Corrosion fatigue crack growth of titanium alloys in aqueous environments

The rate of fatigue crack propagation for Ti-6Al-6V-2Sn and Ti-6 A1-4V in aqueous environments has been measured as a function of solution chemistry, frequency, and stress wave form. Depending on the specific encironment, three types of fatigue crack growth rate behavior have been observed as a function of frequency. Crack growth rates increase with decreasing frequency in distilled water, while addition of Na₂SO₄ produces frequency-independent behavior. In solutions containing chloride or bromide ions, a reversal in frequency-dependence takes place at ΔK_scc. Below this transition ΔK level, crack growth rates decrease with decreasing frequency due to passive film formation at the crack tip. Above ΔK_scc corrosion fatigue crack growth is due to SCC under cyclic loading. The ΔK transition in fatigue is lower than the static stress corrosion threshold because of repeated rupture of the passive film at the crack tip, approaching K_Isco only for very slow cycling frequencies. This paper is based upon a thesis submitted by D. B. Dawson in partial fulfillment of the requirements of the degree of Doctor of Science at Massachusetts Institute of Technology.     

Intrinsic fatigue crack growth

The influences of microstructure and deformation mode on inert environment intrinsic fatigue crack propagation were investigated for Al-Li-Cu-Mg alloys AA2090, AA8090, and X2095 compared to AA2024. The amount of coherent shearable δ (Al₃Li) precipitates and extent of localized planar slip deformation were reduced by composition (increased Cu/Li in X2095) and heat treatment (double aging of AA8090). Intrinsic growth rates, obtained at high constantK _max to minimize crack closure and in vacuum to eliminate any environmental effect, were alloy dependent;da/dN varied up to tenfold based on applied ΔK or ΔK/E. When compared based on a crack tip cyclic strain or opening displacement parameter (ΔK/(σ_ys E)^1/2), growth rates were equivalent for all alloys except X2095-T8 which exhibited unique fatigue crack growth resistance. Tortuous fatigue crack profiles and large fracture surface facets were observed for each Al-Li alloy independent of the precipitates present, particularly δ, and the localized slip deformation structure. Reduced fatigue crack propagation rates for X2095 in vacuum are not explained by either residual crack closure or slip reversibility arguments; the origin of apparent slip band facets in a homogeneous slip alloy is unclear. Better understanding of crack tip damage accumulation and fracture surface facet crystallography is required for Al-Li alloys with varying slip localization.     

Micromechanism of accelerated near-threshold fatigue crack growth in Al2O3/Al-Cu composite at elevated temperature

Fatigue crack growth behavior of a peak-aged Al₂O₃/Al-Cu composite was examined at 150 °C and compared to the behavior at room temperature (RT). At 150 °C, fatigue crack growth rates showed strong dependence on loading time. At short loading time, when stress-intensity range was decreased to approach fatigue threshold, crack growth rates at 150 °C were comparable to those measured at RT. Prolonged fatigue testing at near-threshold crack growth rates resulted in oscillations of crack growth rate until the fatigue crack growth behavior was stabilized to become similar to that in an overaged composite. Measurement of the matrix hardness at different distances from the crack plane and transmission electron microscopy examination of the fatigue specimen have shown that the matrix microstructure at the tip of the fatigue crack underwent overaging during prolonged testing in the near-threshold regime. Consequently, the fatigue fracture mechanism was modified, a lower crack closure developed, and the fatigue threshold reduced to that of the overaged composite.     

