表面缺陷对单晶高温合金高周疲劳性能的影响

在定向凝固炉中采用螺旋选晶法制备了一种单晶高温合金试棒，标准热处理后加工成旋转弯曲高周疲劳试样，试样中间位置用电火花加工成不同尺寸的孔洞以模拟叶片的表面缺陷，在980 ℃、应力分别为400 MPa和500 MPa条件下，研究表面孔洞对合金高周疲劳性能的影响，用扫描电镜分析了疲劳试样的断口形貌.结果表明，与标准试样相比，带有孔洞合金的高周疲劳寿命都有不同程度的降低，随着表面孔洞尺寸增大，合金的疲劳寿命逐渐减小.在合金试样的高周疲劳断口上可见疲劳源区、裂纹扩展区和瞬断区.相对于标准试样，带有孔洞试样疲劳源除了试样表面，还有表面孔洞，所有试样都为多源疲劳断裂.与高温下拉伸持久的断裂机制不同，高温下旋转弯曲高周疲劳为类解理断裂.      

等静压铍材高周疲劳寿命研究

采用拉-拉加载方式对等静压铍材进行高周疲劳试验,根据试验数据绘制了S-N曲线,获得其疲劳极限,并运用扫描电镜观察疲劳断口形貌。结果表明,等静压铍材在指定疲劳寿命10~7次下的疲劳极限为385 MPa;疲劳裂纹萌生于铍材试样边缘处,裂纹穿过晶界与相邻晶粒内的微裂纹连接合并长大;铍材试样发生瞬时解理断裂,形成主要由疲劳源和放射区构成的断口形貌;疲劳源尺寸随最大循环应力的减小而增大。     

激光沉积TA15钛合金高周疲劳性能研究

为了研究激光沉积TA15钛合金的高周疲劳性能,在720,760和800 MPa应力水平下进行了室温高周疲劳(HCF)测试并分析了疲劳断口,结果显示激光沉积TA15钛合金具有较高的高周疲劳寿命,720 MPa下疲劳源区断口形貌表现为很高的组织敏感性,敏感尺寸达单个α片层尺寸单元,800 MPa下敏感性较弱,只达到α片层集束尺寸单元。部分试样有气孔存在,气孔范围在20～40μm,气孔的存在使试样的高周疲劳寿命呈现不同于锻件的明显分散性,表现为同一应力水平下寿命量级的分散,且气孔的大小与位置对疲劳寿命有不同程度的影响。直径较大和距离表面越近的气孔对疲劳寿命损伤越大。采用数值模拟方法研究发现尺寸大、距离表面近的气孔应力集中系数大,导致裂纹萌生寿命降低。虽然有气孔的存在,但激光沉积TA15钛合金仍具有优异的疲劳性能,这与取向随机、尺寸细小的片层组织有关,细小的显微组织增加裂纹的萌生阻力,提高高周疲劳强度。     

Ti-2Al-2.5Zr合金低温下单向与循环变形行为

采用管材试样研究了低温下 (- 196℃ )Ti 2Al 2 .5Zr合金的循环变形行为。分析了应力比、应力幅、循环周次、加载速率等对合金循环变形行为和疲劳性能的影响。建立了循环和单调加载方式下合金的应力—应变曲线方程和疲劳寿命预测方程。根据不同循环周次下疲劳试样的金相分析和断口观察 ,讨论了合金的循环变形规律和疲劳断口特征     

Ti-2Al-2．5Zr合金低温下单向与循环变形行为

采用管材试样研究了低温下(-196℃)Ti-2m-2．5Zr合金的循环变形行为。分析了应力比、应力幅、循环周次、加载速率等对合金循环变形行为和疲劳性能的影响。建立了循环和单调加载方式下合金的应力一应变曲线方程和疲劳寿命预测方程。根据不同循环周次下疲劳试样的金相分析和断口观察，讨论了合金的循环变形规律和疲劳断口特征。     

