共查询到19条相似文献,搜索用时 203 毫秒
1.
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. 相似文献
2.
研究了双尺寸板条组织Ti-22Al-25Nb合金在650℃和700℃下的高周疲劳行为,采用升降法测试了合金的高温高周疲劳强度极限。当应力比R=-1,循环周次Nf=107次时,650℃和700℃的疲劳强度极限分别为470 MPa和400 MPa。由于不同阶段的高周疲劳裂纹在高温条件下暴露的时间不同导致试样断口表面的氧化产物不同,从而使得高温高周疲劳试样的断口上呈现出不同颜色的区域。通过颜色的变化发现疲劳裂纹既可萌生于试样表面,也可萌生于次表面,并且高周疲劳裂纹在次表面形核的试样具有更高的疲劳寿命。此外,研究发现双尺寸板条组织在高温高周疲劳损伤过程中以胞状析出的形式发生B2→β+O相变,形成组织中的不均匀区域,促使疲劳裂纹在此优先形核。 相似文献
3.
4.
7475-T7351铝合金抗疲劳性能研究 总被引:2,自引:0,他引:2
采用旋转弯曲疲劳试验、轴向加载疲劳试验、疲劳裂纹扩展速率试验等疲劳性能测试方法,研究了7475-T7351铝合金厚板的疲劳性能.并通过透射电镜(TEM)和扫描电镜(SEM)分析了该合金的显微组织和疲劳断口形貌.结果表明:7475-T7351铝合金具有良好的耐疲劳损伤性能,光滑试样(Kt=1)在室温旋转弯曲和高温轴向加载条件下的疲劳极限分别为180.0和345.0 MPa,缺口试样(Kt=2.2)在室温旋转弯曲加载条件下的疲劳极限为91.9 MPa;合金厚板材料在高温下缺口敏感性有所降低;国产材料裂纹扩展速率随应力比增加而增大,裂纹扩展门槛值减小;国产7475铝合金与进口材料在裂纹稳定扩展阶段裂纹扩展行为基本相当;在近门槛值附近不同应力比下的裂纹扩展门槛值略有差别. 相似文献
5.
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. 相似文献
6.
研究了[011]取向的镍基单晶高温合金在750~980℃温度范围和200~680 MPa应力下的蠕变断裂特征.在扫描电镜上对各种实验状态下的蠕变断口和纵向剖面进行了详细观察.研究发现:在低温750℃和中温870℃不同初始蠕变应力条件下,枝晶间区亚晶界处不规则γ'/γ界面是裂纹主要萌生场所,这些已萌生的裂纹在与外加应力轴垂直的(011)面上沿〈110〉和〈100〉两个方向扩展;980℃不同初始应力条件下,裂纹主要在合金中显微疏松孔洞处萌生,沿与外应力轴垂直的方向扩展.观察750℃和870℃不同应力状态蠕变试样的纵向剖面,对亚晶界区不规则γ'相面积分数的测量和计算表明,用面积分数表征该合金[011]取向在中低温状态下的蠕变损伤程度是可行的. 相似文献
7.
介绍了基于直流电压降法测量蒸汽发生器传热管690合金轴向疲劳裂纹扩展速率的销加载拉伸方法.该方法与其他方法相比较,可以直接采用原始管状材料,在线连续测量管状试样在不同应力强度因子下的疲劳裂纹扩展.通过对标准紧凑拉伸试样的类比分析,建立传热管试样的销加载拉伸模型,并对该模型进行电学和力学有限元模拟分析,确定直流电压降数据采集方法.验证试验采用核电蒸汽发生器用690合金传热管,分别研究了室温和高温325℃空气中载荷和温度对材料疲劳裂纹扩展速率的影响,试验结果采用Paris-Erdogan公式进行拟合,吻合度较好.扫描电镜下观察端口形貌,疲劳裂纹的扩展为穿晶形式,在穿晶断口上观察到明显的疲劳辉纹和微塑性区. 相似文献
8.
测定了国产和进口核电站凝汽器用钛焊管在室温大气和人工海水100℃水蒸汽介质中的疲劳性能,同时采用扫描电子显微镜对疲劳断口进行了观察,探讨了钛焊管的蒸汽腐蚀疲劳机理。研究结果表明:国产钛焊管与进口钛焊管的疲劳寿命较为接近,水蒸汽氛围大幅度降低了钛焊管的疲劳寿命;疲劳试样的裂纹均萌生于钛焊管焊缝区域的外表面,多为多源萌生,裂纹扩展均以条纹机理为主;断口处有少量二次裂纹,未发现沿晶断裂及周期解理断裂特征,静断区断口形貌为韧窝;从蒸汽腐蚀疲劳试样中,未能观察到明显的腐蚀产物;钛焊管的蒸汽疲劳腐蚀是交变应力和电化学腐蚀交互作用的结果。 相似文献
9.
