Korrosionsverhalten der Legierung Al-Li-Cu-Mg

Corrosion Behaviour of the Al-Li-Cu-Mg Alloy The localized corrosion behaviour of the Al-Li-Cu-Mg alloy 8090-T651 (25 mm thick plate) was investigated using salt spray tests, EXCO tests and tests according to the MIL-H-6088F specification. These accelerated laboratory tests indicate that the material is susceptible to intergranular and exfoliation corrosion. The stress corrosion cracking behaviour was studied under constant load, constant deformation and slow strain rate conditions. The material was found to be resistant against stress corrosion cracking in the long transverse direction. In the short transverse direction a threshold stress below 50 MPa was determined. The same values were measured for the conventional alloys 2014-T351 and 2024-T351. The fracture surfaces of specimens failed due to stress corrosion cracking were examined using a scanning electron microscope. Fractographic features attributed to corrosion were discussed.     

Susceptibility of 7050-T7451 electron beam welded specimens to stress corrosion

Susceptibility to stress corrosion tests were carried out on electron beam welded specimens made from 7050-T7451 aluminium alloy. As a comparison, specimens made from base material were tested too. The resistance of the welded material was high: the tensile properties were only slightly lower than those of the base material. After 30 day exposure to a corrosive environment (alternate immersion in a 3.5% NaCl solution), the tensile properties of the welded material were considerably reduced, while the same properties were only slightly affected in the base material. The combined effect of stress and corrosion was only slightly detrimental for the base material and very detrimental for the welded material. At the lowest stress level tested, about 25% of the ultimate stress, the welded specimens failed after a mean life of 90 days. Considerable residual stresses associated with the welding process were measured in a plate. A test was performed to verify the possibility of stress corrosion cracking promoted by the welding residual stresses. In actual fact, no cracks were observed, but the corrosion rate increased, particularly in the areas affected by the higher residual stresses.     

时效对Al-Li(2091)合金应力腐蚀行为的影响

本文研究了在不同时效状态下Al-Li(2091)合金在3％NaCl+0.5％H_2O_2溶液中应力腐蚀行为;并与传统的2024(CZ)合金进行了比较。结果表明,2091合金具有较好的抗应力腐蚀能力。利用扫描电镜与透射电镜分别对应力腐蚀断口和合金显微组织进行了观察和分析;对应力腐蚀开裂的机制进行了初步探讨。     

Corrosion fatigue of extruded magnesium alloys

Corrosion fatigue tests were carried out on extruded AZ31 (3% Al, 1% Zn, 0.3% Mn, Mg—the rest), AM50 (5% Al, 0.4% Mn, Mg—the rest) and ZK60 (5% Zn, 0.5% Zr, Mg—the rest) Mg alloys in air, NaCl-based and borate solutions. N_sol/N_air ratios (the relative fatigue life) were used for the analysis of the corrosion fatigue behavior of Mg alloys in various environments, where N_sol and N_air are the numbers of cycles to failure in the solution and in air, respectively. Extruded ZK60 alloy reveals very high fatigue and corrosion fatigue properties in comparison with other alloys. However, it has the lowest relative fatigue life (N_sol/N_air 10⁻³–10⁻²) or the highest sensitivity to the action of NaCl-based solutions in comparison with that of AM50 and AZ31 alloys (N_sol/N_air 10⁻²–10⁻¹). Under the same stress, the corrosion fatigue life of extruded alloys is significantly longer than that of die-cast alloys (N_sol for extruded AM50 in NaCl is two to three times longer than that of die-cast AM50).     

Effect of Calcium Treatments and Strain Rate on Reheat Cracking of Vanadium-Modified 2.25Cr-1Mo Steel

Vanadium-modified (V-Mod) 2.25Cr-1Mo steel has been studied to determine the effect of calcium treatments and strain rate on susceptibility to reheat cracking of the heat-affected zone in low-alloy steels for reactor applications. The susceptibility to reheat cracking was determined by tensile tests in the strain-rate range of 1 × 10⁻⁴/s to 3 × 10⁻⁶/s in the temperature region of 293K to 1023K. The grain size of tensile specimens was about 200μm. For the low-sulfur V-Mod steel, calcium treatment dramatically increased the total elongation and reduction of area at 923K at high strain rates. However, the beneficial effect on the ductility decreased as the strain rate decreased. For the V-Mod steel with commercial sulfur-content level, the addition of calcium was not very effective in decreasing the susceptibility to reheat cracking, independent of the strain rates investigated. The results are described and discussed.     

