Dissolution Condensation Mechanism of Stress Corrosion Cracking in Liquid Metals: Driving Force and Crack Kinetics
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L EMBRITTLEMENT AS STRESS CORROSION CRACKING PROBLEM
CRYSTAL plasticity and strength are environmentally sensitive properties. Along with corrosion and stress corrosion cracking (SCC), complex but well amenable to scientific interpretation electrochemical phenomena, more abstruse nonelectrochemical effects in crystal plasticity were observed for solids of different molecular nature when under simultaneous action of tensile stress and wetting liquids. The most famous among these effects is liquid metal embrittlement (LME), which describes premature failure of metals under simultaneous action of tensile stress and wetting liquid metals. LME creates potential reliability concerns in many technologies, including nuclear energy, welding, soldering, protective coatings, and gas industry.[1–10] The fact that LME was reported not only for polycrystals but also for single-crystalline[1–5] and amorphous metals[11] suggests that the phenomenon can not be explained in terms of either grain boundary (GB) diffusion/grooving modified by stress or adsorptionenhanced dislocation injection/mobility. LME was reported to most strongly manifest itself in solid metal (SM)–liquid metal (LM) couples, which form simple eutectic phase diagrams without any intermetallics;[9] this suggests that the concept of formation and fracture of brittle surface films does not apply to LME.
EVGENY E. GLICKMAN, Professor, Faculty of Engineering, Tel Aviv University, Ramat Aviv, 69978, Tel Aviv, Israel. Contact e-mail: [email protected] Manuscript submitted April 12, 2010. Article published online December 1, 2010 250—VOLUME 42A, FEBRUARY 2011
Figure 1 taken from Reference 5 shows that LME involves subcritical crack growth, the typical feature that distinguishes SCC from crack nucleation–controlled brittle fracture below the ductile-brittle transition temperature in inert atmosphere. Crack kinetics observations have made clear that LME is not a ductile-brittle transition but a special case of time-dependent process of SCC. The subcritical growth under LME starts at an apparent threshold stress intensity KTH that can be as small as ~0.1 MPaÆm½; the process of crack extension spans almost all the lifetime tF for the precracked specimens and changes to very fast, purely mechanical failure when the stress intensity K approaches the fracture toughness KC of the solid in inert atmosphere. Similar to SCC in aqueous solution, the LME subcritical crack velocity plots ln V(K) at K > KTH often show a plateau with the velocity VII that can be as high as ~0.1 m/s and only weakly, if at all, dependent on K (Figure 1). In terms of very high VII and small KTH, LME is the striking example of SCC. Even more challenging is a description of driving forces and kinetic mechanisms of LME. Thermodynamic driving force for aqueous corrosion and SCC is exothermic effect, Hox, of metal oxidation at the anodic sites generated at the metal surface.[12] The electrical current flow between anodic and cathodic sites determines a loss of metal and the rate of local dissolution at the crack tip.
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