Mechanistic studies of stress-corrosion cracking in austenitic stainless steels and in copper-zinc alloys

This thesis focuses on the fractographic aspects of stress corrosion cracking (SCC) as a function of microstructural and mechanical variables to provide a better understanding of SCC mechanisms. Stainless steels (316L and 310) and 70Cu-30Zn underwent SCC tests in a MgCl2 aqueous solution at 154°C and in concentrated ammonia solutions at room temperature respectively.; A series of tensile tests in air on specimens of 316L and 310 stainless steels partially cracked by SCC were carried out at different displacement rates and different time intervals between SCC tests and post-SCC (PSCC) tensile tests. Absorbed hydrogen from previous SCC tests was shown to be involved in the tensile fracture by the presence of crystallographic cracking. In 316L stainless steel, the PSCC crystallographic cracking appeared both in the d -ferrite and in the austenite ( g ), at the SCC crack tips and ahead of SCC crack tips. The PSCC cracking in completely austenitic 310 stainless steel was less crystallographic.; The SCC cracking of the d -ferrite phase in ST orientated specimens of 316L stainless steel presented interfacial fracture along the d - g interfaces and transgranular fracture within the d -ferrite. Many wavy slip traces and crack arrest lines were found on the d -ferrite side of the interfacial boundaries.; The average cracking planes of SCC was {lcub}110{rcub} with the microscopic propagation directions of ⟨110⟩ and ⟨111⟩ in the d -ferrite. The crystallography of PSCC in d -ferrite was determined as {lcub}100{rcub}⟨100⟩, {lcub}100{rcub}⟨110⟩ and {lcub}100{rcub}⟨210⟩.; Precracked 70Cu-30Zn specimens were tested in a deoxygenated ammonia solution. The SCC of 70Cu-30Zn in the annealed condition showed both intergranular and transgranular fracture. Heavy cold work greatly increased the amount of transgranular fracture and changed intergranular fracture to quasi-intergranular fracture.; Crack propagation velocity appeared to increase with increasing K in the annealed condition, while the plateau region where the crack propagation velocity is independent of K was more clear in the cold worked condition.; SCC propagation was studied by imposing small periodic load pulses on the essential constant applied load. The load pulses often induced cracking pattern transitions in 70Cu-30Zn. Transitions from intergranular fracture to quasi-intergranular fracture and to transgranular fracture occurred along or near the crystallographic orientations on the grain boundaries. (Abstract shortened by UMI.)...