An investigation of Stress corrosion cracking in MgAZ61 alloy in 3.5% NaCl + 2% K2CrO4 aqueous solution at room temperature

Effect of specimen orientation, heat treatment and applied potential on the stress corrosion susceptibility of magnesium AZ61 (Mg-6.3% Al-0.5% Zn-0.20% Mn) alloy in an aqueous 3.5% NaCl + 2% K₂CrO₄ solution at room temperature was investigated. Stress corrosion times to failure were measured at different values of initial stress intensities using single edge (pre) cracked sheet tensile specimens and a modified tensometer. It was observed that while the specimen orientation has a significant effect on the measured values of stress corrosion threshold stress intensity, K_Iscc, the effect of varying the quench rate during heat treatment was minimal. Polarization measurements both in stressed and unstressed conditions, failed to reveal any significant effect of the applied stress intensity on the anodic polarization behavior of the alloy. However, measurements made under four different potentiostatic conditions showed a considerable increase in stress corrosion times to failure of the alloy. The results, together with fractographic observations of fractured specimen are discussed in terms of the mechanisms of stress corrosion cracking, in magnesium alloys.     

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.     

Stress corrosion cracking behaviour of the duplex Fe-10Al-29Mn-0.4C alloy in 20% NaCl solution at 100° C

The stress corrosion cracking behaviour of the duplex Fe-10 Al-29 Mn-0.4 C alloy having two phases ( and) in an aqueous 20% NaCl solution (100° C) has been investigated using both the static constant load and dynamic slow strain rate tests. The constant load test shows that the duplex alloy investigated is immune to stress corrosion cracking in a 20% NaCl solution. However, the slow strain-rate test reveals that this alloy is susceptible to stress corrosion cracking at the stabilized corrosion potential, and also at potentials anodic and cathodic to this potential. Furthermore, the metallographic cross section of this duplex alloy after slow strainrate testing shows that the secondary cracks propagate transgranularly through the ferrite grains at and above the stabilized corrosion potential. However, the cracks propagate transgranularly in both the ferrite and austenite grains and also at the austenite-ferrite grain boundaries when the applied potential is cathodic to the stabilized corrosion potential.     

7A52铝合金焊缝应力腐蚀性能研究

本文用20mm厚的7A52铝合金板材,采用钨极氩孤焊接工艺焊接,用慢应变速率实验方法,应变速率为1．58×10^-6s^-1,在3．5％氯化钠溶液和惰性气体中,研究了7A52铝合金焊接接头的应力腐蚀性能。试验结果表明：7A52铝舍金焊缝对应力腐蚀较为敏感,在3．5％氯化钠溶液中应力腐蚀断裂均发生在焊缝熔合线附近区,而在惰性气体中试样断裂发生在焊缝中间部位。     

Mechanical properties of Friction Stir Processed 2618/Al2O3/20p metal matrix composite

The effect of Friction Stir Processing (FSP) on the mechanical properties of 2618 aluminium alloy reinforced with 20% of alumina particles aluminium alloy has been studied in the present paper. The material was processed into the form of sheets of 7 mm thickness after T6 treatment and was tested in tension and fatigue at room temperature.

Tensile tests were also performed at higher temperatures and different strain rates in the nugget zone, in order to analyse the superplastic properties of the recrystallized material and to observe the differences with the parent materials as a function of the strong grain refinement due to the Friction Stir Process. The high temperature behaviour of the material was studied, in longitudinal direction, by means of tensile tests in the temperature and strain rate ranges of 400–500 °C and 10⁻³–10⁻¹ s⁻¹, respectively.

Fracture surfaces of the deformed fatigue test specimens were comprehensively examined in a scanning electron microscope equipped with field emission gun to determine the macroscopic fracture mode and characterize the fine-scale topography and microscopic mechanisms governing fatigue fracture.

The mechanisms governing fatigue life, cyclic deformation and fracture characteristics are analysed in function of magnitude of applied stress, intrinsic micro structural evolution and material deformation behaviour.     

