Effects of Ag addition on the microstructures and properties of Al-Mg-Si-Cu alloys

Effects of Ag addition on the microstructures, aging characteristics, tensile properties, electrochemical properties, and intergranular corrosion (IGC) properties of Al-1.1Mg-0.8Si-0.9Cu-0.35Mn-0.02Ti alloy were investigated using scanning electronic microscopy and transmission electronic microscopy. The aging process of Al-Mg-Si-Cu alloys was accelerated by the addition of Ag. The strength of peak-aged Al-Mg-Si-Cu alloys was enhanced by Ag addition because of the high density of β"- and L-phase age-hardening precipitates. The corrosion performance of the Al-Mg-Si-Cu alloy is closely related to the aging conditions and is independent of the Ag content. The IGC susceptibility is serious in the peak-aged alloy because of the continuous distribution of Cu-rich Q-phase precipitates along grain boundaries. Ag addition reduces the size of the grain-boundary-precipitate Q phase and the width of the precipitate-free zone and thus results in decreased IGC susceptibility of Al-Mg-Si-Cu alloys.     

服役经历对7N01铝合金断裂力学性能的影响

材料在服役过程中的可靠性和耐久性是结构安全评估的基础,针对某型高速列车上服役多年的7N01铝合金材料,进行实验室空气环境下断裂力学性能参数测试,并分析服役前后疲劳裂纹扩展行为差异.结果表明:服役经历促使材料的各项断裂力学性能退化,呈规律性下降趋势;服役材料稳态疲劳裂纹扩展阶段da/d N-ΔK关系曲线呈折线现象,分段拟合方法能够精确描述其疲劳裂纹扩展行为.微观形貌的结果显示:服役后7N01铝合金的疲劳裂纹早期扩展区、稳定扩展区和快速断裂区的断裂表面依次为脆性断裂特征、塑性疲劳条带和脆性疲劳条带混合模式及沿晶和穿晶的混合断裂形貌.     

Al-Cu-Li合金轧制厚板的 疲劳性能及断裂机理研究

利用疲劳试验机、扫描电子显微镜(SEM)和透射电子显微镜(TEM)等设备研究了95 mm厚的Al-Cu-Li合金热轧板材不同方向的组织特征、疲劳性能和断裂机理.结果表明:Al-Cu-Li合金热轧厚板的力学性能和疲劳性能均存在各向异性.LT向(横向)的力学性能和疲劳性能均优于L向(纵向)和ST向(高向),L向(纵向)的力学性能较ST向更好,但疲劳性能较ST向更差.Al-Cu-Li合金厚板的疲劳裂纹源主要出现在试样表面、近表面的夹杂物和晶界处.裂纹扩展过程中,LT向裂纹扩展路径较L向更曲折,ST向具有典型的解理特征,瞬断区形貌与静态拉伸断口相似.晶界越多,裂纹扩展阻力越大,LT向和ST向的未溶第二相能够有效阻碍裂纹扩展,导致其疲劳性能优于L向.     

Strain localization and damage development in 2060 alloy during bending

The microstructure evolution and damage development of the third-generation Al–Li alloy 2060 (T8) were studied using in situ bending tests. Specimens were loaded with a series of punches of different radii, and the microstructure evolution was studied by scanning electron microscopy, electron backscatter diffraction, and digital image correlation (DIC) methods. The evolution of the microscopic fracture strain distribution and microstructure in 2060 alloy during bending was characterized, where the dispersion distribution of precipitates was recorded by backscattered electron imaging and later inputted into a DIC system for strain calculations. The experimental results showed that strain localization in the free surface of bent specimens induced damage to the microstructure. The region of crack initiation lies on the free surface with maximum strain, and the shear crack propagates along the macro-shear band in the early stages of bending. Crack propagation in the later stages was interpreted on the basis of the conventional mechanism of ductile fracture.     

