晶粒尺寸对纳晶双峰材料断裂韧性的影响

为了描述由纳晶基体和粗晶颗粒组成的纳晶双峰材料的断裂韧性,通过建立一个粘聚力模型来研究纳晶双峰材料的临界应力强度因子K_(IC)(表征材料断裂韧性)。考虑到纳晶双峰材料的一个典型情况:裂纹位于2个纳晶颗粒的交界面处,裂纹尖端与粗晶粒的晶界相交,假设粘聚区的尺寸等于纳晶颗粒的尺寸d。裂纹的钝化和扩展过程受位错和粘聚力的共同影响,刃型位错是从粘聚力裂纹的尖端发射,该过程对裂纹产生屏蔽效应。模型计算结果显示:当粗晶颗粒尺寸D确定时,K_(IC)随着纳晶材料晶粒尺寸d的增大而增大;当纳晶材料晶粒尺寸d确定时,K_(IC)随着粗晶材料晶粒尺寸D的增大而增大;相对于纳晶颗粒的尺寸,断裂韧性对粗晶晶粒的尺寸更加敏感。     

Microstructural model of intergranular fracture during tensile tests

A microstructural model of intergranular fracture in textured materials is presented. In this model, the material is represented by a two-dimensional microstructure with non-regular polygonal grains which represents material's texture and grain shape measured in experiments or calculated from Monte Carlo simulations. The grain boundary character, grain boundary energy, and fracture stress are assigned to each grain boundary according the grain boundary character distribution. Intergranular fracture susceptibility is analyzed by defining the probability of finding a continuous path along the grain boundaries which are intrinsically susceptible to fracture. In this analysis the orientations of the grain boundary with respect to the applied or residual tensile stress axis is considered. The probability of intergranular fracture for each grain boundary depends on the intergranular fracture resistance, the interface orientation relative to the stress axis, and a value of the tensile stress acting on the grain boundary. The crack arrest distance and the fracture toughness are calculated in terms of the frequency of low-energy grain boundaries, fracture stress of low-energy grain boundary, angle distribution of grain boundary interfaces, and anisotropy of grain shape. The results indicate that the fracture toughness increases and the crack arrest distance decreases dramatically with increasing the frequency of the low-energy grain boundaries. Lowering the grain boundary energy can improve the fracture toughness and decrease the crack arrest distance. The angle distribution of grain boundary interfaces and the grain shape factor are also very effective in controlling the fracture toughness. High fracture toughness of polycrystalline materials is related to the presence of a high frequency of low-energy boundaries which are resistant to fracture. The best fracture toughness for brittle materials can be achieved by controlling the frequencies of the low-energy grain boundaries, the grain boundary character, and the boundary inclination.     

Scaling of discrete dislocation predictions for near-threshold fatigue crack growth

Analyses of the growth of a plane strain crack subject to remote mode I cyclic loading under small scale yielding are carried out using discrete dislocation dynamics. Plastic deformation is modelled through the motion of edge dislocations in an elastic solid with the lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation being incorporated through a set of constitutive rules. An irreversible relation is specified between the opening traction and the displacement jump across a cohesive surface ahead of the initial crack tip in order to simulate cyclic loading in an oxidizing environment. Calculations are carried out with different material parameters so that values of yield strength, cohesive strength and elastic moduli varying by factors of three to four are considered. The fatigue crack growth predictions are found to be insensitive to the yield strength of the material despite the number of dislocations and the plastic zone size varying by approximately an order of magnitude. The fatigue threshold scales with the fracture toughness of the purely elastic solid, with the experimentally observed linear scaling with Young’s modulus an outcome when the cohesive strength scales with Young’s modulus.     

