The Early Stage of Dislocation Process around a Crack Tip Observed by HVEM-Tomography in Silicon Single Crystals

Abstract

Crack tip dislocations in silicon single crystals were observed by combining high-voltage electron microscopy and electron tomography. Cracks were introduced by an indentation method and dislocations were introduced around the crack tip by keeping the indented sample at high temperatures for several hours. The number of dislocations emitted from the crack tip was controlled by changing the holding time of the indented specimen at high temperatures. The dislocations observed were characterized in detail. It was found that primary emitted dislocations all had the same Burgers vectors and that some dislocation segments cross-slipped around the crack. The local stress intensity factor due to dislocations was calculated, basing on the dislocation character obtained in this study, indicating that emitted dislocations shields mode I, II and III stress intensity at the crack tip. After the emission of the number of those dislocations, dislocations with another Burgers vector were emitted around the crack. It was found that those dislocations accommodate mode II and III stress components which are the excess shielding fields due to the dislocations primary emitted from the crack tip.

Mechanisms of dislocation multiplication at crack tips

“…Although it has long been known that predeformation and the availability of dislocations in a material strongly influence fracture toughness, only recently attention has been directed towards the mechanisms of interaction of existing dislocations with the crack tip. In-situ experiments by X-ray topography [29][30][31][32] or transmission electron microscopy and tomography [33][34][35] elucidate how dislocations interact with a crack-tip in otherwise dislocation free Silicon. While many dislocations either move away from the crack tip or do not seem to interact strongly, some individual dislocations impinging on the crack front stimulated the generation of large numbers of new dislocations on multiple glide systems [29][30][31][32].…”

High-Voltage Scanning Transmission Electron Microscopy: A Tool for Structural Characterization of Micrometer-Thick Specimens

“…18,19) However, some of the dislocations are curved or bend gradually, which is owing to the fact that the P-N force is weakened at high temperatures, and these dislocations were emitted from the crack tip introduced by indentation followed by annealing at 1123 K. Examples of detailed characterization of the dislocations around the crack tip in Si(001) can be found in the literature. 20) Figure 4 shows a composite BF-STEM image including the overall area of the wedge-shaped Si specimen, where the dent-shaped marker is indicated (double-arrowhead in figure). The contrast of each image was adjusted locally to clarify the dislocations, where the specimen becomes thicker from the left (1 µm) to the right (9 µm) of the image.…”

60 years of dislocations in ceramics: A conceptual framework for dislocation mechanics in ceramics

“…99 For MgO dislocation motion has been set into context with fracture toughness in the 1960s as well. 81,[100][101][102][103][104] While in single crystals emission of dislocation from the crack tip (at lower velocities) has been observed in MgO (and NaCl) [105][106][107][108][109][110][111][112][113][114][115][116][117] including a functional plastic zone, 117 interaction with grain boundaries was found to cause partially detrimental microcracking in polycrystalline MgO. 81,[100][101][102][103][104] Interestingly, the idea of dislocations being related to fracture toughness of ceramics was not much regarded in the wave of studies on improving fracture toughness of ceramics in the 1980s and 90s.…”

“…For MgO dislocation motion has been set into context with fracture toughness in the 1960s as well 81,100–104 . While in single crystals emission of dislocation from the crack tip (at lower velocities) has been observed in MgO (and NaCl) 105–117 including a functional plastic zone, 117 interaction with grain boundaries was found to cause partially detrimental microcracking in polycrystalline MgO 81,100–104 …”