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Fatigue behavior of AW7075 aluminum alloy in ultra-high cycle fatigue region
Materials Science & Engineering A 774 (2020) 138922
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Fatigue behavior of AW7075 aluminum alloy in ultra-high cycle fatigue region � a, *, I. Kub�ena a, L. Tr�sko b, V. Horník a, L. Kunz a S. Fintova a b
� �zkova 22, 616 62, Brno, Czech Republic Institute of Physics of Materials, The Czech Academy of Sciences, Zi � � � Research Centre of the University of Zilina, University of Zilina, Univerzitn� a 8215/1, 010 26, Zilina, Slovakia
A R T I C L E I N F O
A B S T R A C T
Keywords: Fatigue Ultra-high cycle fatigue Fatigue crack initiation Fatigue damage AW7075
Advanced electron microscopy methods were used with the aim to explain the differences in the response of strengthened AW 7075 – T6511 aluminum alloy to fatigue loading at 5 Hz and 20 kHz. The shift of the S–N curve to higher number of cycles to fracture for the specimens tested at high frequency was experimentally determined. This effect is not connected with a change of the fatigue crack initiation mechanism and site from the surface to material interior. The absence of slip markings and cracking of primary intermetallic particles on the surface of cycled specimens were characteristic features. A difference in the dislocation density and dislocation arrange ment in the vicinity of the fatigue crack initiation sites was shown to be the only observable effect. However, consistent with the strengthened structure of the alloy, no specific dislocation structure due to the cyclic loading was observed, regardless of the loading frequency and stress amplitude.
1. Introduction Lightweight Al alloys are irreplaceable materials in many engineer ing areas, mainly in transportation and civil engineering. Aluminum alloys are widely used because their properties can be easily tailored by heat treatment processing to desired application. Processing and heat treatment of Al alloys, however, often results in an anisotropy of me chanical properties. Direct effects are attributed to the orientation of crystals and slip systems with respect to the applied stress and grain morphology. Indirect effects are attributed to work hardening and pre cipitation during processing, including orientation of precipitates with respect to slip systems, the distribution of dislocation densities in differently orientated slip systems and the corresponding distribution of precipitates [1]. Fatigue strength of materials in the high cycle region is pre determined by the fatigue crack initiation. This phenomenon is even more pronounced in the case of the ultra-high cycle fatigue. There are two different mechanisms of fatigue crack initiation. Surface fatigue crack initiation is dominant in the low and high cycle fatigue region of homogeneous materials where no internal defects are present. Internal fatigue crack initiation is characteristic for the very high number of cycles to failure, i.e. in the high and ultra-high cycle fatigue regions or in materials containing internal defects or regions with different
deformation properties than exhibits the majority of the microstructure [2–4,14,15]. The fatigue crack initiation mechanisms and corresponding models for low and high cycle fatigue region are available in literature (e.g. surface stress assisted models [5,6]; vacancy models [7–10]; and micromechanical models [11]). However, the knowledge as well as experimental data and observation of the crack initiation in the ultra-high cycle fatigue region is still rare. The fatigue testing in this region has to be performed at high loading frequencies and thus ultra sonic fatigue devices have to be used to perform an experiment in a reasonable time. However, the influence of the very high loading fre quency on the crack initiation process has not been sufficiently explored. The influence of the loading frequency was shown to be negligible for some materials (Nb, Ti–6Al–7Nb), whereas for others a strong impact was observed (Ta, Ti) [12]. Obviously, an influence of the testing fre quency does not simply depend on the type of the crystallographic lat tice only; it is a more complex problem. The influence of the loading frequency on the dislocation dynamics was already discussed in the literature [13]. It is also obvious that increasing frequency is connected to more difficult dislocation motion. The microstructure of strengthened 7XXX series Al alloys consists of solid solution grains with intermetallic phases (constituent) particles created during solidification. The coarse intermetallic particles (from one to several tenths of micrometers) are composed of the elements with
* Corresponding author. E-mail address: [email protected] (S. Fintov� a). https://doi.org/10.1016/j.msea.2020.138922 Received 2 August 2019; Received in revised form 2 January 2020; Accepted 3 January 2020 Available online 7 January 2020 0921-5093/© 2020 Elsevier B.V. All rights reserved.