C1-controlled creep crack growth by grain boundary cavitation

Quasi steady-state creep crack growth is widely associated with the nucleation and growth of voids on grain boundaries ahead of the crack tip. In this paper, a micromechanics-based constitutive law is used to study the velocity-dependent fracture toughness of porous solids under extensive creep conditions. Void growth and coalescence in the fracture process zone is modeled by a nonlinear viscous microporous strip of cell elements. Under steady-state crack growth, two dissipative processes contribute to the macroscopic fracture toughness: the work of separation in the fracture process zone, and creep dissipation in the background material. Under extensive creep conditions, the competition between these two processes produces an inverted U-shaped C¹–velocity curve. The effects of rate sensitivity, initial porosity as well as hydrogen attack on fracture toughness are studied. The numerically simulated fracture toughness vs. crack velocity curves show good agreement with existing experimental results.     

高强度低焊接裂纹敏感性钢焊接热影响区的冲击韧度

采用埋弧自动焊接方法焊接高强度低焊接裂纹敏感性钢,分析了高强钢焊接热影响区中不同微区的显微组织特征与冲击韧度之间的关系.焊接接头粗晶区和细晶区的显微组织分别为粗大的粒状贝氏体和细小的准多边形铁素体组织,其-20℃的平均冲击吸收功分别为45 J和170 J.粗晶区中粒状贝氏体的有效晶界为原始奥氏体晶界,晶内存在大量的小角度晶界和亚晶界,有效晶粒尺寸较大,冲击韧度显著降低;细晶区中准多边形铁素体的平均有效晶粒尺寸约为5.3μm,大角度晶界可以有效阻碍了裂纹的扩展,具有较好的冲击韧度.     

γ-TiAl中<110>倾斜晶界断裂行为的分子动力学模拟(英文)

采用分子动力学(MD)方法研究γ-Ti Al合金中<110>对称倾斜界面的断裂行为,模拟在不同温度与应变速率下垂直界面方向的拉伸变形。结果表明:晶粒的相对取向及晶界特定的原子结构是影响位错形核临界应力的两个主要因素。取向差角度大于90°的Σ3(111)109.5°、Σ9(221)141.1°和Σ27(552)148.4°界面,位错在晶界处形核和扩展;取向差角度小于90°的Σ27(115)31.6°和Σ11(113)50.5°界面,无位错在晶界处形核,当应力达到峰值后界面直接断裂。γ-Ti Al双晶的断裂机制为微裂纹在界面处的形核及沿界面扩展;不同取向差界面的区别在于裂纹前端有无塑性区增韧。     

In situ TEM observations of fast grain-boundary motion in stressed nanocrystalline aluminum films

Free-standing nanocrystalline Al thin films have been strained in situ in a transmission electron microscope at room-temperature. Extensive grain-boundary migration accompanies the in situ loading and has been observed to occur preferentially at crack tips and only in the presence of the applied stress. This grain growth precedes dislocation activity, and measured boundary velocities are greater than can be explained by diffusive processes. The unambiguous observations of stress-assisted grain growth are compatible with recently proposed models for stress-coupled grain-boundary migration. The growth occurs in a faceted manner indicative of preferential boundaries. The fast collapse of small grains with sizes of 30–50 nm demonstrates the unstable nature of a nanocrystalline structure. Clearly observable shape changes testify to the effectiveness of grain-boundary migration as a deformation mechanism, and preferential grain growth at crack tips resulted in efficient crack tip blunting, which is expected to improve the films’ fracture toughness.     

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。     

Out-of-Plane Constraint Effect on the Fracture Toughness of Single Edge Notch Tension SpecimensEI北大核心CSCD