10.
11.
The effects of air and vacuum on the fatigue behavior of a nickel-base superalloy, Mar-M200, in single crystal form were investigated.
Between 800° and 1400°F fracture is entirely in the Stage I mode in air and vacuum, and fatigue life is unaffected by environment.
At 1700° F in both environments, fracture is predominantly in the Stage II mode and fatigue life in air is greater than that
in vacuum. At both temperatures, fatigue cracking in air is internally initiated, whereas in vacuum cracking is generally
initiated at the specimen surface. Identical fatigue lives in air and in vacuum between 800° and 1400° F are attributed to
the fact that internally initiated cracks in air are actually propagating in a high vacuum, surface cracking being inhibited
by dynamic oxidation of emerging surface slip offsets. The subsurface portion of the Stage I fracture surface produced in
air tests and all of the Stage I fracture produced in vacuum tests shows a dimpled structure, whereas the Stage I fracture
surface produced while the crack propagation is in air shows a cleavage appearance. At 1700° F, bulk oxidation of surface
initiated cracks interferes with the plastic blunting mechanism of Stage II crack growth normally observed at this temperature,
internally initiated cracks causing ultimate failure. Shorter lives in vacuum are thought to result from enhanced Stage II
surface crack propagation.
Formerly with Materials Engineering and Research Laboratory, Pratt and Whitney Aircraft, Middletown, Conn. 相似文献
12.
Fatigue crack propagation in nickel-base superalloys at low and intermediate temperatures occurs predominantly in the Stage I mode, along {111} slip planes. Cracking normally starts at an external surface and the Stage I fracture surface has a cleavage appearance. Both of these factors indicate that the environment may play an important role in this mode of propagation. To assess the role of environment in Stage I fracture and to determine the mechanism of failure, fatigue tests were run in air and vacuum on single crystals of low-carbon MAR-M200. The fatigue life at room temperature is significantly greater in vacuum than in air, and the improvement in life increases as the stress range is reduced. Fatigue crack propagation in specimens tested in air and in vacuum is entirely in the Stage I mode, but only the specimens tested at low stress ranges in air have a cleavage appearance. In vacuum and at high applied stress levels in air, fracture surfaces have a matte appearance with fewer fracture steps and river lines. At high magnifications, a dimpled structure is observed on these fracture surfaces. The fatigue life in air can be attributed to a faster rate of crack growth resulting from oxygen adsorption at the crack tip. A model for Stage I fatigue crack propagation in planar slip materials is presented which is an extension of the Griffith-Orowan criterion to cases where localized cleavage occurs at a crack tip in fatigue. 相似文献
13.
An electron fractrographic study was made of opposing crystallographic (Stage I) fatigue fracture surfaces of specimens of a single crystal nickel-base superalloy, low carbon MAR-M200, that were tested at room temperature. In regions near the crack initiation site, features on both surfaces are the same. In regions at some distance from the initiation site significant differences in the type and extent of microfeatures were observed. For a crack propagating in an upward direction, irregular markings were observed on the upper fracture surface and regularly spaced slip offsets and slip band cracks were observed on the lower surface. These observations are explained by a consideration of the elastic stress field and the resultant glide forces on all possible slip systems surrounding a crack growing at an angle to the principal stress direction. Additionally the results are used to support a previously proposed model for Stage I fatigue crack propagation. 相似文献
14.
Study on notch fracture of TiAl alloys at room temperature 总被引:7,自引:0,他引:7
J. H. Chen G. Z. Wang R. Cao J. Zhang 《Metallurgical and Materials Transactions A》2004,35(2):439-456
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. 相似文献
15.
《钢铁研究学报(英文版)》2015,(7)
Very high cycle fatigue behavior(107-109 cycles)of 304 Laustenitic stainless steel was studied with ultrasonic fatigue testing system(20kHz).The characteristics of fatigue crack initiation and propagation were discussed based on the observation of surface plastic deformation and heat dissipation.It was found that micro-plasticity(slip markings)could be observed on the specimen surface even at very low stress amplitudes.The persistent slip markings increased clearly along with a remarkable process of heat dissipation just before the fatigue failure.By detailed investigation using a scanning electron microscope and an infrared camera,slip markings appeared at the large grains where the fatigue crack initiation site was located.The surface temperature around the fatigue crack tip and the slip markings close to the fracture surface increased prominently with the propagation of fatigue crack.Finally,the coupling relationship among the fatigue crack propagation,appearance of surface slip markings and heat dissipation was analyzed for a better understanding of ultrasonic fatigue damage behavior. 相似文献
16.
