X-Ray Study on Stress-Corrosion Cracking of 7-3 Brass in Ammoniacal Atmosphere

Abstract

Specimens of annealed 7-3 brass were subjected to constant loads in a corrosive environment of gaseous ammonia. The crystal deformation caused in the process of stress corrosion cracking was examined by two diffraction methods, viz., the X-ray profile analysis and the X-ray microbeam diffraction method, and the progress of stress corrosion cracking was discussed in terms of microscopic parameters supplied by these X-ray methods and by optical microscopic observations.The following is the summary of the results obtained:(1) The half-value breadth of the diffraction profile is little changed in the early period, increases gradually in the middle stage and increases sharply near the final fracture in the process of stress corrosion cracking. This change is similar to the change in specimen elongation.(2) The microscopic stress obtained from the half-value breadth by Hall's analysis increases and the particle size decreases as the stress corrosion cracking progresses. The results obtained by the Fourier analysis of the (311)-reflection profile after Garrod are similar to those by Hall's analysis.(3) Slight changes in micro-lattice strain, total misorientation and subgrain size, which are determined from X-ray microbeam diffraction patterns, are observed of the unnotched specimens. The X-ray microbeam diffraction patterns at the stress corrosion fractured surfaces of the notched specimens differ from those at the mechanical brittle or ductile fractured surfaces. It can be surmised, therefore, that cracks mainly propagates mainly by electrochemical attacks.(4) It seems from the optical microscopic observations near the notch roots of specimens that initial cracks are formed by the joining of microcracks which appear mainly along grain boundaries. The main crack, which is also intergranular, propagates by the joining of microcracks which have been formed in front of the main crack.