时效近等原子比钛镍形状记忆合金TEM分析

应用TEM分析技术对一种富Ni钛镍形状记忆合金时效后的组织进行了研究 ,结果表明 ,在不同温度下时效析出物的相结构和形态是不同的 .在 6 73～ 773K温度时 ,析出物都是Ti1 1 Ni1 4 相 ,随时效温度的提高 ,析出物的形态由细点状变为透镜状截面的椭圆板 ,这种相与B2母相是共格的 ,它的应变场使R相变与马氏体相变分离 ,Ti1 1 Ni1 4 相的析出会改善钛镍形状记忆合金的性质 .此外 ,在更高温度下时效合金将析出稳定的TiNi3相 ,该相与基体没有共格关系 ,同时导致基体镍含量下降 ,提高了马氏体转变温度 .     

粉末高温合金FGH97疲劳裂纹扩展行为

测定不同晶粒尺寸、γ'相以及不同Hf含量的粉末高温合金FGH97在650℃高温条件下的疲劳裂纹扩展速率,并将其与FGH95和FGH96两代粉末合金的疲劳裂纹扩展速率进行对比. 用定量分析的方法对FGH97合金在疲劳断裂各个阶段的行为特征进行分析. 较大晶粒尺寸的FGH97合金具有较低的裂纹扩展速率,合理的二次和三次γ'相匹配析出,可以获得较高的疲劳寿命;Hf元素的添加使合金的整体疲劳寿命增大;FGH97合金与FGH95和FGH96相比,具有较高的疲劳裂纹萌生抗力,更低的高温疲劳裂纹扩展速率.     

High temperature tensile properties,fracture behaviors and nanoscale precipitate variation of an Al-Zn-Mg-Cu alloy

In order to assess Al-Zn-Mg-Cu alloys as potential high temperature structural materials,the hardness,tensile properties and fracture behaviors of 7085 Al alloy were investigated at various temperatures from room temperature to 175℃.High-resolution transmission electron microscopy was used to investigate the evolutions of precipitates at different temperatures,particularly on the relationship between microstructural evolution and tensile strength.The results reveal that both the microstructure and mechanical properties of the alloy are quite sensitive to the environmental temperature.As the temperature increases,the hardness and strength decrease while the elongation and reduction of area increase.As tensile testing temperature rises,the strain hardening exponent(n) decreases due to the thermal softening effect.The fracture mode of the alloy transforms from mixture of intergranular and transgranular fracture to completely transgranular dimples when tensile testing temperature reaches 150℃.The precipitate sequence during high temperature tests is coincident with that of aging.With the increase of tensile testing temperature,the mean precipitate radius grows larger,and the distribution of grain boundary precipitates transforms from continuous to discontinuous.     

回火对微合金管线钢疲劳裂纹扩展行为的影响

采用MTS858电液伺服万能试验机、扫描电镜及透射电镜研究回火对一种高强度微合金管线钢疲劳裂纹扩展行为的影响。研究结果表明:回火可提高微合金管线钢疲劳裂纹扩展的门槛值,降低疲劳裂纹扩展速率,但对裂纹扩展稳态区的扩展速率影响不大;回火使碳氮化物沉淀析出、晶间马氏体/奥氏体(M/A)组元由岛状转变为点状及细条状,形成马氏体薄膜结构,阻碍变形和裂纹在材料中扩展,增加裂纹的偏折程度;在控轧控冷终冷温度进行2～4 h回火热处理,可以提高微合金管线钢强韧性和抗疲劳裂纹扩展能力。     

Failure analysis of 18CrNi3MoA drilling tails

The fracture of a drilling tail made of 18CrNi3MoA steel in the exchanging water hole was analyzed in terms of inclusions, prior austenite grain size, carburized layers, and fatigue fracture morphology by means of optical microscopy, quantitative metallography, and scanning electron microscopy (SEM). Fatigue crack initiation and propagation on the drilling tail were also studied. The results showed that the fracture on the drilling tail was not induced by inclusions and the distribution inhomogeneity of prior austenite grain size. Instead, because the outside surface of the exchanging water hole was not continuously geometric, there was a great deal of stress concentration in those areas under continuous twisting, axial impact, and corrosion of mineral water. Thus three crack sources emerged in these areas. Initial cracks centered on these sources spread from the outside surface to the inside surface, and eventually the drilling tail ruptured. Furthermore, the fractograph of the region around the crack sources exhibited a typical ductile dimple fracture pattern, and cracks initiated on the outside surface of the carburized layers of the exchanging water hole. Three crack sources could be the sources of fatigue crack propagation. Based on the analysis of fatigue crack propagation, it was further demonstrated that fatigue damage originated from the outside surface of the exchanging water hole.     