Fatigue behavior of maraging steel 300

The cyclic stress-strain curves, the low cycle and high cycle fatigue lives and the fatigue crack growth rates of annealed (1 h 820°C) and aged (3 h 480°C) maraging steel 300 were determined. Incremental step testing and stable hysteresis loop tip measurements were used to determine the cyclic σ-ε curves. Both annealed and aged maraging steels were found to cyclically soften at room temperature over a plastic strain range from 0.1 to 20 pct. The S-N curves were determined from 10 to 10⁷ cycles to failure by plastic strain controlled low cycle fatigue tests performed in air and load controlled high cycle fatigue tests performed in dry argon. The test results compared very well with the theoretical lifetime predictions derived from Tomkins’ theory. Fatigue crack growth rates were measured in air and dry argon for the annealed and aged alloys. Crack growth rates of annealed maraging steel were found to be equal to those of aged maraging steel at rates between 10^-7 and 10^-5 in./cycle. A significant difference in crack growth rates in the two environments was found at low stress intensity factor ranges, indicating a high susceptibility to corrosion fatigue in the presence of water vapor. The mechanisms of cyclic softening in the two alloys are discussed in terms of dislocations rearrangement in the annealed alloy and dislocation-precipitate interactions in the aged alloy.     

Fatigue crack growth through alloyed niobium, Nb-Cr2Nb, and Nb-Nb5Si3 in situ composites

Fatigue cracks were grown through several niobium-based materials. For Nb-Cr-Ti composition materials, the single-phase alloy represented the matrix of two in situ composites with about 22 and 38 vol pct Cr₂Nb. Grain boundaries were coated with intermetallic in the lower-volume fraction material, while the 38 vol pct Cr₂Nb composite consisted of mainly spherical, dispersed intermetallic. The Nb-10Si composite was composed of about 28 vol pct primary Nb₅Si₃, with most of the matrix alloy in “fiberlike” shapes due to extrusion. Crack growth rates through the composites were generally faster than for unalloyed Nb, roughly in proportion to the volume fraction of intermetallic, although differences in microstructure make this comparison difficult. The presence of intermetallic greatly alters deformation of material near the crack tip. Particles of Cr₂Nb were broken during the crack growth process, leading to increased crack growth rates. These results suggest microstructural modifications that could be expected to enhance fatigue crack growth resistance.     

Fatigue crack growth behavior of Ti-6Al-6V-2Sn in methanol and methanol-water solutions

Differences in the corrosion fatigue crack growth behavior of anα–β titanium alloy in chloride-containing aqueous and methanol environments are reported, and discussed in relation to differences in repassivation behavior for the two types of environments. Experiments have been conducted with various solution mixtures of water (a passive film-forming environment) and methanol (a nonfilm-forming environment) to define the role of repassivation in controlling fracture modes and crack growth rates at different frequencies. The critical event in determining whether the repassivation process can suppress environmental fatigue fracture is the interaction between the rate of exposure of fresh metal surfaces at the crack tip and the rate at which they can be repassivated. The out-come of this mechano-chemical interaction is shown to be dependent on the frequency and stress intensity(ΔK) level as well as the chemistry of the environment. As a result, differences in repassivation behavior for methanol-water solutions can be correlated with major differences in fatigue crack growth rates and fracture modes at low ΔK levels, whereas repassivation differences have little effect at high ΔK levels. Based on these low-ΔK corrosion fatigue characteristics, methanol solutions are concluded to be far more detrimental to titanium alloys than aqueous solutions.     

An investigation of environmental effects on fatigue crack growth in Q1N (HY80) steel

Fatigue threshold tests have been conducted on through-thickness and semielliptic cracks in laboratory air, vacuum, and salt water at stress ratios(R = K_min/K_max @#@) of 0.2 and 0.7. The effects of stress ratio are rationalized by crack closure concepts. Environmental effects are explained by considerations of the irreversibility of slip at the crack tip and the role of debris on the fracture surfaces. Differences in the fatigue crack growth rates in the three environments are attributed largely to the extent of the irreversibility of slip due to the chemisorption of water/ water vapor at the crack tip. Debris in saltwater solutions is also shown to significantly affect the near-threshold growth through its influence on crack closure and the transportation of environment to the crack tip. formerly Graduate Student, Department of Materials Science and Technology, Cambridge University formerly with the Department of Materials Science and Metallurgy, Cambridge University     

Effects of frequency and temperature on short fatigue crack growth in aqueous environments

