6061-T6铝合金单轴时间相关循环变形行为

对6061-T6铝合金进行系统的单轴应变循环和应力循环实验，揭示该材料在室温和高温下的循环变形行为，讨论环境温度、加载速率、峰，谷值保持对其应变循环特性及棘轮行为的影响。结果表明，6061-T6铝合金表现出弱的循环软化特性，其棘轮行为不仅依赖于平均应力和应力幅值的大小，还依赖于加载历史。尽管该合金的单拉行为对应变率的变化不敏感，但其循环变形行为却体现出明显的时间相关特性，即；应变循环下，在峰／谷值有保持时的响应应力幅值明显小于没有保持时的值，且随着保持时间的增加，响应应力幅值将进一步减小；应力循环下，在峰值有保持时产生的棘轮应变比没有保持时的值大，且随着峰值保持时间的增加及应力率的降低，棘轮应变明显增大。     

Al—Zn—Mg—Cu—Zr（-Sn）合金的强度和疲劳断裂行为

通过拉伸试验和疲劳裂纹扩展试验研究了Al-Zn—Mg—Cu-Zr（--Sn）合金的强度和疲劳断裂行为。运用光学显微镜（0M）、扫描电镜（SEM）和透射电子显微镜（TEM）对试验合金的微观组织进行分析检测。结果表明,Sn的添加可以阻碍固溶时Al-Zr-Mg-Cu-Zr合金晶粒的长大,也使得过时效Al-Zr-Mg-Cu-Zr-Sn合金的晶界无沉淀析出带（PFZ）变窄及晶界析出相变小,因此,提高了合金的抗疲劳裂纹扩展能力。此外,过时效的Al—Zn—Mg—Cu—Zr-Sn合金具有较高的抗拉强度。     

Effects of rare earths on friction and wear characteristics of magnesium alloy AZ91D

The influence of various rare-earth contents on the friction and characteristics of magnesium alloy AZ91D was studied.The results show that the wear r5esistance properties of rare-earth magnesium alloys are better than those of the matrix alloy under the testing conditions.Magnesium alloys undergo transition from mild wear to severe wear.The addition of rare earths refines the structure of alloys,improves the comprehensive behaviors of the magnesium alloys,increases the stability of oxidation films on worn surfaces,enhances the loading ability of rareearth magnesium alloys,and delays the transition from mild wear to severe wear effectively.     

固溶处理和疲劳保载时间对FGH96合金高温疲劳裂纹扩展速率的影响

测定了FGH96合金在650℃空气环境中的疲劳裂纹扩展速率,研究了合金显微结构、固溶冷却速率及保载时间对FGH96合金裂纹扩展速率的影响。结果表明,控制固溶后以适当的方式冷却,使得二次和三次γ′相均匀匹配析出,可以获得具有良好疲劳裂纹扩展抗力的合金组织。FGH96合金的高温疲劳裂纹扩展速率随保载时间的增加而增加,其断裂模式为沿晶断裂。     

Stress-corrosion cracking of aged AlCuMg alloys in NaCl solution

Pitting potentials and stress corrosion life-times of AlCuMg alloys (mainly 2024 alloy) with various ageing structures have been measured in a de-aerated 1M NaCl solution under conditions of controlled potential. The aged alloy, which has the higher susceptibility to stress-corrosion cracking, showed two pitting potentials corresponding to pitting at the grain boundaries and within the grains. The susceptibility of the alloys to intergranular stress-corrosion cracking occurred at potentials above the pitting potential of the grain boundaries. The intergranular stress-corrosion cracking is caused not by the dissolution of the grain boundary precipitates (S phase) but by the pitting dissolution of the solute-denuded zones along the grain boundaries. Aspects of SCC in the alloys are similar to those in the Al-4%Cu alloy without Mg.     

