Atmospheric stress corrosion cracking of a superplastic 7475 aluminum alloy

The influence of different heat treatments upon the atmospheric stress corrosion cracking (SCC) of fine-grained 7475 Al-alloy plates has been investigated. The small size of the matrix precipitates and grain-boundary precipitates (GBPs) was found to be the main cause of atmospheric SCC suscepti-bility. Increasing the size of the matrix precipitates and GBPs by increasing the degree of aging could improve the atmospheric SCC resistance. The size of the matrix precipitates was the major factor affecting the atmospheric SCC resistance when GBPs were larger than a critical size that could nucleate hydrogen bubbles. However, if the size of the GBPs was smaller than this critical size, the improvement of atmospheric SCC resistance due to grain refinement, resulting from a more homo-geneous slip mode, could not be obtained because hydrogen embrittlement became serious. By meas-uring the electrical conductivity, the influence of matrix precipitates, but not that of GBPs, on SCC susceptibility could be obtained. Retrogression and reaging (RRA) treatment could effectively im-prove the atmospheric SCC resistance of T6 temper because RRA temper could produce larger sizes of both the matrix precipitates and GBPs than could T6 tempered condition.     

Stress-corrosion cracking susceptibility of the superplastically formed 5083 aluminum alloy in 3.5 pct NaCl solution

The slow strain rate test (SSRT) method was employed to study the stress corrosion cracking (SCC) susceptibility of the superplastic 5083 Al alloy in a 3.5 pct NaCl solution after superplastic forming and various heat treatments. Experimental results showed that both superplastically formed specimens and specimens subject to the same thermal processes as that used in superplastic forming suffered severe SCC susceptibility, and obvious intergranular fracture surfaces were also observed. Furthermore, scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) analyses demonstrated that the thermal processes of superplastic forming led to continuously distributed precipitation layers of β phase (Mg₂Al₃) at grain boundaries, i.e., sensitization had occurred. However, postforming annealing treatment at 345 °C for 1 hour eliminated the sensitization effect of both specimens. In this case, the SCC susceptibility was alleviated, and the fracture surfaces changed to a transgranular dimpled structure, characteristic of that found in the as-received specimen. From the metallographic observations, it was also seen that a number of cavities appeared at the grain boundaries of the superplastically formed specimen. However, the cavitation effect on SCC susceptibility is minor in comparison with the sensitization effect.     

Stress corrosion cracking of an aluminum alloy under compressive stress

Stress corrosion cracking (SCC) of 7075 aluminum alloy in a 3.5 pet NaCl aqueous solution under compressive stress was investigated using modified WOL notched specimen. The result showed that SCC could occur if an applied compressive displacement was larger than a critical value. Finiteelement analysis indicated that there was a stress concentration and the stress components were negative at the notch tip under the compressive displacement. Since the unloaded displacements were equal but opposite to the loaded ones, no stress relaxation occurred throughout SCC. Thus, the SCC was induced by compressive stress. The threshold stress intensity nucleating SCC from the notch under the compressive applied stress was 27.6 MPa m^1/2, but the corresponding value under tensile stress was 8.3 MPa m^1/2. Besides, the incubation period for SCC under compressive stress was one order of magnitude longer than that under tensile stress in the sameK ₁ The fracture surfaces of SCC under compressive stress were quite different from those under tensile stress. The latter was composed of intergranular but the former was quasi-cleavage characterized by parallel striation pattern. Formerly Student at Beijing University of Iron and Steel Technology     

Stress corrosion cracking of aluminum alloys

Stress corrosion cracking of aluminum alloys is reviewed. An extensive failure analysis shows how many service failures occurred in the aerospace industry over a ten year period and what kind of alloys and stresses led to initiation and propagation of stress corrosion cracks which caused these service failures. The paper contains most of the results of stress corrosion tests with aluminum alloys that have been obtained to date with fracture mechanics techniques. Stress corrosion crack growth rate measurements are compared with the results from smooth specimen testing and it is shown that the correlation between the different test results is very satisfactory. The present and limited status of theoretical understanding of stress corrosion cracking is outlined. A major part of the paper is devoted to the results of the latest alloy development. High strength aluminum alloys of dramatically increased stress corrosion resistance are available now. In the near future, stress corrosion resistant alloys of even higher strength might become available. What is still lacking is a detailed understanding of the mechanisms by which stress corrosion cracks initiate and propagate.     