The growth of short fatigue cracks in a NiCrMoV steel forging was examined, under constant applied stress intensity range (ΔK = 31 MPa-m^1/2) in deaerated deionized water and 0.3 M Na₂SO₄ solution, as a function of frequency and temperature. Measurements were also made of the kinetics of electrochemical reactions of bare steel surfaces with the deaerated 0.3 M Na₂SO₄ solution, under free corrosion, to provide for comparison and correlation. Fatigue crack growth rate increased with reductions in frequency and with increases in temperature. The maximum amount of crack growth enhancement by the different environments appeared to be equal, although the crack growth response in deionized water appeared to be consistent with a faster reaction rate. The temperature and frequency dependence for corrosion fatigue crack growth corresponded directly with that for charge transfer between the “bare” and “filmed” metal surfaces under free corrosion. The results showed that shortcrack growth in the aqueous environments is controlled by the rate of electrochemical reactions, and is thermally activated with an apparent activation energy of about 40 kJ/M.     

Oxide-Induced Crack Closure: An Explanation for Near-Threshold Corrosion Fatigue Crack Growth Behavior

The concept of oxide-induced crack closure is utilized to explain the role of gaseous and aqueous environments on corrosion fatigue crack propagationat ultralow, near-threshold growth rates in bainitic and martensitic 2 1/4 Cr-1 Mo pressure vessel steels. It is shown that at low load ratios, near-threshold growth rates are significantly reduced in moist environments (such as air or water), compared to dry environments (such as hydrogen or helium gas), due to the formation of excess corrosion deposits on crack faces which enhances crack closure. Using Auger spectroscopy, it is found that at the threshold stress intensity, ΔK_o, below which cracks appear dormant, the maximum thickness of excess oxide debris within the crack is comparable with the pulsating crack tip opening displacement. The implications of this model to near-threshold fatigue crack growth behavior, in terms of the role of load ratio, environment, and microstructure are discussed. formerly with M.I.T., is with McDonnell-Douglas Corporation, Redondo Beach, CA. formerly with M.I.T., is Associate Professor, Department of Materials Science and Mineral Engineering, and Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720.     

Fatigue crack propagation in two classes of directionally solidified eutectic alloys

Fatigue crack propagation rates were measured in two classes of directionally solidified eutectic alloys under isothermal, stress-controlled cycling at temperatures of 298 to 1311 K. Alloy 73C, a cobalt-base material reinforced by fibers of Cr₇C₃, and γ/γ′ + δ, a nickel-base alloy reinforced by lamellae (platelets) of Ni₃Cb, were grown at solidification rates of 1 and 25 cm/h to achieve significant differences in interfiber and interlamellar spacing (λ). No influence of the spacing of the reinforcing phase on crack growth rates were found for either alloy. In addition, chromium level and perfection of the microstructure had a minimal effect on propagation rates for γ/γ′ + δ. The independence of the fatigue crack growth rates on λ may be associated with the ratio of the cyclic plastic zone diameter at the crack tip to λ. In all instances, this ratio was estimated to be greater than one for the test conditions employed. At the lower temperatures, crack propagation rates in γ/γ′ + δ were up to two orders of magnitude lower than those in Alloy 73C due to crack deflection at interlamellar interfaces and grain boundaries which lowered the effective stress intensity range for opening mode cracking.     

The debonding and fracture of Si particles during the fatigue of a cast Al-Si alloy

Constant-amplitude high-cycle fatigue tests (σ_max=133 MPa, σmax/σy=0.55, and R=0.1) were conducted on cylindrical samples machined from a cast A356-T6 aluminum plate: The fracture surface of the sample with the smallest fatigue-crack nucleating defect was examined using a scanning electron microscope (SEM). For low crack-tip driving forces (fatigue-crack growth rates of da/dN<1 × 10⁻⁷ m/cycle), we discovered that a small semicircular surface fatigue crack propagated primarily through the Al-1 pct Si dendrite cells. The silicon particles in the eutectic remained intact and served as barriers at low fatigue-crack propagation rates. When the semicircular fatigue crack inevitably crossed the three-dimensional Al-Si eutectic network, it propagated primarily along the interface between the silicon particles and the Al-1 pct Si matrix. Furthermore, nearly all of the silicon particles were progressively debonded by the fatigue cracks propagating at low rates, with the exception of elongated particles with a major axis perpendicular to the crack plane, which were fractured. As the fatigue crack grew with a high crack-tip driving force (fatigue-crack growth rates of da/dN>1 × 10⁻⁶ m/cycle), silicon particles ahead of the crack tip were fractured, and the crack subsequently propagated through the weakest distribution of prefractured particles in the Al-Si eutectic. Only small rounded silicon particles were observed to debond while the fatigue crack grew at high rates. Using fracture-surface markings and fracture mechanics, a macroscopic measure of the maximum critical driving force between particle debonding vs fracture during fatigue-crack growth was calculated to be approximately K _max ^tr ≈6.0 MPa √m for the present cast A356 alloy.     