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low solubility in Al. Fe and Si-rich intermetallic particles are usually unaffected by heat treatment, however, Cu and Mg-rich particles can be dissolved during heat treatment. Other components of the strengthened structure are dispersoids, fine intermetallic particles created in the solid state during homogenization processes. Dispersoids are formed by ele ments with low diffusion in Al, ensuring their thermodynamic stability during heat treatment. The fine intermetallic particles prevent recrys tallization processes due to the retardation of the grain boundaries migration. The presence of the combination of coherent Guinier-Preston (GP) zones, metastable semicoherent η’ and equilibrium incoherent η (MgZn2) phases, depending on the exact conditions of the heat treatment is a consequence of the heat treatment. η (MgZn2) was stated as the main hardening phase. Depending on the heat treatment conditions in the case of overaged structure, also the precipitate free zones on the grain boundaries can occur [1,16–21]. The above mentioned inhomogeneities of microstructure were shown to have different influences on the properties of the strengthened AW 7075 alloy under cyclic loading. Hard Fe-containing constituent particles were shown to have detrimental effect on the fatigue proper ties. Initiation of fatigue cracks due to the hard particle cracking and following crack growth to the matrix was presented in literature [22–24]. Contraty to this the softer Mg2Si were not reported as the fa tigue crack initiation sites. In literature [23], also the fatigue crack initiation on voids, defined as cracks in particles or on the parti cle/matrix interface was observed. It is well known, that in the homogeneous materials the fatigue crack initiation is a result of the localization of cyclic plastic deformation due to the formation of a specific dislocation arrangement (cells, ladder-like structures, …), [7,25]. This process is connected with dislocation motion and dislocation distribution which is dependent on microstructure. Precipitates influence the fatigue properties of the strengthened material due to the interaction with dislocations. Based on the precipitates character, two types of dislocation slip can occur [20,26–28]. Inho mogenous slip distribution is allowed due to the ability of dislocations to cut or by-pass the coherent particles. As a result the slip bands formation and subsequent stress concentration and fatigue crack initiation can be observed. The second type of dislocation slip is related to the incoherent particles acting as overcome obstacles for dislocations motion. The distribution of dislocations due to the homogeneously distributed small incoherent precipitates is also homogenous and no specific dislocations arrangement can be created. In addition to the microstructural characteristics, fatigue strength of AW 7075 alloy is strongly influenced also by the reactivity of the ma terial with corrosive environments. Anodic oxidation which increases resistance against corrosion was shown to be detrimental for the fatigue lifetime of the alloy in the low cycle fatigue region. On the other hand, the created oxide layer was shown to be improving fatigue life of the alloy in the high cycle fatigue region [30–32]. The presented study offers experimental results of fatigue life measuremet at two substantially differerent frequencies (5 Hz and 20 kHz) and discussion of the fatigue damage mechanism of the strength ened AW 7075 - T6511 aluminum alloy on the basis of detailed SEM and TEM observations of microstructure in the close vicinity of the fatigue crack initiation sites.
conventional methodology consisting of specimens cutting and molding into Cu based resin, grinding on SiC papers, polishing with 1 μm dia mond pastes using ethanol as lubricant and finished by chemicalmechanical polishing by OP-S suspension. Fuss etchant (7.5 ml HF, 8 ml HNO3, 25 ml HCl and 1000 ml H2O) was used to reveal material microstructure. Backscattered electron imaging (BSE) was adopted to reach chemical contrast and more precise imaging of individual micro structural features. Chemical analysis of large intermetallic particles present at boundaries of elongated grains was performed by energydispersive X-ray spectroscopy (EDS). The atomic percentage of indi vidual chemical elements was compared with literature. Electron backscatter diffraction (EBSD) was used for analysis of grain orientation and individual grains were determined by 15� grain misorientation. Focused ion beam (FIB) technique was applied for fatigue crack initia tion mechanism analysis including observations of local microstructural changes and formation of surface relief and early cracks. Lamellas pre pared by FIB were extracted from the cross-section of the fatigue tested specimens in the vicinity of the fatigue crack initiation sites in the di rection perpendicular to the loading axis. Electron channeling contrast imaging (ECCI) technique was adopted to reveal the sub-grains structure and fine precipitates distribution within the primary material grains. A standard procedure consisting of grinding and etching was used for the preparation of specimens for the dislocation structure analysis of the material by transmission electron microscopy (TEM) using JEOL JEM2100F microscope. The microstructure of AW 7075 alloy after heat treatment consisted of grains of the solid solution of Zn in Al with a large number of inter metallic phase particles, Fig. 1a. Elongation of grains in the direction of extrusion was a characteristic feature, Fig. 1a. The Al2CuFe, Al2CuMgZn, and Al7Cu2Fe intermetallic phase (constituent) particles were mostly observed at grain boundaries of the elongated grains. Due to the heat treatment, small sub-grains in the large elongated primary grains, Fig. 1b, and a large number of fine particles precipitated in the grains during the aging, Fig. 1c, were created. A pronounced grain elongation was found in the extrusion direction, whereas the individual grains are oriented randomly, Fig. 1d. The deeper analysis confirmed that the subgrains in large elongated primary grains were created during the extrusion. Observation of conventionally prepared foil by TEM revealed low dislocation density and pinning of dislocations by small precipitates, Fig. 1e–f. No specific dislocation arrangement was found by TEM. Typically, the T651 heat treatment results in creation of GP zones and precipitation of η’ phase in the AW 7075 alloy [17–19]. The ultimate tensile strength, σUTS, of the investigated AW 7075 – T6511 aluminum alloy is 631 MPa, elongation is 4.9 % and the hardness is 175 HV10 [33]. Dynamic elastic modulus is 71 GPa and the density is 2800 kg∙m 3. High-frequency fatigue tests (20 kHz) were performed using piezo electric ultrasonic system made by Lasur®. Fatigue tests were performed at symmetrical cyclic loading in tension-compression with stress ratio R ¼ -1. Dry and clean pressurized air was used for specimen cooling in order to prevent changes in the microstructure due to specimen heating. The specific geometry of the specimen following the requirements of the ultrasonic system with a diameter of 4 mm is shown in Fig. 2. The precise length of the specimen was adjusted to reach the resonance frequency of the system during the ultra-high cycle fatigue tests. The specimens and loading axis was in the direction of the material extrusion. Low-frequency tests were performed on Zwick/Roell Amsler HC25 servohydraulic testing machine at symmetrical tension-compression cyclic loading at cycle asymmetry ratio R ¼ -1 with the nominal fre quency of 5 Hz. The tests were performed in laboratory air and at temperature 23 � 1 � C. The same specimen geometry was used for both loading frequencies. The middle part of the machined specimens was ground and polished (grinding by SiC papers and polishing by 1 μm diamond paste with ethanol used as a lubricant) and finished by chemical-mechanical pol ishing by OP-S suspension with the aim to observe and describe the
2. Experimental material and procedures AW 7075 – T6511 aluminum alloy provided in the form of extruded bars with 15 mm in diameter and 1.5 m in length was used as the experimental material. Solution heat treatment and stress-relieving by stretching and artificial aging with minor straightening after aging were applied on experimental material (heat treatment marked as T6511). Tescan LYRA 3 XMU FEG/SEM x FIB scanning electron microscope (SEM) was used for the metallographic analysis of the material micro structure, fractographic analysis and investigation of the fatigue crack initiation. Specimens for metallographic analysis were prepared by 2
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Fig. 1. Typical microstructure of AW 7075 – T6511 extruded aluminum alloy.
Fig. 2. Geometry of the specimen for fatigue tests.
fatigue crack initiation. 3. Results
Fig. 3. S–N curves for AW7075 – T6511 aluminum alloy tested under low and high frequency.
3.1. S–N curves
low loading frequency and the highest stress amplitude (5 Hz, 350 MPa), the multiple fatigue crack initiation was observed, Fig. 4a. In all the other cases, only one fatigue crack initiation site was observed. The very rough fracture surface was characteristic for all the broken specimens, Figs. 4a–c and g-i. This can be explained by the elongated microstructure (texture) of the extruded material. After the primary crack growth in the plane almost perpendicular to the loading axis a high slope of the fracture surface was a typical feature. Two different locations of the fatigue crack initiation were found. The fatigue crack initiation was observed either on the specimen surface (Figs. 4e, f, k, and l) or in its nearest vicinity (Figs. 4d and j). The maximum depth of crack initiation site with respect to the surface was about 10 μm. This was observed, however, only in the case of the low cycle fatigue, Nf < 105 cycles (for both the loading frequencies). When the fatigue life was above 105 cycles, only surface fatigue crack initiation
Fig. 3 shows that the lifetime at 20 kHz is generally higher than that at 5 Hz. The difference increases with decreasing stress amplitude. Different slope of the S–N curves fitted to the experimental points is obvious. Fatigue endurance limit defined on the basis of 1 � 107 cycles for specimens tested at 5 Hz makes 220 MPa, while the fatigue endur ance limit determined for 20 kHz for the same number of cycles is 275 MPa. The specimen tested at the stress amplitude of 220 MPa at 20 kHz reached more than 1 � 109 cycles. Fatigue endurance limit based on 1 � 1010 cycles for specimens tested at 20 kHz is 200 MPa. 3.2. Fractographic observation The same appearance of the fatigue fracture surface was character istic for all the broken specimens, regardless of the loading frequency and the reached number of cycles to the fracture. Only in the case of the 3
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Fig. 4. Fatigue fracture surfaces and fatigue crack initiation places of AW 7075 – T6511 extruded aluminum alloy tested at 5 Hz and 20 kHz.
took place (Figs. 4e, f, k and l). The detailed inspection of the loaded specimens did not reveal any signs of the localization of the cyclic plastic deformation in a form of slip markings on the polished surface. This was observed in all the cases regardless of the loading frequency and the applied stress amplitude.
corresponding cuts, it follows that the fatigue crack initiation site for the specimen tested at 350 MPa at 5Hz was located approximately 5 μm below the surface, Fig. 5a. At lower stress amplitudes applied at 5 Hz, Fig. 5b and c, crack initiation site was at the specimen surface. A similar observation was performed on the specimens tested at 20 kHz. Subsurface fatigue crack initiation, approximately 5 μm below the surface, was observed when the stress amplitude of 350 MPa was applied, Fig. 6a. Surface fatigue crack initiation was observed at lower stress amplitudes at 20 kHz, Figs. 6b and c. EBSD analysis performed at the sections in the vicinity of the fatigue fracture initiation sites revealed the random grain orientation for indi vidual specimens, Figs. 5d–f and Figs. 6d–f. Large grains with no internal sub-grains seem to be preferable sites for the fatigue crack initiation. BSE and ECCI analysis did not show any influence of the cyclic
3.3. Fracture profile and adjacent microstructure With the aim to investigate the mechanism of the fatigue crack initiation and changes of microstructure in the nearest vicinity of the fatigue cracks, advanced SEM techniques EBSD and ECCI were adopted. Cuts of the broken specimens parallel to the loading axis through the fatigue crack initiation sites are shown in Figs. 5 and 6. From a combi nation of fracture surface observation by SEM and observation of 4
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Fig. 5. Characterization of the microstructure evolution in the crack initiation place vicinity for specimens tested at 5 Hz.
