Effect of high-temperature pre-precipitation on mechanical properties and stress corrosion cracking of Al-Zn-Mg alloys

1　INTRODUCTIONMediumandhighstrengthAl Zn Mgseriesaluminumalloysarethe primaryweldingstructurematerialsofaerocrafts ,transportationvehiclesandmilitaryequipments[1,2 ] ,duetoitsbetterweldabilityandexcellenttechnologicalproperty .Buttheexten siveutilizationofthesealloysishamperedbyitspoorstresscorrosioncracking (SCC)resistance[3,4 ] .Ac cordingly ,manystudieshavebeenconcernedwithsuchaproblemashowtoimprovestresscorrosionre sistanceofAl Zn Mgalloyswithnon deterioratedstrength[3,58] .Atpres…     

Strengthening-toughening of 7xxx series high strength aluminum alloys by heat treatment

1　INTRODUCTIONAerospaceand groundtransportationvehicleswithreducingstructuralmasscanenhancecarrycapa bilityandflexibility ,decreasefuelconsumption ,andextendflightvoyageevidently .Highstrengthstruc turalmaterialswithlightdensityplayapivotalroleonreducingmassinstructure .Presently ,confrontedwithcompetitionofresinbasedcomposites ,alu minumalloysas primarystructuralmaterialsforspacecraftandgroundtransportationvehiclesarede velopinginlinesofsuperhighstrength ,corrosionen durance ,lowdensitya…     

时效工艺对2519A铝合金力学性能及抗应力腐蚀开裂性能的影响

通过常温拉伸实验、慢应变拉伸应力腐蚀实验、极化曲线测试及透射电镜等研究了不同时效工艺对2519A合金的力学性能和抗应力腐蚀开裂性能的影响。结果表明:与传统的2519A-T87合金相比,再时效时间为19 h的2519A-T9I7合金同时具有优异的力学性能和良好的抗应力腐蚀开裂性能。这是由于2519A-T9I7合金在T9I6态的基础上延长了再时效时间,使晶界析出相聚集球化,相间距增加,破坏了腐蚀通路,提高了抗应力腐蚀开裂性能。     

双重淬火对7055铝合金组织性能的影响

通过拉伸性能测试、晶间腐蚀和剥落腐蚀性能测试,金相显微镜、透射电子显微镜研究了双重淬火对7055铝合金组织及性能的影响.结果表明:合适的双重淬火可调控晶界和晶内析出状态,使合金时效后晶界上的析出相呈断续分布,晶内沉淀强化相均匀、弥散、细小析出,保证合金高强度的同时,提高晶间和剥落腐蚀性能.     

7050铝合金晶界偏析与应力腐蚀,腐蚀疲劳行为的研究

研究了不同时效状态的７０５０铝合金晶界偏析与应力腐蚀开裂及湿空气环境下疲劳裂纹扩展的关系，并采用修正的化学方法计算了晶界偏析对断裂应力的影响。结果表明：随着时效程度的增加，晶界上Ｍｇ偏析浓度减小，捕获Ｈ的能力减弱，晶界断裂应力的百分数随之下降，因而合金的抗应力腐蚀和腐蚀疲劳性能提高。     

Influence of heat treatment regimes on microstructures and fracture characteristics of 7055Al alloy containing Ag

Tensile properties, fracture characteristics and microstructures of 7055 aluminum-based alloy containing Ag after T6, T73 and RRA treatment were investigated. The results show that RRA treatment retains strength of 7055-T6 with higher electrical conductivity close to that of 7055-T73 alloy, but its elongation decreases greatly.SEM fractographs reveal that intergranular cracking and shear-type transgranular cracking are both presented on the fracture appearance of 7055-T6 specimen. After T73 treatment, the fractographs mainly consist of dimple-type transgranular cracking with minor intergranular cracking. For 7055-RRA specimen, intergranular cracking domi-nates with minor dimples on the fracture surface. The type and size of precipitates, width of grain boundary and the ability of precipitates to impede dislocation motion vary with heat treatment regimes. Three frature models were built on the basis of microstructural analyses.     

