Environmentally Induced Crack Propagation Behavior of 2219 Aluminum: Microstructural Effects

Abstract

Alloy 2219 has been evaluated under corrosion fatigue conditions. The effect of the microstructures present in the T851 and T6 conditions on crack propagation rates has been determined. Tests were performed on compact tension specimens in air and in NaCl solutions at 23 and 70°C. The corrosion fatigue behavior of the material under these conditions was evaluated by studying the crack propagation kinetics and also crack fractography.

The results of this study show that 2219 aluminum is resistant to corrosion fatigue in aqueous solutions containing up to 10% NaCl. However, the material is more resistant in the T6 than in the T851 condition. In the absence of stress corrosion cracking, the initial stages of the crack growth were characterised by ductile fracture while a mixed fracture mode dominated the final stages of the growth. At low loading frequencies (0.1 Hz), crack branching and blunting were observed when tests were performed at 70°C. © 1997 Canadian Institute of Mining and Metallurgy. Published by Elsevier Science Ltd

Résumé On a évalué l'alliage 2219 sous des conditions de fatigue due à la corrosion. On a déterminé l'effet des microstructures présentes dans la condition T851 et la condition T6 sur le taux de propagation de crique. On a conduit des épreuves sur des échantillons de traction compacts, à l'air et dans des solutions de NaCl à 23 et à 70°C. On a évalué le comportement de fatigue due à la corrosion du matériau sous ces conditions en étudiant la cinétique de propagation de crique ainsi que la fractographie de criques.

Les résultats de cette étude montrent que l'aluminium 2219 est résistant à la fatigue due à la corrosion, en solutions aqueuses contenant jusqu'à 10% de NaCl. Cependant, le matériau est plus résistant sous la condition T6 que sous la condition T851. En absence de fissuration par corrosion sous tension, les étapes initiales de la croissance. À basses fréquence de charge (0.1 Hz), on a observé une ramification et un émoussement des criques, lorsque les épreuves avaient lieu à 70°C.     

Growth rate models for short surface cracks in AI 2219-T851

Rates of fatigue propagation of short Mode I surface cracks in Al 2219-T851 are measured as a function of crack length and of the location of the surface crack tips relative to the grain boundaries. The measured rates are then compared to values predicted from crack growth models. The crack growth rate is modeled with an underlying assumption that slip responsible for early propagation does not extend in significant amounts beyond the next grain boundary in the direction of crack propagation. Two models that contain this assumption are combined: 1) cessation of propagation into a new grain until a mature plastic zone is developed; 2) retardation of propagation by crack closure stress, with closure stress calculated from the location of a crack tip relative to the grain boundary. The transition from short to long crack growth behavior is also discussed.     

The effect of heat treatments on the corrosion fatigue properties of 13 Pct Chromium Stainless Steel in 3 Pct NaCI Aqueous Solution

The effect of austenitizing or tempering temperature on the corrosion fatigue properties of 13 pct chromium stainless steel was studied. Three pct NaCI aqueous solution was used as the corrosive environment, and the results were compared with the atmospheric fatigue properties. Strong influence of the tempering temperature on the S-N and FCP behavior of this blading material was found. The damage ratios (corrosion fatigue limit divided by endurance limit) of these various heat treated specimens became very low by this environment. Especially, extremely low corrosion fatigue strength of the specimen tempered at 600 °C was noticed. This microstructure was strategically used to clarify the reduction of pH inside the corrosion pits which were generally formed at the fatigue crack initiation sites. FCP data in the corrosive environment showed higher resistance than the atmospheric ones at the stress intensities below 18 MPa · m^1/2, and which is opposite to the generally known influence of the corrosive environments. As for the fractographic feature, an appearance of the intergranular facets was especially noticed in NaCI aqueous solution environment. The fraction of this intergranular cracking was obtained as a function of the stress intensity factor.     

A study of creep crack growth in 2219-T851

Creep crack growth rates were measured in high strength 2219-T851 aluminum alloy with a computerized fully automated test procedure. Crack growth tests were performed on CT specimens with side grooves. The experimental set-up is described. During a test, the specimen is cyclically loaded on a servohydraulic testing machine under computer control, maintained at maximum load for a given hold time at each cycle, unloaded, and then reloaded. Crack lengths are obtained from compliance measurements recorded during each unloading. It is shown that the measured crack growth rates per cycle do represent creep crack growth rates per unit time for hold times longer than 10 seconds. The validity of LEFM concepts for side-grooved specimens is reviewed, and compliance and stress intensity factor calibrations for such specimens are reported. For the range of testing conditions of this study, 2219-T851 is shown to be creep brittle in terms of concepts of fracture mechanics of creeping solids. It is found that, under these testing conditions, a correlation exists between the creep crack growth rates under plane strain conditions and the stress intensity factor (da/dt =A K ^3.8 at 175 °C) for simpleK histories in a regime of steady or quasi-steady state crack growth. The micromechanisms of fracture are determined to be of complex nature. The fracture mode is observed to be mixed inter- and transgranular, the relative amount of intergranular fracture decreasing asK andda/dt increase. Formerly Graduate Student, Massachusetts Institute of Technology, is Ingenieur de l’Armement, ETCA, 94114 Arcueil Cedex, France.     

