Low-Cycle Fatigue Behavior of a Nickel Base Single Crystal Superalloy at High Temperature

Rare Metal Materials and Engineering Volume 44, Issue 2, February 2015 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2015, 44(2): 0288-0292.

ARTICLE

Low-Cycle Fatigue Behavior of a Nickel Base Single Crystal Superalloy at High Temperature Shui Li1, 1

Liu Ping2 2

Shenyang Ligong University, Shenyang 110159ˈChina; Shenyang Liming Aero-Engine (Group) Corporation Ltd, Shenyang 110043, China

Abstract: Total strain controlled low cycle fatigue tests have been performed at 760 and 870 oC on a nickel base single crystal supperalloy and its fatigue behavior and microstructural character have been studied. The dislocation characteristics and the fracture surface observation were evaluated through scanning electron microscopy and transmission electron microscopy, respectively. Results show that the fatigue life for specimens increases with the decreased strain amplitude. The homogeneous secondary Ȗƍ particles distribution in the matrix is beneficial for enhancement of fatigue resistance. The formation of persistent slip bands (PSBs) enhances the fatigue crack initiation at micropores and the propagation along the PSBs. At 870 oC and higher strain amplitude, the locally higher stress concentration resulted from dislocation tangling, and progressive coarsening promotes a relatively soon initiation of fatigue microcracks. Key words: nickel base single crystal superalloy; low cycle fatigue; microstructure; dislocation

Single crystal nickel-base superalloys, containing a large volume fraction of intermetallic Ni3(Al,Ti) phase (usually more than 65%~70%) are widely used as blade and vane materials in modern gas turbine and aeroengine [1-3]. Blades and vanes experience complex thermomechanical loading including long periods of isotheral creep and creep with superimposed vibration, i.e. interaction of high cycle fatigue with high temperature creep [4,5]. In addition, low cycle fatigue (LCF) at elevated temperature is an important consideration factor in the design of turbine components such as disks and turbine blades. Life prediction calculations for single crystal turbine blades must take account of the possible growth of cracks during the service life of a component, and a material design objective should be to minimize crack growth rate under both creep and fatigue loading conditions. Microstructural changes during service and their influence may be dealt with failure mechanisms and life-time must also be considered. Various approaches seem to work well for some alloy systems, but not for others and a universally applicable fatigue law has not been discovered. This is probably due to the fact that in different classes of materials,

different forms of damage can occur. In practical applications, the microstructures such as the size, number and morphology of J c particles, density and configuration of dislocations, undergo changes in the highly localized plastic deformation regions [6,7]. Therefore, it is very necessary to study the microstructural modifications and fracture mechanisms at elevated temperature. The aim of this paper is to present some results concerning the LCF-behavior and the fracture characteristics, and to show how they depend on cyclic strain amplitude in a newly developed nickel base single crystal superalloy.

1

Experiment

The single crystal nickel-base superalloy with [001] orientation was prepared by a crystal selection method in a vacuum directional solidification furnace under a high thermal gradient. All specimens were within 8q deviating from [001] orientation. The composition (wt%) was 6.0Al, 2.1Ti, 7.8Cr, 5.3W, 2.2Mo, 5.5Co, 3.2Ta, Ni bal. As-cast bar received a solution treatment at 1300 oC for 3 h, and two-step aging treatment, consisting of heat treatment at 1080 oC for 6 h and

Received date: April 1, 2014 Foundation item: National Natural Science Foundation of China (50171045) Corresponding author: Shui Li, Ph. D., Professor, School of Mechanical Engineering, Shenyang Ligong University, Shenyang 110159, P. R. China, Tel: 0086-24-24193189, E-mail: [email protected] Copyright © 2015, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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2.1 Low cycle fatigue Fig.1 shows the dependence of the cycle number to failure Nf on the total strain range ǻİt for the test alloy at 760 and 870 o C. Lower strain amplitude and lower test temperature exhibit a pronounced improvement in LCF compared to higher temperature and higher strain amplitude. For the sake of simplicity, the results for these two test temperatures were expressed by connected straight lines, which do not involve any theoretical consideration. As can be seen from Fig.1, the connected lines show a general trend that the fatigue life increases with decreasing of test temperature at the same cyclic strain amplitude. Fig.2 shows the cyclic stress response behavior for specimens tested at 760 and 870 oC at various strain amplitudes. A quite significant difference in the shapes of cyclic stress response curves, is found between the specimens tested at 760 oC and higher 870 oC. At the lower temperature of 760 oC, the specimens show an initially rapid cyclic hardening followed by a longer regime where the stress amplitude falls off less rapidly. At the higher temperature of 870 oC, the earlier scatter lines exhibit a state of maintaining nearly constant stress for a period and thereafter there is a steady decline in stress. This hardening can result from fatiguing of a worked hardening structure, dislocation tangling or structural stabilities. The reasons for the observed cyclic hardening and softening behavior will be discussed later in light of microstructural observations. Finally near the end of the test when cracks are present between the extensometer probes the stress drops off again very rapidly. 2.2 Dislocation structure Fig.3 is a set of TEM images showing the typical dislocation structure after low cycle fatigue tested at 760 oC. It is revealed that the dislocation are on the J/J c interfaces; there is little evidence of cutting through the J c particles (Fig.3a). This proves that the fatigue deformation is mainly concentrated in the J matrix. The dislocations move through the material by bowing out through the J channels on ^111` planes; a part of

