Nudeating and propagating of nanocrack forming in dislocation-free zone (DFZ) for a brittle material, TiAl alloy, was studied through in situ tensile test in TEM and analyzed using micro-fracture mechanics. The result...Nudeating and propagating of nanocrack forming in dislocation-free zone (DFZ) for a brittle material, TiAl alloy, was studied through in situ tensile test in TEM and analyzed using micro-fracture mechanics. The results show that many of dislocations can be emitted from a crack tip when the applied stress intensity KIa is larger than the stress intensity for dislocation emission KIe = 1.4MPa·m1/2 and a DFZ is formed after reaching equilibrium. The stress in a certain site in the DFZ, which is an elastic zone and is thinned gradually through dislocation multiplication and motion in the plastic zone, may be equal to the cohesive strength, resulting in initiating of a nanocrack in the DFZ or sometimes at the notch tip. The nanocrack forming in the DFZ is stable and can propagate into a cleavage microcrack through multiplication and movement of dislocations in the plastic zone under constant displacement condition.展开更多
The Ti-36Nb-2Ta-3Zr-0.350 （mass fraction, %） （TNTZO） alloy was produced by cold isostatic pressing and sintering from elemental powders, followed by hot and cold deformation. The effects of deformation process on ...The Ti-36Nb-2Ta-3Zr-0.350 （mass fraction, %） （TNTZO） alloy was produced by cold isostatic pressing and sintering from elemental powders, followed by hot and cold deformation. The effects of deformation process on microstructures and mechanical properties were investigated using the SEM, TEM, OM and the universal material testing machine. Results show that the alloy can be easily hot forged and cold swaged due to the fine-grained microstructure. Only after cold swaging by 85%, the alloy shows the typical ＂marble-like＂ structure. And thecold deformation is accompanied by stress-induced a＂ phase transformations. Moreover, both the strength and the ductility of the alloy are significantly improved by hot and cold working.展开更多
To investigate the relation between material＇s cychc plastic behaviour and fatigue crack growth, a new model is proposed. The model incorporated the two intrinsic properties of material＇ s cyclic plastic and crack ...To investigate the relation between material＇s cychc plastic behaviour and fatigue crack growth, a new model is proposed. The model incorporated the two intrinsic properties of material＇ s cyclic plastic and crack tip＇ s deformation dislocation to interpret fatigue crack threshold. The relation between material＇ s cyclic hardening parameters （cyclic hardening amplitude and cyclic hardening rate） and fatigue threshold is studied. Fatigue threshold is determined based on the dislocation-free zone （DFZ） model, the theory of cohesive zone and the cyclic deformation behaviour. The results show that fatigue threshold increases with the decrease of the amplitude of cyclic hardening and is independent of cyclic hardening rate, but fatigue crack growth rate increases with the increase of cyclic hardening rate.展开更多
For large diarneter silicon single crystal, the solid-liquid growth interface is necessary to be a coneaveshape with a certain radius range. If the change of the radius of growth interface is not in this limited range...For large diarneter silicon single crystal, the solid-liquid growth interface is necessary to be a coneaveshape with a certain radius range. If the change of the radius of growth interface is not in this limited range,the growth of DF (dislocation free) sinsle crystal is very difficult. The growth of FZ-Si single crystal was stud-ied. It is found that the growth speed ( 2. 5～2. 7 mm/min) as well as the rotation speed (3. 5 r/min) for theΦ100 mm crystal can be smaller . comparing with the Φ76. 2 mm crystal with the same coil. In order to satisfythe demand of large diameter crystal . the size of coil should be large enough, and the shape should satisfy theneed of the growth interface of crystal. With the increasing of diameter , the heating power , the anode voltageand the strength of electric field within the coil should be increased, and Ar pressure in surrounding circum-stance should also be higher , from 1. 96 × 1 0￣4 Pa to 4. 90 × 10￣4 Pa.According to the above growth factors, three rods of Φ100 mm FZ-Si single crystal were grown success-fully , the weights are 8～10 kg. When the diameter of crystal cone is increased to a limited size, “remeltingarca” will occur in the surface of the crystal , which cause a failure of growing DF crystal , this reason may bethat the recrystalliztion direction has been chansed , as it does.展开更多
The results of tensile test in TEM for a brittle material, Ti-24Al-11Nb alloy, show that a lot of dislocations can also be emitted from a brittle crack tip and a dislocation-free zone (DFZ) is formed. The DFZ is an el...The results of tensile test in TEM for a brittle material, Ti-24Al-11Nb alloy, show that a lot of dislocations can also be emitted from a brittle crack tip and a dislocation-free zone (DFZ) is formed. The DFZ is an elastic zone with large strain and the stress in the DFZ may reach the cohesive strength of the material provided that the crack tip is sharp enough and the applied stress is large enough, resulting in initiating of a nanocrack in the DFZ or sometimes at the crack tip. The nanocrack formed in the DFZ is stable and will propagate into a deavage microcrack through multiplication and movement of dislocations in the plastic zone. Increasing the applied stress, the crack can stably propagate continuously or discontinuously.展开更多
Rice and Thomson assumed that no dislocation emission takes place from thecrack tip during brittle crack propagation. Experimental observations for Si andFe-Si have shown that dislocations are emitted during cleavage ...Rice and Thomson assumed that no dislocation emission takes place from thecrack tip during brittle crack propagation. Experimental observations for Si andFe-Si have shown that dislocations are emitted during cleavage crack propagation.Argon suggested that the transformation from brittle fracture to ductile fracture insilicon is controlled by the number of dislocations emitted from the crack tip ratherthan by dislocation nucleation from the tip. However, Chiao assumed that展开更多
基金the National Natural Science Foundation of China
文摘Nudeating and propagating of nanocrack forming in dislocation-free zone (DFZ) for a brittle material, TiAl alloy, was studied through in situ tensile test in TEM and analyzed using micro-fracture mechanics. The results show that many of dislocations can be emitted from a crack tip when the applied stress intensity KIa is larger than the stress intensity for dislocation emission KIe = 1.4MPa·m1/2 and a DFZ is formed after reaching equilibrium. The stress in a certain site in the DFZ, which is an elastic zone and is thinned gradually through dislocation multiplication and motion in the plastic zone, may be equal to the cohesive strength, resulting in initiating of a nanocrack in the DFZ or sometimes at the notch tip. The nanocrack forming in the DFZ is stable and can propagate into a cleavage microcrack through multiplication and movement of dislocations in the plastic zone under constant displacement condition.