loading on the evolution of the microstructure in terms of grain or subgrain coarsening, regardless of the loading frequency, Figs. 5g–i and Figs. 6d–i.
mechanism of the fatigue crack initiation at substantially different fre quencies. No signs of the localization of the plastic deformation in terms of the slip markings, development of surface relief or primary interme tallic particle cracking were observed (Figs. 4b, c, h, and i) even in the case when the fatigue cracks initiated just at the specimen surface. This holds for both the low and high frequency and for all the stress ampli tudes used. The absence of the slip markings can be attributed to a large amount of the small precipitates present in the small sub-grains that were created during production and heat treatment of the material. As re ported in Refs. [20,26,27], the interaction of dislocations with the strengthening precipitates depends on their character in respect to the matrix (coherent, semicoherent or incoherent), their size and distribu tion in the matrix. In the case of the examined AW 7075 – T6511, the metastable semicoherent η’precipitate particles were created in the microstructure due to the heat treatment applied, [17–19]. These pre cipitates act as obstacles for the dislocation motion. As a result, homo geneous stress distribution in the material occur and no specific dislocation arrangement as a result of the cyclic loading can be observed by TEM. The high density of obstacles for dislocation movement pre vents formation of long slip bands, which create intrusions and extru sions on the surface. The interactions between dislocations and precipitates were observed by TEM. Locally, the dislocations pinning on precipitates was observed, and no precipitates were cut by dislocations, regardless of the loading frequency and the stress applied. This excludes applying the fatigue crack initiation models based on the development of surface relief in the form of intrusions and extrusions. Due to the homogeneous distribution of the stress in the material and absence of localization of the cyclic plastic deformation, grain boundaries, or large intermetallic particles remains as the only possible places for the fatigue crack initiation. This mechanism can be quite complicated, because it depends also on the particles distribution and orientation, grain
3.4. Dislocation structure TEM foils extracted from the broken specimens near the fatigue crack initiation sites were examined with the aim to describe and compare dislocation structures after cycling at 5 Hz and 20 kHz. Dislocation structures shown in Fig. 7 do not exhibit any signs of localization of the cyclic plasticity in terms of 3D dislocation arrangement (cells, ladderlike structure etc.), regardless of the testing frequency. Locally, dislo cations pinning on precipitates was observed. The effect of frequency is only reflected in the dislocation density and dislocation re-arrangement. Specimens tested at 20 kHz, Figs. 7d–f, seems to exhibit higher dislo cation density when compared to the specimens tested at 5 Hz, Figs. 7a–c. 4. Discussion The surface quality and the surface state generally strongly influence the fatigue lifetime, especially in the high and ultra-high cycle fatigue region. In this work, the fatigue endurance limit of 250 MPa for 108 cycles for 20 kHz loading frequency was determined, Fig. 3. The speci mens used in this study was carefully polished. The value 250 MPa is substantially above 170 MPa reported for the identical material in Ref. [33], where the gauge length of the specimens was only conven tionally machined. The comparison of both results proves the high sensitivity of AW 7075-T6511 on the surface quality in the high and ultra-high cycle fatigue region. Carefully polished specimens with the identical geometry were used for the fatigue testing at both the frequencies with the aim to explore the 5
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Fig. 6. Characterization of the microstructure evolution in the crack initiation place vicinity for specimens tested at 20 kHz.
Fig. 7. Dislocation structure of cycled specimens analyzed in the crack initiation place vicinity for specimens tested at 5 Hz and 20 kHz, TEM.
