Stress-corrosion-cracking behavior of heat-treated Al−Zn−Mg−Cu alloy with temperature

Stress corrosion cracking is a typical fracture process for metals and alloys. Among aluminum alloys, the Al−Zn−Mg−Cu group is characterized by high performance as far as its mechanical properties are concerned. However, it is susceptible to stress corrosion cracking. Within this large group, the AA-7075-type alloy is most extensively used in manufacturing light structural components, mainly in the aircraft industry. We study the characteristics of the AA-7075 alloy in a 3.5% NaCl aqueous solution at 20 and 80°C. Stress corrosion cracking was observed for all analyzed AA-7075 temper conditions in different tested environments. It was very rapid at 80°C, especially for the T6-aging condition. In all studied cases, the maximum and minimum susceptibility to stress corrosion cracking were exhibited by the AA-7075-T6 temper and T73 temper, respectively. As compared to AA-7075-T6, the AA-7075-RRA temper has better resistance to stress corrosion cracking but its mechanical properties are lower. E.T.S.I. Industriales, Universidad Politécnica de Madrid, Spain. Published in Fizyko-Khimichna Mekhanika Materialiv. Vol. 35, No. 4, pp. 59–62, July–August, 1999.     

The mechanics and mechanisms of fracture in stress corrosion cracking of aluminium alloys

A fracture mechanics approach to stress corrosion cracking is highlighted. The mechanisms of stress corrosion cracking is presented. Experiments on 2024 and 7075 aluminium alloys are carried out to determine their mechanical properties, microstructure and plane strain fracture toughness (K_IC). Stress corrosion cracking tests, namely, cantilever beam tests as well as wedge opening loading tests using sea water as a corrosive medium, are conducted to establish the critical stress intensity factor for stress corrosion cracking (K_ISCC) for each aluminium alloy. It is found that the K_ISCC is in the range of (1/5) to (1/6) of the plane strain fracture toughness, K_IC, depending on the alloy. The scanning electron microscopy of fracture surfaces reveals a great dependence of the cracking and/or pit severity on the applied stress intensity factor. A brief discussion on the dislocation's role in stress corrosion cracking is given.     

Evaluation of stress corrosion resistance and corrosion fatigue fracture behavior of ultra-high-strength P/M Al–Zn–Mg alloy

Quasi-static tensile tests in air and slow strain rate tests (SSRTs) in a 3.5% NaCl solution were conducted in an ultra-high-strength P/M Al–Zn–Mg alloy fabricated through powder metallurgy. Attention is also paid to fatigue strength and fatigue crack growth behavior in laboratory air and in a 3.5% NaCl solution. The alloy has extremely high strength of about 800 MPa. However, elongation at break remains small, at about 1.3%. The final fracture occurs by a macroscopically flat crack normal to the tensile axis, with little reduction in area and little shear lip on the periphery of a smooth sample. However, it fails microscopically in a ductile manner, with dimples. Dimple size is less than 1 μm, because the grain size of the alloy is extremely small. Strengthening mechanisms operating in the alloy are: small grains, sufficient metastable η′ phase in a matrix, and intermetallic compound acting as a fiber reinforcement. The SSRT strength in a 3.5% NaCl solution decreases slightly at a very low strain rate, that is smaller than those observed in aluminum alloys sensitive to stress corrosion. This means that the crack initiation resistance to stress corrosion is superior. However, under cyclic loading, the corrosion fatigue strength becomes lower than that conducted in air, because pitting corrosion on a sample surface acts as a stress concentrator. Crack initiation site of quasi-static and fatigue failure of the alloy is at inclusions, and hence, it is essential to decrease inclusions in the alloy for the improvement of the mechanical properties. Fatigue crack resistance of the alloy is inferior to conventional Al–Zn–Mg alloys fabricated by ingot metallurgy, because the fatigue fracture toughness, or ductility, of the alloy is inferior to other Al alloys, and intergranular cracking promotes crack growth. However, no influence of 3.5% NaCl solution on corrosion fatigue crack growth is observed, although an investigation is required into whether stress corrosion crack growth occurs or not, and at the same time, and of corrosion fatigue crack growth behavior at lower stress intensity. The fracture surface and crack initiation sites are closely examined using a high-resolution field emission type scanning electron microscope, and the fracture mechanisms of the alloy are discussed.     

7075铝合金的力学与电化学交互作用

用慢应变速率拉伸(SSRT)技术研究了7075铝合金在应力腐蚀过程中的力学与电化学交互作用.结果表明,外加极化会提高7075铝合金的应力腐蚀敏感性,这种通过极化而改变铝合金表面电化学反应从而影响断裂应力的现象是一种电化学-力学效应.然而,对于不同热处理状态的7075铝合金,外加极化对敏感性的影响程度不同.增加拉伸应力,7075-RRA铝合金的阳极极化曲线略向正移,滞后环面积扩大,但并不显著.这种拉伸应力对极化曲线的影响是一种力学-电化学效应,有利于应力腐蚀裂纹的扩展.铝合金在应力腐蚀过程中的电化学作用和力学作用是交互的,彼此促进的.     