The crack-tip stress and strain fields of single edge notch tension (SENT) specimen are similar to those of the full-scale pipe containing surface cracks under longitudinal tension and/or internal pressure. It is well known that material's fracture toughness is not constant, and the specimen size has a significant influence on fracture toughness. It is thus essential to consider the transferability from fracture specimens in laboratory testing to practical structures, i.e., size effects or constraint effects. However, the specimen dimensions for SENT specimens recommended by current design procedures have not validated the out-of-plane constraint effect on the fracture toughness. In this work, the effect of specimen thickness on the crack tip opening displacement (CTOD) of SENT specimen was investigated using an API X90 grade steel. Full-field deformation measurement by digital image correlation (DIC) technique and stretching zone width (SZW) examination were performed to analyze the size effects on fracture toughness. The results show that the critical crack initiation toughness is highly sensitive to specimen thickness, and decreases significantly as specimen thickness increases until the thickness-to-width ratio (B/W) equals to 4, beyond which the effect of specimen thickness becomes relatively weak. As the specimen thickness increases, the maximum longitudinal strain and stretching zone width decrease sharply, and the location of high-strain zones changes significantly; when B/W >= 3, strain is initiated from the area oppo-site the cracked side rather than from the crack tip, indicating a strong loss of plasticity for thicker specimens. A dimension size is recommended for the fracture toughness testing to take the out-of-plane constraint into account for SENT specimen.     

Atomistic simulations on intergranular fracture toughness of copper bicrystals with symmetric tilt grain boundaries

The intergranular fracture toughness of Cu bicrystals with symmetric tilt grain boundaries was investigated using atomistic simulations. Mode I fracture of Cu bicrystals with an intergranular crack was considered. The boundary conditions were specified by the near-tip displacement fields obtained based on linear elastic fracture mechanics (LEFM). Based on the energy interpretation of the energy release rate, a two-specimen method was adopted to determine the fracture toughness. The simulation results of the fracture toughness matched well with those determined using LEFM. In contrast to the toughness obtained using the Griffith energy criterion, the atomistic simulation results for the same bicrystal were not constants, but dependent on the crack-tip circumstances. This behavior was mainly associated with the different local stress conditions and fracture patterns observed for the different models.     

EFFECT OF CREEP FRACTURE TOUGHNESS ON CRACK INITIATION AND GROWTH

本文研究了蠕变断裂韧性对二种低合金耐热钢蠕变裂纹开裂和扩展的影响。试验表明:随着蠕变断裂韧性提高,抗蠕变裂纹开裂和扩展能力增加。材料呈韧性或脆性状态时,蠕变裂纹萌生和扩展过程不同。韧性状态时,裂纹为穿晶和晶界二种混合形式:穿晶裂纹可在晶内碳化物处发生,或在晶界上形核后向晶内扩展,晶界裂纹仍是由晶界上空洞形成和相互连接而成,裂纹可沿晶界和晶内扩展,但不连续。脆性状态时,裂纹沿晶界发生,它是由晶界形成空洞和相互连接而成,扩展仅沿晶界发生。     

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.     

Microstructural effects on discrepancy between toughness under sharp cracks and blunt notches in Ti-added 8Ni-13Co secondary hardening steel

In this study, the toughness under sharp cracks and blunt notches is investigated in terms of the prior austenite grain size (PAGS) and inclusion particles as a function of the austenitizing treatment in a Ti-added 13Co-8Ni secondary hardening steel. For the quantitative analyses of the inclusion and precipitate particles, small-angle neutron scattering analyses are conducted under austenitizing conditions of 1050 °C, 1200/1050 °C, and 1200/1200 °C; the impact toughness values are 32, 30, and 24 J for each austenitizing condition, respectively. In contrast, the fracture toughness values under the same conditions are 66, 78, and 103 MPa·m^1/2. Thus, the fracture toughness significantly improves under 1200/1200 °C austenitizing conditions with coarse PAGS; however, the impact toughness deteriorates. The adverse effect of the grain size on the toughness under sharp cracks and blunt notches is elucidated in terms of the effective microstructural factors that control the fracture process inside the plastic zone, the size of which varies with the notch sharpness. In particular, through considering the density of the slip bands as a function of the grain size in the small confined plastic zone before the sharp crack, the complicated problem regarding an increase in the fracture toughness with an increasing grain size is described from micromechanical and microstructural perspectives.     

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.     

