Environment Induced Cracking Of Metals

Identification of consensus issues is undoubtedly controversial. Many of the written comments from participants suggested that no true consensus was reached on any particular aspect of environmental cracking! This camp believes that while we may have progressed somewhat in our appreciation o the various environmental cracking phenomena since the Firminy meeting, we have not reached any true understanding of the phenomena as a whole. This view proposes that continued, often unsubstantiated and qualitative, debate on the details of a variety of mechanisms hinders our progress toward material and component performance predictions. This situation, more than anything else, makes for frustration within the engineering design community. A second camp of participants concludes that substantial progress has been made since Firminy. Much of the speculation of the 1970s has been replaced by quantitative experimental evidence. Quantitative and testable models are emerging, with focus on specific embrittlement mechanisms that are relevant to classes of alloys and environments. Meaningful life predictions may be developed to include environmental cracking. The critical importance of crack chemistry and crack-tip process-zone deformation and fracture has spawned new models and experimental probes. Those with this view do not believe that all problems are solved, but rather that the field is generally on the right track. Useful information is being made available for the user, but improvements and new ideas are, of course, still required.

This report deals with the stress-corrosion cracking of aluminum alloys, and it represents an effort by DMIC to expand on the information contained in DMIC Memorandum 202, 'Stress-Corrosion Cracking of Aluminum Alloys', dated February 15, 1965. DMIC Report 228 begins by presenting a comprehensive definition of stress-corrosion cracking. This is followed by sections dealing with (1) the historical development and growth in awareness of the problem, (2) the mechanisms involved, and (3) the theory of stress-corrosion cracking. A section on experimental techniques is presented. These techniques include test methods used to determine the susceptibility of alloys to stress-corrosion cracking, as well as more refined methods of studying the fundamental mechanisms of the problem. Different evaluation methods, applicable to obtaining the different objectives of stress-corrosion testing, are also presented. All of the foregoing serve as background to the sections on stress-corrosion-cracking behavior of aluminum alloys and preventive measures. (Author).

This research program has included two thrusts. The first addressed environment-induced embrittlement in a parallel study of stress corrosion cracking and metal-induced embrittlement. This work has examined (1) mechanical properties as influenced by embrittling environments, (2) fractography and crystallography or transgranular cracking, (3) the mechanics of cracking, (4) the extent and role of local plastic flow, and (5) local chemistry within stress corrosion and metal-induced cracks. The embrittlement of iron aluminide alloys by air was addressed by determining the effect of water and hydrogen upon the mechanical properties. Slow strain rate testing in aqueous environments was carried out at controlled anodic and cathodic potentials. The effect of cathodically charged hydrogen and the effect of subsequent baking were measured. Environmental susceptibility was measured as affected by alloy composition, microstructure and degree of ordering.

In order to predict when and how a material will crack under environmental conditions, engineers used to subject it to pressure for a long time, up to a year in cases; about 20 years ago, a testing method was developed that slowly but inexorably increases the pressure, which seems to cut down the ti