Applied Stress Affecting the Environmentally Assisted Cracking

A.K. VASUDEVAN 0 0 A.K. VASUDEVAN, Scientific Officer, is with the Office of Naval Research , Code-332, 875, North Randolph Street, Arlington, VA 22203 . Contact Stress corrosion cracking (SCC) is affected by the mode of applied stress, i.e., tension, compression, or torsion. The cracking is measured in terms of initiation time to nucleate a crack or time to failure. In a simple uniaxial loading under tension or compression, it is observed that the initiation time can vary in orders of magnitude depending on the alloy and the environment. Fracture can be intergranular or transgranular or mixed mode. Factors that affect SCC are solubility of the metal into surrounding chemical solution, and diffusion rate (like hydrogen into a tensile region) of an aggressive element into the metal and liquid metallic elements in the grain boundaries. Strain hardening exponent that affects the local internal stresses and their gradients can affect the diffusion kinetics. We examine two environments (Ga and 3.5 pct NaCl) for the same alloy 7075-T651, under constant uniaxial tension and compression load. These two cases provide us application to two different governing mechanisms namely liquid metal embrittlement (7075-Ga) and hydrogen-assisted cracking (7075-NaCl). We note that, in spite of the differences in their mechanisms, both systems show similar behavior in the applied K vs crack initiation time plots. One common theme among them is the transport mechanism of a solute element to a tensile-stress region to initiate fracture. - (a) Active path dissolution (APD or AD) involving dissolution of metallic elements, sometimes preferentially at slip bands or at grain boundaries (GBs) or of specific precipitates at the boundaries[2]; (b) Liquid metal embrittlement (LME) involving a liquid metal penetrating into GB or dissolving the host metal to induce embrittlement[3]; (c) Hydrogen-assisted cracking (HAC) involving hydrogen-enhanced decohesion, hydrogen-enhanced localized plasticity (HELP), adsorption-induced dislocation emission or reduction in cohesive energy, particularly, as in Fe-, Ti-, Ni-, and Al-based alloys[4]; In the literature, there has been greater emphasis on the kinetics of crack growth than on the initiation thresholds, even though damage involves both the initiation of a crack and its growth. Not much experimental data are available on the initiation/incubation times for SCC. Incubation time appears to depend on the tensile or compressive stress at the crack tip. In particular, under uniaxial tension or compression, the initiation time varies significantly with alloyenvironment system. It is observed that SCC initiation times under tensile stress can be one to two orders of magnitude shorter compared to that under compression load. For a crack to initiate under compression, there has to be a local tensile stress at some point away from the notch. Even if this crack forms, it will not grow until the crack-tip driving force becomes tensile. The current article examines the role of applied stress on the EAC crack initiation time in terms of applied stress intensity factor, Kapp, and the factors controlling the crack initiation time. II. EARLY EXPERIMENTAL OBSERVATIONS Early investigators[5] had observed that SCC in stainless steels could occur also under compression load, using the horseshoe-type specimens. They observed that corrosion occurs on the compressive side of a bent sample, albeit at a lower rate than on the tensile side. Since then, there have been several qualitative experimental observations, using bent sheet samples, showing that SCC can occur under compressive stress in 304 stainless steels. Similar results were observed in other steels. For example, SCC occurs under compression in boiling 42 pct MgCl2 solution[6] in 1015 mild steel, and boiling nitrate solution in 1017 mild steels.[7] It was observed that the time to nucleate a SCC crack under far field compression was about one to two orders of magnitude longer than that under tension. The results were attributed to slower dissolution under compression than in tension.[6,7] Dissolution occurs on freshly created surface, when the passive films are broken by local plastic deformation. The process can be continuous on a macroscopic scale. The dissolution of a fresh surface is slower under compression than in tension. Alternatively, slower dissolution under compression can be independent of the slip deformation process. On the other hand, if the incubation time and the related stress effects are due to HAC, then two possibilities can exist: (1) HAC crack initiation occurring close to the crack/notch tip; and (2) crack initiation occurring ahead of the crack/ notch tip. HAC can occur at or ahead of the crack tip depending on where the peak tensile hydrostatic stress is located. For the case in which HAC occurs ahead of the crack tip, hydrogen diffusion via GB or matrix is needed; in this case, H-diffusion kinetics can be a controlling factor. On the other hand, for HAC occurring at the crack/notch tip, no H-diffusion into metal is required. HAC at the crack tip can be reaction-rate controlled if H-generation is slow, or surface diffusion is controlled along the crack/notch surface if H-generation is fast. The relative kinetics of crack-tip reaction and the associated surface diffusion vs the diffusion of hydrogen into the metal become the governing factor in determining the incubation time for crack formation as well as for crack growth kinetics[8]. In either case, the relative roles of applied tensile or compressive stresses need to be understood and characterized. MATERIALS AND EXPERIMENTS A set of data was published by Chu et al.[9,10] under uniaxial tension and compression loads for 7075-T651. The tests on the 7075 alloy were performed in two different environments: (1) liquid gallium, and (2) 3.5 pct NaCl solution at pH = 3.5. The fractographic analyses of SCC failures under tension and compression were also reported for each of the alloys.[9,10] The mechanical properties for 7075-T651 were yield strength (YS) = 510 MPa, and strain hardening coefficient n = 0.113. 7075-T6 alloy deforms by mixed planar and wavy slip. In the following sections, for convenience, we label the two main alloyenvironment systems as: 7075-Ga and 7075-NaCl. In all the tests, the modified wedge opening load (WOL) samples[10] under plane stress (2.5 mm thick with notch radius q = 0.1 mm) were used without precracking for the case of 7075-NaCl. 7075-Ga test was under plane strain (10-mm-thick samples with q = 0.15 mm). The crack initiation time was monitored using clip gage and optical methods with accuracy ranging from 10 to 30 lm in crack length measurement. See original papers[9,10] for details. Test ing in un-precracked condition has some advantages. For notches of finite radius, the peak stress rmax, remains the same, except for a change in sign, for both tension and compression loadings. In all the tests reported, the n (...truncated)