Stress corrosion cracking of aluminum alloys

Stress corrosion cracking of aluminum alloys is reviewed. An extensive failure analysis shows how many service failures occurred in the aerospace industry over a ten year period and what kind of alloys and stresses led to initiation and propagation of stress corrosion cracks which caused these service failures. The paper contains most of the results of stress corrosion tests with aluminum alloys that have been obtained to date with fracture mechanics techniques. Stress corrosion crack growth rate measurements are compared with the results from smooth specimen testing and it is shown that the correlation between the different test results is very satisfactory. The present and limited status of theoretical understanding of stress corrosion cracking is outlined. A major part of the paper is devoted to the results of the latest alloy development. High strength aluminum alloys of dramatically increased stress corrosion resistance are available now. In the near future, stress corrosion resistant alloys of even higher strength might become available. What is still lacking is a detailed understanding of the mechanisms by which stress corrosion cracks initiate and propagate. Brown Boveri Research Center, Baden, Switzerland This paper is based on an invited presentation made at a symposium on “Advances in the Physical Metallurgy of Aluminum Alloys” held at the Spring Meeting of TMS-IMD in Philadelphia, Pennsylvania, on May 29 to June 1, 1973. The symposium was co-sponsored by the Physical Metallurgy Committee and the Non-Ferrous Metals Committee of TMS-IMD.     

Al-Zn-Mg系铝合金应力腐蚀性能

研究了热处理制度和时效工艺的改变对Al-Zn-Mg系铝合金的组织结构、力学性能和应力腐 蚀性能的影响。研究结果表明:高温预析出可以改变Al-Zn-Mg系铝合金晶界的析出相大小和分 布,从而改善其抗应力腐蚀性能;在T6和T612种人工时效条件下,预析出的合金的抗应力腐蚀 性能均好于无预析出的合金。在自然时效状态下,引入超声波,对无预析出合金的应力腐蚀性能 进行了初步探索,发现超声波可以提高合金的抗应力腐蚀性能,而对合金的硬度无影响。     

Stress corrosion cracking of an Al-Li alloy

Stress corrosion cracking (SCC) has been studied in an Al-Li alloy with variables of orientation of specimen, heat treatment, and applied potentials. The distribution of the electrochemical potential resulting from precipitate clusters was measured, and the hydrogen content on the specimen surface was detected. The results showed that the SCC susceptibility under the peak- aged (PA) condition was higher than that under the natural (NA) and overaged (OA) conditions. The transverse (TL) specimen was more susceptible to SCC propagation than the longitudinal (LT) specimen. The SCC susceptibility and the hydrogen content on the specimen surface were dependent on the applied potentials. The hydrogen content increased when the applied potential changed to positive or negative directions. There was a critical hydrogen content, below which local anodic dissolution (LAD) plays an important role, above which hydrogen embrittlement (HE) plays an important role.     

Stress corrosion cracking of an Al-Li alloy

Stress corrosion cracking (SCC) has been studied in an Al-Li alloy with variables of orientation of specimen, heat treatment, and applied potentials. The distribution of the electrochemical potential resulting from precipitate clusters was measured, and the hydrogen content on the specimen surface was detected. The results showed that the SCC susceptibility under the peakaged (PA) condition was higher than that under the natural (NA) and overaged (OA) conditions. The transverse (TL) specimen was more susceptible to SCC propagation than the longitudinal (LT) specimen. The SCC susceptibility and the hydrogen content on the specimen surface were dependent on the applied potentials. The hydrogen content increased when the applied potential changed to positive or negative directions. There was a critical hydrogen content, below which local anodic dissolution (LAD) plays an important role, above which hydrogen embrittlement (HE) plays an important role.     

Cavity distribution pattern in a superplastically deformed aluminum alloy

Cavitation in superplastically formed AA7475 aluminum alloy by gas pressure has been investigated. Two systems of cavity stringers on the diametrical section of the pressure-formed domes were observed by using optical microscopy under normal light, polarized light, and dark field. Qualitative analyses have shown that the cavity stringers are oriented 35 ∼ 60 deg to the midplane of the sheet, and the spacing of the cavity stringers decreases with increasing strain. The explanation of this new observation of cavity morphology is given from a viewpoint of cooperative grain boundary sliding (CGBS). Formerly Associate Professor with the Department of Materials Science, Ufa Aviation Institute     

二氧化硫复合盐雾环境下2024-T351铝合金应力腐蚀开裂

通过SO₂复合盐雾试验模拟工业污染海洋大气环境,结合有限元模拟分析、扫描电镜/能谱仪、光电子能谱分析等技术研究2024-T351铝合金在弹性应力区间的应力腐蚀开裂行为.结果表明:应力腐蚀开裂行为优先发生在2024-T351铝合金C型环的顶部应力集中区域;疏松的腐蚀产物层的形貌经历了由细棒状、团絮状到板块状的变化;试验6 h就可以监测到裂纹,进行到480 h的时候有贯穿裂纹形成,720 h的时候试样完全断裂;裂纹为穿晶和沿晶混合机制,主裂纹以穿晶机制沿C型环法线扩展,二次裂纹沿晶界扩展.     