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Fig.1 Relational curves between ǻİt and Nf at 760 and 870 oC 800 Max Tensile Stress/MPa

Results

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at 850 oC for 24 h, both followed by air cooling. The process of heat treatment produced the precipitates of regular cuboidal Jc particles aligning along <100> in the J matrix. The cuboidal J c possesses a volume fraction of about 70%, and an average size of about 450 nm. The LCF behavior was investigated using longitudinal specimens at 760 and 870 oC at a strain rate H 5 u 103 s 1 . All the specimens were tested under total strain control with a triangular waveform in the strain amplitude r0.3ar1.2% range at a strain ratio R=İmin/İmax= –1 using an EHF-100KN-20L servo-hydraulic mechanical fatigue machine. Foils for transmission electron microscopy (TEM) analysis were obtained from gauge sections of specimens in the perpendicular direction with respect to load axis. Fracture surfaces were examined by means of scanning electron microscopy (SEM).

Total Strain Range, ǻİ/%

Shui Li et al. / Rare Metal Materials and Engineering, 2015, 44(2): 0288-0292

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Cycle to Failure, Nf Fig.2 Cyclic stress response curves for three different strain amplitude fatigue tests at 760 and 870 oC: (a) ǻİt=0.6%, (b) ǻİt=0.8% , and (c) ǻİt=1.0%

them are annihilated upon encountering with the dislocations of the opposite sign, and another part occupy J/J c interfaces to relief the misfit stresses. A micrograph of the cycle medium-

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Shui Li et al. / Rare Metal Materials and Engineering, 2015, 44(2): 0288-0292

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Fig.3 TEM images of dislocation configuration after LCF test at 760 oC: (a) N=680 cycle, ǻİt=0.4%; (b) N=6000 cycle, ǻİt=0.6%; (c) N=1700 cycle, ǻİt=0.8%

term structure is shown in Fig.3b. The microstructure consists of a high density of dislocations and a large number of hyperfine secondary Ȗ c particles which are spherical and homogeneously distributed in the Ȗ matrix channel. The presence of the secondary Ȗc is thought to be related to the cyclic plastic deformation occurring during the fatigue loading process and high temperature [3]. Near the area marked “A”, the dislocations are bowing through the narrow channel and spreading out in the channel-absent hyperfine secondary Ȗc particles, whereas in the regions marked “B” only a few dislocations are present locally in the channels containing numerous hyperfine secondary J c particles. It is suggested that the hyperfine secondary J c particles would retard the dislocation bowing by preventing the dislocations from gliding in the matrix channel. One additional structural feature was observed, namely persistent slip bands shown in the Fig.3c. At the arrow a long and straight persistent slip bands (PSBs) are going right through both the matrix and the Ȗc precipitates. Such bands have never been found in the specimens in the early and medium-term cycles. The bands lie along the ^111` slip planes. The angle between the ^111` slip planes and the plane of the foil (001) is 54.7°. That is why the fatigue slip bands appear in the foil as broad bands. The PSBs seem to be filled with a large amount of dislocations. The precipitates can not change to a rafting structure. The occurrence of the PSBs dislocations in fatigue specimen is presumably related to the plastic instability. The deformation structure of the specimen at 870 oC with different strain amplitudes is shown in Fig.4. The dislocations can be characterized to be matrix dislocations, interfacial dislocations and misfit dislocations networks. These dislocation networks can reduce the mobility of the dislocations in the matrix channels and inhibit the partial dislocation entering into precipitates. As can be seen from Fig.4, the coarsening of the J c morphology takes place in the early of fatigue process. A relatively high-density dislocation tangles can be seen in the

