Mechanism of high-temperature oxidation effects in fatigue crack propagation and fracture mode for FGH97 superalloy
来源期刊:Rare Metals2019年第7期
论文作者:Chao Xu Zhi-Hao Yao Jian-Xin Dong Yu-Kun Jiang
文章页码:642 - 652
摘 要:The high-temperature fatigue crack growth behaviors in powder metallurgy(P/M)Ni-based superalloy FGH97 for turbine disk application were investigated at different temperatures(650,700 and 800 ℃)in air using a combination a servohydraulic test system,fractographic and microanalytical investigations.It is found that there is a temperature-sensitive region in which the fatigue life of FGH97 alloy decreases sharply.To further evaluate the crack propagation mode and oxidation effects,interruption experiments were conducted at 700 and 300 ℃,respectively.The results indicate that the reduction of the fatigue lifetime for FGH97 takes place when the fracture mechanism transforms from a predominantly transgranular mode to an intergranular one as the temperature increases.Although the microstructures and mechanical properties may vary with the temperature,they are not the dominating factors contributing to the temperature sensitivity of fatigue property for FGH97.It is the oxidation that governs the fatigue crack growth behaviors in air at elevated temperature.The enhanced thermal activity of oxygen and certain active metal elements result in accelerated oxidation reaction.The brittle oxide intrusions formed at the crack tip and grain boundaries of crack frontier lead to grain boundary weakness,which is responsible for the transformation of crack growth mode and degradation of the fatigue property of FGH97 alloy.
稀有金属(英文版) 2019,38(07),642-652
Chao Xu Zhi-Hao Yao Jian-Xin Dong Yu-Kun Jiang
School of Materials Science and Engineering, University of Science and Technology Beijing
作者简介:*Zhi-Hao Yao,e-mail: xc158158@163.com;
收稿日期:30 August 2017
基金:financially supported by the National Natural Science Foundation of China (No.51371023);
Chao Xu Zhi-Hao Yao Jian-Xin Dong Yu-Kun Jiang
School of Materials Science and Engineering, University of Science and Technology Beijing
Abstract:
The high-temperature fatigue crack growth behaviors in powder metallurgy(P/M)Ni-based superalloy FGH97 for turbine disk application were investigated at different temperatures(650,700 and 800 ℃)in air using a combination a servohydraulic test system,fractographic and microanalytical investigations.It is found that there is a temperature-sensitive region in which the fatigue life of FGH97 alloy decreases sharply.To further evaluate the crack propagation mode and oxidation effects,interruption experiments were conducted at 700 and 300 ℃,respectively.The results indicate that the reduction of the fatigue lifetime for FGH97 takes place when the fracture mechanism transforms from a predominantly transgranular mode to an intergranular one as the temperature increases.Although the microstructures and mechanical properties may vary with the temperature,they are not the dominating factors contributing to the temperature sensitivity of fatigue property for FGH97.It is the oxidation that governs the fatigue crack growth behaviors in air at elevated temperature.The enhanced thermal activity of oxygen and certain active metal elements result in accelerated oxidation reaction.The brittle oxide intrusions formed at the crack tip and grain boundaries of crack frontier lead to grain boundary weakness,which is responsible for the transformation of crack growth mode and degradation of the fatigue property of FGH97 alloy.
Keyword:
FGH97; Fatigue crack growth; Temperature sensitivity; Fracture mode; Oxidation;
Received: 30 August 2017
1 Introduction
Powder metallurgy (P/M) Ni-based superalloys have been often used for many high-temperature components in landbased gas turbine,aircraft engine indlustries,and chemical process industries
The key components of aero-engine operating at a harsh environment have to endure high temperature,high-pressure and serious corrosion atmosphere
Among various factors that affect thermal fatigue resistance of Ni-based superalloys,temperature is a significant one;it influences the fatigue property in many ways
Table 1 Nominal chemical compositions of FGH97 (Ni being matrix)
Zheng
For now,there are a lot of researches at home and abroad,involving the impact of temperature on the fatigue crack propagation behavior of superalloys;however,the research with respect to temperature sensitivity and its fundamental principles are not systematic.Thus,the fatigue crack growth behaviors of FGH97 at different temperatures were analyzed firstly in this paper.The mechanical properties,microstructures and fatigue surface morphologies of FGH97 specimens tested at different temperatures were observed.The interruption experiments were carried out to investigate the crack propagation mode and oxidation at the crack tips and grain boundaries of the samples.Finally,the mechanism of the existence of temperature-sensitive region and the transition law of fatigue fractures were concluded.