Stress corrosion cracking of stainless steels

The similarities and differences in the stress corrosion cracking response of ferritic and austenitic stainless steels in chloride solutions will be examined. Both classes of materials exhibit a cracking potential: similar transient response (to loading) of the potential in open circuit tests or the current in potentiostatic tests and similar enrichment of chromium and depletion of iron in the film associated with localized corrosion processes. The ferritic steels are more resistant to localized corrosion than are the austenitic steels, which is responsible for the difference in the influence of prior thermal and mechanical history on cracking susceptibility of the two types of steel. Similarities in the fractography of stress corrosion cracks and those produced by brittle delayed failure during cathodic charging of the ferritic steels indicate that hydrogen embrittlement is involved in the failure process.     

Stress corrosion cracking of beta-brasses in water

Stress corrosion cracking (SCC) ofβ ^’-phase brasses in water at 20 °C was studied in four Cu-Zn binaries and one Cu-Zn-Sn ternary alloy using slow strain rate tensile tests and load relaxation SCC of notched rods. Electron diffraction techniques were used to identify phases on the fracture surfaces. All alloys were susceptible to SCC and it was found that decreasing the electron/atom(e/a) ratio of theβ ^’-phase promoted transgranular SCC and increased the rate of cracking. The most rapid crack propagation rates were associated with alloy compositions giving rise to a strain induced martensite transformation during SCC. Formerly Graduate Student at the University of British Columbia     

Stress corrosion cracking of zirconium cladding tubes: III. Effect of the alloy strength

The stress corrosion cracking resistance of zirconium T110, E635, and Zirkaloi-4 alloys of different strengths provided by different mechanisms of hardening (solid solution, precipitation, and strain hardening) is studied. It was shown that the stress corrosion resistance of the tubes decreases when the alloy strength increases, in particular, at strain hardening.     

Stress corrosion cracking behaviors of RE-containing ME21 magnesium alloy processed by equal-channel angular pressing

A commercial as-cast ME21 magnesium alloy containing rare-earth (RE) element was processed by equalchannel angular pressing to obtain fine-grained micro structure. Stress corrosion cracking (SCC) behaviors of the fine-grained samples were studied by slow-strain-rate testing in air, distilled water and Hanks’solution at the strain rate of 1×10~(-6) s~(-1). All samples show a relatively low SCC sensitivity in distilled water but a great SCC tendency in Hanks’ solution. The microscopic observations of the fracture surfaces and the side surfaces reveal obvious active anodic dissolution and hydrogen embrittlement cracks, which indicate the higher SCC susceptibility in Hanks'solution. The fine-grained microstructure with more crystal defects promotes the passivation process of the oxide film and restrains the hydrogen induced cracking of the ME21 magnesium alloy, leading to the higher general corrosion resistance as well as SCC resistance.     

Stress corrosion cracking relation with dezincification layer-induced stress

Brass foil with a protective layer formed on one side was deflected during corrosion in an ammonia solution under various applied potentials, and then corrosion-induced stress generated at brass/dezincification layer under different potentials could be measured. At the same time, susceptibility to stress corrosion cracking (SCC) of brass in the ammonia solution under various applied potentials was measured by using a single-edge notched specimen. At open-circuit potential, both corrosion-induced tensile stress and susceptibility to SCC (I _σ) had a maximum value. Both tensile stress σ _p and susceptibility I _σ decreased slightly with decreasing potential under anodic polarization, but reduced steeply with a decrease in potential under cathodic polarization. At the cathodic potential of − 500 mV_SCE, corrosioninduced stress became compressive because of the copper-plating layer; correspondingly, susceptibility to SCC was zero. Therefore, the variation of SCC susceptibility with potential is consistent with that of the corrosion-induced additive stress.     

The effect of microstructure and environment on fatigue crack closure of 7475 aluminum alloy

The effects of slip character and grain size on the intrinsic material and extrinsic closure contributions to fatigue crack growth resistance have been studied for a 7475 aluminum alloy. The alloy was tested in the underaged and overaged conditions with grain sizes of 18 μm and 80 μm. The fracture surface exhibited increased irregularity and planar facet formation with increased grain size, underaging, and tests in vacuum. These changes were accompanied by an increased resistance to fatigue crack growth. In air the 18 μm grain size overaged material exhibited relatively poor resistance to fatigue crack growth compared with other microstructural variants, and this was associated with a lower stress intensity for closure. All materials exhibited a marked improvement in fatigue crack growth resistance when tested in vacuum, with the most significant difference being ˜1000× at a ΔK of 10 MPa m^1/2 for the 80 μm grain size underaged alloy. This improvement could not be accounted for by either an increase in closure or increased crack deflection and is most likely due to increased slip reversibility in the vacuum environment. The intrinsic resistance of the alloy to fatigue crack growth was microstructurally dependent in vacuum, with large grains and planar slip providing the better fatigue performance.     

