Dissolution-facilitating Dislocation Emission, Motion andInitiation of SCC in 310 Stainless Steel

Using a special constant deflection device, the in situ TEM observations of dislocation emission, motion and the initiation of nanocrack in 310 stainless steel in water were carried out. The results show that SCC in thin foil specimen of 310 stainless steel can occur in water. A nanocrack initiated in the DFZ or at the blunt crack tip in corrosion solution after dislocation emission, multiplication and motion, which were induced by the localized anodic dissolution, reached a critical condition. The nanocrack propagated by a cleavage mode because of the influence of the corrosion solution.

In-situ TEM Observation of Dislocation Emission, Multiplication and Motion Induced by Anodic Dissolution in Type 310 Stainless Steel

A special constant deflection device for TEM has been designed and then the dislocation configuration change ahead of a loaded crack tip after corrosion or anodic dissolution for some time but before the initiation of SCC can be observed in TEM. The results showed that anodic dissolution during SCC of a type 310 stainless steel in deionized water could promote dislocation emission, multiplication and motion before the initiation of SCC.

In-situ Observation of Initiation and Propagation of Cleavage Microcrack in TiAl

Nucleating and propagating of nanocrack formed in dislocation free zone (DFZ) for the brittle TiAl alloy has been studied through in-situ tensile test in TEM and analyzed using microfracture mechanics. The results show that a lot of dislocations can be emitted from a crack tip when the applied stress intensity K-la is larger than the stress intensity for dislocation emission K-le = 1.4 MPa . m(1/2) and a dislocation free zone, which sometimes is a close zone, can form after reaching equilibrium. The DFZ is a elastic zone with large strain and then the stress in the DFZ might equal to the cohesive strength sigma(th) because the crack tip is still sharp. When K-la is larger than the stress intensity for nanocrack nucleation K-li = 2.4 MPa . m(1/2), the stress within a certain range in the DFZ would equal to sigma(th) and then a nanocrack initiates in the DFZ or sometimes at the notch tip. The nanocrack formed in the DFZ is stable and can propagate a small distance in cleavage mode through multiplication and movement of dislocation in the plastic zone, during keeping constant displacement. Increasing K-la, can make the crack stably propagate continuously or discontinuously and it means that the stress intensify for crack propagation, K-lp, is larger than K-li. Therefore, K-le [K-li [K-lp [K-lC.

Stress Corrosion Cracking and Blue Brittleness of Rotor Steels in High Temperature Aqueous Solutions

The tensile behaviors of two bolt steels 20Cr12NiMoWV and 25Cr2MoV, and rotor steel 30Cr2Ni4MoV used in a nuclear power plant were studied at different temperatures up to 280℃ in air and aqueous solution. The results showed that blue brittleness occurred near 230℃ and 180℃ for steels 30Cr2Ni4MoV and 25Cr2MoV, respectively. No blue brittleness was found for the steel containing higher Cr content. Susceptibility to stress corrosion cracking (SCC) for the steels in aqueous solution at the test temperatures was very low except at the blue brittleness temperature. At the blue brittleness temperatures, however, the susceptibility to SCC was very high.

Investigation of stress corro-sion-enhanced plastic deformation using atomic force microscope

Atomic force microscope (AFM) was used to study in situ the change in slip bands during stress corrosion of brass in the ammonia solution prior to initiation of stress corrosion crack. The result showed that additive slip bands appeared ahead of a loaded notch tip during stress corrosion prior to initiation of stress corrosion crack after the constant deflection specimen was precrept for 48 h and then polished. The average height and spacing of slip bands generated during stress corrosion were about three times less or larger, respectively, than those during loading in air.

The in-situ TEM observation of chemisorption facilitating dislocation emission, multiplication and motion

THE reduction in ductility and fracture stress of many metals tested in certain liquid metal envi-ronments is generally known as liquid metal embrittlement（LME）.The resistance to fractureR=（1-v2）KIC2（L）/E,calculated by the fracture toughness KIC（L）in LME,is often 1—3orders of magnitude greater than the surface energy of the material,γ.Orowan’s equationof R=2γeff=γ+γP must be used in considering LME rather than Griffith’s equation of R=2γ,where γP is plastic work done.The experiments in LME showed that γP=（10—1 000）γ.It has been shown that there is localized plastic deformation in brittle failure induced bythe chemisorption of liquid metal.