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。     

2397合金高周疲劳性能及裂纹萌生扩展行为

研究了2397-T87铝锂合金的高周疲劳性能及裂纹萌生扩展行为。结果表明:在应力比R=0.1时,2397-T87铝锂合金L方向、LT方向和ST方向光滑试样(K_t=1.0)的疲劳寿命极限分别约为192,243和151 MPa;缺口试样(K_t=3.0)的疲劳寿命极限分别约为72,78和70 MPa。其疲劳裂纹主要萌生于试样表面,以及氧化物、夹杂等脱落形成的空洞,Al(CuFeMn)第二相杂质粒子。驻留滑移带(PSB)和晶粒取向对其疲劳裂纹早期扩展有重要影响。     

7050铝合金FSW接头微区低周疲劳裂纹扩展行为

对5 mm厚铝合金7050搅拌摩擦焊接头微区进行了低周疲劳裂纹扩展试验，研究了焊核区、前进侧及后退侧热力影响区的疲劳裂纹扩展行为. 结果表明，焊核区的疲劳裂纹扩展速率最快，前进侧热力影响区次之，后退侧热力影响区最慢. 焊核区疲劳裂纹呈沿晶与穿晶混合方式扩展，热力影响区的裂纹主要以穿晶方式进行扩展. 裂纹偏转或裂纹产生分支是热力影响区疲劳裂纹扩展速率降低的原因. 疲劳裂纹扩展初期，焊核区断口有疲劳辉纹出现；热力影响区断口并没有找到疲劳辉纹，而是出现了轮胎压痕花样. 疲劳裂纹稳态扩展期，焊核区和热力影响区断口均有疲劳辉纹.     

Q960E热影响粗晶区疲劳寿命与ΔKth值的关系分析

基于模拟焊接热循环试验及疲劳裂纹扩展试验,对动载结构用高强钢Q960E热影响粗晶区进行了多种应力幅值作用下的疲劳寿命研究.通过得到Paris方程建立了不同焊接热模拟工艺下疲劳裂纹扩展门槛值（ΔK_th）随不同交变载荷下疲劳寿命值的近似线性关系.利用场发射扫描电镜中背散射衍射功能（EBSD）对疲劳裂纹扩展试样中的裂纹尖端进行了晶体学取向分析及扩展机制讨论.结果表明,在应力幅值ΔP固化后,疲劳寿命N随ΔK_th的增大而增加,其延寿微观机理在于组织中的亚结构取向存在差异,所形成的大角度晶界（≥15°）可有效迫使裂纹转向,从而提高材料的疲劳寿命.     

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 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.     

Microstructure and fatigue properties of plasma transferred arc alloying TiC-W-Cr on gray cast iron

In this paper, TiC-W-Cr powders were alloyed on grey cast iron by plasma transferred arc (PTA). The alloying samples were characterized the microstructure, microhardness, fatigue life and fatigue crack growth. From the results, it is indicated that two distinguishing region: alloying zone, heat affected zone are formed on the surface after PTA alloying. The alloying zone mainly consists of primary austenite, martensite, a eutectic of (Fe,Cr)₇C₃ carbide and austenite as well as the uniformly distributed un-melted TiC particles. PTA alloying TiC-W-Cr eliminates the stress concentration at the edge of graphite and produced hard carbide, resulting in frequent crack deflection. As a result, the Weibull distribution of fatigue life demonstrates that PTA alloying TiC-W-Cr exhibits longer lives compared to matrix and PTA hardening without reinforcement, but more scattered. In addition, on the basis of the careful observation of fatigue crack growth, it is shown that the fatigue crack growth rate could be retarded by PTA alloying TiC-W-Cr at low stress intensity.     

Fatigue crack growth behavior of a solid solution-strengthened nickel-base superalloy (Incoloy 825)

Fatigue crack growth behavior of a solid solution-strengthened nickel-base superalloy (Incoloy 825)* was investigated. The investigation also examined the influence of heat treatment on resultant microstructures and the near-threshold fatigue crack growth behavior. In addition, the influence of load ratios(R), material strength, and grain size on fatigue threshold was studied. Compact tension specimens prepared from Incoloy 825 with transverse-longitudinal (TL) orientation in the as-received, as well as two different heat treated conditions, were used. The heat treatment studies revealed a peak hardness condition after solution treatment at 1200 °C for 1/2 hr, followed by aging at 600 °C for 434 hr. Among all the heat treated conditions, the fatigue threshold was the highest and the near-threshold crack growth rate was lowest in this peak aged condition. Fatigue threshold values were observed to decrease with an increase in load ratio, whereas an increased grain diameter resulted in a higher fatigue threshold. An earlier mathematical model was found applicable to characterize the relationship between load ratio and fatigue threshold. Preferential etching of grain boundary suggests formation of a thin film of carbide precipitation along the grain boundary region in the aged specimens. This carbide precipitation facilitated intergranular crack growth in these samples, resulting in higher roughness-induced crack closure. The highest fatigue threshold in the peak aged condition can be attributed to this large roughness-induced crack closure process. Incoloy 825 is a trademark for products of Huntington Alloys, Inc.     

