Dislocation emission from the crack tip in copper under mode II loading is simulated with molecular dynamics method. After 26 partial dislocations are emitted and then relaxed to reach the equilibrium under the consta...Dislocation emission from the crack tip in copper under mode II loading is simulated with molecular dynamics method. After 26 partial dislocations are emitted and then relaxed to reach the equilibrium under the constant displacement, the double pile-ups (including an inverse pile-up and a pile-up) are formed. i.e., the first dislocation is piled up before the obstruction, and the last dislocation is piled up ahead of the crack tip. These results conform to the TEM observations.展开更多
The effect of fine precipitates on the initiation and propagation of micro cracks and the length of dislocation free zone (DFZ) in a low alloy Cr Mo V steel was studied by in situ TEM observation for the specimens bot...The effect of fine precipitates on the initiation and propagation of micro cracks and the length of dislocation free zone (DFZ) in a low alloy Cr Mo V steel was studied by in situ TEM observation for the specimens both in embrittled and toughened conditions. The results show that in the embrittled specimen, dislocations emitted from the crack tip can easily slip by cutting through the precipitates and move away from the crack tip region, so that a long DFZ forms and the brittle fracture occurs via cleavage cracks initiating and propagating. The size of DFZ mainly depends upon the diameter and the interparticle distance of the precipitates in the matrix. In the toughened specimens, the size of DFZ extremely changes with the interparticle distance. Mobile dislocations are emitted from the crack tip and bypassing the particles. The interaction between the dislocations and the particles results in the dislocation pile up and tangle, leading to diminishing of the DFZ. The crack tip is easy to blunt or chang the propagating direction and the crack propagation shows a jagged path.展开更多
An investigation has been made of the disloca- tion distribution and dislocation free zone near the crack tip in bulk Fe-3% Si single crystal during deformation in SEM.It has been found that a number of dislocations w...An investigation has been made of the disloca- tion distribution and dislocation free zone near the crack tip in bulk Fe-3% Si single crystal during deformation in SEM.It has been found that a number of dislocations were emitted from the crack tip during deformation.After that,the dislocations moved rapidly away from the crack tip,which indi- cated that they were strongly repelled by the stress field at the crack tip.Between the crack tip and the plastic zone there is a region of dislocation-free, which is referred to as dislocation-free zone (DFZ). The length of DFZs is roughly estimated 100 μm which is much longer than that found in thin foil specimen.The variation of dislocation density as a function of the distance from the crack tip was measured,which showed that the dislocations are inversely piled up in the plastic zone.The length of DFZs increased with both the length of pre-crack and the amplitude of applied stress.展开更多
Nucleating of a microcrack within nanometres and blunting into a void are studied through in situ tensile test for thin foil specimens of type 310 steel by transmission electron microscopy (TEM). The mechanism of micr...Nucleating of a microcrack within nanometres and blunting into a void are studied through in situ tensile test for thin foil specimens of type 310 steel by transmission electron microscopy (TEM). The mechanism of microcrack nucleation is analyzed by means of microfracture mechanics. The results show that a main crack tip may be still sharp after dislocations are emitted from the crack tip and dislocation free zone (DFZ) forms so that the local stress near the crack tip in the DFZ is high enough to equal the cohesive strength and then a microcrack initiates in the DFZ. The microcracks initiating in the DFZ will blunt into voids through the movement o. the existing dislocations and/or those emanated from the crack tip and nearby dislocation sources. The discontinuous voids resulting from the blunt of the microcracks in the DFZ will link with each other and with the main crack, resulting in a zig-zag crack propagation. A microcrack will nucleate at the main crack tip and the new crack will blunt rather than propagate continuously under the condition of constant displacement. The repeat of these processes leads to continuative propagation of ductile crack.展开更多
The changes of microstructures after the exhaustion of dislocation glide ability but before cracking, as well as microcrack initiation by them, were studied in detail byin-situ transmission electron microscopy (TEM) f...The changes of microstructures after the exhaustion of dislocation glide ability but before cracking, as well as microcrack initiation by them, were studied in detail byin-situ transmission electron microscopy (TEM) for pure copper, aluminum and iron. Thinning occurred in the thicker regions through dislocation gliding in the three metals. After that, {111} 〈112〉 twinning or tensile cracking took place in thinned zones in fcc metals. In the case of twinning, new microcracks were initiated along another {111} plane by the high stress concentration near the growing tip of the twin, while in iron (bcc), many nanocracks were initiated in the thinned areas just ahead of the crack front, resulting in misorientation microstructure from which microcracks or microvoids were developed.展开更多
基金The project supported by the National Natural Science Foundation of China
文摘Dislocation emission from the crack tip in copper under mode II loading is simulated with molecular dynamics method. After 26 partial dislocations are emitted and then relaxed to reach the equilibrium under the constant displacement, the double pile-ups (including an inverse pile-up and a pile-up) are formed. i.e., the first dislocation is piled up before the obstruction, and the last dislocation is piled up ahead of the crack tip. These results conform to the TEM observations.
