Cu双晶的循环形变行为与疲劳裂纹萌生：Ⅱ.疲劳裂纹萌生与早期扩展

用扫描电镜观察了受应变疲劳载荷作用的Ｃｕ双晶物的表面形貌，发现晶界是疲劳形变双晶是有利的裂纺萌生地点，在滑移带撞击晶界的地方，特别是在几条粗滑移带共同撞击晶界的地方观察到许多疲劳微裂纹；并且发现与平行晶界双晶相比，垂直晶界双晶有有利于疲劳裂纹沿晶界作早期扩展。     

A crystallographic mechanism for fatigue crack propagation through grain boundaries

A crystallographic model is proposed which takes into account both crack-plane twist and tilt effects on crack retardation at grain boundaries. The twist and tilt angles of the crack-plane deflection at a grain boundary are the key factors that control the path and growth rate of a short crack. Because of crack-plane twist, the area between the traces on the grain-boundary plane of the crack planes across the boundary has to be fractured in order for the crack to propagate through the boundary. This presents significant resistance to crack growth. As the area to be fractured increases with the extent of crack growth beneath the surface of observation, the grain boundary could still resist crack growth after the crack tip has passed the grain boundary on the surface, until the crack propagates through the whole boundary below the surface. A grain boundary with a large twist component could cause a short crack to arrest or branch. Studies of short fatigue crack growth in an Al–Li 8090 alloy plate provide evidence that supports the model.     

EFFECT OF GRAIN BOUNDARY ON FATIGUE BEHAVIOUR OF Al BICRYSTALS

The effect of grain boundaries on cyclic deformation and fatigue crack growth in aluminumbicrystals has been studied.The effect of a grain boundary is restricted in a narrow area,termed as grain boundary affecting zone(GBAZ),where the incompatible plastic straintreats internal stress which conversely promotes inhomogeneous slip in the area and grainboundary cracking.As an extended stage I crack initiated from a notch approaches the grainboundary under a constant cyclic stress,the crack front branches splits into several pieces,meanwhile,the growth rate of the crack reduces to a minimum value at the center of theGBAZ.Such microstructure-sensitive growth of extended stage I cracks is mainly attributedto the grain boundary-induced crack tip sheilding.     

HIGH TEMPERATURE CRACK PROPAGATION AND FRACTUREOF SUPERALLOYS

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High-cycle fatigue performance of solution-treated metastable-β titanium alloys

The effect of grain boundary misorientation on the high-cycle fatigue performance of solution-treated, metastable-β titanium alloys was investigated. Initial damage during cyclic deformation was associated with the formation of coarse, planar slip bands, these often propagating through several grains without obstruction or redirection when intersecting with a grain boundary. This “continuous” slip through several grains was associated with the presence of a significant number of “low-angle” grain boundaries. Fatigue crack initiation was associated with crack initiation at intersecting planar slip bands at the free surface. The increase in operative slip length occasioned by the presence of low-angle grain boundaries lead to enhanced crack initiation and reduced lifetime. Fatigue crack propagation was characterized by step-like features formed through the interaction of the propagating crack and the coarse slip bands present in the plastic zone ahead of the crack tip. The direction of local fatigue crack propagation was also minimally affected when crossing low-angle grain boundaries.     

GH536高温合金焊接接头疲劳裂纹萌生与扩展原位试验研究

采用原位疲劳试验方法,实时观察了GH536焊接接头疲劳裂纹的萌生和扩展行为,从而揭示了GH536焊接接头疲劳裂纹的萌生和扩展机制:疲劳加载过程中,位错沿滑移带在晶界前沿塞积,晶界阻碍位错运动,裂纹沿滑移带开裂,萌生疲劳裂纹;疲劳裂纹扩展初期,受单滑移的交替作用,裂纹呈“Z”字型向前扩展,随后裂纹的扩展逐渐以主应力控制为主,垂直于加载方向、平直向前扩展;GH536合金焊接接头组织中的晶界和碳化物会阻碍疲劳裂纹的扩展。     

