航空AZ31镁合金腐蚀疲劳性能研究

研究了AZ31镁合金在3.5 wt%Na Cl水溶液中的腐蚀疲劳寿命及腐蚀疲劳断口。结果表明:相比空气中的疲劳极限,AZ31镁合金试样在3.5 wt%Na Cl水溶液中的腐蚀疲劳极限下降了34.1%。AZ31镁合金的腐蚀疲劳裂纹萌生于多源腐蚀坑。在Paris公式的基础上,结合试验结果,提出了修正后的腐蚀疲劳模型,其适用于中高应力幅下的疲劳寿命计算。     

轧制AZ31B镁合金腐蚀疲劳过程中的声发射信号分析

利用全数字声发射系统研究了轧制AZ31B 镁合金腐蚀疲劳过程中的声发射信号.结果表明,轧制AZ31B镁合金在NaCl溶液中的腐蚀疲劳过程主要存在4种声发射源,其中与腐蚀相关的两种信号分别对应于阳极溶解和阴极析氢,前者属于板平面内激励源,产生扩展波信号;后者属于板平面外激励源,产生弯曲波.与载荷相关的两种信号分别对应于塑性变形的连续型信号和裂纹扩展阶段高载荷阶段出现的裂纹扩展信号.腐蚀相关的声发射信号存在于整个疲劳过程,而塑性变形信号只发生在疲劳过程中特定的应力阶段.     

激光熔修AZ31B镁合金焊趾区材料的疲劳性能

疲劳性能是评估镁合金焊接结构寿命的重要指探.AZ31B镁合金TIG焊趾存在的微细裂纹与几何不连续,是在役焊接结构疲劳裂纹萌生的主要原因.为减轻接头的应力集中和改善其疲劳性能,采用CO2激光热源重熔AZ31B镁合金焊趾.研究发现:Mg17Al12在近熔合线的热影响区中沿晶界析出,而在焊缝中呈弥敌分布.此外,因受脆晶相的作...     

微弧表面处理对AZ31B镁合金耐腐蚀及耐腐蚀疲劳性能的影响

采用微弧表面处理技术(微弧氧化MAO和微弧复合MCC)在AZ31B镁合金基体上制备出不同断面结构的防护涂层。通过电化学腐蚀及腐蚀疲劳测试方法,研究了MAO、MCC涂层的电化学腐蚀及腐蚀疲劳性能。结果表明,生长10 min的MAO涂层具有较好的耐电化学腐蚀性能。MAO涂层表面存在微孔和微裂纹,在应力条件下微孔和微裂纹作为疲劳断裂的裂纹萌生点,可加速裂纹的萌生与扩展,使其腐蚀疲劳寿命相较AZ31B合金基体降低了55%。而具有MCC涂层的AZ31B合金试样腐蚀疲劳极限为(64.0±5.4) MPa,比AZ31B合金基体提高了59%。在低应力载荷下(＜80 MPa),微弧复合涂层试样的腐蚀疲劳强度得到明显提高。     

固溶/时效处理对体育器械用AZ80镁合金性能的影响

通过硬度测试、室温拉伸试验、室温疲劳试验,研究了AZ80镁合金在轧制、固溶、时效以及固溶+时效4种状态下的力学性能和疲劳寿命。结果表明:固溶处理对AZ80镁合金硬度提高效果不大,时效处理、固溶+时效处理均可以提高AZ80镁合金硬度,AZ80镁合金经固溶+时效处理后硬度比轧制态提高38%,效果最为明显。固溶处理使AZ80镁合金强度降低,伸长率提高,时效处理、固溶+时效处理使其强度提高,伸长率降低。在较低应变振幅(0.4%)条件下,热处理AZ80镁合金疲劳寿命小于轧制态。在中高应变振幅(0.6%、0.8%、1.0%)条件下,热处理提高了AZ80镁合金疲劳寿命,其中时效处理试样疲劳寿命最佳。在超高应变振幅(1.2%)条件下,热处理对提高AZ80镁合金疲劳寿命效果不明显。     