钛合金慢应变速率拉伸应力腐蚀行为研究

对TA31和TC4ELI两合金分别在干燥空气、3.5% NaCl环境下进行慢应变速率拉伸试验。结合应力-应变曲线以及断口宏观、微观组织分析两种材料的应力腐蚀敏感性。结果表明:在35℃、3.5%(w,下同) NaCl溶液、应变速率为10-6/s条件下,TC4ELI合金应力腐蚀敏感性较高,在5×10-7/s和5×10-6/s的应变速率下应力腐蚀敏感性较低;TA31合金在上述3种应变速率下应力腐蚀敏感性均较低;在3.5% NaCl环境中,TA31合金断口形貌较空气中试验的断口形貌无明显的变化,TC4ELI则出现较明显的解理面。      

High strain rate superplastic behaviour of powder-metallurgy processed 7475Al+0.7Zr alloy

Superplasticity was investigated in powder-metallurgy (PM) processed 7475Al+0.7Zr alloy. Strain-rate-change (SRC) tests were carried out at various temperatures to examine the relationship between strain rate and flow stress. After the compensation by threshold stress, the superplastic flow was found to be well correlated with lattice diffusivity in aluminium, like that in the ingot-metallurgy (IM) processed 7475Al alloy having a coarser grain size. Large tensile elongations of up to 1000% could be obtained at a very high strain rate near 10⁻¹ s⁻¹ and at 515°C. Short fibre formation was observed after the superplastic deformation. This formation seemed to be related to liquid formation on the grain boundaries and similar evidences were found over a wide range of temperature, not necessarily near the incipient melting point.     

Stress corrosion cracking susceptibility of ultrafine grained AZ31

In this study, stress corrosion cracking (SCC) behavior of AZ31 magnesium alloy was carried out using slow strain rate testing (SSRT) technique in 3.5 wt% NaCl solution. The influence of microstructural scale on the stress corrosion behavior was investigated in AZ31 alloy with three different mean grain sizes. Single-pass and two-pass friction stir processing (FSP) was employed to obtain fine grain and ultrafine grain microstructures, respectively. For FSP, SSRT specimens were extracted from the processed region. SSRTs were carried out in air and solution at an initial strain rate of 10⁻⁶/s. A significant decrease in the ultimate tensile strength was observed for FSP specimens tested in chloride solution as compared to specimens tested in air. More than 75 % loss in total elongation was observed for the specimens tested in chloride solution as compared to the ones tested in air. In comparison with base material, lower time to failure was observed for processed samples. The higher SCC susceptibility of processed microstructure is attributed to increased hydrogen adsorption and favorable basal texture.     

CRACK NUCLEATION AND PROPAGATION IN BLADE STEEL MATERIAL

Stress corrosion cracking and corrosion fatigue of 12 Cr steel in sodium chloride solution has been investigated. Tests have been performed in air at room temperature and in aqueous solution with 22% NaCl at 80°C. The influence of corrosion pits on crack nucleation has been investigated. On fracture surfaces tested in environment (22% NaCl solution), crack initiation was observed in correspondence with corrosion pits; in this case fatigue life can be described using a fracture mechanics approach. The ΔK value for crack nucleation from a pit in rotating bending fatigue tests is very low in air (about 3 MPa√m). The results of slow strain rate tests on smooth specimens show that there is a threshold stress intensity, K_ISCC, of about 15 MPa√m and a plateau in stress corrosion crack growth rate of about 10^-5mm/s.     

Effect of hydrogen charging on the mechanical properties of medium strength aluminium alloys 2091 and 2014

Cathodic hydrogen charging in 3·5% NaCl solution altered the mechanical properties of 2091-T351 (Al-Cu-Li-Mg-Zr) determined by a slow (10⁻³/s) strain rate tensile testing technique. UTS and YS decreased in the case of 2091-T351 and 2014-T6(Al-Cu-Mn-Si-Mg) with increase in charging current density. Elongation showed a decrease with increase in charging current density for both the alloys. However, elongation occurring throughout the gauge length in uncharged specimens changed over to localized deformation, thus increasing the reduction in area in charged specimens. A transition in fracture mode from surface (brittle) to the core (ductile) was observed. The presence of hydrogen increased the hardness, mostly indicative of solution strengthening and it decreased with depth confirming the existence of hydrogen concentration gradient. The effects were similar in 2014-T6, but to a slightly smaller extent.     