Experimental approach to dimple fracture mechanisms under short pulse loading

Dimple fracture mechanisms are discussed for three kinds of aluminum alloys on the basis of an experimental approach and a finite element (FEM) analysis. The void growth and coalescence process was observed by an optical microscope and a scanning electron microscope. The fractographic observation for aluminum alloys 7075-T651 and 6061-T651 showed that several large voids called a dominant void are nucleated at inclusion sites or the second-phase particles ahead of the crack tip and followed by fine voids initiation leading coalescence of the dominant voids with the crack tip. On the other hand, in aluminum alloy 2017-T3, voids are nucleated very close to the crack tip and directly coalesce with the crack tip. FEM computation results suggested that the void nucleation and growth process is closely related to the triaxial stress state ahead of the crack tip.     

基于灰色理论的高强铝合金应力腐蚀开裂预测模型的建立与应用

采用3.5%NaC1溶液中预制裂纹的方法测试了2124高强铝合金的应力腐蚀裂纹扩展长度随时间的变化,获得了裂纹扩展速率随腐蚀时间的变换规律及应力腐蚀断裂韧性界限值,并对断口进行分析.根据裂纹扩展的基本规律,运用灰色理论GM(1,1)模型,依据2124铝合金应力腐蚀开裂裂纹扩展长度的原始数据进行了灰色预测,并对预测结果进...     

The interaction between pitting corrosion,grain boundaries,and constituent particles during corrosion fatigue of 7075-T6 aluminum alloy

Concomitant corrosion fatigue research was performed on 7075-T6 aluminum alloy to gain an increased understanding of how microstructure influences pit growth, pit-to-crack transition, and critical crack propagation to fracture. Two thicknesses of rolled sheet and an extrusion of 7075-T6 aluminum alloy were etched and subjected to concomitant corrosion fatigue in a 3.5% sodium chloride solution. Testing was interrupted at various intervals to obtain information on pit generation, growth, and potential cracking. Results indicated that microstructure has a significant influence on pit-to-crack transition and fatigue crack propagation. Constituent particles competed with corrosion pits as critical crack nucleation sites, with some affecting the critical crack by either nucleation of additional cracking or linkage with the main crack. Post-fracture analysis confirmed the presence of noncritical cracks within the corroded region, related to pitting and constituent particles.     

A stochastic model of fatigue crack propagation under variable-amplitude loading

This paper presents a stochastic model of fatigue crack propagation in ductile alloys that are commonly encountered in mechanical structures and machine components of complex systems (e.g. aircraft, spacecraft, ships and submarines, and power plants). The stochastic model is built upon a deterministic state-space model of fatigue crack propagation under variable-amplitude loading. The (non-stationary) statistic of the crack growth process for center-cracked specimens is obtained as a closed form solution of the stochastic differential equations. Model predictions are in agreement with experimental data for specimens fabricated from 2024-T3 and 7075-T6 aluminum alloys and Ti-6Al-4 V alloy subjected to constant-amplitude and variable-amplitude loading, respectively. The stochastic model of crack propagation can be executed in real time on an inexpensive platform such as a Pentium processor.     

Zusammenwirken von Spannungsrißkorrosion und Ermüdung bei der Schwingungsrißkorrosion eines hochfesten Stahles

Conjoint Action of Stress Corrosion Cracking and Fatigue on Corrosion Fatigue of a High Strength Steel The corrosion fatigue characteristics of a high strength, martensitic steel in 0.5 n NaCl solution is investigated with regard to the fatigue and stress corrosion cracking behaviour of the material. Test parameters are stress ratio and frequency, testing is carried out with fracture mechanics methods, the crack surfaces are examined fractographically. An analysis of the results reveals that corrosion fatigue in high strength steel is caused by fatigue or by stress corrosion cracking, depending on the kinetics of the two processes. Fatigue and stress corrosion cracking do not act cumulative or additive. Instead, the kinetically faster process causes crack advance. The crack growth characteristics are interpreted with respect to the fractographic appearance of the crack surfaces. Corrosion fatigue cracks propagate either intergranular relative to the prior austenite grain boundaries as stress corrosion cracks do or transgranular like fatigue cracks, depending on the crack growth rates of the two processes. Fatigue and stress corrosion cracking do not interact, at least in a measurable degree, because of the different crack path of the two fracture processes. Results can be assessed quantitatively with the “process competition model”.     