文摘The effect of fine precipitates on the initiation and propagation of micro cracks and the length of dislocation free zone (DFZ) in a low alloy Cr Mo V steel was studied by in situ TEM observation for the specimens both in embrittled and toughened conditions. The results show that in the embrittled specimen, dislocations emitted from the crack tip can easily slip by cutting through the precipitates and move away from the crack tip region, so that a long DFZ forms and the brittle fracture occurs via cleavage cracks initiating and propagating. The size of DFZ mainly depends upon the diameter and the interparticle distance of the precipitates in the matrix. In the toughened specimens, the size of DFZ extremely changes with the interparticle distance. Mobile dislocations are emitted from the crack tip and bypassing the particles. The interaction between the dislocations and the particles results in the dislocation pile up and tangle, leading to diminishing of the DFZ. The crack tip is easy to blunt or chang the propagating direction and the crack propagation shows a jagged path.
文摘An investigation has been made of the disloca- tion distribution and dislocation free zone near the crack tip in bulk Fe-3% Si single crystal during deformation in SEM.It has been found that a number of dislocations were emitted from the crack tip during deformation.After that,the dislocations moved rapidly away from the crack tip,which indi- cated that they were strongly repelled by the stress field at the crack tip.Between the crack tip and the plastic zone there is a region of dislocation-free, which is referred to as dislocation-free zone (DFZ). The length of DFZs is roughly estimated 100 μm which is much longer than that found in thin foil specimen.The variation of dislocation density as a function of the distance from the crack tip was measured,which showed that the dislocations are inversely piled up in the plastic zone.The length of DFZs increased with both the length of pre-crack and the amplitude of applied stress.
基金Project supported by the National Natural Science Foundation of China.
文摘Nucleating of a microcrack within nanometres and blunting into a void are studied through in situ tensile test for thin foil specimens of type 310 steel by transmission electron microscopy (TEM). The mechanism of microcrack nucleation is analyzed by means of microfracture mechanics. The results show that a main crack tip may be still sharp after dislocations are emitted from the crack tip and dislocation free zone (DFZ) forms so that the local stress near the crack tip in the DFZ is high enough to equal the cohesive strength and then a microcrack initiates in the DFZ. The microcracks initiating in the DFZ will blunt into voids through the movement o. the existing dislocations and/or those emanated from the crack tip and nearby dislocation sources. The discontinuous voids resulting from the blunt of the microcracks in the DFZ will link with each other and with the main crack, resulting in a zig-zag crack propagation. A microcrack will nucleate at the main crack tip and the new crack will blunt rather than propagate continuously under the condition of constant displacement. The repeat of these processes leads to continuative propagation of ductile crack.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 59471048, 59725104, 19891180).
文摘The changes of microstructures after the exhaustion of dislocation glide ability but before cracking, as well as microcrack initiation by them, were studied in detail byin-situ transmission electron microscopy (TEM) for pure copper, aluminum and iron. Thinning occurred in the thicker regions through dislocation gliding in the three metals. After that, {111} 〈112〉 twinning or tensile cracking took place in thinned zones in fcc metals. In the case of twinning, new microcracks were initiated along another {111} plane by the high stress concentration near the growing tip of the twin, while in iron (bcc), many nanocracks were initiated in the thinned areas just ahead of the crack front, resulting in misorientation microstructure from which microcracks or microvoids were developed.