预拉伸对峰时效2297铝锂合金应力腐蚀性能的影响

在峰时效前对2297铝锂合金试样施加不同变形量的预拉伸,利用慢应变速率拉伸 (SSRT)、扫描电子显微镜 (SEM)、透射电镜 (TEM) 等手段,研究峰时效前不同预拉伸量对应力腐蚀敏感性的影响。结果表明,5组不同预拉伸量的样品经过峰时效处理后,预拉伸量为5%的样品应力腐蚀敏感因子为5.9%,抗应力腐蚀性能最好;预拉伸量为12.5%的样品,应力腐蚀敏感因子为41.4%,抗应力腐蚀性能最差;随着预拉伸量的增加,晶粒尺寸差逐渐减小,晶粒内部针状T₁相析出量逐渐增大,并且不断细化;T₁相的尺寸、数量以及分布的均匀性这3种因素的匹配度对合金强度及抗应力腐蚀性能有决定性的影响。     

Spannungsrißkorrosion von hochlegierten Manganstählen in wäßrigen Chloridlösungen

Stress corrosion cracking of high alloy manganese steels in aqueous chlorides In tensile tests made without applied current in aerated solutions a stabilization of the austenitic structure by increasing Mn and N contents yields increased times to failure. The potential-time-to-failure curves determined by potentiostatic tensile tests reveal a compley joint action of constitution and passivation behaviour of the steels. The intercrystalline stress corrosion cracking of the steel X 40 MnCr 19 with chromium carbide precipitations at the grain boundaries can be attributed to an electrochemical differentiation of the chromium-depleted grain boundary region. Steels of this type are characterized by a pronounced sensitivity to intercrystalline stress corrosion cracking the precipitation annealed state, and by a certain sensitivity to transcrystalline corrosion cracking after solution annealing. Low carbon Mn steels containing up to 4 % Cr are susceptible to transcrystalline stress corrosion cracking irrespective of the heat treatment. As to the temperature dependence of times-to-failure, constitution and layer formation have different effects. Increasing the Cr content to 8 % gives rise to a transition from stress corrosion cracking to pitting type corrosion. In terms of electron optics, an increased chromium content gives rise to a changed dislocation pattern, so that there may be an effect of the type of gliding processes on stress corrosion, The increased stress corrosion resistance of MnCr steels containing at least 8% Cr may be due to the lower height Of the gliding step and to an increasing tendency to repassivation of damaged surface layers.     

淬火速度对7085铝合金显微组织和应力腐蚀的影响(英文)

采用力学性能测试、慢应变速率拉伸实验,结合扫描电镜和透射电镜及电化学测试等方法,研究淬火速度对7085铝合金组织和应力腐蚀性能的影响。结果表明,随着淬火速度的降低,合金晶界析出相的尺寸和间距增大,晶界析出相的Cu含量降低;合金的抗应力腐蚀性能随着淬火速度的减少先增强后减弱。晶界析出相的尺寸和分布以及Cu含量是影响合金抗应力腐蚀性能的主要因素。     

Comparison of microstructure and corrosion properties of Al-Zn-Mg-Cu alloys 7150 and 7010

The influence of coarse Cu-bearing particles, matrix and subgrain boundary precipitates on the stress corrosion susceptibility of the Al-Zn-Mg-Cu alloys was investigated. The strength of 7150 alloy is about 15 MPa higher than that of 7010 alloy. The 7010 alloy exhibits higher resistance to stress corrosion cracking as compared with the 7150 alloy. The coarse Cu-bearing particles are detrimental to the resistance to stress corrosion cracking. The increase of size of matrix and subgrain boundary precipitates decreases the susceptibility of stress corrosion. The anodic dissolution and hydrogen embrittlement govern the cracking process. The severity of stress corrosion cracking is shown to be related to the coarse Cu-bearing particles, matrix and subgrain precipitates in Al-Zn-Mg-Cu alloys.     

Special aspects of localized High-temperature corrosion

As in aqueous corrosion a localized corrosive attack can be important in high-temperature corrosion of metallic materials and become, unvoluntarily, a lifetime determining factor for combustion systems. Deposit-induced corrosion is the most important form of localized corrosion on alloy 800 and other materials in advanced coal combustion systems. Coal-ash deposits may cause a sulfidation/oxidation reaction which propagates preferentially along grain boundaries into the underlying metallic material. Preferred grain boundary corrosion may also occur in the walls of the fuel nozzle tubes of gas combustion systems. In this case, an oxidation reaction takes place. The lifetime of the tubes can be increased by use of a special alloy 601 H grade. Pitting may be observed in coal combustion systems. It has also been observed on alloy 601 after service in ceramic firing kilns where it is promoted by the presence of chlorine. Additionally, pitting occurs under carburizing conditions (metal dusting) as does cracking being similar in appearance to stress corrosion cracking in aqueous corrosion. Compared to deposit-induced and grain boundary corrosion, crevice corrosion is of minor importance in high-temperature corrosion. Only one example has been identified so far on alloy 600 but which may also be interpreted as a kind of deposit-induced corrosion.     