高铌TiAl合金的高温断裂韧性

利用紧凑拉伸试样通过预制疲劳裂纹研究近片层组织Ti-45Al-8Nb-0.2W-0.2B-0.1Y合金和全片层组织Ti-45Al-7Nb-0.2W-0.2Hf-0.3B-0.15C合金在750℃下的断裂韧性,并分析两种组织合金的断口形貌.结果表明,近片层组织和全片层组织高铌TiAl合金750℃时的断裂韧性分别为19.54和31.58 MPa·m1/2,且近片层组织疲劳裂纹开始萌生时的最大疲劳载荷明显低于全片层组织.断口分析表明近片层组织中裂纹主要在等轴γ晶中萌生,裂纹扩展方式包括沿γ晶、穿γ晶及沿片层、穿片层;全片层组织中裂纹主要在垂直于加载方向的片层间萌生,裂纹以沿片层与穿片层的混合方式进行扩展且伴有二次裂纹的萌生.      

不同烧结条件下Fe-Cu-Ni-Mo-C合金的弯曲疲劳性能及断口分析

以部分扩散预合金Fe-2Cu-2Ni-1Mo-1C粉末为原料,利用模壁润滑温压技术与3种烧结工艺制备合金材料,研究不同烧结工艺下合金的疲劳性能。结果表明:3组不同烧结工艺制备的试样在104~109循环周次下的应力幅值-循环周次(S-N)曲线均为一条连续下降的曲线,不存在传统疲劳概念上的疲劳极限,只存在条件疲劳极限;在107循环周次下,3组试样的条件疲劳极限分别为280、264和239 MPa。断口分析发现,3组试样的疲劳裂纹均萌生在试样表面的棱角处,且均表现为多源萌生;疲劳裂纹扩展以穿晶断裂为主,不同的组织对裂纹的扩展有明显的影响;扩展区存在典型的解理和疲劳辉纹形貌;断裂区有塑性韧窝出现。     

FGH95合金高低周复合疲劳行为

为了解高周振动应力对FGH95合金低周疲劳过程的影响,对FGH95合金进行高低周复合疲劳和低周疲劳试验。试验结果表明：在低周疲劳过程中叠加的高周振动明显降低了FGH95合金的低周疲劳寿命;且叠加的高周振动应力越大,FGH95合金的低周疲劳寿命降低程度越大。宏观和体视显微镜下分析的试样断口形貌显示：断口呈平坦区和倾斜区两大部分,表面粗糙,有明显的反光小平面。最后,着重用扫描电子显微镜（SEM）分析疲劳裂纹扩展不同阶段的形貌特征,并对不同试验条件下的相同裂纹扩展阶段的裂缝形貌特征进行比较。     

32Cr3MoVE轴承钢旋弯疲劳特性及裂纹萌生扩展行为

摘要：采用光滑漏斗状试样对32Cr3MoVE轴承钢进行旋转弯曲疲劳测试，研究了32Cr3MoVE轴承钢旋转弯曲疲劳性能及裂纹萌生扩展行为。采用升降法测得其疲劳极限为860MPa，疲劳断口SEM观察并统计破断试样结果表明：疲劳破坏68.7%是由于非金属夹杂起裂，18.8%由表面加工缺陷起裂，125%为表面粗糙度起裂。当加载应力低于980MPa时，疲劳断裂主要是由于内部非金属夹杂引起的，高于980MPa时，疲劳断裂主要是由于表面粗糙度引起的。表面加工缺陷和表面粗糙度引起的最大应力强度因子分别为3.05和2.97MPa·m1/2，容易引发疲劳裂纹。非金属夹杂物尺寸在5.30～5.90μm范围内，局部应力从859.35MPa升至977.75MPa时，疲劳寿命从1.96×105降低到1.58×105；非金属夹杂物局部应力在840～900MPa范围内，夹杂物尺寸从2.28μm升至5.83μm时，疲劳寿命从1.10×106降低到1.96×105。     

