Richard P. Gangloff

Effect of artificial aging on the fatigue crack propagation resistance of 2000 series aluminum alloys

Artificial aging performed to impart higher strength generally degrades the fatigue crack propagation (FCP) resistance of naturally aged 2xxx-series aluminum alloys. This behavior is examined for commercial AA2024-T351 and a naturally aged Al–Cu–Mg–Li alloy stressed in high-humidity air. Environmental fatigue crack growth rate decreases with initial-artificial aging, then increases monotonically with increasing aging time. Overaging does not improve cracking resistance. This effect of microstructure is pronounced at low stress intensity factor range and persists for high stress ratio conditions that minimize crack closure. For both alloys, the highest resistance to fatigue crack growth correlates with the presence of artificial aging intensified solute clusters, and absence of distinct precipitates, as evidenced by electron microscopy and small-angle neutron scattering (SANS). Increased crack growth rates correlate with the dissolution of clusters and/or the formation of an increasing amount of S precipitates for AA2024 and T1 precipitates for the Li-bearing alloy. The fundamental effects of very fine-scale clusters and precipitates on cyclic-slip mode and environment-sensitive crack tip damage are unresolved.  2001 Elsevier Science Ltd. All rights reserved.

Environmental fatigue of an Al-Li-Cu alloy: part I. Intrinsic crack propagation kinetics in hydrogenous environments

Deleterious environmental effects on steady-state, intrinsic fatigue crack propagation (FCP) rates(da/dN) in peak-aged Al-Li-Cu alloy 2090 are established by electrical potential monitoring of short cracks with programmed constant ΔK andKmaxI loading. Such rates are equally unaffected by vacuum, purified helium, and oxygen but are accelerated in order of decreasing effectiveness by aqueous 1 pct NaCl with anodic polarization, pure water’ vapor, moist air, and NaCl with cathodic polarization. Whileda/dN depend on ΔK4.0 for the inert gases, water vapor and chloride induce multiple power laws and a transition growth rate “plateau.” Environmental effects are strongest at low ΔK. Crack tip damage is ascribed to hydrogen embrittlement because of acceleratedda/dN due to parts-per-million (ppm) levels of H2O without condensation, impeded molecular flow model predictions of the measured water vapor pressure dependence ofda/dN as affected by mean crack opening, the lack of an effect of film-forming O2, the likelihood for crack tip hydrogen production in NaCl, and the environmental and ΔK-process zone volume dependencies of the microscopic cracking modes. For NaCl, growth rates decrease with decreasing loading frequency, with the addition of passivating Li2CO3 and upon cathodic polarization. These variables increase crack surface film stability to reduce hydrogen entry efficiency. Small crack effects are not observed for 2090; such cracks do not grow at abnormally high rates in single grains or in NaCl and are not arrested at grain boundaries. The hydrogen environmental FCP resistance of 2090 is similar to other 2000 series alloys and is better than 7075.

Localized deformation and elevated-temperature fracture of submicron-grain aluminum with dispersoids

Abstract Advanced aluminum alloys with thermally stable submicron grains, fine dispersoids, and metastable solute are limited uniquely by reduced ductility and toughness at elevated temperatures. The mechanism is controversial. Experimental results for cryogenically milled powder metallurgy Al extrusion (with 3 vol.% of 20 nm Al 2 O 3 , a 0.5 μm grain size, but no solute) establish that uniaxial tensile ductility, plane strain crack initiation fracture toughness K JICi , and tearing resistance T R decrease monotonically with increasing temperature from 25 to 325 °C. Fracture is by microvoid processes at all temperatures; reduced toughness correlates with changed void shape from spherical to irregular with some faceted walls. Strain-based micromechanical modeling predicts fracture toughness, and shows that temperature-dependent decreases in K JICi and T R are due to reduced yield strength, elastic modulus, and intrinsic fracture resistance. Since CM Al does not contain solute such as Fe, dynamic strain aging is not necessary for low-toughness fracture at elevated temperature. Rather, increased temperature reduces work and strain rate hardening between growing primary voids, leading to intravoid instability and coalescence at lowered strain. Decreased strain rate hardening is attributed to increased mobile dislocation density due to dislocation emission and detrapping from dispersoids in dynamically recovered dislocation-source-free grains.