boundary orientation to the loading axis and neighbouring grains slip systems orientation. In the vast majority of cases investigated in this study, a surface fa tigue crack initiation was observed. In a small number of events, fatigue crack initiation was observed just below the surface. In these cases, it
can be assumed that the primary intermetallic phase particles could act as the stress concentrators an thus increase the probability of the fatigue crack initiation in these areas, Figs. 4d and 4j. Payne et al. [22] observed that the fatigue cracks initiated due to the cracking of the iron-based particles present in the matrix. Following cyclic loading led to the 6
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crack propagation to the matrix. Contrary to the observation of Payne et al., [22], authors in the present study did not observe any particular intermetallic phase particles debonding or particle cracking. On the other hand, also Bozek et al. in [24] proposed a model calculating the probability of the fatigue crack initiation on the Fe-rich intermetallic particles. The surface initiation of fatigue cracks is generally influenced also by the environment. In our case, the environment at low frequency loading was the laboratory air and the laboratory temperature. The temperature of the specimens tested at 20 kHz was controlled by a cooled and dried air and did not exceed on the specimen surface 30 � C. Though the environmental conditions were at both frequencies different, no influ ence of the cooling medium on the fatigue crack initiation in sense of cavitation or oxidation was observed. The macroscopic appearance of the fatigue fracture surface was the same for both the testing frequencies. After initiation and primary crack growth in a plane perpendicular to the loading axis, the macroscopic crack plane inclines to the angle of 45–60� to the specimen axis. This finding corresponds to observation published in literature [33,34]. The fracture surface relief correlates to the material microstructure elonga tion in the extrusion direction. Since no difference in the fatigue cracks initiation mechanism (i.e. surface/internal) at low and high loading frequency was observed by SEM on the fracture surfaces and specimen surfaces, TEM analysis of the microstructure as close as possible to the crack initiation site was per formed. Specimens for TEM were prepared by both, conventional and FIB procedures. Foils prepared by the conventional procedure did not show any specific dislocation arrangement indicating localization of cyclic plastic deformation, which could result in the crack initiation. TEM specimens prepared by the conventional way (etching) do not, obviously, contain areas representing the very close vicinity of the initiated crack. Contrary to this, FIB technique allows preparing sitespecific TEM foils. Therefore this technique was adopted to prepare TEM foils from the grains where fatigue cracks initiated. Observation of FIB foils similarly did not show any clear signs of localization of cyclic plasticity. Only a small difference in the dislocation arrangement and density between the specimens tested at 5 Hz and 20 kHz was observed. Finer dislocation structure and higher dislocation density were observed in the case of the specimens tested at 20 kHz when compared to the specimens tested at 5 Hz. The difference could be connected with lower probability of dislocation movement due to the higher deformation rate at 20 kHz comparing to the 5 Hz loading. Consequently, higher local stress is required for the dislocation movement in the case of higher loading rate [13]. Lower dislocation activity at higher testing frequency results in reduced creation of dislocation sources, in diminished multi plication of dislocations and consequently in reduced tendency to rearrangement of dislocation structure. This explains the observed shift of the S–N curve for high frequency loading.
amplitude range used); particularly the grain size, particle distribution or the grain orientation. The only clear observable effect was slightly different dislocation density and dislocation arrangement in the nearest vicinity of the initiated fatigue cracks, however, no specific dislocation structure related to the crack initiation was found. Detailed TEM ob servations of the dislocation structure close to the fatigue crack initia tion sites, indicate pinning of dislocations on precipitates. Author contributions �: Conceptualization, Methodology, Validation, Investiga S. Fintova tion, Resources, Writing - Original Draft, Writing - Review & Editing, Visualization, Supervision, Project administration I. Kub� ena: Conceptualization, Methodology, Investigation, Writing Review & Editing L. Tr�sko: Conceptualization, Resources V. Horník: Investigation L. Kunz: Conceptualization, Writing - Review & Editing Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This research has been supported by the Ministry of Education, Youth and Sports of the Czech Republic under the project m-IPMinfra (CZ.02.1.01/0.0/0.0/16_013/0001823) and the equipment and the base of research infrastructure IPMinfra were used during the research ac tivities. The research has been supported also by the Science Grant Agency of the Slovak Republic under the project No. 1/0029/18. References [1] H. Hu, X. Wang, Effect of heat treatment on the in-plane anisotropy of as-rolled 7050 aluminum alloy, Metals 6 (2016) 79, https://doi.org/10.3390/met6040079. [2] H. Mughrabi, Specific features and mechanisms of fatigue in the ultrahigh-cycle regime, Int. J. Fatigue 28 (11) (2006) 1501–1508, https://doi.org/10.1016/j. ijfatigue.2005.05.018. [3] T. Nicholas, High Cycle Fatigue: A Mechanics of Materials Perspective, Elsevier Science, 2006. [4] C. Wang, A. Nikitin, A. Shanyavskiy, C. Bathias, An understanding of crack growth in VHCF from an internal inclusion in high strength steel, in: Crack Paths, Gruppo Italiano Frattura, Gaeta, Italy, 2012, pp. 367–374. [5] J.G. Antonopoulos, L.M. Brown, A.T. Winter, Vacancy dipoles in fatigued copper, Philos. Mag. 34 (4) (1976) 549–563, https://doi.org/10.1080/ 14786437608223793. [6] T. Zhai, J.W. Martin, G.A.D. Briggs, Fatigue damage in aluminium single crystals. I. On the surface containing the slip Burgers vector, Acta Metall. Mater. 43 (10) (1995) 3813–3825, https://doi.org/10.1016/0956-7151(95)90165-5. [7] J. Pol� ak, Cyclic Plasticity and Low Cycle Fatigue Life of Metals, Elsevier, 1991. [8] J. Pol� ak, J. Man, Mechanisms of extrusion and intrusion formation in fatigued crystalline materials, Mater. Sci. Eng. A 596 (2014) 15–24, https://doi.org/ 10.1016/j.msea.2013.12.005. [9] J. Pol� ak, J. Man, Fatigue crack initiation – the role of point defects, Int. J. Fatigue 65 (2014) 18–27, https://doi.org/10.1016/j.ijfatigue.2013.10.016. [10] U. Essmann, U. G€ osele, H. Mughrabi, A model of extrusions and intrusions in fatigued metals I. Point-defect production and the growth of extrusions, Philos. Mag. A 44 (1981) 405–426, https://doi.org/10.1080/01418618108239541. [11] T.H. Lin, S.R. Lin, Micromechanics theory of fatigue crack initiation applied to time-dependent fatigue, in: F.J. T (Ed.), Fatigue Mechanisms, ASTM International, 1979, pp. 707–728, https://doi.org/10.1520/STP35911S. [12] M. Papakyriacou, H. Mayer, C. Pypen, H. Plenk Jr., S. Stanzl-Tschegg, Influence of loading frequency on high cycle fatigue properties of b.c.c. and h.c.p. metals, Mater. Sci. Eng. A 308 (1–2) (2001) 143–152, https://doi.org/10.1016/S09215093(00)01978-X. [13] P.M. Anderson, J.P. Hirth, J. Lothe, Theory of Dislocations, Cambridge University Press, 2017. [14] C. Bathias, L. Drouillac, P. Le François, How and why the fatigue S–N curve does not approach a horizontal asymptote, Int. J. Fatigue 23 (Supplement 1) (2001) 143–151, https://doi.org/10.1016/S0142-1123(01)00123-2.
