含两种尺度微裂纹的高强铝合金的断裂韧性模型

基于高强铝合金在断裂过程中萌生不同尺度微裂纹的机制,用断裂力学建立两种尺度微裂纹影响应力应变场的规律,导出高强铝合金拉伸延性与两种尺度微裂纹的关系,由断裂韧性与拉伸延性的关系建立了高强铝合金断裂韧性与双级微裂纹的非线性关系模型。通过模型解析,分析两种尺度微裂纹体积分数对高强铝合金断裂韧性的影响规律。结果表明:随着一级微裂纹体积分数的增加,材料的断裂韧性开始迅速下降,然后缓慢降低;在较大尺度微裂纹之间萌生小尺度微裂纹,将显著降低合金的断裂韧性。将高强铝合金的结晶相作为一级微裂纹,将弥散相和粗大析出相作为二级微裂纹,预测高强铝合金断裂韧性随两种尺度相(微裂纹)的变化,其规律与实验结果较为吻合。利用模型解析与实验验证结果,提出了改善高强铝合金断裂韧性的组织控制方向。     

高速列车用高强铝合金焊接接头疲劳裂纹的扩展特性

使用疲劳试验机测试A7N01铝合金焊接接头的疲劳裂纹扩展速率,采用光学显微镜对焊接接头的显微组织进行分析,使用扫描电镜对疲劳断口形貌进行研究。结果表明:母材的显微组织为时效状态的α(Al)基体与粗大一次相、弥散二次强化相以及少量的夹杂物构成的轧制状态组织;热影响区的显微组织为α(Al)基体以及少量未固溶的一次相组织,焊缝的显微组织为α(Al)基体与离异共晶组织。焊接接头的显微组织以及第二相粒子尺度不同导致了疲劳裂纹扩展特性不同。随着应力比的增加,疲劳裂纹扩展速率均呈增大趋势。裂纹尖端塑性区内的第二相粒子与基体的不协调塑性变形可促进疲劳裂纹的扩展,影响疲劳裂纹的扩展速率。     

含有不同尺度量级第二相的高强铝合金断裂韧性模型

基于合金中不同尺度第二相对其塑性的影响以及塑性与韧性指标之间的人在联系，建立了高强铝合金中粗大第二相颗粒，中间尺寸第二相颗粒以及时效强化第二相颗粒形态与其断裂韧性间的多元非线性关系解析模型。借助于该模型的理论分析，可以得出不同尺度第二相颗粒尺寸，体积分数改变对高强铝合金断裂韧性的定量影响，并且可以解释铝合金板材断裂韧性的取向效应以及加工变形对断裂韧性的弱化作用，更进一步地能够给出在确保铝合金高强度的前提下改善其断裂韧性的优化方案。     

含铒铝合金显微组织对疲劳裂纹扩展的影响

对含铒铝合金板材进行了不同的热处理,分析了各板材的显微组织对疲劳裂纹扩展和力学性能的影响。结果表明:晶粒的长/径比越小,板材的疲劳裂纹扩展速率越慢;且随着疲劳裂纹扩展速率的降低,裂纹宽度增加且裂纹路径曲折;稀土元素Er的加入,在合金中形成Al_3(Er,Zr)第二相粒子,其强烈地钉扎位错,阻碍位错运动,以减少位错在晶界处的聚集产生的应力集中,降低板材的疲劳裂纹扩展速率。     

A7N01铝合金退火处理后焊接接头疲劳裂纹扩展特性

针对高速列车底架结构采用A7N01铝合金的焊接接头进行了疲劳裂纹扩展速率试验,利用扫描电镜以及光学显微镜方法,研究了母材、热影响区及焊缝微观组织的演化,晶粒中弥散强化相、粗大强化相、粗大析出相以及杂质相等第二相粒子对疲劳裂纹扩展抗力的影响,进而研究A7N01铝合金焊接接头不同区域的疲劳裂纹扩展机制,为轨道车辆的安全可靠性评估提供基础性数据。     

微观组织对2E12铝合金疲劳裂纹扩展的影响

研究室温大气环境下2E12铝合金微观组织与疲劳裂纹扩展行为之间的关系,并分别利用光学显微镜、透射电镜和扫描电镜对合金的微观组织及疲劳断口进行分析。结果表明:合金的疲劳裂纹扩展速率与合金中微米级第二相粒子的体积分数成正比,且随着亚微米级第二相粒子尺寸的增加及晶粒尺寸的减小,合金的疲劳裂纹扩展速率明显降低,尤其是在Pairs区及以后,其值降低30%;在低速扩展区,合金断口呈现脆性的准解理断裂特征,在中速扩展区,合金出现清晰的疲劳条纹。     