Prediction of the crack extension under fretting wear loading conditions

Fretting is associated with small amplitude oscillatory movements between two surfaces in contact. One possible consequence of fretting is the formation and subsequent growth of cracks at the edges of the contact. This paper presents an experimental investigation of the cracking behaviour under fretting loading of two different aluminium alloys: 2024-T351 and 7075-T651. Systematic and controlled experiments with a cylinder-flat contact under partial slip fretting conditions were carried out. A model which combines both crack nucleation and propagation processes is used to predict the crack extension throughout the life of the component. The direction of crack propagation experimentally observed was taken into account by the model. Furthermore, an analytical prediction of crack nucleation based on the process volume approach is made. The predictions of both crack extension and nucleation are compared with the experimental results, and show good agreement.     

Dynamic Tensile Fracture Behaviours of Selected Aluminum Alloys under Various Loading Conditions

Experiments investigating dynamic tensile fracture were performed on the extruded rods of 2024‐T4 and 7075‐T6 aluminum alloys under varying loading conditions. The initial yield stress and fracture strain of 7075‐T6 alloy obtained in spilt Hopkinson tension bar tests are higher than that of 2024‐T4 alloy. But the initiation fracture toughness and spall strength of 2024‐T4 alloy are higher than those of 7075‐T6 alloy in three‐point bending and plate impact experiments, which indicates that 2024‐T4 alloy has better crack initiation tolerance and stronger spall failure resistance. Based on metallurgical investigations by using optical and scanning electron microscopes, it is revealed that the microstructure has a profound effect on the dynamic tensile fracture mechanism of each aluminum alloy. The 2024‐T4 alloy is relatively brittle due to voids or cracks nucleated at many coherent CuMgAl₂ precipitate phases in the grain interiors, and the fracture mode is predominantly transgranular. The 7075‐T6 alloy exhibits relatively ductile fracture because voids or cracks growth is partly intergranular along the grain boundaries and partly transgranular by void formation around coarse intermetallic particles. The obvious differences of damage distribution and void coalescence mechanisms for 2024‐T4 and 7075‐T6 alloys under plate impact are also discussed.     

Determining cyclic corrosion cracking resistance for titanium alloys with allowance for electrochemical conditions at the fatigue corrosion crack tip

Published data are examined on how various factors affect fatigue crack growth rates. Basic diagrams have been constructed for the cyclic cracking resistance in Ti-6AI-4V and Ti-6AI-6V-2Sn alloys in air, distilled water, and 3.5% NaCl for use in working-life calculations. Appropriate heat treatment can produce two microstructures in a titanium alloy, one of which has the largest cyclic cracking resistance, while in the second, the cracks grow at the lowest rate. The cyclic corrosion cracking resistance for a titanium alloy should be determined in relation to the state of stress and strain and to the electrochemical conditions at the corrosion fatigue crack tip, while the variations in fatigue crack growth rate for a given stress intensity factor in a corrosive medium are due to differing electrochemical conditions at the crack tip during the testing on different specimens. Basic diagrams can be derived for titanium alloys by using a physically sound methodology developed previously for steels, which is based on invariant diagrams for cyclic cracking resistance in air and in the corresponding medium, which can be constructed in relation to extremal working and electrochemical conditions at corrosion-fatigue crack tips.Translated from Problemy Prochnosti, No. 12, pp. 3–11, December, 1993.     

ENVIRONMENTAL EFFECTS ON FATIGUE FRACTURE MODE TRANSITIONS OBSERVED IN ALUMINIUM ALLOYS

Abstract— Fatigue crack propagation tests were carried out in different environments on 7075–T6 and 2024–T3 centre-cracked sheet specimens. Observations were made on the macroscopic transition from tensile mode to shear mode. The transition is suppressed by an aggressive environment, whereas it is promoted by an inert environment. As a consequence there is no unique correlation between the state of stress and the mode of cracking. Both the state of stress and the environment have a significant effect on the mode of cracking. A simple model for the effect of environment on fatigue crack growth is presented. The implications for crack growth under corrosion fatigue conditions are discussed.     

The mechanism of hydrogen embrittlement: the stress interaction between a crack, a hydrogen cluster, and moving dislocations

Mechanisms of dissolvent anodic chemical reaction and hydrogen embrittlement have been proposed as stress corrosion cracking (SCC) mechanisms. The former is feasible for the case of plastic deformation dominant metals (low-yield stress), and the latter is for high-strength metals such as high-strength steels. However, in spite of low-yield stress, a discontinuous cleavage-like fracture is sometimes observed during SCC for ductile fcc alloys, which concerns the interaction between dislocations and the hydrogen cluster. The problem of when these mechanisms will be dominant remains. In this paper, the stress corrosion cracking model on the basis of hydrogen diffusion and concentration toward the elastic-plastic stress field around a crack and the interaction of dislocations and hydrogen around a crack tip are proposed to clarify the mechanism of stress corrosion cracking for ductile and brittle materials. We conducted numerical analyses using these proposed models.     