挤压态AZ31镁合金的疲劳行为研究

通过外加总应变幅控制的疲劳试验和断口形貌分析,确定了挤压态AZ31镁合金的循环应力响应行为、循环变形行为、疲劳寿命行为和疲劳断裂机制。结果表明,在外加总应变幅控制的疲劳加载条件下,挤压态AZ31镁合金呈现明显的循环应变硬化现象和拉-压不对称循环变形现象,其弹性应变幅、塑性应变幅与断裂时的载荷反向周次之间的关系可分别用Basquin和Coffin-Manson公式来描述,断口上的疲劳裂纹的萌生和扩展均以穿晶模式进行。     

变面轧制AZ31镁合金高周疲劳行为

本文研究了变面轧制AZ31镁合金的高周疲劳行为,并与传统挤压态AZ31合金的疲劳行为进行对比。结果表明,变面轧制可以显著提高AZ31镁合金的强度,相应的疲劳强度也从75MPa提升至90MPa。在高应力幅区,孪生是主要的变形机制,Basquin公式可用于描述应力-寿命关系(S-N)。然而,在低应力幅区,疲劳变形过程受位错机制控制,此时的寿命偏离S-N曲线并趋于长寿命区。由于循环塑性区的存在,在断裂表面附近存在大量的变形孪晶。传统挤压AZ31合金断口表面较为粗糙,且大量孪晶存在;而变面轧制材料的断口表面光滑。     

AZ91镁合金的应变疲劳行为研究

研究了铸态和挤压态AZ91镁合金的室温应变疲劳行为。结果表明，铸态镁合金表现为循环应变硬化，而挤压态镁合金可呈现循环应变硬化、软化或循环稳定。与此同时，对两种状态的AZ91镁合金的应变寿命行为和循环应力-应变行为进行了分析测试，确定了相应的室温应变疲劳参数。此外，利用扫描电子显微镜观察分析了铸态和挤压态AZ91镁合金的疲劳裂纹萌生和扩展行为。     

绿色缓蚀剂SDDTC对AZ31B镁合金的缓蚀作用及吸附行为

采用极化曲线、电化学阻抗谱、红外光谱及扫描电镜等方法研究了二乙基二硫代氨基甲酸钠(SDDTC)对AZ31B镁合金在3.5%(质量分数)NaCl溶液中的缓蚀作用及吸附行为。结果表明,SDDTC能有效抑制AZ31B镁合金在NaCl介质中的腐蚀,属阴极抑制为主的混合型缓蚀剂。当SDDTC浓度为5 mmol·L~(-1)时,缓蚀效果最好。SDDTC在AZ31B镁合金表面发生物理吸附,符合Langmuir吸附模型。吸附在表面的SDDTC形成较为致密的保护膜,有效抑制了AZ31B镁合金的腐蚀。     

高应变速率挤压下AZ31B合金的压缩断裂机制

采用分离式Hopkinson压杆装置测试研究了挤压态AZ31B镁合金的动态压缩断裂行为,用光学显微镜和扫描电镜观察分析了AZ31B镁合金的动态压缩断口形貌.结果表明,AZ31B镁合金沿挤压方向压缩时,断口形貌呈韧-脆混合的断裂特征,裂纹经多级萌生和扩展,最终导致断裂,宏观断裂面与压缩方向成45°角,为典型的剪切破坏.     

Corrosion Fatigue Behavior of 7A85 Aluminum Alloy Thick Plate in NaCl Solution

The S–N curves of 7A85-T7452 aluminum alloy in laboratory air and in neutral 3.5 wt% NaCl solution were obtained by axial fatigue tests. Results show that the detrimental effect of the aggressive solution was not noticeable at high-cyclic-stress regions, but the effect was significant at low-stress region. Corrosion fatigue mechanism was discussed by corrosion morphology analysis, fracture surface analysis and microstructure characterization. It was found that the corrosion fatigue crack commonly initialed at the localized intergranular corrosion site. TEM analysis showed that the microstructures of 7A85-T7452 aluminum alloy were characterized by fine and homogeneously distributed matrix precipitates, as well as continually distributed anodic grain boundary precipitates. The types of microstructures are the reason for its intergranular corrosion susceptibility. The corrosion fatigue process of 7A85 aluminum alloy in 3.5 wt% NaCl solution can be divided into four stages: the crack initiation stage, the stable growth stage with low and high growth rate and the final rupture stage. The sodium chloride solution mainly affected the crack initiation stage and the stable growth stage with low growth rate, and when the crack growth rate reached a threshold, the effect was reduced.     