Creep effects on crack growth in a Mo–Si–B alloy

Crack growth behavior during monotonic and cyclic loading at elevated temperature is affected by creep and/or by environment and, therefore, high-temperature toughness and fatigue response can be temperature and loading-rate dependent. This paper reports on the effect of loading rate on fracture toughness at high temperatures for a two-phase Mo–Si–B alloy; the observed response is understood by examining the interaction of the advancing crack with the microstructure, and the evolution of microstructure ahead of the crack tip as a consequence of the crack-tip field. Parallel studies were also performed under cyclic loading conditions by subjecting compact tension specimens to sinusoidal and trapezoidal loading waveforms. In certain cases, the microstructure ahead of the crack tip revealed several instabilities (recrystallization, grain growth and creep cavitation). Finite element analysis revealed strain localization ‘pockets’ ahead of the crack tip, which are thought to provide the driving force for the observed microstructural instabilities.     

Al双晶体的晶界疲劳效应SCIEI

通过Al双晶体疲劳试验,研究了晶界对循环变形和疲劳裂纹扩展的影响。实验结果表明,晶界疲劳效应集中体现于晶界影响区,表现为不相容塑性应变在该区内产生内应力,并由此可激发次级滑移,甚至产生晶界裂纹;在常幅循环应力下,随超Ⅰ阶段裂纹逐渐接近晶界,裂纹分叉、前沿碎裂,扩展速率降低,直至在晶界影响区中央达到一最小值。本文提出了晶界诱发裂纹顶端屏蔽机制,并以此解释了上述超Ⅰ阶段裂纹对微观组织敏感的扩展行为。     

A unified model of environment-assisted cracking

Environment-assisted cracking is caused by enhanced crack-tip plastic deformation. Lattice resistance to shear slip is caused by the periodic fluctuation of the misfit energy across a slip plane. Lu et al. have shown that hydrogen reduces the misfit energy, slip resistance and resistance to dislocation motion. The 1s orbital of a hydrogen atom in a metal is not full, and the electron of the atom moves and reacts as a valence electron. The resistances to shear slip and dislocation generation at a crack tip are reduced by hydrogen electrons and hydrogen nuclei. Crack-tip plastic deformation is thus enhanced, and crack growth rate is increased. The model of reduced resistance to crack-tip dislocation generation and enhanced crack-tip plastic deformation can be extended to liquid metal-assisted cracking and stress corrosion cracking if the valence electron(s) of a liquid metal or chemical reduces the misfit energy and the slip resistance sufficiently.     

Intergranular environmentally assisted cracking of Alloy 182 weld metal in simulated normal water chemistry of boiling water reactor

Following characterization of grain boundaries in Alloy 182 weld metal, intergranular environmentally assisted cracking (IGEAC) growth behavior of the alloy in simulated normal water chemistry (NWC) of boiling water reactor (BWR) was tested by employing a crack growth rate test. The grain boundary was found consist of about 72% random boundaries, 21% low-angle boundaries and a few coincident boundaries.The correlation between the grain boundary characters and IGEAC was discussed. Based on the IGEAC growth rates obtained in the test and evaluations of the crack tip strain rate, the parameter of the slip-dissolution/oxidation model that embodies the kinetics of oxidation/repassivation in the crack-tip environment of Alloy 182 in BWR-NWC was quantitatively determined.     

EH36钢埋弧焊焊缝金属组织临界断裂韧性

通过对典型海洋结构用钢EH36埋弧焊焊缝金属进行单边缺口拉伸试验(single edge notched tensile,SENT)研究焊缝金属中针状铁素体(acicular ferrite,AF)和先共析铁素体(proeutectoid ferrite,PF)的临界断裂韧性. 结果表明,AF的断裂韧性要高于PF,主要原因在于AF内部具有较高密度的位错及位错能,在外力作用下,裂纹尖端极易发生塑性变形而钝化;相反,PF组织较纯且均匀,位错密度较小,裂纹尖端并未发生钝化. 这种组织的不均匀性导致了焊缝金属断裂韧性较大的分散性,主要归结于试样预制裂纹尖端位置的微观组织性能.