铒对Al-4.7Mg-0.7Mn-0.1Zr合金疲劳强度的影响

研究了Al-4.7Mg-0.7Mn-0.4Er-0.1Zr合金板材的室温高周疲劳性能,用升降法测试并计算出该合金在应力比R=0.1,循环寿命为1×107条件下的疲劳强度为293.6 MPa.用扫描电镜观察了疲劳试样断口的形貌特征,分析了疲劳裂纹产生的原因和疲劳断裂过程.用TEM观察了合金中析出相的形貌,分析了铒的存在形式.结果表明:添加铒以后,在合金内部形成了纳米级球形Al3Er粒子,合金的室温疲劳强度得到明显提高.     

High-temperature low-cycle fatigue of a gamma titanium aluminide alloy Ti-46Al-2Nb-2Cr

The low-cycle fatigue (LCF) behavior of a gamma titanium aluminide alloy Ti-46Al-2Nb-2Cr in fully lamellar (FL) and nearly lamellar (NL) microstructural conditions is studied at 650 °C and 800 °C, with and without hold times. At 650 °C and 800 °C, the alloy in either condition exhibits cyclic stability at all strain levels studied, excepting the NL structure which shows slight cyclic hardening at higher strain levels at 650 °C. Fracture in the FL condition occurs by a mixed mode comprising delamination, translamellar fracture, and stepwise fracture. On the other hand, fracture occurs mostly by translamellar mode in the NL condition. At both test temperatures, the alloy in the FL condition obeys the well-known Manson-Coffin behavior. The fatigue resistance of the alloy at 650 °C in the FL condition is very much comparable to, while in the NL condition it is superior to, that of Ti-24Al-llNb alloy. At 650 °C, a 100-second peak tensile strain hold doubles the fatigue life of the alloy in the FL condition, while a 100-second hold at compressive peak strain or at both tensile and compressive peak strain degrades fatigue life. The observed hold time effects can primarily be attributed to mean stress. Irrespective of the nature of the test, the hysteretic energy (total as well as tensile) per cycle remains nearly constant during the majority of its life. The total and tensile hysteretic energy to fracture, at both test temperatures, increase with cycles to failure, and the variation follows a power-law relationship. Formerly NRC Senior Resident Associate, Wright Laboratory.     

Influence of microstructure on crack-tip micromechanics and fracture behaviors of a two-phase TiAl alloy

The tensile deformation, crack-tip micromechanics, and fracture behaviors of a two-phase (γ + α₂) gamma titanium aluminide alloy, Ti-47Al-2.6Nb-2(Cr+V), heat-treated for the microstructure of either fine duplex (gamma + lamellar) or predominantly lamellar microstructure were studied in the 25 °C to 800 °C range.In situ tensile and fracture toughness tests were performed in vacuum using a high-temperature loading stage in a scanning electron microscope (SEM), while conventional tensile tests were performed in air. The results revealed strong influences of microstructure on the crack-tip deformation, quasi-static crack growth, and the fracture initiation behaviors in the alloy. Intergranular fracture and cleavage were the dominant fracture mechanisms in the duplex microstructure material, whose fracture remained brittle at temperatures up to 600 °C. In contrast, the nearly fully lamellar microstructure resulted in a relatively high crack growth resistance in the 25 °C to 800 °C range, with interface delamination, translamellar fracture, and decohesion of colony boundaries being the main fracture processes. The higher fracture resistance exhibited by the lamellar microstructure can be attributed, at least partly, to toughening by shear ligaments formed as the result of mismatched crack planes in the process zone.     