Corrosion fatigue behavior of epoxy-coated Mg–3Al–1Zn alloy in NaCl solution

The corrosion fatigue behavior of epoxy-coated Mg–3Al–1Zn alloy was investigated in air and 3.5 wt%NaCl solution. Epoxy coating as a new method was used to improve the corrosion fatigue property of the material.Results show that the fatigue limit(FL) of the coated specimens is higher than that of the uncoated specimens in3.5 wt% NaCl solution because of the strengthening and blocking functions of the epoxy coating. The FL of the coated specimens in 3.5 wt% NaCl solution is as high as that in air. It implies that the coated specimens are not as sensitive to the environment as the magnesium alloy. The low tensile strength and the short elongation of the pure epoxy coating lead to that the fatigue crack of the coated specimen is always initiated from the epoxy-coating film Pores and pinholes accelerate the fatigue crack initiation process. Pinholes are caused by the corrosion reactions between the epoxy coating and the NaCl solution.     

300M钢腐蚀疲劳裂纹萌生的超载特性

本文对３００Ｍ钢在空气与３．５％ＮａＣｌ溶液中疲劳裂纹萌生的周期超载和单次超载特性进行了试验研究与理论分析。试验结果表明，３．５％ＮａＣｌ溶液中，单次超载所致腐蚀疲劳裂纹迟滞萌生的迟滞强度远小于周期超载的对应值。３．５％ＮａＣｌ溶液与空气中，超载迟滞强度均随超载比增大或等效应力幅水平的提高而增大，在腐蚀疲劳裂纹萌生寿命估算中，只考虑单次超载的迟滞规律，会得出大大保守的寿命估计，只有将单次超载和周期     

Corrosion fatigue crack propagation behavior of A7N01P-T4 aluminum alloy welded joints from high-speed train underframe after 1.8 million km operation

Using the potentiodynamic polarization analysis, the fatigue crack propagation behavior of A7N01P-T4 aluminum alloy metal inert gas welded joints cut from a high-speed train underframe after 1.8 million km operation was studied in air and in a 3.5 wt% NaCl solution. The fracture surface and crack growth path were analyzed using optical microscopy, scanning electron microscopy, and electron backscattered diffraction. The results reveal that the corrosion fatigue crack growth rate of an A7N01P-T4 welded joint in a 3.5 wt% NaCl solution is higher than that in air. Furthermore, the corrosion fatigue crack growth rate is noted to be the fastest in the heat-affected zone, followed by the base metal, whereas it is the slowest in the weld metal, which is consistent with the corrosion resistance of the A7N01P-T4 joints. The second phase is observed to exhibit a significant influence on the corrosion fatigue crack propagation path. The cracks are noted to grow toward the soft orientation and have obvious plastic deformation during the propagation process, which indicates that the anodic dissolution is the main cause of the corrosion fatigue crack growth.     

Effect of corrosive environment on the fatigue crack initiation and propagation behavior of Al 5454-H32

Fatigue behavior of aluminum alloy 5454- H32 was studied under laboratory air and 3 % NaCl solution environment using smooth cylindrical and notched plate specimens. Presence of 3 % NaCl environment during fatigue loading drastically reduced alloy fatigue life. The deleterious effect was pronounced in both types of specimens in the long- life regions, where the fatigue lives were lowered by as much as a factor of 10. However, the sharply notched specimens showed only a modest reduction in fatigue life in corrosive environment. The severe influence of the corrosive environment in the long- life (low- stress) regime cannot be explained merely by the early initiation of the fatigue crack from surface pits; the environmental contribution in the early crack growth regime must also be considered an important factor. Fracture surface studies revealed extensive pitting and some secondary cracking in the crack initiation region. It was shown that lowered fatigue life in Al 5454- H32 occurs by early initiation of fatigue cracks from surface pits. In addition, a corrosion pitting and secondary cracking process may be operative in the small crack growth region. This could have enhanced the early crack growth rate and thus contributed to the lower fatigue life in the long- cycle region.     