含有不同尺度量级第二相的高强铝合金拉伸延性模型

基于合金中不同尺度第二相在其断裂过程中的作用机制及位错理论 ,建立了高强铝合金中结晶相、沉淀相以及析出强化相性质与其拉伸延性间的多元非线性关系模型。结果表明对于 2 0 2 4铝合金的拉伸延性 ,该模型的预测值与相应的实验值吻合良好。同时借助于该模型的理论分析 ,可以得到在确保高强铝合金强度不降低的前提下提高其延性的优化方案。     

退火温度对Al-4.2Cu-1.4Mg合金的微观结构和力学性能的影响

针对航空用高强Al-4.2Cu-1.4Mg铝合金,研究了退火温度对微观组织结构、力学性能和疲劳裂纹扩展速率的影响。采用金相显微镜、扫描电镜和能谱分析手段对合金的微观组织和析出相种类进行了分析。结果表明:退火温度从380℃提高到440℃后,T3态板材中的晶粒尺寸不断细化,第二相Al2CuMg和Al2Cu的体积分数逐渐减少,因此疲劳裂纹扩展速率呈下降趋势。另外,随着中间退火温度的升高,成品板材的拉伸强度有少量降低,而伸长率则有所提高。     

1500 MPa级贝氏体/马氏体复相高强钢的疲劳断裂特性

对一种C—Si—Mn—Cr合金钢，通过900℃奥氏体化20min，空冷及280与370℃回火2h，获得抗拉强度为1500MPa的新型无碳化物贝氏体／马氏体(B／M)复相组织高强钢．采用C—T试样进行疲劳实验，测定了疲劳裂纹扩展速率(da／dN)及疲劳门槛值(△Kth)，利用扫描电镜观测了疲劳裂纹在疲劳循环过程中的扩展路径，分析断口形貌与显微组织间的关系．结果表明，这种无碳化物B／M复相高强钢具有较高的△Kth值，并且能明显地降低da／dN，其原因在于B／M复相组织高强钢独特的精细组织结构及疲劳裂纹尖端的闭合抗力的提高．     

2050铝合金的疲劳裂纹扩展行为研究

研究取样方向、应力比以及微观组织对2050铝合金疲劳裂纹扩展速率的影响。结果表明:取样方向和应力比对疲劳裂纹扩展速率在近门槛区和快速扩展区的影响显著,而在稳态扩展区的影响则相对较小;L-T取样方向的速率最低,T-L取样方向的速率次之,S-L取样方向的速率最高,这主要与合金的晶粒取向、织构取向和第二相粒子取向有关;应力比为0.5的裂纹扩展速率高于应力比为0.1的速率,这在近门槛区主要与裂纹闭合效应有关,在快速扩展区主要受最大应力场强度因子Kmax的影响。     

淬火条件对砂铸A356合金疲劳裂纹扩展速率的影响

在相同固溶和时效条件下，选择3种不同温度的水对砂铸A356合金进行了淬火处理，并对这3种不同淬火态下的合金进行了拉伸及疲劳裂纹扩展性能研究。结果表明，随淬火介质温度的降低，淬火冷却速率提高，合金的强度增加，塑性下降；当应力比R较低时，淬火条件对合金门槛区的裂纹扩展有明显影响，降低淬火介质温度可以提高合金的疲劳裂纹扩展阻力。此外，不同淬火态合金的疲劳扩展均明显地表现出与应力比R的相关性，这种相关性可以用裂纹闭合来说明。     

The effects of gas nitriding on fatigue behavior in titanium and titanium alloys

Fatigue behavior has been studied on gas-nitrided smooth specimens of commercial pure titanium, an alpha/beta Ti-6Al-4V alloy, and a beta Ti-15Mo-5Zr-3Al alloy under rotating bending, and the obtained results were compared with the fatigue behavior of annealed or untreated specimens. It was found that the role of the nitrided layer on fatigue behavior depended on the strength of the materials. Fatigue strength was increased by nitriding in pure titanium, while it was decreased in the Ti-6Al-4V and Ti-15Mo-5Zr-3Al alloys. Based on detailed observations of fatigue crack initiation, growth, and fracture surfaces, the improvement and the reduction in fatigue strength by nitriding in pure titanium and both alloys were primarily attributed to enhanced crack initiation resistance and to premature crack initiation of the nitrided layer, respectively.     

Fatigue crack growth behavior of laser-annealed IN 718 alloy in hydrogen

The effect of hydrogen embrittlement on the fatigue crack growth of IN 718 plate and laser-annealed specimens in hydrogen containing environment were investigated. Although the differences in tensile strength and impact toughness between solution-annealed (S) and aged (A) IN 718 specimens were significant, the experimental results indicated that both specimens within the low ΔK regime exhibited a similar fatigue behavior. As the ΔK increased above 30 MPa , the solution-annealed specimen revealed a higher fatigue crack growth rate (FCGR) than the aged one. In general, the IN 718 alloy had a low sensitivity to hydrogen-enhanced fatigue crack growth, independent of hydrogen sources. Residual compressive stresses ahead of the crack tip were responsible for the improved resistance to fatigue crack growth in a laser-annealed specimen. For alloys with similar strength, IN 718 alloy trapped a huge amount of hydrogen in the matrix showing a less susceptibility to hydrogen-enhanced fatigue crack growth in comparison with the maraging steel. Additionally, fatigue-fractured appearance near crack initiation sites reveals quasi-cleavage fracture in embrittled specimens.     