Metallurgical control of fatigue crack propagation in superalloys

Low-cycle fatigue life of turbine engine disk alloys is determined by the initiation and propagation of fatigue cracks. Performance improvements can be achieved through the combination of clean melting technology, to reduce the defect size, and a new generation of high-strength superalloys with fatigue cracking resistance. Metallurgical control of fatigue crack propagation in high-strength superalloys becomes feasible only through a clear understanding of the fatigue cracking mechanism, as well as the micro-structure/property relationships. Many metallurgical parameters have been identified to control the fatigue cracking resistance at high temperatures. One of the most effective methods, applicable to all high γ′ content superalloys, is to modify the grain boundary structure by means of a controlled cooling from a supersolvus solutioning. The precipitation reaction occurring on the grain boundaries during cooling generates a serrated structure that exhibits a good stress oxidation resistance for fatigue cracking.     

Effect of Microstructure and Environment on Static Crack Growth Resistance in Alloy 706

The relationship between thermo-mechanical processing, resultant microstructure, and mechanical properties has been of interest in the field of metallurgy for centuries. In this work, the effect of heat treatment on microstructure and key mechanical properties important for turbine rotor design has been investigated. Specifically, the tensile yield strength and crack growth resistance for a nickel-iron based superalloy 706 has been examined. Through a systematic study, a correlation was found between the processing parameters and the microstructure. Specifically, differences in grain boundary and grain interior precipitates were identified and correlated with processing conditions. Further, a strong relationship between microstructure and mechanical properties was identified. The type and orientation of grain boundary precipitates affect time-dependent crack propagation resistance, and the size and volume fraction of grain interior precipitates were correlated with tensile yield strength. It was also found that there is a strong environmental effect on time-dependent crack propagation resistance, and the sensitivity to environmental damage is microstructure dependent. Microstructures with η decorated grain boundaries were more resistant to environmental damage through oxygen embrittlement than microstructures with no η phase on the grain boundaries. An effort was made to explore the mechanisms of improving the time-dependent crack propagation resistance through thermo-mechanical processing, and several mechanisms were identified in both the environment-dependent and the environment-independent category. These mechanisms were ranked based on their contributions to crack propagation resistance.     

超声强化2D12铝合金疲劳断裂过程的微观分析

采用超声强化工艺对2D12铝合金进行表面处理,借助金相显微镜和扫描电镜,并结合原位跟踪测量裂纹长度的方法,对强化后的疲劳断裂行为进行了研究。结果表明,超声强化后2D12铝合金表面晶粒得到细化,疲劳源主要产生于试样表面,仅个别向内部转移,强化后疲劳寿命提升了约8倍。其原因一方面是强化过程在试样表面引入了残余压应力,由于裂纹扩展过程倾向于连接裂纹扩展路径上的缺陷,所以残余压应力的存在效降低了裂纹在两缺陷间的扩展速率;另一方面晶粒细化导致晶界密度增加,加强了对裂纹扩展的阻碍,从而有益于提高裂纹扩展寿命。     

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。     

EFFECT OF GRAIN BOUNDARY CARBIDE ON CREEP CRACK GROWTH IN Fe-Cr15-Ni25 ALLOY

Creep crack growth behavior of Fe-Cr15-Ni25 alloys with different grain boundary featureshas been studied.Cavities nucleate at triple junctions of grain boundaries for the single phasealloy and at grain boundary carbide for the alloy with grain boundary carbide.Grain bounda-ry carbide particles are obstacles to cavity growth and coalescence,and therefore increase thecreep crack growth resistance greatly.     