Crack closure load development for surface microcracks in Al 2219-T851

The development of crack closure load with increasing crack length for noncrystallographic transgranular surface microcracks produced by cyclic fatigue of Al 2219-T851 is studied for two environmental relative humidities (5 and 30 pct). Closure loads are found to be initially low for short cracks and increase with subsequent crack propagation. The increase in closure with crack length is faster if the humidity is low or if the initiation crack size is large, as determined by the size of the surface intermetallic particle initiation site. At 30 pct humidity it is possible to associate the closure load increase observed to a decreasing crack planarity with increasing crack length.     

Fatigue crack growth in an as-rolled dual-phase steel

Fatigue crack propagation behaviour of an as-rolled dual-phase steel is investigated over a wide range from 10^?10 to 10^?6 m/cycle in a 3.5% NaCI solution and laboratory air under different load ratios. It was found that the as-rolled dual-phase steel studied in the present investigation shows good resistance to fatigue crack growth in laboratory air. The threshold values decrease with increasing stress ratios, which is consistent with the competition model proposed previously. The threshold value obtained using a 3.5% NaCI solution is higher than that in the laboratory air owing to the wedge effect of corrosion products within the crack. Fatigue crack growth rates in the higher ΔK range obey the Paris formula in each case within this study.     

Fatigue crack propagation in rapidly solidified aluminum alloys at 25 °C and 300 °C

The fatigue crack propagation performance of two rapidly solidified aluminum alloys was investigated in air at 25°C and 300°C. The results show that the crack propagation rates for continuous cycling tests of Al-8Fe-4Ce and Al-4.7-Fe-4.7Ni-0.2Cr alloys were similar at 25°C. Although the crack propagation rates of both alloys were increased at 300°C, the Al-Fe-Ce alloy exhibited the greater resistance to crack propagation. The inclusion of a tensile hold time in the fatigue loading cycle at 300°C produced an increase in the crack propagation rates for both alloys over the rates for continuous cycling. The fatigue crack propagation performance of the rapidly solidified alloys was not found to be superior when compared with the fatigue crack propagation performance of a wrought aluminum alloy tested under the same conditions. Transmission and scanning electron microscopy study of the tested specimens revealed that the crack propagation mode was primarily transgranular, with the metastable dispersoid particles providing impenetrable barriers to dislocation motion.     

The effects of microstructure on elevated temperature crack growth in nickel-base alloys

The effects of microstructure on the fatigue and creep crack growth of Waspaloy and P/M Astroloy were evaluated at 650°C. In Waspaloy, changes in γ′ size and distribution did not markedly affect fatigue crack growth. An increase in fatigue crack growth rate occurred at low test frequencies and was associated with a transition to intergranular crack propagation. In P/M Astroloy, a coarser grain size lowered the fatigue crack growth rate. Serrated grain boundaries, though beneficial under creep loading, have no effect in fatigue.     

The noncontinuum crack tip deformation behavior of surface microcracks

The crack tip opening displacement (CTOD) of small surface fatigue cracks (lengths of the grain size) in Al 2219-T851 depends upon the location of a crack relative to the grain boundaries. Both CTOD and crack tip closure stress are greatest when the crack tip is a large distance from the next grain boundary in the direction of crack propagation. Contrary to behavioral trends predicted by continuum fracture mechanics, crack length has no detectable effect on the contribution of plastic deformation to CTOD. It is apparent from these observations that the region of significant plastic deformation is confined by the grain boundaries, resulting in a plastic zone size that is insensitive to crack length and to external load.     