Effects of sic content on fatigue crack growth in

An experimental study has been conducted with the purpose of examining the fatigue crack growth characteristics of cast aluminum alloy matrix composites reinforced with different vol- ume fractions of silicon carbide particles. Particular attention has been paid to developing com- posite microstructures with similar matrix aging condition, precipitation, matrix strength, reinforcement particle size distribution, and interfacial characteristics but with different con- trolled amounts of reinforcement particles. Fatigue crack growth experiments have been con- ducted using constant stress amplitude methods with a fixed load ratio as well as constant K_max control involving a varying load ratio. The development of crack closure and the microscopic path of the crack through the composite microstructure are monitored optically and using the electron microscope in an attempt to examine the mechanisms of fatigue fracture. The results indicate that an increase in SiC content results in the suppression of striation formation in the ductile matrix. Although ductile matrix failure involving the formation of striations in the low SiC content composite or of void growth in the high SiC content composite is evident, the results also show that fracture of the reinforcement particles plays a significant role in dictating the rates of fatigue crack growth. Detailed quantitative analyses of the extent of particle fracture as a function of the reinforcement content have been performed to elucidate the mechanistic origins of fatigue resistance. The propensity of particle fracture increases with particle size and with the imposed value of stress intensity factor range. While discontinuously reinforced metal- matrix composites with predominantly matrix cracking are known to exhibit superior fatigue crack growth resistance as compared to the unreinforced matrix alloy, the tendency for particle fracture in the present set of experiments appears to engender fatigue fracture characteristics in the composite which are inferior to those seen in the unreinforced matrix material. Particle fracture also results in noticeable differences in the microscopic fracture path and causes a reduction in crack closure in the composites as compared to that in the matrix alloy. The results of this work are discussed in light of other related studies available in the literature in an attempt to develop a mechanistic perspective on fatigue crack growth resistance in metal-matrix composites.     

An investigation of the fatigue and fracture behavior of a Nb-12Al-44Ti-1.5Mo intermetallic alloy

This article presents the results of a study of the fatigue and fracture behavior of a damage-tolerant Nb-12Al-44Ti-1.5Mo alloy. This partially ordered B2 + orthorhombic intermetallic alloy is shown to have attractive combinations of room-temperature ductility (11 to 14 pct), fracture toughness (60 to 92 MPa√m), and comparable fatigue crack growth resistance to IN718, Ti-6Al-4V, and pure Nb at room temperature. The studies show that tensile deformation in the Nb-12Al-44Ti-1.5Mo alloy involves localized plastic deformation (microplasticity via slip-band formation) which initiates at stress levels that are significantly below the uniaxial yield stress (∼9.6 pct of the 0.2 pct offset yield strength (YS)). The onset of bulk yielding is shown to correspond to the spread of microplasticity completely across the gage sections of the tensile specimen. Fatigue crack initiation is also postulated to occur by the accumulation of microplasticity (coarsening of slip bands). Subsequent fatigue crack growth then occurs by the “unzipping” of cracks along slip bands that form ahead of the dominant crack tip. The proposed mechanism of fatigue crack growth is analogous to the unzipping crack growth mechanism that was suggested originally by Neumann for crack growth in single-crystal copper. Slower near-threshold fatigue crack growth rates at 750 °C are attributed to the shielding effects of oxide-induced crack closure. The fatigue and fracture behavior are also compared to those of pure Nb and emerging high-temperature niobium-based intermetallics.     