航空航天铝合金腐蚀疲劳研究进展

铝合金因具有高的比强度、比模量,好的加工性能及焊接性能,在航空航天领域应用广泛.而腐蚀疲劳是造成航空航天材料失效的重要原因之一,因其危害性高、破坏性强且难以提前预测等特点,受到了广泛关注.铝合金腐蚀疲劳问题一直是飞机日历寿命研究中的重点问题,随着可重复使用航天器理念的提出,多次空天往返和地面修复过程也使腐蚀疲劳问题在可...     

Al-6Mg-0.8Zn-0.5Mn-0.2Zr-0.2Er合金高强耐损伤工艺与微观组织研究

以Al-6Mg-0.8Zn-0.5Mn-0.2Zr-0.2Er合金为基础,对该材料的冷轧态,温轧态,完全退火态进行拉伸测试和疲劳裂纹扩展速率测试。运用电子背散射衍射(EBSD),透射电镜(TEM),扫描电镜(SEM)对合金的原始组织、疲劳断口、裂纹扩展路径进行观察,研究微观组织对材料拉伸性能及疲劳裂纹扩展速率的影响。结果表明:温轧态屈服强度高,裂纹扩展抗力大,实现了高强高耐损伤性能的匹配。这主要是由于温轧态轧制过程中发生动态回复,位错缠结规整化,具有较多的亚晶界,该种组织模式对材料的屈服强度和疲劳裂纹扩展抗力均有提高。     

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。     

Effect of microstructure and load ratio on fatigue crack growth behavior of advanced Al-Cu-Li-Mg-Ag Alloys

Fatigue crack growth (FCG) behavior of recently developed three Al-Cu-Li-Mg-Ag alloys, Weldalite 049, X 2095 and MD 345, was examined in air at load ratios of 0.1 and 0.75. It was found that all three alloys showed better resistance to fatigue crack growth than conventional high strength Al alloys. The morphologies of crack growth paths were generally linear, but some showed deflection and branching. And the alloys revealed rough and transgranular fracture surfaces. Among the factors contributing to the excellent resistance of Al-Cu-Li alloys to fatigue crack growth are enhanced slip reversibility and high surface roughness causing a high crack closure level, thus reducing ΔK_eff for crack extension. The fatigue threshold decreased and fatigue crack growth rates increased significantly with increasing the load ratio. This is caused by the decrease in crack closure level at high load ratio. But the fracture mode did not show a significant change with increasing the load ratio.     

EFFECTS OF TRACE Zr ON FATIGUE CRACK GROWTH RATE OF HIGH DAMPING Zn-Al ALLOY

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Effect of electromagnetic bulging on fatigue behavior of 5052 aluminum alloy

The effect of electromagnetic bulging on the fatigue behavior of the 5052 aluminum alloy was investigated through tensile–tensile fatigue testing. The intriguing finding is that the bulged specimens exhibited enhanced fatigue strength as depicted by maximum stress vs the number of cycles until failure (S–N) curves, by comparison with these original aluminum alloys. Although the fatigue process of the original and budged alloys follows the same mechanism with three distinct steps, namely, crack initiation at a corner of the tested samples, stable crack propagation with typical fatigue striations and finally catastrophic fracture with dimple fractographic features. The typical crack propagation rate vs stress intensity factor range (da/dN–ΔK) curves derived from the spacing of striations reveal a lower crack propagation rate in the bulged specimens. The enhancement of fatigue strength in electromagnetically bulged aluminum alloy is further rationalized in-depth on the basis of strain hardening and dislocation shielding effect.     

Impurity levels and fatigue lives of pseudoelastic NiTi shape memory alloys

In the present work we show how different oxygen (O) and carbon (C) levels affect fatigue lives of pseudoelastic NiTi shape memory alloys. We compare three alloys, one with an ultrahigh purity and two which contain the maximum accepted levels of C and O. We use bending rotation fatigue (up to cycle numbers >10⁸) and scanning electron microscopy (for investigating microstructural details of crack initiation and growth) to study fatigue behavior. High cycle fatigue (HCF) life is governed by the number of cycles required for crack initiation. In the low cycle fatigue (LCF) regime, the high-purity alloy outperforms the materials with higher number densities of carbides and oxides. In the HCF regime, on the other hand, the high-purity and C-containing alloys show higher fatigue lives than the alloy with oxide particles. There is high experimental scatter in the HCF regime where fatigue cracks preferentially nucleate at particle/void assemblies (PVAs) which form during processing. Cyclic crack growth follows the Paris law and does not depend on impurity levels. The results presented in the present work contribute to a better understanding of structural fatigue of pseudoelastic NiTi shape memory alloys.