5. Conclusions The fatigue life of AW 7075-T6511 aluminum alloy under symmet rical tension-compression cycling at 5 and 20 kHz frequency was experimentally determined. It was found that the fatigue life at the 20 kHz frequency continuously decreases in the interval from 107to 1010 cycles to failure. The fatigue endurance limit of 220 MPa determined for 107 cycles at the loading frequency of 5 Hz increased to 275 MPa for the loading frequency of 20 kHz. The fatigue endurance limit at a high frequency based on 1 � 1010 cycles was 200 MPa. No influence of the loading frequency and stress amplitude on the fatigue crack initiation site (surface/subsurface) was observed. Subsur face fatigue crack initiation was characteristic for high stress amplitudes (350 MPa), while surface crack initiation was characteristic for stress amplitudes resulting in fatigue life higher than 105 cycles. The microstructure of the investigated alloy remained practically unchanged due to cycling at low and high frequency (in the stress 7
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[15] I. Marines, X. Bin, C. Bathias, An understanding of very high cycle fatigue of metals, Int. J. Fatigue 25 (9–11) (2003) 1101–1107, https://doi.org/10.1016/ S0142-1123(03)00147-6. [16] J.K. Park, A. Ardell, Correlation between microstructure and calorimetric behavior of aluminum alloy 7075 and AlZnMg alloys in various tempers, Mater. Sci. Eng. A 114 (1989) 197–203, https://doi.org/10.1016/0921-5093(89)90859-9. [17] P.N. Adler, R. DeIasi, Calorimetric studies of 7000 series aluminum alloys: II. Comparison of 7075, 7050 and RX720 alloys, Metall. Trans.A 8 (1977) 1185–1190, https://doi.org/10.1007/BF02667404. [18] J.M. Papazian, Calorimetric studies of precipitation and dissolution kinetics in aluminum alloys 2219 and 7075, Metall. Trans.A 13 (1982) 761–769, https://doi. org/10.1007/BF02642389. [19] B.A. Behrens, F. Nürnberger, C. Bonk, S. Hübner, S. Behrens, H. Vogt, Influences on the formability and mechanical properties of 7000-aluminum alloys in hot and warm forming, J. Phys. Conf. Ser. 896 (2017), 012004, https://doi.org/10.1088/ 1742-6596/896/1/012004. [20] G. Lütjering, J. Albrecht, C. Sauer, T. Krull, The influence of soft, precipitate-free zones at grain boundaries in Ti and Al alloys on their fatigue and fracture behavior, Mater. Sci. Eng. A 468–470 (2007) 201–209, https://doi.org/10.1016/j. msea.2006.07.168. [21] T.S. Srivatsan, E.J. Lavernia, The presence and consequences of precipitatefree zones in an aluminium-copper-lithium alloy, J. Mater. Sci. 26 (1991) 940–950, https://doi.org/10.1007/BF00576770. [22] J. Payne, G. Welsh, R.J. Christ, J. Nardiello, J.M. Papazian, Observations of fatigue crack initiation in 7075-T651, Int. J. Fatigue 32 (2) (2010) 247–255, https://doi. org/10.1016/j.ijfatigue.2009.06.003. [23] C.Q. Bowles, J. Schijve, The role of inclusions in fatigue crack initiation in an aluminum alloy, Int. J. Fract. 9 (2) (1973) 171–179, https://doi.org/10.1007/ BF00041859. [24] J.E. Bozek, J.D. Hochhalter, M.G. Veilleux, M. Liu, G. Heber, S.D. Sintay, A. D. Rollett, D.J. Littlewood, A.M. Maniatty, H. Weiland, R.J. Christ, J. Payne, G. Welsh, D.G. Harlow, P.A. Wawrzynek, A.R. Ingraffea, A geometric approach to