Fatigue Crack Growth Analysis Under Spectrum Loading in Various Environmental Conditions

The fatigue process consists, from the engineering point of view, of three stages: crack initiation, fatigue crack growth, and the final failure. It is also known that the fatigue process near notches and cracks is governed by local strains and stresses in the regions of maximum stress and strain concentrations. Therefore, the fatigue crack growth can be considered as a process of successive crack increments, and the fatigue crack initiation and subsequent growth can be modeled as one repetitive process. The assumptions mentioned above were used to derive a fatigue crack growth model based, called later as the UniGrow model, on the analysis of cyclic elastic–plastic stresses–strains near the crack tip. The fatigue crack growth rate was determined by simulating the cyclic stress–strain response in the material volume adjacent to the crack tip and calculating the accumulated fatigue damage in a manner similar to fatigue analysis of stationary notches. The fatigue crack growth driving force was derived on the basis of the stress and strain history at the crack tip and the Smith–Watson–Topper (SWT) fatigue damage parameter, D = σ_maxΔε/2. It was subsequently found that the fatigue crack growth was controlled by a two-parameter driving force in the form of a weighted product of the stress intensity range and the maximum stress intensity factor, ΔK ^p K _max ^1?p . The effect of the internal (residual) stress induced by the reversed cyclic plasticity has been accounted for and therefore the two-parameter driving force made it possible to predict the effect of the mean stress including the influence of the applied compressive stress, tensile overloads, and variable amplitude spectrum loading. It allows estimating the fatigue life under variable amplitude loading without using crack closure concepts. Several experimental fatigue crack growth datasets obtained for the Al 7075 aluminum alloy were used for the verification of the proposed unified fatigue crack growth model. The method can be also used to predict fatigue crack growth under constant amplitude and spectrum loading in various environmental conditions such as vacuum, air, and corrosive environment providing that appropriate limited constant amplitude fatigue crack growth data obtained in the same environment are available. The proposed methodology is equally suitable for fatigue analysis of smooth, notched, and cracked components.     

Hydrogen embrittlement of a cyclically deformed high strength Al alloy

High cycle fatigue experiments have been performed on a 7075 Al alloy, principally in the T6 temper in dry air, distilled water 0.5N NaCl and 0.5N Na₂SO₄ solutions as functions of cathodic charging and catalyst poisoning of the hydrogen evolution reaction. All aqueous solutions appreciably lowered fatigue resistance with Cl^-ion producing the greatest reduction in resistance and SO₄ ⁼ion behaving essentially in the same manner as distilled water. Under cathodic charging conditions fatigue resistance is significantly reduced and both Cl^- and SO₄ ⁼solutions produce similar fatigue lives. A catalyst poison (As) added to Cl^- solutions reduces fatigue resistance relative to neutral Cl^- solutions. Fractography of specimens fatigued in aqueous environments shows that a significant amount of cleavage and quasicleavage occurs, the extent of these features being apparently a function of hydrogen available to the alloy free surface and to the tips of growing cracks. On the basis of these observations, it is suggested that corrosion fatigue of 7075 alloy is essentially a hydrogen embrittlement phenomenon where the low diffusivity of hydrogen is counterbalanced by the fact that hydrogen need only be present in the alloy free surface for crack initiation and in the plastic zone of growing cracks for propagation.     