The fractal dimension along the crack propagation direction on the fatigue fracture surface of a dual-phase steel was investigated by both vertical section profile method and secondary electron line scanning method. Results from the vertical section profile method showed that during the crack propagation, the fractal dimension of the fracture surface increases with increasing stress intensity factor, however, the secondary electron line scanning method presented somewhat different results of fractal dimension and was proved not feasible in all kinds of fracture surfaces, so this method is not suggested in calculating the fractal dimension of the fracture surface. 相似文献
17.
E. R. Golubovskiy I. L. Svetlov N. V. Petrushin S. A. Cherkasova M. E. Volkov 《Russian Metallurgy (Metally)》2010,(10):941-947
Single-crystal samples of nickel superalloys containing rhenium or rhenium plus ruthenium are subjected to low-cycle fatigue
(LCF) tests under rigid cycle conditions at temperatures of 850 and 1050°C. It is found that the single crystals of the alloy
containing rhenium with ruthenium have higher LCF resistance at 104 cycles as compared to the alloy containing only rhenium. At a test temperature of 850°C, volume stress concentrators in the
form of pores or their clusters represent fatigue crack nucleation zones; at 1050°C, surface corrosion cracks are the main
fracture zones. The fatigue microcrack growth rate is anisotropic: it is higher in the [001] direction and lower in the [011]
direction. 相似文献
18.
W. L. Haworth V. K. Singh R. K. Mueller 《Metallurgical and Materials Transactions A》1980,11(2):219-229
Changes in optical correlation intensity (I)c are observed during fatigue cycling of 2024-T3 aluminum alloy. TheI
c measurements are made by transmitting light scattered from the specimen surface through a holographic filter containing information
about the surface topography at an earlier time. Topographic changes such as slip band development, microcracking, and crack
propagation are observed and recordedin situ during fatigue cycling of individual specimens and cause corresponding changes in correlation intensity. A three-stage curve
log (I)c
vs number of fatigue cycles is observed for both unnotched and notched specimens. The overall shape of the curve is not affected
by the applied stress levels in constant amplitude tests. Thein situ metallographic observations confirm that region A of the correlation intensity curve corresponds to progressive roughening
of the specimen surface caused by slip during the early part of the fatigue life, together with a rapid increase in the number
of microcracks of the order of a few micrometers in length. Few metallographic changes are observed during region B of the
curve, where the correlation intensity remains relatively constant. The accelerating loss of correlation intensity in region
C of the curve arises from the elastic and plastic displacements which occur as a crack or cracks grow beyond about 10 μm
in length. The metallographic observations also show that for both notched and unnotched specimens, the correlation intensity
readings in region C are sensitive to factors such as crack branching, crack-tip plasticity, and changes in crack growth direction
as well as to the overall increase in crack length. The total loss of correlation intensity from the beginning of fatigue
cycling to the development of a crack about 800 μm in length can be more than eight orders of magnitude at the present sensitivity
of our experiments. The optical correlation technique is an extremely sensitive method of detecting remotely, in air, fatigue
damage, and the propagation of fatigue cracks from ten to several hundred micrometers in length. The correlation intensity
curve provides an indication of developing fatigue damage and impending fatigue failure in individual specimens, and detects
the onset of crack propagation with no prior knowledge of the presence or precise location of particular flaws or cracks. 相似文献
19.
R. B. Scarlin 《Metallurgical and Materials Transactions A》1976,7(10):1535-1541
High cycle fatigue crack growth rates have been measured in the cast nickel-base alloy IN 738 LC in directionally-solidified
form, at room and high temperature and for crack propagation both parallel and perpendicular to the solidification direction.
The resistance of this material to crack propagation has been compared with conventionally-cast material of the same composition.
The considerable differences in observed growth rates may be understood in terms of the effects of chemical segregation, crack
branching and crystallographic fracture. In particular, the high-temperature cyclic fracture toughness for crack growth perpendicular
to the solidification direction is higher than in conventionally-cast material as cracks tend to deviate along the segregated
interdendritic regions. However the room temperature threshold stress intensity amplitude is low because fatigue crack growth
occurs on definite crystallographic planes. 相似文献