垂直晶界和倾斜晶界Cu双晶的疲劳开裂行为及其机制

用扫描电镜（ＳＥＭ）研究了一种垂直晶界和两种倾斜晶界Ｃｕ双晶的疲劳开裂行为及其机制。这三种双晶组元晶体的取向均为「１３４」。结果表明,沿晶界的疲劳开裂是Ｃｕ双晶疲劳破坏的主要形式,但垂直晶界和倾斜晶界双晶疲劳裂纹萌生的机制有所不同。垂直晶界双晶沿晶疲劳裂纹主要由驻留滑移带撞击晶界面产生,而倾斜晶界双晶疲劳裂纹的萌生是由晶界两侧晶粒的滑移台阶而引起的应力集中所致。造成这种差别的原因同两种双晶的活动滑     

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。     

Ti-15-3板材中晶界特征及其对疲劳裂纹萌生与扩展的影响

研究Ti-15-3合金冷轧板晶界特征及其对疲劳滑移带萌生和短裂纹扩展的影响.结果表明:Ti-15-3合金冷轧板中晶界大多为大角度晶界,小角度晶界很少.在本文统计范围内,相邻晶粒取向差在40-～60-的范围内超过60%,小于15-的只占4%;大角度晶界有利于疲劳滑移带的萌生,对短裂纹穿越有阻碍作用,在小角晶界附近未发现有滑移带产生,但短裂纹很容易穿过小角度晶界.     

单晶Nb循环变形过程中疲劳裂纹的萌生及其机制

用光学显微镜，扫描电镜和透射电镜观察了Ｎｂ单晶在循环变形后的表面形貌和饱和时的错组态。发现疲劳裂纹的萌生和表面形变带有关，当滑移线和形变带方向平行时，在双形变带的公共皱折处很容易萌生疲劳裂纹，已萌生裂纹呈交滑移特征，在裂纹萌生处附近，有很多由交滑移引起的滑移台阶，根据我们的实验结果，从理论上对Ｍｏｔｔ的疲劳裂纹萌生模型进行修正，也为Ｍｏｔｔ模型提供了实验依据，     

2524-T34合金疲劳裂纹的萌生和扩展行为

通过四点弯曲疲劳试验研究2524-T34板材的疲劳性能,借助金相和扫描电镜观察疲劳裂纹的萌生和扩展行为。结果表明:2524合金具有良好的疲劳性能,疲劳强度达到屈服强度的80%以上;疲劳裂纹主要在第二相粒子以及第二相粒子/基体界面萌生,裂纹扩展过程中的偏转与晶界的阻碍有关;相邻晶粒内两个有利滑移面之间的位向差是控制裂纹通过晶界扩展的重要因素。     

[112]铝单晶体的疲劳损伤与裂纹萌生

对「１１２」双滑移取向铝单晶体进行了对移压疲劳试验，详细观察和对比研究了正表面和侧表面上的疲劳损伤与裂纹萌生过程，在正表面上两个主滑移系滑移台阶是相互平行行的，呈典型的持滑转移带损伤形貌；     

[12]铝单晶体的疲劳损伤与裂纹萌生

对[112]双滑移取向铝单晶体进行了对称拉压疲劳试验，详细观察和对比研究了正表面和侧表面（包含两个主滑移系Burgers矢量）上的疲劳损伤与裂纹萌生过程.在正表面上两个主滑移系滑移台阶是相互平行的，呈典型的持续滑移带（PSB）损伤形貌；而在S面上则始终未见PSB,但有晶体学关系不明确的呈波纹形不均匀分布的形变花样产生，并发展导致微裂纹的萌生,其尺寸可与正表面上的PSB裂纹相匹比。由此进一步证实了循环形变累积所致的内部组织损伤是疲劳裂纹萌生的基本因素．和单滑移取向单晶体相比，在复滑移条件下的显著交滑移过程明显地促进了试样的疲劳损伤和裂纹萌生.     