2 Experimental
2.1 Materials
The material used in the investigation was P/M nickelbased superalloy FGH97 developed in China for turbine disk application,and similar to Russian EP741NP superalloy except for the optimized composition,Table 1 gives the nominal chemical composition of the alloy.FGH97alloy was manufactured through plasma rotating electrode process (PREP) milling+hot isostatic pressing (HIP)method
The FGH97 samples underwent traditional metallographic procedure using silicon-carbon paper for grinding from 60 up to 2000 grits,followed by polishing with 2.5-and 0.5-μm diamond paste.The polished surface was subsequently chemical corrosion by erodent (5 g CuCl2+100 ml HCl+100 ml C2H5OH) for 30 s,the grain structure could be observed through 9XB-PC optical microscope (OM).The surface was disposed by electrolytic polishing in solution of 20 vol%H2SO4+80 vol%CH3OH with 28-30 V input voltage for 3-5 s afterwards.The SUPRA 55 field emission scanning electron microscope (FESEM) was employed to observeγ'phase in detail.After experiencing a fatigue test,the fatigue fracture surface analysis was performed using JEOL-7600F scanning electron microscope (SEM) on cracked specimens after cleaning with ethyl alcohol by ultrasonic cleaner.
The morphology of grain structures andγ'phases of FGH97 is shown in Fig.1,with an average grain size of30-40μm,and the rectangular secondary-precipitatedγ'phases closely distribute in the matrix.
2.2 Fatigue crack growth tests
The fatigue crack growth tests were conducted using a servo hydraulic test system CMT5204GL in accordance with standard of ASTM E647-81.A resistance-type furnace,attached to the testing machine,was used to heat the specimens.Test temperature fluctuation was maintained within±3℃and the thermal gradient across both the width and length of the specimen did not exceed±3℃.According to the standards of JB/T8189-1999 and ASTM E647-81,standard compact tension (CT) specimen,as shown in Fig.2,was employed in this study.Each specimen was installed correctly in the displacement-controlling mode,maintained at an appropriate minimum load for elimination of thermal expansion during heating to the required test temperature.The tests commenced once the temperature had stabilized in about 30 min.
Fig.1 Micro structures of FGH97 alloy specimen:a OM image of grain size and b FESEM image ofγ'phase
Fig.2 Schematic illustrations of a CT specimen for crack propaga-tion test,where a0 representing initial notch length and B representing thickness (unit:mm)
Fig.3 Fatigue load waveform figure
Studies
2.3 Interruption experiments
To study the mechanism of oxidation and its effects on crack growth at different temperatures,the interruption experiments involving the polished CT specimen were carried out.The experimental processes and parameters were the same as that of the fatigue crack growth tests.The experiment would be stopped once the crack reached a desired length.After cooling to room temperature,the CT specimen was unload and washed by ultrasonic cleaning.It could be further examined by SEM equipped with energydispersive spectrometry (EDS) to reveal the path of crack propagation and oxidation at/ahead of the crack tip.
3 Results and discussion
3.1 Fatigue crack propagation behaviors
The experimentally determined points describing the behaviors of crack growth in the investigated FGH97 alloy at different temperatures are presented in the fatigue crack growth life (a-N) curves and FCGR (da/dN-ΔK) curves,as shown in Fig.4,where a,N,andΔK are crack length,fatigue life and stress intensity factor range,respectively.Note that the latter ones are double logarithmic curves.The FCGR increases as theΔK value increasing,following the Paris law.It can be seen that the test temperature has a huge effect.With the increase in temperature,fatigue life decreases,and FCGR tends to accelerate.However,in different temperature ranges,the decreasing tendency of fatigue life and the increasing tendency of FCGR have obvious differences.For instance,with temperature rising from 700 to 800℃,the FCGR is nearly two orders of magnitude higher than that in the range of 650-700℃.
Fig.4 Lifetime and FCGR curves of FGH97 alloy at different temperatures:a a-N and b da/dN-ΔK
Considering the fatigue crack growth curves of GH4720Li alloy at various temperatures were conducted by Nai et al.
According to the Paris power law,the FCGR curves consist of three different areas:the near threshold zone,the stabilized extension zone (Paris zone) and the transient zone.Fatigue cycles of the three areas for FGH97 at different temperatures are obtained by taking the derivative of a-N curves,as Fig.6 shows.It is found that the fatigue lives of both the near threshold zone and Paris zone decline dramatically in the temperature-sensitive region,which indicates that the fatigue property for FGH97 alloy is deteriorated.