horizontal, perpendicular and gable channels in Fig.4b. This observation strongly suggests that dislocation tangling represents the sites for the initiation and early propagation of fatigue cracks. 2.3 Fractograph The fracture surfaces vary significantly with different strain amplitudes and loading temperature. The analysis on the fractograph of the fatigue tested at 760 oC and lower strain amplitude in Fig.5b shows the crack initiation on a relatively large pore (white arrow) lying near the specimen surface. It is highly probable that the cracks initiate due to the interaction a

202=g 220

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400 nm Fig.4 TEM images of dislocation configuration after LCF tested at 870 oC: (a) N=190 cycle, ǻİt=1.0%; (b) N=2000 cycle, ǻİt=0.6%

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o

of the persistent slip bands with the pores. Fatigue striations were observed on the fracture surfaces of the fatigued specimens tested under 0.6% strain amplitude at 760 oC. Above observation proves that the fatigue crack propagation would be the decisive fracture mechanism of the low temperature fatigued specimens. At 870 oC under low ǻİt and higher ǻİt, the appearances of fracture surfaces are altered little. As shown in Fig.5a, the most evident feature on the fracture surfaces is the significant dimple-like fracture. The single specimens contain micropores and the dimple like appearance can be attributed to the micropores. These micropores act as the origin of the fatigue ductile fracture.

3

Discussion

In the present study, it is found that the number of cycles is closely related to testing temperature and strain amplitude. Under the condition of 760 oC and different ǻİt (Fig.3), the homogeneous hyperfine secondary Ȗƍ particles play a significant role in process of fatigue deformation. The microstructure clearly shows that the channels with hyperfine secondary Ȗƍ particle contain very few dislocations. The above fact confirms that hyperfine secondary Ȗƍ particles are effective barriers to dislocation movement[8]. As a result, the cyclic stress response curves under lower strain amplitude at 760 oC hold a longer cyclic hardening period compared with that at 870 oC. In the Fig.2b, the number of cycles at 760 and 870 oC under the same strain amplitude were measured to be 3218

and 2034, respectively; the former is nearly one and a half of the later one. On one hand, the homogenous secondary Ȗƍ particles distribution facilitates a homogeneous dislocation structure in the alloy, and such a deformation structure could retard crack propagation along the plane perpendicular to the stress axis. On the other hand, for higher strain amplitude the interaction of locally higher stress concentration with high temperature cycle fatigue leads to a formation of the PSBs running through the J matrix and the Jc precipitates. This results in a stress wave and shortening of the fatigue life (Fig.2b black arrow), as the PSBs represent the sites for the initiation and early propagation of fatigue cracks. Further the specimens tested at 760 oC exhibit a higher resistance against fatigue compared to that at 870 oC. At 870 oC, the localization of the cyclic plastic deformation into the horizontal, vertical and gable channels already existing dislocation, this results in the occurrence of high density dislocation tangling. In Fig.4, the coarsened particles are associated with increased ducility and the interfacial dislocations reduce the matrix/precipitate strain energy. The rafted Ȗƍ structure results in a reduction in fatigue resistance. A consequence of the progressive coarsening and dislocation tangling formation is the enhanced fatigue crack initiation at micropores and shortening of the lifetime at higher strain amplitude.

4

Conclusions

1) The fatigue deformation mechanism is closely related to the loading temperature and strain amplitude. At 760 oC under different strain amplitudes, the interaction of the cyclic plastic deformation and high temperature induce the formation of the secondary Ȗƍ phase in matrix channel. A homogeneous secondary Ȗƍ particles distribution in the matrix is beneficial for enhancement of fatigue resistant. At 870 oC under higher strain amplitude, the curves of cyclic stress response shows initial hardening followed by a softening regime in which the stress falls off rapidly. This softening is due to the rafting structure and high density dislocation tangling. 2) The fracture crack mode exhibits a strong difference for lower and higher loading temperatures. At 760 oC, the PSBs formation enhances the fatigue crack initiation at micropores and propagation along the PSBs. At 870 oC under higher strain amplitude, the locally higher stress concentration results from dislocation tangling and progressive coarsening promotes a relatively soon initiation of fatigue microcracks.

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