Stress corrosion cracking of high‐strength steels

The threshold stress intensity of stress corrosion cracking (SCC) in the NaCl solution, K_ISCC, has been measured for five low alloy steels. The effects of yield strength, alloy elements, microstructure and grain size on K_ISCC were studied. The results showed that K_ISCC decreased exponentially with increasing yield strength, σ_ys, i.e., K_ISCC = 1.38 · 10⁶exp(‐8.26 · 10^‐3σ_ys) for 40CrMoV steel and K_ISCC = 1.42 · 10⁶exp(‐4.66 · 10^‐3σ_ys) for 30CrMnSiNi steel. For low‐alloy high‐strength steels with σ_ys = 1400 MPa, the effect of alloy elements, microstructure and grain diameter larger than 7 μm on K_ISCC was little. The threshold stress intensity of hydrogen‐induced cracking during dynamical charging for 40CrMoTi steel decreased linearly with the logarithm of the concentration of diffusible hydrogen, C₀, i.e., K_IH = 31.3‐9.1lnC₀. This equation was also applicable to SCC of a high‐strength steel in aqueous solution, and in this case, C₀ is constant. The critical hydrogen enrichment concentration, C_th, necessary for SCC of high‐strength steel in water decreased exponentially with the increase in yield strength. It was possible to deduce the relationship between K_ISCC and σ_ys, i.e., K_ISCC = Ak₁exp(‐k₂σ_ys), where A = 3RT√πρ /2(1 + ν) , k₁ and k₂ are constants, which depend upon the compositions and microstructure of the steel as well as the test conditions.     

Stress corrosion cracking of stainless steels in NaCl solutions

The metallurgical influences on the stress corrosion resistance of many commercial stainless steels have been studied using the fracture mechanics approach. The straight-chromium ferritic stainless steels, two-phase ferritic-austenitic stainless steels and high-nickel solid solutions (like alloys 800 and 600) investigated are all fully resistant to stress corrosion cracking at stress intensity (K₁) levels ≤ MN • m-^3/2 in 22 pct NaCl solutions at 105 °C. Martensitic stainless steels, austenitic stainless steels and precipitation hardened superalloys, all with about 18 pct chromium, may be highly susceptible to stress corrosion cracking, depending on heat treatment and other alloying elements. Molybdenum additions improve the stress corrosion cracking resistance of austenitic stainless steels significantly. The fracture mechanics approach to stress corrosion testing of stainless steels yields results which are consistent with both the service experience and the results from testing with smooth specimens. In particular, the well known “Copson curve” is reproduced by plotting the stress corrosion threshold stress intensity (AT_ISCC) vs the nickel content of stainless steels with about 18 pct chromium. Formerly with the BBC Brown Boveri Company, Baden, Switzerland     

Stress corrosion cracking mechanisms of alloy 600 polycrystals and single crystals in primary water—Influence of hydrogen

This article investigates the mechanisms governing the process of alloy 600 stress corrosion cracking (SCC). Several critical points have been selected. First, the deleterious influence of cathodic polarization on alloy 600 SCC resistance has been assessed by slow strain rate tests (SSRTs) in primary water at 360 °C. The effects on crack initiation and propagation have been distinguished. Second, a global hydrogen embrittlement of alloy 600 has also been studied at different temperatures from 25 °C to 360 °C. Finally, the use of alloy 600 single crystals allowed clear separation of the crack initiation and crack propagation mechanisms. Transgranular SCC propagation has been precisely observed and described. The possible mechanisms for SCC initiation and propagation on polycrystals are then discussed.     

Stress corrosion cracking behavior of friction-stir-welded Al 6061-T651

In the present study, the stress corrosion cracking (SCC) behavior of friction-stir-welded AI 6061-T651 alloy was examined of −650 mV vs Ag/AgCl using a slow strain rate testing technique. The resistance to SCC was correlated to the percent change in tensile elongation with exposure to 3.5 pct NaCl aqueous solution with respect to the reference environment. It was demonstrated the the SCC resistance of friction-stir-welded Al 6061-T651 was considerably higher than that of parent material at an anodically applied potential. In friction-stir-welded Al 6061-T651 alloy, the stress corrosion cracks occur only locally in the boundary region between the dynamically recrystallized zone (DXZ) and the heat affected zone (HAZ) regions. However, the HAZ has much lower strength properties compared with the rest of the material, and thus, fracture occurs there despite the increase in stress intensity due to corrosion at the DXZ and HAZ boundary. Eventually, the tensile fracture in friction-stir-welded A1 6061-T651 was relatively unaffected by the SCCs formed in 3.5 pct NaCl aqueous solution.