Fig.5 Lifetime of FGH97 and GH4720Li at different temperatures
Fig.6 Lifetime comparison of three stages of FGH97 alloy
If an aero turbo-engine equipped with a FGH97 alloy turbine disk is in service,and operates at the safe application temperature,it will be a longer service life.However,its service life will shorten tremendously when running at the temperature locating in the sensitive range;hence,the safe operation of aircraft or equipment will be threatened seriously.Fatigue performance is a crucial indicator for turbine disk superalloys,which determines whether the alloys can be used for a long time.Therefore,it is necessary to study not only the effects of temperature on fatigue performance for FGH97 alloy,but also the essential reason of temperature sensitivity for fatigue life.
3.2 Microstructural characteristic and mechanical property
In high temperature,changes in micros true tures
Fig.7 FESEM images ofγ'precipitates for FGH97 after fatigue tests at a 650℃and b 800℃
When the temperature elevates,mechanical properties related to thermal activation will deteriorate,especially for yield strength (σy) and elastic modulus (E)
Based on the mentioned method in Ref.
Table 2 Elastic modulus (E) and yield strength (σy) of FGH97 alloy at different temperatures
Fig.8 da/dN-AKnorm curves of FGH97 alloy after modification of E andσy
The FCGR of RR1000 superalloy under fatigue loading(1 s dwell,0.25 Hz) at temperatures ranged from 550 to775℃in vacuum was carried out by Li et al.
3.3 Fracture morphology
Appearance of fatigue fracture morphology tested at different temperatures was observed by SEM to analyze the reason that fatigue crack growth behavior decreases with the increase in temperature.Figure 9 presents the fatigue fracture morphologies of FGH97 alloy at 650,700 and800℃,and scale lines of the crack length,fatigue cycles andΔK values are marked in the images according to fatigue crack propagation curves.Note that the crack growth direction is from bottom to top for all images,as indicated by the red arrow.
Figure 9a illustrates the fracture surface corresponding to the fatigue test at 650℃,resulting in a transgranular manner.Only a thin oxide layer is observed on the fracture surface.Red mark on the left scale represents the piding line between the near threshold zone and Paris zone.Even after the Paris area,the crack growth mode is still totally transgranular.This relative smooth fracture surface is associated with the crack traversing inpidual grains without significant deflection at grain boundaries.Clear radiation ridge and brittle material behavior were characterized by the river pattern within the whole picture.Besides,the secondary cracks are rarely found.
As Fig.9b shows,at 700℃,the crack growth of FGH97 is still dominated by transgranular mode,but presents a coarser surface compared with the fracture at650℃.In near threshold zone,the fracture surface is smoother,then its roughness increases gradually with crack propagating,accompanied by a handful of secondary cracks,including secondary transgranular cracks and secondary intergranular cracks.Jiang et al.
The fatigue fracture surface at 800℃presented in Fig.9c shows a noticeable difference in contrast to the surfaces at lower temperatures.The crack extends along grain boundaries even from the beginning,it is characterized by the intergranular fracture mode,with a tremendously rough surface.Decohesion of grain boundaries is readily apparent,presenting a crystal-sugar-like fracture morphology.Besides,a thick oxide layer and a large number of secondary intergranular cracks can be observed,and some of the secondary cracks are longer and more open.The FCGR is very fast at 800℃,indicating that the cohesion strength of grain boundaries has been seriously weakened.
According to the fatigue life curves and fracture morphologies of FGH97 at different temperatures,it is found that the sharp reduction of fatigue life in temperaturesensitive region is closely related to the transition of fatigue fracture mode.The relationship between temperature-fatigue life curve and fracture mode is schematically illustrated in Fig.10,and temperature-sensitive region pides the curve into three sections.FGH97 alloy shows a completely transgranular fracture mode at the temperature that is lower than sensitive region and tends to have a longer fatigue life.When the temperature is higher and on the right side of that region,the fatigue crack advances from the beginning in an intergranular manner,therefore,the FCGR is rapid and fatigue life is very short.However,in the temperature-sensitive region,the fracture mode changes obviously,from transgranular to completely intergranular fracture,accompanied by a sharp decline in fatigue life.It is precisely because of the occurrence of intergranular fracture,the FCGR of FGH97 increases.The difference of microscopic mechanisms of crack growth at various temperatures also account for the differences in slops of FCGR curves.