高钴钼低碳不锈轴承钢的拉压疲劳裂纹萌生与扩展

采用QBG-50型高周疲劳试验机对高钴钼不锈轴承钢进行轴向拉压疲劳试验,研究了其在室温和500℃下的疲劳裂纹萌生扩展行为。结果表明：室温条件下试验钢的拉压疲劳极限强度为785 MPa, 500℃条件下试验钢的拉压疲劳极限强度为550 MPa,较室温降低了30.0%。通过扫描电镜对疲劳断口观察发现,室温下试验钢的疲劳裂纹起裂类型为单源起裂,起裂源为驻留滑移带、表面加工缺陷和夹杂物,高温下试验钢的起裂类型为单源起裂和多源起裂,起裂源为驻留滑移带、夹杂物和表面加工缺陷。室温下疲劳裂纹萌生于内部驻留滑移带,而500℃下疲劳裂纹多形成在表面或接近表面的滑移带。拉压裂纹萌生寿命占总寿命的97.88%以上。     

取向差对SRR99双晶高温合金拉伸性能的影响

通过对具有不同取向差的SRR99双晶高温合金试样进行室温拉伸实验,系统比较了取向差对试样拉伸行为、表面形貌、断裂方式和断口形貌的影响.结果表明:取向差为4°的晶界对双晶的拉伸性能几乎没有影响;当取向差达到10°时,晶界的作用开始明显显现;而取向差为16°和18°的晶界对双晶的力学性能影响最大.扫描电镜观察发现:滑移易于贯穿4°取向差晶界,最后沿滑移带开裂,断口呈明显的滑移特征;取向差为10°的双晶在滑移带撞击下,裂纹沿滑移带和晶界同时扩展,最后率先沿滑移带失稳断裂,断口上除有滑移特征,还呈现枝晶间开裂特征;而取向差为16°和18°的双晶,在屈服初期便萌生晶界裂纹,最后沿晶界快速发生断裂,断口枝晶间开裂特征明显.     

Cr-Mn-Si-Ni高强度钢疲劳裂纹的萌生行为

通过扫描电镜原位疲劳试验,研究Cr-Mn-Si-Ni高强度钢疲劳裂纹的萌生行为,观察并记录从滑移带自然萌生的小裂纹和其发展到宏观裂纹的过程。研究结果表明：在低周疲劳情况下,不是所有萌生的裂纹都会持续扩展,大部分微裂纹在疲劳早期即已萌生,但是裂纹发展很慢,导致失效的裂纹快速扩展在疲劳晚期发生,微裂纹长度与循环数之间也有一定的关系。     

2A97铝锂合金的疲劳裂纹萌生及早期扩展行为(英文)

研究2A97铝锂合金的疲劳裂纹萌生及早期扩展行为。在室温条件下,采用光滑试样进行疲劳测试,其中最大应力为恒定值,应力比R为0.1,频率f为40 Hz。利用金相显微镜、透射电镜、扫描电镜及电子背散射衍射等手段对合金的微观组织进行分析,研究合金的疲劳裂纹萌生及早期扩展行为与其微观组织的关系。结果表明:2A97合金的疲劳裂纹主要萌生于试样表面的杂质相和粗大第二相处;其疲劳裂纹的早期扩展行为主要受晶粒结构与位错或滑移带共同作用的影响。当相邻晶粒的错配度接近于其晶内的最优滑移面的位向差时,大角度晶界强烈阻碍滑移带的运动,从而导致裂纹分叉和偏折。     

铜双晶体中晶界及组元晶体的疲劳寿命

本文对具有单,双滑移取向组元晶体的近生趣晶界铜双晶体进行拉－拉循环变形,得出铜双晶体中晶界及组元晶体的Ｓ－Ｎ曲线,比较了铜双晶体晶界及组元晶体疲劳寿命的差别,发现在相同的应力幅下晶界的疲劳寿命明显低于组元晶体的疲劳寿命。     

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-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.     