Environmental fatigue of an Al-Li-Cu alloy: Part II. Microscopic hydrogen cracking processes

Microscopic fatigue crack propagation (FCP) paths in peak-aged unrecrystallized alloy 2090 are identified as functions of intrinsicda/dN- δK kinetics and environment. The FCP rates in longitudinal-transverse (LT)-oriented 2090 are accelerated by hydrogen-producing environments (pure water vapor, moist air, and aqueous NaCl), as defined in Part I. Subgrain boundary cracking (SGC) dominates for δK values where the cyclic plastic zone is sufficient to envelop subgrains. At low δK, when this crack tip process zone is smaller than the subgrain size, environmental FCP progresses on or near 100 or 110 planes, based on etch-pit shape. For inert environments (vacuum and He) and pure O2 with crack surface oxidation, FCP produces large facets along 111 oriented slip bands. This mode does not change with δK, and T1 decorated subgrain boundaries do not affect an expectedda/dN- δK transition for the inert environments. Rather, the complex dependence ofda/dN on δK is controlled by the environmental contribution to process zone microstructure-plastic strain interactions. A hydrogen embrittlement mechanism for FCP in 2090 is supported by similar brittle crack paths for low pressure water vapor and the electrolyte, the SGC and 100/110 crystallographic cracking modes, the influence of cyclic plastic zone volume (δK), and the benignancy of O2. The SGC may be due to hydrogen production and trapping at T1 bearing sub-boundaries after process zone dislocation transport, while crystallographic cracking may be due to lattice decohesion or hydride cracking.

Critical issues in hydrogen assisted cracking of structural alloys

Abstract : Both internal and hydrogen environment assisted cracking continue to seriously limit high performance structural alloys and confound quantitative component prognosis. While intergranular H cracking assisted by impurity segregation can be minimized, other mechanisms promote IG cracking and transgranular H cracking modes have emerged; new alloys suffer serious H cracking similar to old materials. Micromechanical models of crack tip H localization and damage by decohesion predict important trends in threshold and subcritical crack growth rate behaviour. H diffusion appears to limit rates of cracking for monotonic and cyclic loading; however, uncertain%adjustable parameters hinder model effectiveness. It is necessary to better define conditions within 0.1-5 micronmeter of the crack tip, where dislocations and microstructure dominate continuum mechanics, and chemistry is localized. Nano-mechanics modeling and experimental results show very high levels of H accumulated in the crack tip fracture process zone, as necessary for interface decohesion. Contributing mechanisms include high crack tip stresses due to dislocation processes such as strain gradient plasticity, as well as powerful H production and trapping proximate to the electrochemically reacting crack tip surface. New sub- micrometer resolution probes of crack tip damage will better define features such as crack path crystallography (EBSD + Stereology) and surface morphology (high brightness, dual detector SEM), local H concentration (%IDS and NRA), and validate crack tip mechanics modelling (micro-Laue x-ray diffraction and EBSD).

Crack tip modeling of hydrogen environement embrittlement: Application to fracture mechanics life prediction☆

Abstract An integrated fracture mechanics life prediction prodedure has been advanced over the past 20 years to mitigate hydrogen-environment-enhanced subcritical crack propagation in steels. Extensive date define monotonic load threshold stress intensities and corrosion fatigue growth kinetics for ferritic and martensitic steels of a wide variety of strengths and exposed to gaseous and aqueous environments. Broadly predictive crack tip models are evolving based on advances in elastic-plastic fracture mechanics, micromechanics and occluded crack chemistry. Hydrogen environment sensors, in sity crack propagation monitors and prototype testing enable evaluations and modifications of fracture mechanisc predictions. While the approach provides major advances towards reliable material usage in energy systems, uncertainties remain; specific issues are identified for future research.