Monitoring of corrosion fatigue cracking using harmonic analysis of current responses induced by cyclic stressing

The response of a current induced by sinusoidal stress within an elastic deformation was measured during corrosion fatigue tests to monitor the course of corrosion fatigue cracking and to evaluate corrosion fatigue damage. The test material was an iron bar subjected to a passive potential in borate buffer solution containing 5 mM NaCl. Harmonic analysis was used to analyze the current response and to extract valuable parameters that were associated with crack initiation and corrosion fatigue damage. The current response before crack initiation consisted of only the fundamental current without any harmonics. Higher harmonic amplitudes of the current response, especially the second and third harmonics, appeared and increased after crack initiation. In addition, the phase shift between the strain and the fundamental current response to the sinusoidal stress decreased from nearly 90° towards 0° as harmonic amplitudes of the current response increase. The results indicate that the amplitudes of the harmonic current response and the phase shift are the optimal parameters for detecting crack initiation and evaluating corrosion fatigue damage.     

Corrosion fatigue crack growth behaviour of naval steels

Fatigue crack growth behaviour of two varieties of HSLA steels used in naval structural applications have been evaluated in air and 3.5% NaCl solution. In air both the HSLA steels showed similar resistance to fatigue crack growth. However, in 3.5% NaCl, the fatigue crack growth resistance of HSLA-80 steel was superior to that of HSLA-100. The apparent inferiority of HSLA-100 to corrosion fatigue crack growth resistance is attributed to rapid film formation and rupture, and occurrence of planar modes of failure. Effect of R-ratio on air fatigue and corrosion fatigue crack growth behaviour is rationalised by the concept of crack closure. Effect of cyclic frequency on corrosion fatigue behaviour is examined. It is noted that the mechanism of corrosion fatigue crack growth for the two HSLA steels changes with attendant change in the Paris slope. This leads to increase or decrease of crack growth rates, depending up on the ΔK range of interest.     

2024高强铝合金腐蚀小裂纹行为研究

本文研究了2024-T62高强铝合金在带有腐蚀预损伤的基础上的腐蚀小裂纹行为。小裂纹试验采用单边缺口拉伸试样（SENT）,在试样半圆形缺口根部预制直径大小为100～300μm的单腐蚀坑,然后在3．5％NaCl溶液中进行疲劳小裂纹试验。采用QUESTAR长焦距显微镜与摄像机相结合的QUESTAR—CCD监测腐蚀小裂纹的萌生及扩展情况。试验结果表明,小裂纹绝大多数萌生在缺口根部腐蚀坑位置,并呈半椭圆形的表面裂纹;该材料在R=0.06恒幅载荷下,小裂纹效应不明显;对应可观测到的最小裂纹长度为6～60μm,裂纹起始寿命约占疲劳寿命的15％～25％。     

火焰喷焊对碳钢疲劳寿命的影响

对三种不同基材状态的45~3钢经表面火焰喷焊Ni60后的疲劳性能及腐蚀疲劳性能变化的研究,表明强度高的调质态基材喷焊后的疲劳极限下降7%,而正火态和轧态基材喷焊后疲劳极限提高14%。当有3.5%NaCl腐蚀介质存在时,不管基材强度高或低,表面喷焊都使材料的腐蚀疲劳强度(10~7寿命)提高30%以上。文中通过断口金相分析阐明喷焊后基材表面组织结构变化及其对疲劳裂纹形成及扩展的影响,讨论了Ni60喷焊层在腐蚀疲劳过程的行为,为喷焊工艺在工业中的应用提供了依据。     

Effect of corrosive environment on fatigue property and crack propagation behavior of Al 2024 friction stir weld

In this investigation, 2024 aluminium alloy plates were friction stir welded, a sequence of experiments was performed including fatigue and crack propagation tests in air, under pre-corrosion and in a 3.5% NaCl solution, in combination with fractography analyses of near-threshold region, Paris region and finial fracture region with the aid of scanning electron microscopy (SEM). Results showed that the corrosive environment caused a dramatical decrease in fatigue lives of FS welds, the corrosion fatigue lives of FS welds were almost a half of those of the as-welded specimens. The crack growth rates in FS welds were higher than their counterparts in base materials, under the corrosive environment, the crack growth rate differences between base materials and FS welds become increasingly apparent with the increase of stress intensity factor range ΔK, but the pre-corrosion process had little effect on the FS welds' crack propagation behavior except for shortening the crack initiation lives greatly.