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modeling microstructurally small fatigue crack formation: I. Probabilistic simulation of constituent particle cracking in AA 7075-T651, Model. Simul. Mater. Sci. Eng. 16 (2008), 065007, https://doi.org/10.1088/0965-0393/16/6/065007. H. Mughrabi, Damage mechanisms and fatigue lives: from the low to the very high cycle regime, Procedia Engineering 55 (2013) 636–644, https://doi.org/10.1016/ j.proeng.2013.03.307. G. Lütjering, H. D€ oker, D. Munz, Microstructure and fatigue behavior of Al-alloys, in: International Conference of the Strength of Metals and Alloys, in: Proceedings of the Third International Conference on the Strength of Metals and Alloys, vol. 1, 1973, pp. 427–431. Cambridge, England. G. Lütjering, S. Weissmann, Mechanical properties of age-hardened titaniumaluminum alloys, Acta Metall. 18 (7) (1970) 785–795, https://doi.org/10.1016/ 0001-6160(70)90043-X. U. Messerschmidt, Dislocation Dynamics during Plastic Deformation, Springer Berlin Heidelberg, 2010. B. Hadzima, F. Nový, L. Tr�sko, F. Pastorek, M. Jambor, S. Fintov� a, Shot peening as a pre-treatment to anodic oxidation coating process of AW 6082 aluminum for fatigue life improvement, Int. J. Adv. Manuf. Technol. 93 (2017) 3315–3323, https://doi.org/10.1007/s00170-017-0776-1. E. Cirik, K. Genel, Effect of anodic oxidation on fatigue performance of 7075-T6 alloy, Surf. Coat. Technol. 202 (21) (2008) 5190–5201, https://doi.org/10.1016/j. surfcoat.2008.06.049. K. Genel, Environmental effect on the fatigue performance of bare and oxide coated 7075-T6 alloy, Eng. Fail. Anal. 32 (2013) 248–260, https://doi.org/10.1016/j. engfailanal.2013.03.020. L. Tr�sko, M. Guagliano, O. Bokůvka, F. Nový, Fatigue life of AW 7075 aluminium alloy after severe shot peening treatment with different intensities, Procedia Engineering 74 (2014) 246–252, https://doi.org/10.1016/j.proeng.2014.06.257. L. Tr�sko, S. Fintov� a, F. Nový, O. Bokůvka, M. Jambor, F. Pastorek, Z. Florkov� a, M. Oravcov� a, Study of relation between shot peening parameters and fatigue fracture surface character of an AW 7075 aluminium alloy, Metals 8 (2018) 111, https://doi.org/10.3390/met8020111.
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Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Fatigue behavior of AW7075 aluminum alloy in ultra-high cycle fatigue region � a, *, I. Kub�ena a, L. Tr�sko b, V. Horník a, L. Kunz a S. Fintova a b
� �zkova 22, 616 62, Brno, Czech Republic Institute of Physics of Materials, The Czech Academy of Sciences, Zi � � � Research Centre of the University of Zilina, University of Zilina, Univerzitn� a 8215/1, 010 26, Zilina, Slovakia
A R T I C L E I N F O
A B S T R A C T
Keywords: Fatigue Ultra-high cycle fatigue Fatigue crack initiation Fatigue damage AW7075
Advanced electron microscopy methods were used with the aim to explain the differences in the response of strengthened AW 7075 – T6511 aluminum alloy to fatigue loading at 5 Hz and 20 kHz. The shift of the S–N curve to higher number of cycles to fracture for the specimens tested at high frequency was experimentally determined. This effect is not connected with a change of the fatigue crack initiation mechanism and site from the surface to material interior. The absence of slip markings and cracking of primary intermetallic particles on the surface of cycled specimens were characteristic features. A difference in the dislocation density and dislocation arrange ment in the vicinity of the fatigue crack initiation sites was shown to be the only observable effect. However, consistent with the strengthened structure of the alloy, no specific dislocation structure due to the cyclic loading was observed, regardless of the loading frequency and stress amplitude.