Thermomechanical fatigue behaviours of a third generation γ-TiAl based alloy

In order to clarify the behaviours of thermomechanical fatigue (TMF) of a third generation γ-TiAl based alloy, the influence of related microstructural instability during TMF process on stress–strain response, fatigue life and fracture way under in-phase (IP) and out-of-phase (OP) loading mode was investigated. Cyclic softening at high temperature (>700 °C) arises from the dissolution of α₂ lamellae and recrystallization of γ phase. Cyclic hardening at low temperature (<550 °C) is caused by strong interaction between dislocations. As temperature increases, the mean stress and remained plastic strain range increase, leading to severe TMF damage. Owing to the formation of superfine γ grains in IP condition, a superimposed effect of creep and fatigue damage contributes to the TMF failure. OP loading mode brings about the coarsening of primary equiaxed γ grains. Fatigue damage displays the intergranular fracture and transgranular cleavage fracture ways of coarse γ grains.     

A damage-coupled multi-axial time-dependent low cycle fatigue failure model for SS304 stainless steel at high temperature

Based on the time-dependent strain cyclic characteristics and fatigue behaviors of SS304 stainless steel under multi-axial cyclic loading at 700 ? C, and in the frame of unified visoco-plastic cyclic constitutive model and continuum damage mechanics theory, the damage-coupled multi-axial time-dependent constitutive model and fatigue failure model were proposed. In the model, the evolution equation of damage was introduced in and the time-dependent effects, e.g. holding time, loading rate, were taken into account. The model was applied to the simulation of whole-life cyclic deformation behaviors and prediction of LCF life for SS304 stainless steel in multiaxial time-dependent low cycle fatigue tests. It is shown that the simulated results agree well with experimental ones.     

Effects of cooling rate and stabilization annealing on fatigue behavior of β-processed Ti-6Al-4V alloys

The effects of stabilization annealing and cooling rate on high cycle fatigue (HCF) and fatigue crack propagation (FCP) behaviors of β-processed Ti64 alloys were examined. After β-process heating above β transus, two different cooling rates of air cooling (β-annealing) and water quenching (β-quenching) were utilized. Selected specimens were then underwent stabilization annealing. The tensile tests, HCF and FCP tests on conducted on the β-processed Ti64 specimens with and without stabilization annealing. No notable microstructural and mechanical changes with stabilization annealing was observed for the β-annealed Ti64 alloys. However, significant effect of stabilization annealing was found on the FCP behavior of β-quenched Ti64 alloys, which appeared to be related to the built-up of residual stress after quenching. The mechanical behavior of β-processed Ti64 alloys with and with stabilization annealing was discussed based on the micrographic examination, including crack growth path and crack nucleation site, and fractographic analysis.     

Effect of Overaging on Fatigue Crack Propagation and Stress Corrosion Cracking Behaviors of an Al-Zn-Mg-Cu Alloy Thick Plate

The fatigue property as well as stress corrosion cracking (SCC) resistance of an Al-Zn-Mg-Cu alloy thick plate in peak-aged and overaged tempers (T7351 and T7651) is systematically investigated by fatigue crack propagation (FCP) test and slow strain rate test (SSRT). Microstructural characterization is examined by transmission electron microscopy and scanning electron microscopy. Results reveal that the T7351 alloy has lower strength but higher electrical conductivity as compared to T7651 alloy. The FCP resistance of T7351 alloy is superior to that of the T7651 alloy due to the coarser precipitates in the highly overaged alloy in which the strain localization is reduced by promoting homogeneous slip. In addition, the SSRT test suggests a higher SCC resistance in T7351 alloy. The enhanced SCC resistance is found to depend on grain boundary precipitate characteristics and crack propagation resistance of the alloys.     

非比例载荷下Al-7Si-0.3Mg合金的循环特性及微观机理

在等效应变幅为0.22%时, 研究了Al--7Si--0.3Mg铸造铝合金在比例、圆形、正方形、菱形、矩形与椭圆形路径下的循环变形行为, 并用TEM观察了疲劳失效试样的位错结构. 结果表明: 在多轴加载条件下, 材料均表现出循环硬化现象, 循环硬化的速率和程度对加载路径有依赖性; 非比例载荷下材料的疲劳寿命远小于比例加载时的寿命, 且非比例加载下的疲劳寿命对各种非比例加载路径有依赖性, 其中圆形路径下疲劳寿命最短; 位错在不同的加载路径下形成不同的组态结构, 位错与强化相、枝晶界及位错间的交互作用是铸造铝合金发生循环硬化的主要原因.     