Fatigue and fracture behavior of a fine-grained lamellar TiAl alloy

The fatigue and fracture resistance of a TiAl alloy, Ti-47Al-2Nb-2Cr, with 0.2 at. pct boron addition was studied by performing tensile, fracture toughness, and fatigue crack growth tests. The material was heat treated to exhibit a fine-grained, fully lamellar microstructure with approximately 150-μm grain size and 1-μm lamellae spacing. Conventional tensile tests were conducted as a function of temperature to define the brittle-to-ductile transition temperature (BDTT), while fracture and fatigue tests were performed at 25 °C and 815 °C. Fracture toughness tests were performed inside a scanning electron microscope (SEM) equipped with a high-temperature loading stage, as well as using ASTM standard techniques. Fatigue crack growth of large and small cracks was studied in air using conventional methods and by testing inside the SEM. Fatigue and fracture mechanisms in the fine-grained, fully lamellar microstructure were identified and correlated with the corresponding properties. The results showed that the lamellar TiAl alloy exhibited moderate fracture toughness and fatigue crack growth resistance, despite low tensile ductility. The sources of ductility, fracture toughness, and fatigue resistance were identified and related to pertinent microstructural variables.     

Thermal-Mechanical fatigue of Ti-48Al-2V alloy and its composite

Thermal-mechanical fatigue (TMF) and isothermal fatigue (IF) of a Ti-48Al-2V alloy and its composite, reinforced with TiB₂ particles, were studied. In-phase TMF testing was conducted under the condition of a minimum temperatureT _min = 100 °C and a maximum temperatureT _max, which ranged from 750 °C to 1400 °C. The applied cyclic stress ranges were 2.8 to 28 MPa and 4.2 to 42 MPa. The IF tests were carried out at aT _max. The TMF and IF lives are longer for lowerT _max and for smaller stress ranges in both the matrix alloy and its composite. The IF life at a givenT _max is shorter than the TMF life in the matrix alloy at all temperatures employed and in the composite only at higher temperatures. At lower temperatures, the TMF and IF lives are essentially the same in the composite. The resistance to TMF is similar in the matrix alloy and the composite, but the IF resistance is greater in the composite than in the matrix alloy. The proposed TMF mechanism is nucleation and growth of voids on interlamellar plate, twin, and grain boundaries; their interlamellar, translamellar, and intergranular linkage; intergranular separation; and disintegration of lamellar structure.     

Influence of Cast Dimension on Rotary Bending High Cycle Fatigue Properties of Single Crystal Superalloy

Samples of DD3 single crystal superalloy with different dimensions were cast in the directionally solidified furnace with high temperature gradient. The effect of cast dimension on the rotary bending high cycle fatigue (HCF) properties of the alloy was investigated at 800 °C in ambient atmosphere. SEM was used to examine the fracture surface and fracture mechanism of the alloy. The results show that the rotary bending HCF properties of the alloy decreases with increasing cast dimension. The cast dimension has little effect on the HCF fracture mechanism of the alloy. The HCF fracture mechanism of the alloy with different cast dimensions is all quasi-cleavage fracture. The fatigue cracks initiated on the surface or near the surface of the specimens. The crack would propagate along {111} octahedral slip planes. Typical fatigue arc and striation formed on fatigue crack steady propagation. The degeneration of HCF properties is due to the increase of dendrite arm spacing and size of γ′ phase particles and maximal microporosity.     

金属间化合物TiAl合金的缺口断裂机制研究

通过TiAl基合金两种组织的四点弯曲断裂实验，研究了TiAl基合金的缺口断裂机制。实验结果表明：缺口试样的断裂过程为几个沿层裂纹直接起裂于缺口根部，并沿着缺口根部的层间扩展；当遇到位向不利的晶粒时，裂纹便停止在这一晶粒前方，随着外加载荷的增加，即应力的增加，裂纹彼此连接并通过穿层裂纹而扩展；裂纹尖端超钝化、裂纹分叉、沿着层间偏转，形成显微裂纹区，裂纹停在障碍晶粒前的边界上为这种材料的韧化机制；裂纹扩展的驱动力是拉伸应力，缺口的存在增加了缺口前沿或者裂纹尖端的应力。     