Fig.9 SEM images for facture morphologies of FGH97 alloy at a 650℃,b 700℃and c 800℃
Fig.10 Diagram of relationship between temperature-cycle and fracture mode
The abrupt change in fracture mode confirms that the role of oxygen is simply to reduce the grain boundary energy in such a way that the crack growth path becomes intergranular
3.4 Research on temperature sensitivity of fatigue life
To study the influences of elevated temperature on fatigue crack propagation,the interruption experiment of FGH97was carried out at 700℃.After a cyclic loading for 23 h,namely 2330 cycles,a crack with a length of about 0.8 mm was produced.Morphology of the crack tip is shown in Fig.11a.The secondary electron micrograph clearly shows that the typical cracking behavior transforms to intergranular,and a small crack has generated in the front of crack tip,demonstrating that the grain boundary strength is reduced remarkably as well.If the crack continues to propagate,the frontier small crack will be linked with the main crack.In plastic deformation zone,grain boundaries vertical to the loading direction suffer from the greatest normal force during fatigue experiments and tend to produce small cracks firstly
Fig.11 FESEM images of fatigue crack surface and associated elemental distribution for FGH97 at 700℃:a crack tip advanced along grain boundary and c elemental distribution crossing crack by linear scanning of EDS;b oxides along flanks of crack and d slip bands connected with main crack
The interruption experiment for FGH97 at 300℃(under otherwise identical conditions) was carried out to research the role of high-temperature oxidization on fatigue crack growth and compares its fracture morphology with that at high temperature.Figure 12 shows the feature of crack tip of FGH97 after a cyclic loading up to 202 h(20,000 cycles,0.7 mm in crack length),the mode of crack growth is quite different to that at 700℃.In Fig.12a,the crack extends through connecting slip bands in plastic deformation zone at crack tip,or simply“bridging”,leaving a zig-zag growth path,as shown in Fig.12b.For most polycrystalline alloys during fatigue,persistent slip bands are regarded as the primary mechanism.As slips accumulate during cyclic loading,plastic deformation manifests into eventual strain localization,leading to crack initiation
The fatigue crack growth will not be affected by oxidization,as a result,the crack grows transgranularly and the specimen has a longer fatigue lifetime.However,oxidization will play a vital role in crack growth when the temperature is high enough,due to the reaction rate and penetration depth of oxygen are temperature-dependent.
Fig.12 Microstructure morphologies of fatigue crack surface for FGH97 at 300℃:a SEM image of crack tip and slip bands,b SEM image of zig-zag crack extending through connecting slip bands,c FESEM image of cracks shearing secondaryγ'phase,and d FESEM image of no sign of oxidization around crack
Fig.13 Fatigue crack growth for three hypothetical scenarios (A,B,C):a da/dN and b apparent activation energy (Eapp)(E1,E2 and E3representing Eapp for fatigue crack growth at different temperatures,respectively
Oxidization mechanism of superalloys can be identified in terms of oxygen long-range and short-range diffusion processes
The fatigue crack growth behaviors of several of superalloys in vacuum and air have been tested
As discussed in detail in Ref.
The apparent activation energy (Eapp
where Rg is the gas constant (8.31 J·mol-1·K-1),
Among the three hypothetical situations (A,B,C)mentioned to illustrate the issues involved,Case C is most similar to the fatigue failure process in the conditions of high temperature and air environment,as shown in Fig.13b.Degradation of mechanical performances is the dominant mechanism for fatigue crack growth at 600℃.However,Eapp reaches 250 kJ·mol-1 and the dominant mechanism switches to oxidization when temperature exceeds 700℃.
From Case C,it can also be found that there exists a temperature-sensitive region,in which Eapp increases rapidly and FCGR raises accordingly (Fig.13a),corresponding to the change of dominant process from degraded mechanical performances to oxidation.Eapp characterizes the thermal diffusion of particles such as atom and vacancy inside the material and their thermally activated reaction with oxygen.Therefore,the underlying mechanism causing the drastic reduction of fatigue property for FGH97 in sensitive range is the enhanced chemical activity of oxygen and active metal elements around crack tip.Then,the oxidation rate increases and grain boundary strength weakens,which is responsible for the transformation of crack advance from transgranular to intergranular mode and the increase of FCGR.
4 Conclusion
The high-temperature fatigue crack growth behaviors in FGH97 alloy for turbine disk application at different temperatures (650,700 and 800℃) in air were investigated using a combination a servohydraulic test system,fractographic and micro analytical investigations.To evaluate the crack propagation mode and oxidation effects,interruption experiments were conducted at 700 and 300℃,respectively.
There exists a temperature-sensitive region in which the fatigue life drops considerably.In this region,the fatigue fracture converts from a predominantly transgranular mode to an intergranular one,resulting in that the FCGR increases and the lifetime declines rapidly.In air and elevated temperature,oxidation is the dominant factor that gives rise to temperature sensitivity,enhanced thermal activity of oxygen and certain active metal elements accelerate the oxidation reaction.Brittle oxides produced in the place of crack tip and grain boundaries of crack frontier weaken the grain boundary strength,leading to that crack propagates intergranularly,thus degrading the fatigue property of FGH97 dramatically.
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