CORRELATION BETWEEN MICROSTRUCTURE AND STRESS CORROSION CRACKING SUSCEPTIBILITY OF 2091 ALLOY

The stress corrosion cracking(SCC) susceptibility of an Al-Li-Cu-Mg-Zr alloy(2091),heat-treated under different regime,has been investigated hy means of tensile tests atslow strain rate.The microstructure has been examined with TEM.The deformationcharacteristic has also been analysed.The stress corrosion resistance of the alloy wasrevealed to be mainly affected by the covering rate of T_2-phase over grain boundariesand by the average distance between T_2-phase particles.SCC resistance can be obviously improved by predeforming the alloy at room temper-ature to reduce the precipitation of T_2-phase along grain boundaries.     

Effect of Cr, Yb and Zr additions on localized corrosion of Al–Zn–Mg–Cu alloy

The effect of Cr, Yb and Zr additions on localized corrosion of Al–Zn–Mg–Cu alloy has been investigated. Additions of Cr, Yb and Zr to Al–Zn–Mg–Cu alloy stabilized the deformation-recovery microstructure with low angle grain boundaries on which grain boundary precipitates distributed discretely. The exfoliation and stress corrosion cracking of Al–Zn–Mg–Cu alloys propagated along the high angle recrystallized grain boundaries. The unrecrystallized Al–Zn–Mg–Cu–Zr–Cr–Yb alloy exhibited higher resistance to stress corrosion cracking and exfoliation corrosion, compared to the partial recrystallized Al–Zn–Mg–Cu–Zr alloy with high angle grain boundaries.     

晶界结构在不同热处理下对铝镁合金腐蚀规律的研究现状

铝镁合金是未来应用汽车领域发展具有很大潜力的一类重要材料.通常,在商品中的铝镁合金,镁的质量分数在0.5%～13%.汽车制造商通常选用镁的质量分数在4.5%以上,然而研究发现,当镁的质量分数超过3%的时候,会引起晶界间腐蚀,从而造成应力腐蚀开裂.此现象未见文献对其进行系统的分析和研究.本文主要介绍铝镁合金在不同的热处理...     

Effects of cooling rate on the precipitation behavior of grain boundary carbide and corrosion resistance of 5Cr15MoV stainless steel

High-carbon and high-chromium alloy steels are prone to pitting and intergranular corrosion, which reduces corrosion resistance. The precipitation behavior of the carbides of high-carbon and high-chromium alloy steels is one of the main factors affecting pitting and intergranular corrosion of stainless steel. In this study, 5Cr15MoV stainless steel was heated to 1,200℃ and then cooled by cooling rates varying from 25 to 150°C/min. The precipitation behavior of grain boundary carbides of 5Cr15MoV steel at different cooling rates, and its effect on the corrosion resistance of materials was studied. The results show that the carbides of 5Cr15MoV steel mainly precipitate along the grain boundaries, which leads to the formation of chromium-depleted zones near the grain boundaries and reduces resistance to intergranular corrosion. It has been found that a higher cooling rate shortens the width of the Cr-depleted zone near the boundary from 0.871 to 0.569 μm, reduces the Cr-concentration gradient near the grain boundary from 36.422% to 12.667%, and suppresses the nucleus growth rate of grain boundary carbides. As the cooling rate increases, the corrosion current density decreases from 13.29 to 2.42 μA/cm². The corrosion rate is the lowest, while the cooling rate is 150°C/min. The corrosion rate decreases from 218.339 to 158.488 mm/a. The phenomenon of intergranular corrosion and pitting corrosion was found to be weakened; and thereby, it is shown that an intensive cooling rate can improve the corrosion resistance of 5Cr15MoV steel.     