Precipitation behaviour of 7000 alloys during retrogression and reaging treatment

Abstract

Retrogression and reaging produces coarsening of grain boundary precipitates and thereby improves resistance to stress corrosion cracking. At the same time it causes pronounced heterogeneous precipitation on dispersoids of E (Al₁₈Cr₂Mg₃) phase inside the grain of 7075 alloy. Such heterogeneous precipitation does not occur on the coherent dispersoids of Al₃Zr phase in 7050 alloy and its absence leads to higher strength compared with 7075 alloy. Supplementary examination of laboratory alloys 7075-Zr and 7075-Cr differing only in transition metal content supports the above result. This effect is probably the reason why retrogression and reaging (T77 heat treatment) is recommended for alloys containing zirconium but not for those containing chromium.

MST/1898     

The mechanism of hydrogen embrittlement: the stress interaction between a crack, a hydrogen cluster, and moving dislocations

Mechanisms of dissolvent anodic chemical reaction and hydrogen embrittlement have been proposed as stress corrosion cracking (SCC) mechanisms. The former is feasible for the case of plastic deformation dominant metals (low-yield stress), and the latter is for high-strength metals such as high-strength steels. However, in spite of low-yield stress, a discontinuous cleavage-like fracture is sometimes observed during SCC for ductile fcc alloys, which concerns the interaction between dislocations and the hydrogen cluster. The problem of when these mechanisms will be dominant remains. In this paper, the stress corrosion cracking model on the basis of hydrogen diffusion and concentration toward the elastic-plastic stress field around a crack and the interaction of dislocations and hydrogen around a crack tip are proposed to clarify the mechanism of stress corrosion cracking for ductile and brittle materials. We conducted numerical analyses using these proposed models.     

On fracture mechanics under complex stress

The majority of linear elastic fracture mechanics investigations since the pioneering work of Irwin and Paris have been carried out under tension-tension loading conditions in sheet metal. However, built up structures have generally been under complex stress conditions and to date very scanty information is available on fracture mechanics parameters under complex stress conditions. The current stale of the art for mixed mode crack tips deformation is reviewed.In order to use linear elastic fracture mechanics methodology to predict crack growth rate in shear webs, an experimental program was initiated. Initial tests on 7075-T6 Aluminum alloy sheet, using a picture frame type specimen, were conducted. The critical stress intensity factors and the rate of crack growth under aforementioned condition are established.     

Engineering property comparisons of 7050-T73651, 7010-T7651 and 7010-T73651 aluminium alloy plate

A comparison of some engineering properties of 7050-T73651, 7010-T7651 and 7010-T73651 plate has been made. The properties investigated were strength, stress corrosion resistance, fracture toughness and fatigue crack propagation resistance under flight simulation loading.

It was found that both 7050 and 7010 are high strength deep hardenable alloys with only minor differences in crack tolerance properties. The fracture toughness of both alloys is equivalent, while 7050 possesses slightly better resistances to stress corrosion cracking and fatigue crack propagation under flight simulation loading.     

Mechanisms of Corosion and Oxidation of Metals and Alloys

Selected topics in field of the study of the mechanisms of corrosion and of oxidation of metals or alloys are presented. The first part reports a new model for the mechanism of the breakaway oxidation of ferritic stainless steels in water vapour. The second part is devoted to the physico‐chemical aspects of oxidation and presents experimental methods useful in the kinetic modelling applied to two alloys, the zircalloy‐4 and an AlMg5 % in the liquid state. In the third part the physical and numerical modelling of the stress corrosion cracking behaviour in face‐centered cubic (fcc) alloys is detailed, which enables the study of the influence of macroscopic parameters (such as the temperature or hydrogen activity) on the fracture process.     

ENVIRONMENTAL EFFECTS ON CRACK PROPAGATION IN ALUMINIUM ALLOYS

Abstract Crack propagation rates have been measured in two aluminium alloys under cyclic and static loading, in air, and in salt solution. On the basis of these results, a model is proposed, whereby corrosion fatigue crack propagation may be interpreted in terms of fatigue and static stress corrosion characteristics. Two interacting processes are operative; one is "stress assisted dissolution", which tends to inhibit mechanical failure by crack blunting and microbranching. The other is "environment assisted fracture" which occurs too rapidly for dissolution to occur. One or other of these processes is always observed to be dominant. This proposal is discussed in relation to other recent models for corrosion fatigue cracking. The effects of frequency, waveform and mean stress variations are also considered.