Effects of precipitates and inclusions on the fracture toughness of hot rolling X70 pipeline steel plates

In order to investigate the fracture toughness, crack tip opening displacement (CTOD) experiments were conducted on two X70 pipeline steel plates with different rolling processes. After the experiments, optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to observe the microstructure and fracture morphology. The effects of precipitates on the fracture toughness and the crack initiation mechanism induced by inclusions were analyzed. The CTOD result shows that the steel with a lower finishing cooling temperature has a higher fracture toughness. Inclusions with different shapes and two kinds of precipitates with different sizes were observed. It can be concluded that precipitates with different sizes have different effects and mechanisms on the fracture toughness. Distinguished from the earlier researches, inclusions enriched in silicon can be also served as the crack initiation.     

A Cu-Cr alloy with nano and microscale Cr particles produced in a water-cooled copper mold

Microstructures have profound effects on the hardness and strength of Cu-Cr alloys. The microstructures of a Cu-Cr alloy cast in a water-cooled copper mold were studied in the present work. The scanning electron microscopy (SEM) results show that there are the copper matrix saturated with chromium, spherical precipitates of chromium separated from liquid phase during cooling before the initiation of solidification, and a eutectic phase in grain boundary areas. To investigate the effect of age-hardening treatment on the microstructures and properties of the material, some samples were subsequently age-hardened in a salt bath and investigated by transmission electron microscopy (TEM). The results show that coherent precipitates with the diameter of 11 nm are detectable in the samples before and after the age-hardening stage. Of course, the volume fraction of coherent precipitates is higher after the aging process.     

690合金传热管疲劳裂纹扩展研究

采用销加载拉伸方法和直流电压降法测试技术,测量了室温和高温325℃空气中3种不同工艺的690合金传热管的疲劳裂纹扩展速率.试验采用Paris-Erdogan公式进行拟合分析,证明了结果的真实性和可靠性.高温加速了疲劳裂纹扩展.由疲劳裂纹扩展速率曲线可以预测出3种690管材在高温325℃下的门槛应力强度因子幅值ΔKth,扫描电子显微镜下观察断口形貌,疲劳裂纹的扩展为穿晶形式,在穿晶断口上观察到明显的疲劳辉纹和微塑性区.     

Fatigue Crack Propagation Behavior of Rubber-toughened Epoxy Resin

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Effects of aging on the microstructure of a Cu-Al-Ni-Mn shape memory alloy

The influence of aging on the microstructure and mechanical properties of Cu-11.6wt%Al-3.9wt%Ni-2.5wt%Mn shape memory alloy (SMA) was studied by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometer, and differential scanning calorimeter (DSC). Experimental results show that bainite, γ_2, and α phase precipitates occur with the aging effect in the alloy. After aging at 300dgC, the bainitic precipitates appear at the early stages of aging, while the precipitates of γ₂ phase are observed for a longer aging time. When the aging temperature increases, the bainite gradually evolves into γ₂ phase and equilibrium α phase (bcc) precipitates from the remaining parent phase. Thus, the bainite, γ₂, and α phases appear, while the martensite phase disappears progressively in the alloy. The bainitic precipitates decrease the reverse transformation temperature while the γ₂ phase precipitates increase these temperatures with a decrease of solute content in the retained parent phase. On the other hand, these precipitations cause an increasing in hardness of the alloy.     

应力集中对2024铝合金高周疲劳寿命的影响

研究了2024铝合金常温下的高周疲劳性能,获得了2024铝合金在不同应力状态下的S-N曲线.分析了疲劳试样的断口形貌和裂纹萌生/扩展机理,以及疲劳试样的组织结构与疲劳性能之间的关系.结果表明,疲劳极限随着应力集中系数的增加而降低,当应力集中系数相同时,疲劳极限随着应力比的减小而降低;2024铝合金的疲劳断口以穿晶断裂为主;第二相颗粒起到了阻碍疲劳裂纹扩展的作用,使2024铝合金的高周疲劳强度得到了明显提高.     

