A special constant deflection device for TEM was used to study the change in dislocation configuration ahead of a crack tip during stress corrosion cracking (XC) of brass in water and of Ti-24Al-IlNb alloy in methanol as well as the initiation of SCC. In situ tensile test in TEM for brass was carried out to compare. The results show that corrosion process itself during SCC can enhance dislocation emission, multiplication and motion as well as a dislocation free zone (DFZ) is formed. When the corrosion - enhanced dislocation emission and motion reaches certain a condition a nanocrack of SCC initiates in the DFZ or from the crack tip. Because of the action of the corrosion solution the nanocrack of SCC propagates into a cleavage or intergranular microcrack rather than blunts into a void like in situ tension in TEM.
1. INTRODUCTION
Many different mechanisms about stress corrosion cracking (SCC) under anodic dissolution control have been proposed ( ?-* I The mechanism that dissolution enhanced localized plasticity results in see ? 6-8 ? is based on the experimental fact that anodic polarization can promote ambient creep for various metals and alloys I?- ) Kaufman indicated that the density of dislocations in a region very close to the fracture surface of SCC was much higher than that far away from the fracture surface ? ? ? Up to now, however, direct proof of corrosion-enhanced dislocation emission. multiplication and motion is lacking. In situ tensile tests in transmission electron microscope (TEM) is the most direct and powerful method for studying dislocation emission, multiplication and motion ? - ? Swann et al 51 studied the dislocation configuration after SCC in TEM, but did not study the effect of corrosion on dislocation emission and motion. Since there is no way to put the solution into TEM, a constant deflection device for TEM has been designed ? ? and then the effect of corrosion on dislocation emission and motion can be investigated based on the change in dislocation configuration ahead of a loaded crack tip after corroded for some time but before the initiation of SCC. The fust aim of the present work is to prove that corrosion process itself can enhance dislocation emission and motion before the initiation of SCC.
In situ tensile tests in TEM showed that for both ductile ? ?*- ? and brittle ? ?+ -? materials, many dislocations could be emitted from a loaded crack tip and moved away from the crack tip, then a dislocation free zone (DFZ) formed under constant displacement, The stress in the elastic DFZ might be up to the cohesive strength and then a nanocrack initiated in the DFZ or from the blunted crack tip ? ?3-?4, ?7-?8 ? In ductile material, the nanocrack would quickly blunt into a void or a notch under constant displacement ? I3 I. In brittle material, the nanocrack would propagate into a cleavage crack rather than blunt ? 17- ? Therefore, the essential difference between ductile and brittle fracture is the different action of the nanocrack after its initiation,
All SCC failures controlled by the anodic dissolution mechanism have macroscopic appearance of brittleness ? I-3 ) in common. Does the ductile/brittle transition induced by SCC obey the same rule? Therefore, the second aim of the present work is to compare the initiation and propagation of a nanocrack of brass in vacuum with that in solution to reveal the nature
