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An electron diffraction and fractographic study of stress corrosion and cracking Birley, Stuart Samuel


An expression was developed, using a kinematical approach, for the relative theoretical intensities of Debye rings formed on electron diffraction patterns obtained from randomly oriented powder aggregates. Calculations of relative theoretical intensities of Debye rings for specific crystal structures were compared with relative visual intensities of Debye rings obtained by high energy electron diffraction from these actual crystal structures. Good correlations were obtained and were taken to justify the theoretical approach. An electron diffraction and fractographic study was made of the transgranular stress corrosion cracking of 310 and 304L austenitic stainless steels in boiling aqueous magnesium chloride solution and of β-brasses (Cu-47.76% Zn, Cu-45.54% Zn, Cu-33.5% Zn-4.5% Sn) in water, and in ammoniacal copper sulphate solution. High energy electron diffraction was applied to protuberances on mechanically fractured and stress corrosion fracture surfaces to detect and identify surface corrosion product films and surface transformation products. Both metallic transformation products and non-metallic phases were detected on the fracture surfaces. Fractographic techniques were employed (1) to characterize the fracture surfaces and (2) to match up the opposite fracture surfaces produced by stress corrosion cracking. The latter technique established whether or not the microstructural features associated with the topographical features were formed before the crack passed. Etching techniques were employed to detect evidence of transformation products adjacent to stress corrosion cracks and the fracture surfaces. The observations pointed to the conclusion that the transgranular stress corrosion crack path in the systems under examination is dictated by the presence of a strain induced transformation product. Moreover, cracking along the interface between the strain induced and parent phases can account for the quasi-cleavage stress corrosion fracture surface topographical features; unusual fracture modes are not required. It was suggested that the conductivity of the oxide product films governs, to some degree, the rate of stress corrosion cracking.

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