Pitting and Stress Corrosion Cracking Susceptibility of Nanostructured Al-Mg Alloys in Natural and Artificial Environments

The stress corrosion cracking (SCC) behavior of two developmental nanocrystalline 5083 alloys with varied composition and processing conditions was studied. The results were compared to a commercial aluminum AA 5083 (H111) alloy. The pitting densities, size and depths, and residual tensile strengths were measured after alternate immersion in artificial seawater and atmospheric exposure under different loading conditions. Optical and scanning electron microscopy (SEM) with EDX was used to analyze the fracture surfaces of failed specimen after removal at selected intervals and tensile testing. One of the nanostructured Al-Mg alloys exhibited significantly superior pitting resistance when compared to conventional microstructured AA 5083. Under conditions where pitting corrosion showed up as local tunnels toward phase inclusions, transgranular cracking was observed, whereas under conditions when pitting corrosion evolved along grain boundaries, intergranular cracking inside the pit was observed. Pit initiation resistance of the nano alloys appears to be better than that of the conventional alloys. However, long-term pit propagation is a concern and warrants further study. The objective of this investigation was to obtain information regarding the role that ultra-fine microstructures play in their degradation in marine environments and to provide insight into the corrosion mechanisms and damage processes of these alloys.     

Effects of Two Different Dynamic Loading Conditions on Spall and Damage of 7075 Aluminum Alloy

The fracture behaviors of the 7075 aluminum alloy under two different dynamic loading conditions are investigated by means of a light-gas gun. The fracture surfaces obtained in the spall test are compared to the fracture surfaces obtained with a blunt projectile struck to the aluminum alloy plate. Optical and scanning electron microscopes are used in the investigation. For the plate-impact test, spall of the target was attributed to intergranular fracture caused by the tensile stress. The fracture behavior during projectile penetration is complex and consists of several fracture modes in addition to that the fracture is also of dynamic character. The penetration process of aluminum alloy target included: plugging stage, the microcracks nucleation stage, and the final tensile fracture stage. Mixed intergranular brittle/ductile fracture was observed, and brittle fracture played a dominate role.     

Fatigue life assessment of weld joints manufactured by GMAW and CW-GMAW processes

The fatigue strength of weld joints manufactured using gas metal arc welding and cold wire GMAW (CW-GMAW) was evaluated under stress-controlled cyclic loading. The material used in this study was class ASTM 131 grade A steel, joined using ER70S wire filler metal. The addition of cold wire led to a decrease in the amount of intergranular ferrite and an increase in hardness in the heat-affected zone. The assessment of fatigue life was performed by using the Weibull distribution and the results revealed that with a 99% reliability the joints produced using the CW-GMAW process have a longer fatigue life, especially as the stress amplitude increases.     

Corrosion behavior of 2195 and 1420 Al-Li alloys in neutral 3.5% NaCl solution under tensile stress

1 Introduction Al-Li alloys, compared with traditional Al alloys, possess many excellent properties, such as lower density, greater elastic modulus and higher specific strength. 2195 Al-Li alloy with high strength and good ability to weld, was applied to …     

含稀土镁合金的摩擦磨损性能

研究稀土对AZ91和AM60镁合金摩擦磨损性能的影响。结果表明:在所研究的范围内,稀土镁合金的摩擦磨损特性明显优于基体合金;含稀土镁合金与不含稀土镁合金的磨损速率都随载荷的增加而增加,AZ91镁合金的耐磨性要远远高于AM60稀土镁合金。磨损机制在实验条件下都相同,均发生由轻微磨损向严重磨损的转变;稀土的加入细化合金组织,改善镁合金的综合性能,增强磨损表面氧化膜的稳定性,提高稀土镁合金的承载能力,有效地延迟由轻微磨损向严重磨损的转变过程。     