2091合金的显微组织与应力腐蚀敏感性

用慢应变速率拉伸试验研究不同热处理状态Al—Li—Cu—Mg—Zr(2091)合金的应力腐蚀敏感性,用透射电镜观察显微组织结构并分析变形特性。结果指出,T_2相在晶界上的复盖程度及T_2相之间的平均间距是影响合金应力腐蚀敏感性的主要因素,室温预形变处理可减少晶界上T_2相的数量,使合金应力腐蚀抗力得到明显提高。     

车体用挤压态Al-6.5Zn-1.55Mg-0.25Cr-0.1Zr合金晶粒取向及应力腐蚀性分析

对含有不同显微组织形态的Al-6.5Zn-1.55Mg-0.25Cr-0.1Zr铝合金开展耐应力腐蚀表征,利用EBSD和TEM处理方法深入探讨了试样发生应力腐蚀开裂的特性。研究结果表明:晶粒形成了许多小角度晶界,大角度晶界基本都是由纤维状晶界构成。以挤压工艺制备的Al-Zn铝合金可以对再结晶过程起到抑制作用,由此减小大角度晶界的比例。有大量亚稳态球形Mg-Zn2颗粒分布于等轴晶组织以及纤维组织晶粒中,还有大量纤维组织出现在亚晶结构中;在纤维组织中还可以观察到一些非常细小的亚晶界析出相,颗粒尺寸只有12 nm。原始组织会减小应力耐腐性,在不同时间的裂纹扩展方向也存在较大差异,在临界深度处变成横向排布的裂纹。大角度晶界成为应力腐蚀裂纹的高效扩展通道,当形成小角度亚晶界后则能够对裂纹扩展发挥抑制作用。     

铝合金应力／应变时效机制的第一性原理研究

基于第一性原理计算方法探讨了铝合金应力／应变时效的可能机制,综合评估了时效温度和外加应力／应变对Al-Sc合金中Al3Sc固溶边界和A1-Cu合金中Al／θ＇＇界面能的潜在影响。计算结果表明：在传统时效过程中引入外加拉应力／应变,声子态密度在高态区有红移现象,可以明显降低溶解熵;同时,导致相图中固溶线上移,表明外加拉应力／应变可降低Al3-Sc在Al-Sc合金中的极限固溶度,从而增加析出相的最大可能体积分数。外加应力／应变对Al-Cu合金中不同取向的Al／θ＇＇,界面形成能有不同程度的影响,这种差别可以通过泊松效应进一步放大,从而影响到Al-Cu合金中析出相的择优取向。这2种机制在应力／应变时效中均可能发挥重要作用。     

冷轧与拉伸复合预变形对2519A铝合金板材应力腐蚀的影响(英文)

采用10-6s-1慢应变拉伸测试手段研究冷轧与拉伸复合预变形对2519A铝合金抗应力腐蚀开裂性能的影响。冷轧7%后再垂直拉伸3%的合金板材抗拉强度和应力腐蚀指数分别为481MPa和0.0429,与冷轧4%后再平行拉伸3%以及冷轧7%后再平行拉伸3%的合金板材相比表现出了更好的力学性能和抗应力腐蚀开裂性能。这主要是由于冷轧7%后再垂直拉伸3%在合金板材中生成了密度更高且分布更均匀的位错组织,使时效后合金板材晶内析出相细小、密集,晶界析出相不连续,晶界无沉淀析出带且较窄。     

Interkristalline,Kornflächen- und Spannungsriß-Korrosion von ausgehärteten AlZnMg 1-Legierungen

Intercrystalline corrosion, grain-phase corrosion and stress-corrosion cracking of aged AlZnMg1 alloys Potentiostatic polarisation curves and stress corrosion crack propagation of a pure and a commercial AlZnMg1-alloy, both peakhardened either by warm or by double ageing, were investigated in airsaturated sodium halide and sulfate solutions at a temperature of 303 K. For this, propagation of crack tips was observed microphotographically under potentiostatic conditions and under constant stress using specimens precracked by fatique corrosion. The pure alloy contents (weight percent): Zn 4,4; Mg 1,26; Fe 0,05; Si 0,006; balance Al, the commercial alloy: Zn 5,05; Mg 1,2; Fe 0,23; Si 0,1; Mn 0,23; Cr 0,2; Cr 0,2; Ti 0,074; Zr 0,12; Cu 0,056; balance Al. In sodium halide solutions either grain boundary and grain attack or only grain attack occurs at a potential region more positive than the respective breakdown potential. At the more negative restpotential no attack can be observed. The influence of concentration of chloride ions on the velocity of discontinuous crack propagation in the pure alloy, which starts without any initiation stage, is different for both heat treatments. No crack propagation is observed in the commercial alloy and in any case in sulfate solutions. The obtained stress corrosion cracking results may be explained by hydrogen embrittlement of the area around the crack tip.