The effect of tempering temperature on near- threshold fatigue crack behavior in quenched and tempered 4140 steel

Fatigue crack growth in compact tension samples of high purity 4140 steel quenched and tempered to various strength levels was investigated. Tempering temperatures of 200, 400, 550, and 700 °C produced yield strengths from 1600 to 875 MPa, respectively. Crack propagation and crack closure were monitored inK-decreasing tests performed underR = 0.05 loading conditions in laboratory air. Results indicated that as the yield strength increased the crack growth rate increased at a given ΔK and ΔK_th decreased. Threshold values varied from 2.8 MPa m^1/2 (200 °C temper) to 9.5 MPa m_1/2 (700 °C temper). Cracks in the 200 °C tempered samples grew by an intergranular mechanism following prior austenite grain boundaries probably caused by hydrogen embrittlement or tempered martensite embrittlement. Tempering above 200 °C produced transgranular fatigue crack growth. The level of crack closure increased with tempering temperature and with crack propagation in a given tempered condition. Crack closure was caused by a combination of plasticity-induced and oxide-induced mechanisms. The use of an effective stress intensity range based on crack closure consolidated the fatigue crack growth curves and the threshold values for all tempering temperatures except 200 °C. Formerly Graduate Research Assistant, Department of Materials Science and Engineering, Stanford University, Stanford, CA. Formerly Professor, Department of Materials Science and Engineering, Stanford University, Stanford, CA.     

Fracture mechanics and surface chemistry studies of fatigue crack growth in an aluminum alloy

Fracture mechanics and surface chemistry studies were carried out to develop further understanding of the influence of water vapor on fatigue crack growth in aluminum alloys. The room temperature fatigue crack growth response was determined for 2219-T851 aluminum alloy exposed to water vapor at pressures from 1 to 30 Pa over a range of stress intensity factors (K). Data were also obtained in vacuum (at < 0.50 μPa), and dehumidified argon. The test results showed that, at a frequency of 5 Hz, the rate of crack growth is essentially unaffected by water vapor until a threshold pressure is reached. Above this threshold, the rates increased, reaching a maximum within one order of magnitude increase in vapor pressure. This maximum crack growth rate is equal to that obtained in air (40 to 60 pct relative humidity), distilled water and 3.5 pct NaCl solution on the same material. Parallel studies of the reactions of water vapor with fresh alloy surfaces (produced either byin situ impact fracture or by ion etching) were made by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The extent of surface reaction was monitored by changes in the oxygen AES and XPS signals. Correlation between the fatigue crack growth response and the surface reaction kinetics has been made, and is consistent with a transport-limited model for crack growth. The results also suggest that enhancement of fatigue crack growth by water vapor in the aluminum alloys occurs through a “hydrogen embrittle ment” mechanism.     

Mechanisms of corrosion fatigue crack propagation in Al-Zn-Mg alloys

Corrosion fatigue crack propagation tests were performed on commercial 7075 alloys. Testing was done in a 3.5 pct sodium chloride solution under constant impressed potential and under reversed anodic-cathodic current conditions. Results indicated that a cathodic potential of -1.400 V vs SCE was sufficient to reduce corrosion fatigue crack growth rates to the level observed in dry argon. By alternately impressing anodic and cathodic currents, it was shown that anodic potentials enhance the crystallographic dependence of the fracture mode, resulting in brittle striations, while cathodic potentials result in ductile striations formed by shear. Modification of the alloy chemistry and lower impurity content resulted in a two-fold reduction in crack growth rates. Thermomechanical treatment of these alloys to refine the grain size proved detrimental. Adding an inhibitor to the sodium chloride solution was found to be the most effective means for reducing corrosion fatigue crack growth rates. A model for the environment-surface interaction is suggested.     

Improvement of the resistance to stress corrosion cracking in austenitic stainless steels by cyclic prestraining

Austenitic stainless steels are known to be sensitive to stress corrosion cracking (SCC) in hot chloride solutions. The aim of the present study is to find improvements in the SCC behavior of 316L-type austenitic stainless steels in 117°C MgCl₂ solutions. Previously, the authors have proposed the “corrosion-enhanced plasticity model” (CEPM) to describe the discontinuous cracking process which occurs in SCC. This model is based on localized corrosion (anodic dissolution, and hydrogen absorption)-deformation (dislocations) interactions (CDI). From the framework of this model, it is proposed that a prestraining in fatigue at saturation decreases the SCC sensitivity. This idea is experimentally confirmed for both crack initiation and crack propagation, through the analysis of the SCC behavior by slow-strain-rate tests of single and polycrystals after different prestraining conditions.     