基金Project(2014CB644002)supported by the National Key Fundamental Research and Development Project of ChinaProject(51301203)supported by the National Natural Science Foundation of China+1 种基金Project(2015CX004)supported by the Innovation-driven Plan in Central South University,Chinasupported by the Outstanding Graduate Project of Advanced Non-ferrous Metal Structural Materials and Manufacturing Collaborative Innovation Center,China
文摘The Ti-36Nb-2Ta-3Zr-0.350 （mass fraction, %） （TNTZO） alloy was produced by cold isostatic pressing and sintering from elemental powders, followed by hot and cold deformation. The effects of deformation process on microstructures and mechanical properties were investigated using the SEM, TEM, OM and the universal material testing machine. Results show that the alloy can be easily hot forged and cold swaged due to the fine-grained microstructure. Only after cold swaging by 85%, the alloy shows the typical ＂marble-like＂ structure. And thecold deformation is accompanied by stress-induced a＂ phase transformations. Moreover, both the strength and the ductility of the alloy are significantly improved by hot and cold working.
基金Sponsored by the Fundamental Research Foundation of Harbin Engineering University(Grant No.HEUFT07007)
文摘To investigate the relation between material＇s cychc plastic behaviour and fatigue crack growth, a new model is proposed. The model incorporated the two intrinsic properties of material＇ s cyclic plastic and crack tip＇ s deformation dislocation to interpret fatigue crack threshold. The relation between material＇ s cyclic hardening parameters （cyclic hardening amplitude and cyclic hardening rate） and fatigue threshold is studied. Fatigue threshold is determined based on the dislocation-free zone （DFZ） model, the theory of cohesive zone and the cyclic deformation behaviour. The results show that fatigue threshold increases with the decrease of the amplitude of cyclic hardening and is independent of cyclic hardening rate, but fatigue crack growth rate increases with the increase of cyclic hardening rate.
文摘For large diarneter silicon single crystal, the solid-liquid growth interface is necessary to be a coneaveshape with a certain radius range. If the change of the radius of growth interface is not in this limited range,the growth of DF (dislocation free) sinsle crystal is very difficult. The growth of FZ-Si single crystal was stud-ied. It is found that the growth speed ( 2. 5～2. 7 mm/min) as well as the rotation speed (3. 5 r/min) for theΦ100 mm crystal can be smaller . comparing with the Φ76. 2 mm crystal with the same coil. In order to satisfythe demand of large diameter crystal . the size of coil should be large enough, and the shape should satisfy theneed of the growth interface of crystal. With the increasing of diameter , the heating power , the anode voltageand the strength of electric field within the coil should be increased, and Ar pressure in surrounding circum-stance should also be higher , from 1. 96 × 1 0￣4 Pa to 4. 90 × 10￣4 Pa.According to the above growth factors, three rods of Φ100 mm FZ-Si single crystal were grown success-fully , the weights are 8～10 kg. When the diameter of crystal cone is increased to a limited size, “remeltingarca” will occur in the surface of the crystal , which cause a failure of growing DF crystal , this reason may bethat the recrystalliztion direction has been chansed , as it does.
基金Project supported by the National Natural Science Foundation of China.
文摘The results of tensile test in TEM for a brittle material, Ti-24Al-11Nb alloy, show that a lot of dislocations can also be emitted from a brittle crack tip and a dislocation-free zone (DFZ) is formed. The DFZ is an elastic zone with large strain and the stress in the DFZ may reach the cohesive strength of the material provided that the crack tip is sharp enough and the applied stress is large enough, resulting in initiating of a nanocrack in the DFZ or sometimes at the crack tip. The nanocrack formed in the DFZ is stable and will propagate into a deavage microcrack through multiplication and movement of dislocations in the plastic zone. Increasing the applied stress, the crack can stably propagate continuously or discontinuously.
基金Project supported by the National Natural Science Foundation of China.
文摘Rice and Thomson assumed that no dislocation emission takes place from thecrack tip during brittle crack propagation. Experimental observations for Si andFe-Si have shown that dislocations are emitted during cleavage crack propagation.Argon suggested that the transformation from brittle fracture to ductile fracture insilicon is controlled by the number of dislocations emitted from the crack tip ratherthan by dislocation nucleation from the tip. However, Chiao assumed that