连续挤压Al-1.1Mg-0.3Cu合金的拉伸性能和加工硬化行为

为了提高Al-1.1Mg-0.3Cu合金线杆的拉伸性能,通过金相、透射电镜、扫描电镜显微组织观察和拉伸试验分别对连续挤压态及拉拔退火态合金的微观组织和拉伸性能进行了研究.结果表明:连续挤压成形工艺有助于进一步改善合金的拉伸性能,与传统拉拔后退火处理工艺相比,通过连续挤压工艺制备的合金组织晶粒细小而均匀,沉淀相和位错密度较少,致使合金的延伸率相对较高而加工硬化率相对较低;此外,相比传统工艺,由连续挤压工艺制备的合金拉伸试样断口形貌中韧窝更深、更细小.     

Fatigue small crack growth threshold determination of a high-Nb TiAl alloy at different temperatures by in-situ observation

The purpose of this paper is to estimate the fatigue crack growth threshold of a high-Nb TiAl alloy at the different temperatures based on scanning electron microscopy （SEM） in-situ observation. The results indicated that the fatigue crack growth threshold △Kth of a nearly lamellar high-Nb TiAl alloy with 8% Nb content at room temperature and 750℃ was determined as 12.89 MPa.m^1/2 and 8.69 MPa.m^1/2, respectively. The effect of the elevated temperature on the fatigue crack growth threshold cannot be ignored. At the same time, the early stage of fatigue crack propagation exhibited multicrack initiation and bridge-link behavior.     

固溶处理对2E12铝合金组织及疲劳断裂行为的影响

通过对2E12铝合金进行493 ℃/20 min和503℃/20 min固溶处理,研究固溶处理对2E12合金室温拉伸及疲劳性能的影响.采用X线衍射分析、扫描电镜和金相分析对合金组织及疲劳断口进行观察.研究结果表明:提高固溶处理温度对合金拉伸性能影响不大,但可明显减少合金中残留相(Al2Cu,A12CuMg)的含量;经503 ℃/20 min处理后合金的拉伸强度较经493℃/20 min处理后合金的拉伸强度略有提高,但其疲劳寿命延长约20％;合金中残留相在疲劳过程中容易开裂形成微小二次裂纹,在主裂纹扩展过程中起到桥接作用,从而加速疲劳裂纹扩展,降低合金疲劳性能.     

Si对Al-Zn-Mg-Cu合金组织、断裂和局部腐蚀行为的影响

采用光学显微镜(OM)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)以及室温拉伸、剥落腐蚀等测试方法,研究了微量的Si对Al-Zn-Mg-Cu铝合金组织、性能和局部腐蚀的影响.研究表明:随着Si含量增加,合金的强度先增加再减小,0.040%Si(质量分数)时合金的强度出现峰值;合金的力学性能各向异性随Si含量的增加,先减小再增大,0.025%Si时,力学性能各向异性最小;随着Si含量的升高,合金横向断裂模式从以穿晶断裂为主向沿晶断裂转变.与合金纵向强度变化幅度比较,Si含量对合金横向强度影响显著.     

7075-T6铝合金搅拌摩擦焊接头腐蚀疲劳断口

利用电子显微镜和扫描电镜对7075-T6铝合金搅拌摩擦焊接头腐蚀疲劳断口进行研究,分析了 7075-T6铝合金搅拌摩擦焊接头腐蚀疲劳裂纹的发展过程。结果表明,搅拌摩擦焊接头腐蚀疲劳裂纹萌生 于焊核区和热力影响区的交界处并逐渐向热力影响区内部扩展,最终延伸并断裂于热影响区。腐蚀疲劳断 口存在多个裂纹源,裂纹源萌生于腐蚀坑处。腐蚀疲劳裂纹扩展分为低速扩展、稳态扩展和快速亚稳态扩展 3个阶段。在低速扩展阶段,产生了很多点蚀坑。在稳态扩展阶段,腐蚀疲劳具有脆性断裂特征。在快速亚 稳态扩展阶段,循环荷载成为裂纹扩展的主要驱动力而腐蚀作用的影响减弱。腐蚀疲劳裂纹扩展的瞬断区 呈现以脆性断裂为主的断裂特征。