Characterization of deformation behavior for waspaloy under creep-fatigue interaction

Hysteresis loops of strain controlled LCF tests for Waspaloy were characterized within the temperature range of 350°C and 600°C. Materials deformation behaviors under symmetric and asymmetric loading conditions were assessed using Chaboche’s viscoplasticity model. The strain hardening and stress relaxation occurring in the loading cycles were estimated. Hardening plays an important role at an early stage of loading, but the stress relaxation becomes dominant at a later stage. The observed change in the slope of hysteresis loops between the first cycle and the second cycle and mean stress relaxation under asymmetric loading conditions may be explained by the evolution of the kinematic hardening variable which depends on the inelastic strain range. The effect of creep on the cyclic shake down is expected to be limited.     

7075铝合金FSW接头腐蚀疲劳性能及断裂特征

以7075-T6铝合金搅拌摩擦焊接头为研究对象,对其显微组织结构、3.5% NaCl(质量分数)溶液腐蚀疲劳寿命和腐蚀疲劳断裂特征进行了研究,分析了7075铝合金搅拌摩擦焊接头的腐蚀疲劳性能及断裂过程.结果表明,7075-T6铝合金搅拌摩擦焊接头腐蚀疲劳S-N曲线方程为lgN=5.845-0.014S,随着应力幅增大,腐蚀疲劳寿命大幅度降低;腐蚀疲劳裂纹起源于接头的热影响区,逐渐扩展最终断裂于接头的焊核区.腐蚀疲劳断口存在多个裂纹源,且受到应力集中作用的影响,裂纹源萌生于腐蚀坑处.高应力作用加剧了试样边角部分的腐蚀损伤,导致边角比平面位置腐蚀程度更严重.裂纹扩展区出现了明显的晶间断裂和疲劳辉纹;在腐蚀介质和交变载荷的共同作用下,裂纹扩展区腐蚀程度最重,晶界处产生了阳极溶解现象并产生了“冰糖块状”和“蚁巢状”的形貌特征;瞬断区产生了大量解理台阶和二次裂纹,为脆性断裂,在第二相粒子分布区域存在孔洞形貌特征.     

挤压变形Mg-x%Al-3%Ni合金的低周疲劳行为

通过对挤压变形Mg--x%Al--3%Ni(x=4, 5, 6, 7, 质量分数)合金进行总应变控制模式的室温疲劳实验, 研究了不同Al含量的挤压变形Mg--x%Al--3%Ni合金的循环应力响应、疲劳寿命和循环应力--应变行为. 结果表明: 不同Al含量的挤压变形Mg--x%Al--3%Ni合金均表现为循环应变硬化; 挤压变形Mg--x%Al--3%Ni合金的应变疲劳寿命与塑性应变幅、弹性应变幅间的关系分别服从Coffin--Manson和Basquin关系式.     

Effect of Hydrogen on the Fatigue Resistance of Titanium Alloy VT6Ch Under Various Loading Conditions

The effect of hydrogen on the fatigue resistance of alloy VT6Ch under various loading parameters (cycle characteristics, loading frequencies, stress concentration factor) is studied for specimens cut from a plate in different directions with respect to the rolling axis and annealed in vacuum under various temperatures. The method of design of experiment is used for determining the relation between the maximum failure stress and the test conditions.     

Ni-Cr-Mo耐蚀合金在CaCl_2-CaF_2熔盐中的腐蚀行为

通过全浸腐蚀实验,采用XRD,OM,SEM和EDS研究了Hastelloy C276和Hastelloy N合金在80%CaCl_2+20%CaF_2(质量分数)熔盐中的腐蚀行为.结果表明,Hastelloy C276和Hastelloy N合金均发生晶间腐蚀,其具体形式为Cr和Mo元素沿晶界发生选择性脱溶腐蚀.由于腐蚀层和腐蚀产物对元素扩散的阻碍作用,Hastelloy C276和Hastelloy N合金在熔盐中的腐蚀速率均逐渐减缓,其腐蚀失重C与腐蚀时间t符合C=Kt~n关系.