EBSD Analysis of Fatigue Crack Growth of 2124 Aluminum Alloy for Aviation

运用电子背散射衍射(EBSD)分析技术对2124-T851铝合金板材的疲劳裂纹扩展进行了分析研究。结果表明,疲劳裂纹扩展以穿晶为主,随晶粒取向的不同而呈现一定的择优性。当裂纹扩展到晶界时,由于相邻晶粒间存在的取向差,裂纹会偏离其正常扩展路径而发生偏转,而晶内的裂纹偏转则更多是因为粗大第二相粒子在循环应力作用下协调变形能力差引起的。裂纹扩展过程中发生裂纹分叉与特定的晶体学方向有关,是由于裂纹尖端多个等效{111}<110>滑移系的同时开动造成的。     

In-situ crack propagation observation in fully lamellar Ti-49%Al alloy

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Effects of precipitates on fatigue crack growth rate of AA 7055 aluminum alloy

The effects of precipitates on the fatigue crack growth rate of AA 7055 Al alloy subjected to different ageing treatments were investigated using transmission electron microscope and fatigue crack growth testing.The results show that the T77 treated samples exhibit the lowest crack growth rate,while the crack growth rate of over-aged samples is the highest.In terms of the model based on the reversibility of dislocation motion within the plastic zone close to the crack tip,the improved crack growth resistance is attributed to many precipitates that are coherent with Al matrix in the under-aged and T77 treated samples.When the precipitate is coherent with the Al matrix,the larger the precipitate is,the slower the fatigue crack grows.The effects of grain boundary precipitates and precipitate free zone on the fatigue crack growth resistance are less significant than those of precipitates within grains of the alloy.     

5CrMnMo钢的热疲劳裂纹研究SCIEI

通过自制Coffin型及自约束型热疲劳试验机研究了5CrMnMc钢经不同热处理后的热疲劳行为。利用扫描电镜、透射电镜等揭示了热疲劳裂纹萌生及其扩展。结果表明,淬火态的5CrMnMo钢绝大多数热疲劳裂纹优先在晶界处萌生;淬回火态的试样裂纹主要萌生于碳化物与基体的脱开处,或形成晶界裂纹;无论是淬火或淬回火态的试样其热疲劳裂纹主要沿着碳化物与基体脱开处及晶界裂纹桥接而扩展。     

海洋柔性立管用钢的疲劳断裂行为

采用GPS100高频疲劳试验机研究了海洋柔性立管用钢在温轧工艺下的疲劳断裂行为。结果表明,海洋柔性立管用钢疲劳寿命随加载应力幅的升高而不断下降;疲劳裂纹产生于试样表面,裂纹源区可见变形带及河流花样,裂纹扩展区存在大量二次裂纹,瞬断区疲劳裂纹呈韧性断裂特征。在疲劳裂纹扩展过程中,裂纹的扩展能力因大角度晶界的铁素体和高强度贝氏体的相互作用而受到限制,同时在晶界处形成的二次裂纹同样可减缓扩展速率,达到提高疲劳寿命的效果。     

不同晶粒尺寸5083铝合金晶间腐蚀和应力腐蚀开裂的差异性

通过冷轧和退火获得具有不同晶粒尺寸(8.7～79.2μm)的5083铝合金板.研究其微观结构、晶间腐蚀(IGC)、应力腐蚀开裂(SCC)和裂纹扩展行为.结果表明,粗晶粒样品表现出更好的抗IGC性能,其腐蚀深度为15μm.慢应变速率测试结果表明,细晶粒样品表现出更好的抗SCC性能,敏感性指数ISSRT为11.2％.此外,...     

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。     

BEHAVIOUR OF INITIATION AND GROWTH OF SHORT FATIGUE CRACKS

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PPB对镍基粉末高温合金裂纹扩展速率的影响

摘 要: 研究了直接热等静压（As-HIP）成形的一种镍基粉末高温合金中原始颗粒边界（PPB）对其裂纹扩展行为的影响。结果表明,粉末高温合金中的原始颗粒边界上析出的碳、氧化物会导致断裂韧性(KIC值)降低,使界面极易萌生裂纹;在裂纹扩展过程中发生沿颗粒间断开,改变扩展棱的走向,并加剧裂纹沿晶界扩展,使裂纹扩展速率增快;沿原始颗粒边界扩展的裂纹尖端的断裂韧度与边界析出物的物理性质、数量、尺寸密切相关,析出物排列越密集,KIC值越小,会加速裂纹在这些薄弱区扩展。