The Electrode Potential Dependence of Environment-Assisted Cracking of AA 7050

Thick plate, peak aged AA 7050 is susceptible to intergranular environment-assisted cracking in near-neutral 0.5 M Na 2 CrO 4 + 0.05 M NaCl solution. The effect of applied electrode potential (E APP ) on the Stage II crack growth rate (da/dt) is complex. While da/dt increases with increasing applied potential, slow growth at E APP less than -0.5 V SCE transitions after incubation to strongly E APP -dependent fast-rate EAC with hysteresis. A large potential gradient exists near the crack tip region, with the tip potential (∼0.8 V SCE ) independent of E APP , suggesting electrochemically decoupled crack tip and wake regions. Crack tip solution acidification (< pH 3.5) occurs from hydrolysis of a concentrated Al-salt solution formed by dissolution during crack advance. These crack chemistry changes are time dependent and cause the hysteresis in da/dt vs. E APP . Hydrogen uptake measured in the wake increases with increasing E APP due to increased acidification and overpotential for proton reduction. The hydrogen environment embrittlement mechanism for environmental cracking in AA 7050-T651 is supported by the correlation between E APP -dependent crack wake H concentration, local crack chemistry, and da/dt.

Intrinsic fatigue crack growth rates for Al-Li-Cu-Mg alloys in vacuum

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 constantKmax 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/(σysE)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.

Aqueous environmental crack propagation in high-strength beta titanium alloys

The aqueous environment-assisted cracking (EAC) behavior of two peak-aged beta-titanium alloys was characterized with a fracture mechanics method. Beta-21S is susceptible to EAC under rising load in neutral 3.5 pct NaCl at 25 °C and −600 mVSCE, as indicated by a reduced threshold for subcritical crack growth (KTH), an average crack growth rate of up to 10 μms, and intergranular fracture compared to microvoid rupture in air. In contrast, the initiation fracture toughness (KICi) of Ti-15-3 in moist air is lower than that of Beta-21S at similar high σYS (1300 MPa) but is not degraded by chloride, and cracking is by transgranular microvoid formation. The intergranular EAC susceptibility of Beta-21S correlates with both α-colonies precipitated at β grain boundaries and intense slip localization; however, the causal factor is not defined. Data suggest that both features, and EAC, are promoted by prolonged solution treatment at high temperature. In a hydrogen environment embrittlement (HEE) scenario, crack-tip H could be transported by planar slip bands to strongly binding trap sites and stress/strain concentrations at α colony or β grain boundaries. The EAC in Beta-21S is eliminated by cathodic polarization (to −1000 mVSCE), as well as by static loading for times that otherwise produce rising-load EAC. These beneficial effects could relate to reduced H production at the occluded crack tip during cathodic polarization and to increased crack-tip passive film stability or reduced dislocation transport during deformation at slow crack-tip strain rates. High-strength β-titanium alloys are resistant, but not intrinsically immune to chloride EAC, with processing condition possibly governing fracture.

Tensile deformation of 2618 and AlFeSiV aluminum alloys at elevated temperatures

The present study experimentally characterizes the effects of elevated temperature on the uniaxial tensile behavior of ingot metallurgy 2618 Al alloy and the rapidly solidified FVS 0812 P/M alloy by means of two constitutive formulations: the Ramberg/Osgood equation and the Bodner-Partom (1975) incremental formulation for uniaxial tensile loading. The elastoplastic strain-hardening behavior of the ingot metallurgy alloy is equally well represented by either formulation. Both alloys deform similarly under decreasing load after only 1-5 percent uniform tensile strain, a response which is not described by either constitutive relation.