铜晶体的疲劳损伤微观机制

对Cu单晶体、双晶体和多晶体疲劳损伤微观机制的总结结果表明：在中、低应变范围Cu单晶体的疲劳裂纹主要沿驻留滑移带萌生,而在高应变范围则沿粗大形变带萌生;Cu双晶体中疲劳裂纹总是优先沿大角度晶界萌生和扩展,而小角度晶界则不萌生疲劳裂纹;对于Cu多晶体,疲劳裂纹主要沿大角度晶界萌生,有时也沿驻留滑移带萌生,而孪晶界面两侧由于滑移系具有相容的变形特征而未观察到疲劳裂纹萌生．     

Effect of crystallographic orientation and grain boundary character on fatigue cracking behaviors of coaxial copper bicrystals

Four coaxial copper bicrystals were employed to study the slip morphologies and fatigue cracking behaviors during cyclic deformation. Three of them had high-angle grain boundaries (GBs) with nearly the same misorientation and one bicrystal had a twin boundary (TB). Different slip bands (SBs) operated near the GBs and TB, generating different dislocation arrangements, which are mainly determined by the crystallographic orientations of the component grains. The GBs suffered impingement or shear damage caused by slip difference from both sides. It is suggested that there is an energy increase in the interfaces between matrix and persistent slip bands (PSBs), GBs and TBs per cycle during cyclic deformation due to the accumulation of lattice defects, which would make the interface unstable. After a certain number of cycles, fatigue cracks initiated firstly at GBs for some bicrystals while fatigue cracking occurred preferentially at PSBs for the others. It is confirmed that the energy growth rate is an increasing function of the shear stress, strain amplitude and strain incompatibility, which results from slip differences on both sides of the interfaces. Interfaces with different energies and strain incompatibilities have different fatigue cracking resistance. It is found that GBs with defective and complex structure, and hence high interfacial energy accompanied by high modulus of the residual GB dislocation (GBD), are preferential sites for fatigue cracking, while the fatigue cracking appeared predominantly at PSBs when the modulus of the residual GBD is lower than that of a perfect dislocation with simple GB structure and low interfacial energy. The present model for the energy can predict well which kind of interface would form cracks preferentially during cyclic deformation in one coaxial bicrystal and which GB would need more cycles to initiate fatigue cracking between coaxial bicrystals with different GB characters.     

冲击疲劳循环形变阶段的晶界行为

研究了低碳钢在低周冲击疲劳循环变阶段的晶界行为,用扫描电子显微镜对试样表面进行了观察,结果表明晶界表出如下特征,滑移线既可连续地跨越晶界,也可受阻于晶界并形成台阶,晶界发生诸如增粗,消失等现象,伴随着形变过程,出现新的亚晶界,晶界的既阻碍滑移,又激发次滑移系开动,讨论了产生这些现象的可能机制。     

Cyclic deformation behavior of non-isoaxial copper tricrystals and bicrystals

The cyclic hardening and saturation behaviors of copper tricrystals and bicrystals were investigated in strain-controlled multiple step tests. The results show that, for the inclined grain boundary (GB) bicrystal with single slip components, the cyclic stress strain (CSS) curve exhibits a plateau or quasi-plateau region, while the CSS curve of tricrystals shows no plateau. Observations of surface morphologies indicated that owing to the strain incompatibility of three grains, at lower strain amplitude the triple junction (TJ) retards obviously the primary slip in grains and makes deformation near it smaller than that near the bicrystal GB, while at higher strain amplitude slip can be distributed near the TJ homogeneously. The probability of crack initiation at the same TJ is closely related to the loading direction. The saturation dislocation structures of tricrystal specimens under the strain amplitude of the last step were explored by the electron channeling contrast technique in SEM (ECC-SEM). Loop patches with persistent slip band (PSB) ladders embedded were found even in the TJ vicinity for all grains. Dislocation-free zones (DFZ) occurred in the vicinity of TJ and GB, and the difference in shapes between them is due to the difference in internal stress field. Misoriented cell structure and dislocation wall structure were found near the crack tip, and the formation of them is associated with the cooperative action of crack tip, GB, grain orientation and the applied strain amplitude.