Study on notch fracture of TiAl alloys at room temperature

In-situ observations of fracture processes combined with one-to-one observations of fracture surfaces and finite-element method (FEM) calculations are carried out on notched tensile specimens of two-phase polycrystalline TiAl alloys. The results reveal that most cracks are initiated and propagated along the interfaces between lamellae before plastic deformation. The driving force for the fracture process is the tensile stress, which is consistent with a previous study.^[1] In specimens with a slit notch, most cracks are initiated directly from the notch root and extended along lamellar interfaces. The main crack can be stopped or deflected into a delamination mode by a barrier grain with a lamellar interface orientation deviated from the direction of crack propagation. In this case, new cracks are nucleated along lamellar interfaces of grains with favorable orientation ahead of the barrier grain. The main crack and a new crack are then linked by the translamellar cleavage fracture of the barrier grain with increasing applied load. In order to extend the main crack, further increases of the applied load are needed to move the high stress region into the ligament until catastrophic fracture. The FEM calculations reveal that the strength along lamellar interfaces (interlamellar fracture) is as low as 50 MPa and appreciably lower than the strength perpendicular to the lamellae (translamellar fracture), which shows a value higher than 120 MPa. This explains the reason why cracks nucleate and preferably extend along the lamellar interfaces.     

Fatigue crack growth mechanisms in Ti6242 lamellar microstructures: Influence of loading frequency and temperature

Fatigue crack growth experiments were carried out on Ti6242 alloy with large colony size. The alloy was heat treated to provide three different lamella size; fine, coarse, and extra coarse. Tests were conducted at two temperatures, 520 °C and 595 °C, using two loading frequencies, 10 and 0.05 Hz. The latter frequency was examined with and without a 300-second hold time. All tests were performed in air environment and at a stress ratio of 0.1. This study shows that at 520 °C, the Fatigue crack growth rate (FCGR) is not significantly influenced by changes in the microstructure. For 0.05 Hz/low ΔK, however, the FCGR is higher in the fine lamellar microstructure and is accompanied by- the appearance of a plateau, which disappears in the extra large lamella microstructure. Furthermore, the addition of a 300-second hold time does not alter the crack growth rate. At 595 °C, while the general level of the FCGR is higher than that at 520 °C, the effects of loading frequency and hold time remain similar to those reported at the lower temperature. Unlike the results at 520 °C, however, the FCGR at low δK is not influenced by variations in lamellar microstructure. Under all test conditions, the fatigue process is predominantly controlled by one single mechanism associated with transcolony fracture and formation of quasi-cleavage facets. The fatigue crack growth results and the associated fracture behavior as obtained in this study are correlated to the crack-tip shear activity and transmission at the α/β interfaces. A general hypothesis accounting for the role of loading frequency, temperature, and microstructure on the observed cracking mechanisms is presented.     

Investigating and Understanding the Cyclic Fatigue,Deformation, and Fracture Behavior of a Novel High Strength Alloy Steel: Influence of Orientation

In this paper, the results of a recent study aimed at understanding the influence of orientation on high cycle fatigue properties and final fracture behavior of alloy steel Pyrowear 53 is presented and discussed. This alloy steel has noticeably improved strength, ductility, and toughness properties compared to other competing high strength alloy steels having a near similar chemical composition and processing history. Test specimens of this alloy steel were precision machined and conformed to the specifications detailed in the ASTM standards for tension testing and stress‐controlled cyclic fatigue tests. Test specimens were prepared from both the longitudinal and transverse orientations of the as‐provided alloy steel bar stock. The machined test specimens were deformed in cyclic fatigue over a range of maximum stress and under conditions of fully reversed loading, i.e., at a load ratio of ?1, and the number of cycles‐to‐failure recorded. The specific influence of orientation on cyclic fatigue life of this alloy steel is presented. The fatigue fracture surfaces were examined in a scanning electron microscope to establish the macroscopic fracture mode and to characterize the intrinsic features on the fatigue fracture surfaces. The conjoint influence of microstructure, orientation, nature of loading, and maximum stress on cyclic fatigue life is discussed.