The Small Fatigue Crack Growth Behavior of an AM60 Magnesium Alloy

The effects of thermomechanical processing and subsequent heat treatment on the small fatigue crack growth (FCG) behavior of an AM60 (Mg-6.29Al-0.28Mn wt pct) alloy were evaluated. The effects of mechanical loading parameters, such as maximum stress and load-ratio, on the small FCG behavior were also determined. Maximum stress did not appear to affect the crack propagation rate of small cracks in the stress and crack size ranges considered. Materials with different microstructures and yield stresses, introduced by different processing conditions, showed similar crack growth rates at equivalent stress intensity factor ranges. The effect of load ratio on small crack growth rates was recorded. Fracture surface characterization suggested that the fatigue crack propagation mechanism was a mixture of transgranular and intergranular cracking. Porosity and other material defects played respective important roles in determining the fatigue crack initiation and propagation behavior.     

Frequency dependent low cycle fatigue crack propagation

The frequency modified Coffin-Manson low cycle fatigue expression phenomenologically describes the influence of the cycling frequency on the fatigue life. This expression relates only to the fatigue life and as such does not enable the separation of the frequency influence on crack nucleation from that on propagation. The approach taken here was to study directly the propagation phase of low cycle fatigue and to this end a frequency modified crack growth expression is presented. The experiments reported here were performed on A286 (an iron-base superalloy) cycled at 1100°F with plastic strain limits. The influence of the cycling frequency is described in terms of two frequency regimes. At the lowest frequencies (below 0.05 cpm) varying the frequency did not change the time to failure. The crack growth rate is thus more a result of stress rupture than fatigue. At higher frequencies both time and cycles determine the crack propagation behavior.     

Stress Corrosion Cracking in Al-Zn-Mg-Cu Aluminum Alloys in Saline Environments

Stress corrosion cracking of Al-Zn-Mg-Cu (AA7xxx) aluminum alloys exposed to saline environments at temperatures ranging from 293 K to 353 K (20 °C to 80 °C) has been reviewed with particular attention to the influences of alloy composition and temper, and bulk and local environmental conditions. Stress corrosion crack (SCC) growth rates at room temperature for peak- and over-aged tempers in saline environments are minimized for Al-Zn-Mg-Cu alloys containing less than ~8 wt pct Zn when Zn/Mg ratios are ranging from 2 to 3, excess magnesium levels are less than 1 wt pct, and copper content is either less than ~0.2 wt pct or ranging from 1.3 to 2 wt pct. A minimum chloride ion concentration of ~0.01 M is required for crack growth rates to exceed those in distilled water, which insures that the local solution pH in crack-tip regions can be maintained at less than 4. Crack growth rates in saline solution without other additions gradually increase with bulk chloride ion concentrations up to around 0.6 M NaCl, whereas in solutions with sufficiently low dichromate (or chromate), inhibitor additions are insensitive to the bulk chloride concentration and are typically at least double those observed without the additions. DCB specimens, fatigue pre-cracked in air before immersion in a saline environment, show an initial period with no detectible crack growth, followed by crack growth at the distilled water rate, and then transition to a higher crack growth rate typical of region 2 crack growth in the saline environment. Time spent in each stage depends on the type of pre-crack (“pop-in” vs fatigue), applied stress intensity factor, alloy chemistry, bulk environment, and, if applied, the external polarization. Apparent activation energies (E _a) for SCC growth in Al-Zn-Mg-Cu alloys exposed to 0.6 M NaCl over the temperatures ranging from 293 K to 353 K (20 °C to 80 °C) for under-, peak-, and over-aged low-copper-containing alloys (<0.2 wt pct) are typically ranging from 80 to 85 kJ/mol, whereas for high-copper-containing alloys (>~0.8 wt pct), they are typically ranging from 20 to 40 kJ/mol for under- and peak-aged alloys, and based on limited data, around 85 kJ/mol for over-aged tempers. This means that crack propagation in saline environments is most likely to occur by a hydrogen-related process for low-copper-containing Al-Zn-Mg-Cu alloys in under-, peak- and over-aged tempers, and for high-copper alloys in under- and peak-aged tempers. For over-aged high-copper-containing alloys, cracking is most probably under anodic dissolution control. Future stress corrosion studies should focus on understanding the factors that control crack initiation, and insuring that the next generation of higher performance Al-Zn-Mg-Cu alloys has similar longer crack initiation times and crack propagation rates to those of the incumbent alloys in an over-aged condition where crack rates are less than 1 mm/month at a high stress intensity factor.     