Stress corrosion cracking of X-70 pipeline steels by eletropulsing treatment in near-neutral pH solution

Slow strain rate tests (SSRT) and environmental scanning electron microscopy (ESEM) were utilized to investigate stress corrosion cracking (SCC) behavior of electropulsed POSCO and Bao Steel X-70 pipeline steels specimens in XJ solution purged with 5% CO₂ + 95% N₂ at the strain rate of 2E-6/s. The results showed that the ultimate tensile strengths (UTS) were raised considerably for the electropulsed specimens. The UTS for electropulsed POSCO X-70 pipeline steel was enhanced much more compared to electropulsed Bao Steel X-70 pipeline steel. In addition, the relationship among UTS, the original grain size and high voltage in electropulsing treatment was revealed. SCC sensitivity increased with the decreasing electrochemical potential, especially at high cathodic potential.     

Elevated temperature creep behavior of three rapidly solidified Al-Fe-Si materials containing Cr, Mn, or Mo

Research on the high temperature creep behavior of three rapidly solidified Al-Fe-X-Si (where X = Cr, Mn or Mo) dispersion strengthened materials with three different alloying compositions has been conducted. Firstly, microstructural examinations have been carried out on the as-received, thermally treated and tested samples. The microstructure consists of a fine Al matrix embedding small round-shaped Al₁₂(Fe,X)₃Si and Al₁₃(Fe,X)₄ dispersoids. Grain sizes ranging from 0.85 to 1.45 μm and dispersoid sizes ranging from 45 to 54 nm were observed. Secondly, tensile tests were performed at high temperature from 573 to 823 K at strain rates ranging from 2.5×10⁻⁶ to 10⁻²s⁻¹. The experimental data exhibited high apparent stress exponent, n_ap, and high activation energy, Q_ap. The rnicrostructure remained stable and fine after testing. The results are analyzed by means of various models used in the literature.     

Strength differential in Al---Li alloy 8090

Cyclic strength differential (SD) data have been derived from the variation of the tensile and the compressive stress amplitude with elapsed cycles during low cycle fatigue (LCF) for the quaternary Al---Li alloy 8090 in the T8E51 condition. LCF test specimens were machined out of the rolled plate such that the direction of stressing was along the longitudinal (L), L+45° and long transverse (LT) directions. The analysis of cyclic stress amplitude data corresponding to the half-life revealed an SD effect at all strain levels in the L+45° and LT directions. In the case of the L direction, at strain levels below 8.5 × 10⁻³, the SD is more than compensated by the Bauschinger effect resulting from the prestretch. The alloy exhibits a similar SD behaviour under monotonic loading conditions. The magnitude of cyclic SD at intermediate strain amplitudes in all the three test directions decreases upon cycling, which behaviour can be attributed to the relaxation of prestretch-related residual stresses leading to a decrease in σ_T only in the L direction and σ_C only in the L+45° and LT directions. As microstructural features vary in the differently heat-treated conditions, a comparison has been made of the monotonic SD data in the stretched and aged (T8E51) condition with those in the solution-treated, in the peak-aged (T6) and in the overaged (T7) conditions.     

Fatigue crack propagation rate at low ΔK of two aluminum sheet alloys, 2024-T3 and 7075-T6

For transport aircraft with long lifetimes, crack growth data to implement the durability life requirements in the low ΔK range are needed. This is the region comprising most of the lifetime for the cracks of interest, and it is also the region where there is little data available. Crack growth data in the form of da/dN vs ΔK vs R at constant amplitude for two primary aircraft aluminum sheet alloys, 2024-T3 and 7075-T6 (clad) were measured in laboratory air and 140°F in the low da/dN region 10⁻⁸ to 10⁻⁵ in./cycle. Crack growth rates were correlated with stress state and with fractographic features.     

A statistical model and experimental study of the strain-rate dependence of the strength of fibres

In the present paper, statistical models of the strain-rate dependence of the strength of fibres and fibre bundles are established and their stress/strain equations are deduced. According to the models, an experimental method of determining mechanical parameters in the stress/strain equations by means of tensile impact loading of the fibre bundles is set up. In order to examine the validity of the models and the experimental method, tensile tests of the E-glass fibre bundles were performed at strain rates ranging from 10⁻⁴ to 10³ s⁻¹. The test results are in good agreement with the models. Hence, the models are reliable and the test method is feasible. The statistical models successfully explain the rate-dependent behaviour of the E-glass fibre.