Micromechanisms of multistage fatigue crack growth in a high-strength aluminum alloy

This study reveals the micromechanisms of fatigue damage formation and evolution with respect to particle topology and grain size and orientation in a rolled 7075-T651 Al alloy. Systematic observations were made of the variations in the fracture surfaces and damage micromechanisms, which were characterized in three fatigue stages: fatigue crack formation, microstructurally/physically small cracks and long cracks. The fatigue crack was formed exclusively at the fractured Fe-rich intermetallic constituent particles preferably located at or near the specimen surface. Large impurities, such as metallic oxides, were also observed to influence the crack nucleation mechanisms. The presence of these impurities close to the nucleation sites was correlated with an approximate 30% reduction in fatigue life. In the microstructurally and physically small crack regimes, the crack front showed a rough localized brittle fatigue fracture along the crack propagation direction in addition to some localized ductile fatigue fracture. Changes in striation size across grain boundaries were clearly observed. In the long crack regime, the fracture surface became rougher but the overall surface tended to be perpendicular to the loading direction, indicating a Mode I fracture. The ramification of the results for a microstructure-based multistage model that comprises crack incubation, small crack growth and long crack growth is discussed in detail.     

Accumulation of microstructural damage due to fatigue of high-strength aluminum alloys

The microstructural features of aluminum alloy 7050-T7451 in the vicinity of fatigue cracks and on the crack path were studied to determine which of these features influence fatigue crack propagation. The studies included characterization of the full spectrum of microstructural and fracture surface features— from the largest (e.g., roughness and grain type) to the smallest (e.g., second-phase particles and dislocations). Of all the features studied, only the second-phase particles were shown to have a definite influence by causing crack deflection. The number of particles encountered by the fatigue cracks were significantly higher than the expected average. The fatigue crack path was predominately transgranular, and there was no change in the dislocation and precipitation structures in the crack-affected zone.     

ZG42CrMo缩松区疲劳裂纹的偏析和微裂纹的形成

测定了ＺＧ４２ＣｒＭｏ高周疲劳裂纹扩展速率，对疲劳裂纹扩展过程进行了金相追踪观察．用ＥＰＭ研究了缩松及其表面微裂纹和硫化物的形态．用ＴＥＭ研究了裂纹顶端位错组态和滑移特征．讨论了缩松对裂纹捕获和偏折对疲劳裂纹扩展的双重影响及微裂纹萌生的可能机制．     

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.     

硬质合金截齿刀在使用中的开裂及疲劳行为分析

借助于扫描电镜,全面分析了截齿硬质合金刀头在使用过程中的开裂及裂纹的疲劳扩展行为。结果表明,硬质合金刀头在使用过程中的开裂是由针焊开裂、热疲劳和冲击疲劳所造成的微裂纹引起的。裂纹扩展的疲劳行为主要源于硬质含金的高Co含量和大的WC颗粒造成的对裂纹扩展的阻碍、裂纹扩展路线的拐折造成的裂纹尖端应力强度因子的降低及刀头承受的交变冲击载荷的共同作用。     

Study on dominant mechanism of high-cycle fatigue life in 6061-T6 aluminum alloy through microanalyses of microstructurally small cracks

The mechanism controlling the fatigue life of a precipitation-hardened Al–Mg–Si alloy (6061-T6) at a high-cycle fatigue (HCF) regime of over 10⁷ cycles was investigated in detail. It was found that over 90% of the total fatigue life was occupied by the growth process of a microstructurally small crack at relatively low stress amplitude. The small crack was often found to be arrested and halted for a long period (more than 10⁶ cycles) before it began to grow again, which resulted in a significantly slow growth process. The small crack was then analyzed not only by the conventional fractography but also by the cross-sectional observation of the crack tip region using a focused ion beam and transmission electron microscopy. These observations, supplemented also by a grain orientation analysis using electron backscattered diffraction, explicitly revealed the following points: (i) the small crack growth observed on the specimen surface is primarily related to facet-type cracking that occurs exclusively at the specimen surface; (ii) the growth direction of the small crack has strong anisotropy (i.e. surface-induced growth); (iii) the facet-type cracking is related to the formation of persistent fine slip bands that accompany no structural change of the matrix. On the basis of these results, the micromechanism of small crack growth and its relation to the concept of fatigue limit at the HCF regime is discussed in detail.     