Effect of Low Temperature on Fatigue Crack Formation and Microstructure-Scale Growth from Corrosion Damage in Al-Zn-Mg-Cu

The strong effect of cold temperature on the fatigue resistance of 7075-T651 is established. As temperature decreases from 296 K to 183 K (23 °C to ?90 °C), the formation life for cracking about pit and EXCO corrosion perimeters increases, microstructure scale crack growth rates decrease in the range from 20 to 500 μm beyond the corrosion topography, and long crack growth rates similarly decline. Fatigue crack surface features correlate with reduced hydrogen embrittlement with decreasing temperature fed by localized H produced during precorrosion for pit and EXCO-proximate cracks, as well as by crack tip H produced by water vapor reaction during stressing for all crack sizes. The importance of the former H source increases with decreasing temperature for cracks sized below 200 μm. Decreasing temperature to 223 K (?50 °C) eliminates the contribution of environmental H through interaction of reduced water vapor pressure in equilibrium with ice and reduced H diffusion. The Knudsen flow model and exposure parameter, $ P_{{{\text{H}}_{2} {\text{O}}}}/f $ , enables improved modeling of temperature dependent crack propagation, but does not fully describe low temperature fatigue behavior due to possible rate limitation by H diffusion. Further decreases in MSC da/dN to 183 K (?90 °C) are related to reduced mobility of the corrosion-precharged H which may associate with vacancies from dissolution. Crack formation, and growth rates correlate with either elastic stress intensity range or cyclic crack tip opening displacement, and are available to predict corrosion effects on airframe fatigue for the important low temperature regime.     

The fatigue and fracture resistance of a Nb-Cr-Ti-Al alloy

The microstructure, fatigue, and fracture behaviors of a cast and heat-treated Nb-Cr-Ti-Al alloy were investigated. The microstructure of the cast alloy was manipulated by annealing at a temperature ranging from 500 °C to 1500 °C for 1 to 24 hours. The heat treatment produced Cr₂Nb precipitates along grain boundaries in all cases except in the 500 °C heat-treated material. Fracture toughness tests indicated low fracture resistance in both the as-cast and heat-treated materials. Fatigue crack growth tests performed on the 500 °C heat-treated material also indicated a low fatigue crack growth resistance. Direct observations of the near-tip region revealed a cleavage-dominated fracture process, in accordance with fractographic evidence. The fracture behavior of the Nb-Cr-Ti-Al alloy was compared to that of other Nb-Cr-Ti alloys. In addition, theoretical calculations of both the unstable stacking energy (USE) and Peierls-Nabarro (P-N) barrier energy are used to elucidate the role of Al additions in cleavage fracture of the Nb-Cr-Ti-Al alloy. The results indicate that an Al alloying addition increases the USE, which, in turn, prevents the emission of dislocations, promotes the nucleation and propagation of cleavage cracks from the crack tip, and leads to a reduction in the fracture toughness.     

A study of the mechanism of stress-corrosion crack propagation in AISI 4340 steel in aqueous 3.5 wt.% NaCl solution

Stress-corrosion (SC) crack propagation in AISI 4340 steel has been studied with 2 mm thick single edge-notched (SEN) specimens under constant load conditions as a function of applied potential and tempering conditions in an aqueous 3.5 wt.% NaCI solution at 30°C. The SC crack lengths were estimated by using the electrical potential method. As the amount of cathodic polarization increased, the SC crack propagation rate increased. Anodic polarization yielded opposite results. These polarization effects on the SC crack propagation are discussed in terms of absorbed hydrogen resulting from a cathodic reaction on the specimen surface. SC cracks propagated by intergranular fracture through most of the inner region, but shear lips were formed at the near subsurface, irrespective of applied potential and tempering temperature. This is explained in terms of the stress state dependency of hydrogen behaviour. Above experimental evidence well supports the theory that SC crack propagation is controlled by the hydrogen embrittlement (HE) process. The SC crack propagation rate decreased in the sequence of 300, 200, and 400°C-tempered specimens. This is discussed as being related to the microstructural and yield stress effects.     

Crack closure load measurements for microcracks developed during the fatigue of Al 2219-T851

A compliance technique is utilized to determine the closure load for surface microcracks of one grain size and smaller produced by fully reversed loading fatigue of Al 2219-T851. Specimens are fatigued in flexure in air at 5, 18, and 45 pct relative humidities. Scanning electron microscopy is then utilized to measure crack compliance for selected microcracks and crack closure load is inferred from the break point in the linear relationship between crack opening and applied stress. For zero applied load the microcracks are found to be partially opened and a linear relationship is found between the closure load measured for the microcracks and the zero load crack opening normalized to crack length. This relationship holds regardless of the ambient humidity during fatigue, although there are significant changes in the zero load crack openings developed with humidity. An empirical relationship between the irregularity of the microcrack propagation path as affected by humidity and crack opening at zero load is also identified, which can be used to estimate crack closure load from crack dimensional parameters measured at the specimen surface.