Fatigue crack growth behavior of Ti-6Al-6V-2Sn in methanol and methanol-water solutions
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titanium alloys are susceptible to stress corrosion cracking (SCC) ~-4 and corrosion fatigue crack growth (CFCG) 5-s in both aqueous and organic solvent solutions. However, there are significant differences in crack growth behavior for these environments, primarily due to differences in repassivation behavior and corrosion-induced fracture modes. Under static loads, SCC of alpha-beta alloys in aqueous solutions (such as salt water) has been attributed to cleavage fracture of the alpha phase on near-basal planes. 9 In these environments, stress corrosion cracks do not initiate in smooth specimens, 2 and in precracked specimens a well-defined threshold K~scc exists below which SCC does not occur. 1~ Cleavage fracture is also observed in organic environments such as methanol, 4,11,~2but at low AK levels it is possible for intergranular attack to occur as well. 2,4,~3,~4This latter fracture mode is accelerated by stress, but can also occur in the absence of stress. 4.t3.~4 Intergranular fracture is associated with crack initiation in smooth specimens, 4 and the absence of a welldefined SCC threshold. 3~ Some of these characteristics also apply to the fatigue behavior of titanium alloys. In S - N (stress vs cycles to failure) fatigue testing of smooth specimens there is little or no effect of an aqueous environment, '5-t8 whereas the endurance limit in methanol solutions is much lower than in air. ~8 This effect can be related to differences in environmental influence on crack initiation, since it is primarily initiation which governs the fatigue threshold stress level. Dawson and Pelloux 7,8 have shown that differences in fatigue crack propagation behavior also occur for methanol and aqueous environments, for example the D. B. DAWSON is Member of Technical Staff, Sandia National Laboratories, Livermore, CA 94550. Manuscript submitted November 6, 1978. METALLURGICAL TRANSACTIONS A
effect of stress cycle frequency on fatigue crack growth rates (FCGR). In aqueous environments FCGR show a " c r o s s o v e r , " and the effect of frequency at low AK is the reverse of the effect at high AK levels (Fig. 1). This reversal has been explained 8 in terms of two competing mechanisms: stress corrosion under cyclic loading, which dominates fatigue crack growth behavior at high AK levels; and the beneficial effect of repassivation, which governs behavior at low AK levels. Crossover at low AK levels and slight increases in F C G R will occur in distilled water, but the effects are greatly accentuated by halide additions such as chloride. For slow cycling frequencies (e.g., 1 Hz), the point of crossover usually occurs with an abrupt increase in FCGR, which has been termed SCC*8
" C r o s s o v e r " behavior does not occur in methanol solutions. Instead, F C G R increase with decreasing frequency over the entire AK range, with the relative rate of increase being greatest at low ~ K levels (Fig. 2). The inverse relationship between F C G R and frequency (also seen above ~ s c c in aqueous solutions) is consistent with a model w
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