低温卷取热轧双相钢的显微组织及疲劳性能

采用扫描电镜、拉-拉疲劳试验机等研究了低温卷取热轧双相钢的显微组织及疲劳性能。结果表明:热轧双相试验钢的疲劳极限约为530 MPa;低温卷取工艺生产的热轧双相试验钢夹杂物平均尺寸多在5 μm以下,晶粒比较细小,马氏体组织较细小且弥散均匀分布,具有良好的综合力学性能。热轧双相试验钢疲劳裂纹源位于样品表面的棱角处,疲劳裂纹扩展区上有大量的韧窝、撕裂棱、疲劳辉纹和二次裂纹,瞬断区以浅韧窝为主,由于铁素体和马氏体发生不同程度的应变,最终二次裂纹在铁素体和马氏体的相界面萌生。二次裂纹虽然萌生但并未扩展,大量二次裂纹分散主裂纹尖端应力集中,可有效降低裂纹扩展的驱动力,降低疲劳裂纹扩散速率,抑制疲劳裂纹扩展,使疲劳强度得到提升。     

Study of the interaction of a short fatigue crack with grain boundaries in a cast Al alloy using X-ray microtomography

Synchrotron Radiation X-ray microtomography is used to visualize and analyze simultaneously the three-dimensional shape of crystallographic grains containing a short fatigue crack in a cast Al alloy. The visualization of the grains is based on the decoration of Al grain boundaries by liquid Ga which serves as a selective contrast agent. The intricate three-dimensional shape of the fatigue crack, as well as the crack stops observed on the sample surface, are correlated to the grain structure of the material. Complementary measurements of the grain orientation on the sample surface by electron backscattering diffraction (EBSD) allow us to discuss and interpret the observations in terms of possible crack propagation mechanisms.     

不同热处理条件下贝氏体钢的微观组织和疲劳裂纹扩展

以贝氏体钢为研究对象,设计了4种热处理工艺,研究了不同热处理工艺下试验钢的显微组织及疲劳裂纹扩展速率。结果表明,热轧态试验钢的微观组织以粒状贝氏体为主,其上分布有少量的板条贝氏体、马氏体和粗大块状M/A岛,残留奥氏体的体积分数为16.2%,但稳定性较差,裂纹能够直接穿过粗大的块状M/A岛继续扩展,疲劳裂纹扩展速率最快。经900 ℃奥氏体化+空冷后,显微组织以板条贝氏体和马氏体为主,M/A岛仍为粗大的块状,残留奥氏体含量减少至12.3%,疲劳裂纹扩展速率略有降低。经900 ℃奥氏体化+380 ℃盐浴等温30 min +空冷后,显微组织以细密、有序的板条贝氏体为主,残留奥氏体含量减少至10.2%,以薄膜状伴生在板条贝氏体间,板条状贝氏体及板条间的残留奥氏体薄膜会使裂纹端钝化、分叉、偏折,阻碍裂纹扩展的能力增强;经350 ℃回火240 min后,显微组织以马氏体和板条贝氏体为主,残留奥氏体含量比热轧态略微降低,为14.9%;而经450 ℃回火240 min后,显微组织以板条状贝氏体为主,其上分布有少量的马氏体,残留奥氏体体积分数减少至8.6%,也以薄膜状伴生在贝氏体板条间,同时有大量的碳化物析出,裂纹扩展速率最慢。     

AZ31镁合金在高周疲劳过程中微观组织和孪晶演变特征

利用扫描电子显微镜（SEM）和电子背散射衍射（EBSD）技术研究了室温条件下AZ31镁合金在不同加载频率（3和30 Hz）和不同应力幅值（90,95,100,105,110 MPa）疲劳变形后的组织演变规律及断口形貌特征。结果表明：随着加载应力增加,基体内残余孪晶数量增加,残余孪晶主要以拉伸孪晶形式存在。随着应力幅值的增加晶粒逐渐细化,这是由于在循环过程中,拉伸孪晶演变诱导晶粒细化。随着应力幅值的增加,织构强度显著减弱,这与试样疲劳后的再结晶机制有关。通过对试样疲劳断口的分析,发现孪晶片层处容易引起裂纹萌生,随着应力的增加,试样中裂纹扩展区面积逐渐减小,在疲劳裂纹扩展区观察到明显的疲劳辉纹。最终断裂区表面粗糙,主要存在韧窝、撕裂脊以及二次裂纹等形貌。在最终断裂区可观察到韧窝,韧窝尺寸随着循环应力的增加,在较高加载频率下,韧窝的尺寸与数量均减小。     

Fatigue crack growth behaviors in Al−Si−Mg sand cast alloys

The fatigue crack growth behavior of Al−Si−Mg sand cast alloys has been investigated with reference to the effects of solidification structure and aging condition. Fatigue crack growth tests have been carried out under constant load amplitude and a stress ratio of R=0.1 using CT specimens. The amount of pores in the matrix was limited by performing HIP treatment. The pores tended to promote deflection of fatigue cracks, which decreased the fatigue crack growth rate at low ΔK regions and increased the number of cycles until final fatigue fracture. Refining and spheroidizing of eutectic Si particles increased the fatigue crack growth rates over a wide range of ΔK up to larger ΔK values. The difference of aging conditions significantly affected the da/dN-ΔK_eff relationship.