situ nanopillar compression test using bright-field TEM imaging. (a) A schematic diagram of the nanopillar compression test performed inside a TEM. (b) The measured stress-time curve for nanopillar A. Two slope changes in the stress-time curve are marked, as well as the selected points I and II. (c) The image frames recorded at the points I and II and the yielding point in (b). (d) A TEM image of nanopillar A taken after the compression. The arrows mark two observed shear bands and nanocrack. The schematic diagram indicates the location and orientation of the observed slip bands as well as the contact surface with the indenter probe.

Figure 2

Observation of granular activities using the difference image method. (a) An example of two bright-field images recorded at 1.0 s time interval apart, and their difference image. (b) The selected difference images obtained during the deformation of nanopillar A. The arrows indicate where the connected granular activities and along the shear bands (S1 and S2) are observed. The time marked on the image is the starting time of the difference image.

Figure 3

Intermittent granular activities detected by the integrated intensity method. (a) Example difference image from Fig. 2 after background noise filtering. Scale bar: 50 nm. (b) Integrated intensity of the filtered difference images as function of time for nanopillar A. (c) A plot of the stress-time curve for the same time period as in (b). Here, $t_1, t_2$, and the yielding point are defined in Figs. 1 and 2. [(d), (e), and (f)] The integrated images from 0 s to $t_1, t_2$, and yielding point, respectively, showing the cumulative granular activities during the different periods of time. The solid arrows and solid lines mark the locations of S1 and S2.

Figure 4

Snapshots of in situ video recorded from for nanopillar B during compressive deformation. Symbol D indicates the dislocation propagation and the development of dislocation activities.

Figure 5

Observation of grain deformation and rotation. (a) Stress-time curve of nanopillar C during compressive deformation. (b) In-plane rotation, (c) lattice strain, and (d) diffraction spot intensity change of the three nanograins with increasing time. The arrows in all four figures point the time of the yielding point in the stress-time curve.

Figure 6

The correlation map of diffraction patterns recorded during nanopillar C deformation. The pixel value at the coordinate $(x,y)$ is the correlation coefficient of two diffraction patterns recorded at time $x$ and $y$. Two width reductions of the red band are found between the turning point and the yielding point, which are indicated by two lines R1 and R2.

Figure 7

Stress drops observed during nanopillar deformation. (a) An example of the load-time curve before yielding. The square in the inset figure marks the enlarged region (b). (b) An example of measuring a load drop from the filtered load-time data. (c) Histogram and (d) CCDF curve of the measured stress drop sizes from 10 nanopillars. The $s^{-1}$ line is drawn for comparison.

Figure 8

The proposed shear banding model in a nanocrystalline ceramic nanopillar. [(a)–(d)] Schematic diagram of shear band formation during compression. (a) Granular activities are initiated in a nanopillar. (b) Modification of local microstructure and initiation of intergranular cracks. Black solid grains represent the grains of high activities. (c) Embryonic shear band is formed when local sites and the stress fields of intergranular cracks are aligned. Some intergranular cracks are developed into nanocracks. (d) Plastic shear is carried out as the shear band structure is completely developed.