Effects of pre-existing grain boundary microvoid distributions on fracture toughness and fatigue crack growth in low alloy steel

The role of dispersions of pre-existing grain boundary microvoids is investigated in fracture toughness and fatigue crack propagation behavior in a low alloy steel. Microvoid damage is achieved by prior exposure of the steel to gaseous hydrogen atmospheres at high temperatures and pressures, where carbon within the steel reacts with ingressed hydrogen to nucleate methane bubbles along prior austenite grain boundaries (hydrogen attack). It is shown that, whereas the crack initiation and crack growth toughness (i.e. K_Ic and the tearing modulus) are severely degraded, even for comparatively mild degrees of microvoid damage, rates of sub-critical crack growth by fatigue remain relatively unaffected. Such results are interpreted in terms of a mutual competition between microstructural damage generated by the grain boundary microvoids, which promotes crack growth by lowering the intrinsic resistance of the microstructure, and the resulting tortuous crack paths, which extrinsically retard crack growth at low stress intensities by lowering the local crack tip “driving force” (crack tip shielding). As shielding effects are minimized at high stress intensities, the degradation in intrinsic toughness is related to changes in ductility by means of a stress-modified critical strain model for ductile fracture, where the presence of small microvoid clusters is shown to promote coalescence through the easier onset of plastic strain localization. Fatigue behavior, conversely, is dominated by extrinsic shielding mechanisms and is modeled in terms of two-dimensional models of crack deflection and roughness-induced crack closure.     

Fatigue of stainless steel in hydrogen

The fatigue crack growth rates of two austenitic stainless steel alloys, AISI 301 and 302, were compared in air, argon, and hydrogen environments at atmospheric pressure and room temperature. Under the stresses at the crack tip the austenite in type 301 steel transformed martensitically to a’ to a greater extent than in type 302 steel. The steels were also tested in the cold worked condition under hydrogen or argon. Hydrogen was found to have a deleterious effect on both steels, but the effect was stronger in the unstable than in the stable alloy. Cold work decreased fatigue crack growth rates in argon and hydrogen, but the decrease was less marked in hydrogen than in argon. Metallographic, fractographic, and microhardness surveys in the vicinity of the fatigue crack were used to try to understand the reasons for the observed fatigue behavior.     

High-cycle fatigue-life of the cast nickel base-superalloys in 738 LC and IN 939

The high cycle fatigue (HCF) properties of two cast nickel base-superalloys, IN 738 LC and IN 939, were investigated using both fracture mechanics samples and smooth specimens. The crack propagation behavior was studied in terms of linear fracture mechanics at RT and at 850 °C. In addition to the influence of temperature, the influences of frequency, mean stress, and environment (vacuum, air, sulfidizing atmosphere) were studied. At 850 °C, the fatigue thresholds were found to be higher in air than in vacuum. This could be explained by crack branching. The high scatter of fatigue crack propagation rates could be related also to this phenomenon. The S/N curves at 850 °C can be predicted treating crack growth from casting pores as the predominant failure mechanism. At RT the same method is not as successful. The reason for this may be that crack growth laws measured on long, branched cracks are not applicable to short, unbranched cracks. At RT, no significant influence of frequency on S/N-curves and fatigue crack growth rates was observed for frequencies up to 20 kHz.     

On the development of crack closure and the threshold condition for short and long fatigue cracks in 7150 aluminum alloy

In an attempt to analyze the behavior of physically “short” cracks, a study has been made of the development, location, and effect of crack closure on the behavior of fatigue cracks arrested at the “long” crack threshold stress intensity range, ΔK _TH , in underaged, peak aged, and overaged microstructures in a 7150 aluminum alloy. By monitoring the change in closure stress intensity,K _cl, during thein situ removal of material left in the wake of arrested threshold cracks, approximately 50 pct of the closure was found to be confined to a region within ∼500 μm of the crack tip. Following wake removal, previously arrested threshold cracks recommenced to propagate at low load ratios even though nominal stress intensity ranges didnot exceed ΔK _TH , representing the behavior of physically short cracks emanating from notches. No such crack extension at ΔK _TH was seen at high load ratios. With subsequent crack extension, crack closure was observed to redevelop leading to a deceleration in growth rates. The development of such closure was found to occur over crack extensions comparable with microstructural dimensions, rather than those associated with local crack tip plasticity. Such results provide further confirmation that the existence of a fatigue threshold and the growth of physically short cracks are controlled primarily by crack closure, and the data are discussed in terms of the micro-mechanisms of closure in precipitation hardened alloy systems. Formerly Graduate Student with the Department of Mechanical Engineering, University of California, Berkeley