1. Introduction Lightweight Al alloys are irreplaceable materials in many engineer ing areas, mainly in transportation and civil engineering. Aluminum alloys are widely used because their properties can be easily tailored by heat treatment processing to desired application. Processing and heat treatment of Al alloys, however, often results in an anisotropy of me chanical properties. Direct effects are attributed to the orientation of crystals and slip systems with respect to the applied stress and grain morphology. Indirect effects are attributed to work hardening and pre cipitation during processing, including orientation of precipitates with respect to slip systems, the distribution of dislocation densities in differently orientated slip systems and the corresponding distribution of precipitates [1]. Fatigue strength of materials in the high cycle region is pre determined by the fatigue crack initiation. This phenomenon is even more pronounced in the case of the ultra-high cycle fatigue. There are two different mechanisms of fatigue crack initiation. Surface fatigue crack initiation is dominant in the low and high cycle fatigue region of homogeneous materials where no internal defects are present. Internal fatigue crack initiation is characteristic for the very high number of cycles to failure, i.e. in the high and ultra-high cycle fatigue regions or in materials containing internal defects or regions with different
deformation properties than exhibits the majority of the microstructure [2–4,14,15]. The fatigue crack initiation mechanisms and corresponding models for low and high cycle fatigue region are available in literature (e.g. surface stress assisted models [5,6]; vacancy models [7–10]; and micromechanical models [11]). However, the knowledge as well as experimental data and observation of the crack initiation in the ultra-high cycle fatigue region is still rare. The fatigue testing in this region has to be performed at high loading frequencies and thus ultra sonic fatigue devices have to be used to perform an experiment in a reasonable time. However, the influence of the very high loading fre quency on the crack initiation process has not been sufficiently explored. The influence of the loading frequency was shown to be negligible for some materials (Nb, Ti–6Al–7Nb), whereas for others a strong impact was observed (Ta, Ti) [12]. Obviously, an influence of the testing fre quency does not simply depend on the type of the crystallographic lat tice only; it is a more complex problem. The influence of the loading frequency on the dislocation dynamics was already discussed in the literature [13]. It is also obvious that increasing frequency is connected to more difficult dislocation motion. The microstructure of strengthened 7XXX series Al alloys consists of solid solution grains with intermetallic phases (constituent) particles created during solidification. The coarse intermetallic particles (from one to several tenths of micrometers) are composed of the elements with
* Corresponding author. E-mail address: [email protected] (S. Fintov� a). https://doi.org/10.1016/j.msea.2020.138922 Received 2 August 2019; Received in revised form 2 January 2020; Accepted 3 January 2020 Available online 7 January 2020 0921-5093/© 2020 Elsevier B.V. All rights reserved.
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low solubility in Al. Fe and Si-rich intermetallic particles are usually unaffected by heat treatment, however, Cu and Mg-rich particles can be dissolved during heat treatment. Other components of the strengthened structure are dispersoids, fine intermetallic particles created in the solid state during homogenization processes. Dispersoids are formed by ele ments with low diffusion in Al, ensuring their thermodynamic stability during heat treatment. The fine intermetallic particles prevent recrys tallization processes due to the retardation of the grain boundaries migration. The presence of the combination of coherent Guinier-Preston (GP) zones, metastable semicoherent η’ and equilibrium incoherent η (MgZn2) phases, depending on the exact conditions of the heat treatment is a consequence of the heat treatment. η (MgZn2) was stated as the main hardening phase. Depending on the heat treatment conditions in the case of overaged structure, also the precipitate free zones on the grain boundaries can occur [1,16–21]. The above mentioned inhomogeneities of microstructure were shown to have different influences on the properties of the strengthened AW 7075 alloy under cyclic loading. Hard Fe-containing constituent particles were shown to have detrimental effect on the fatigue proper ties. Initiation of fatigue cracks due to the hard particle cracking and following crack growth to the matrix was presented in literature [22–24]. Contraty to this the softer Mg2Si were not reported as the fa tigue crack initiation sites. In literature [23], also the fatigue crack initiation on voids, defined as cracks in particles or on the parti cle/matrix interface was observed. It is well known, that in the homogeneous materials the fatigue crack initiation is a result of the localization of cyclic plastic deformation due to the formation of a specific dislocation arrangement (cells, ladder-like structures, …), [7,25]. This process is connected with dislocation motion and dislocation distribution which is dependent on microstructure. Precipitates influence the fatigue properties of the strengthened material due to the interaction with dislocations. Based on the precipitates character, two types of dislocation slip can occur [20,26–28]. Inho mogenous slip distribution is allowed due to the ability of dislocations to cut or by-pass the coherent particles. As a result the slip bands formation and subsequent stress concentration and fatigue crack initiation can be observed. The second type of dislocation slip is related to the incoherent particles acting as overcome obstacles for dislocations motion. The distribution of dislocations due to the homogeneously distributed small incoherent precipitates is also homogenous and no specific dislocations arrangement can be created. In addition to the microstructural characteristics, fatigue strength of AW 7075 alloy is strongly influenced also by the reactivity of the ma terial with corrosive environments. Anodic oxidation which increases resistance against corrosion was shown to be detrimental for the fatigue lifetime of the alloy in the low cycle fatigue region. On the other hand, the created oxide layer was shown to be improving fatigue life of the alloy in the high cycle fatigue region [30–32]. The presented study offers experimental results of fatigue life measuremet at two substantially differerent frequencies (5 Hz and 20 kHz) and discussion of the fatigue damage mechanism of the strength ened AW 7075 - T6511 aluminum alloy on the basis of detailed SEM and TEM observations of microstructure in the close vicinity of the fatigue crack initiation sites.
conventional methodology consisting of specimens cutting and molding into Cu based resin, grinding on SiC papers, polishing with 1 μm dia mond pastes using ethanol as lubricant and finished by chemicalmechanical polishing by OP-S suspension. Fuss etchant (7.5 ml HF, 8 ml HNO3, 25 ml HCl and 1000 ml H2O) was used to reveal material microstructure. Backscattered electron imaging (BSE) was adopted to reach chemical contrast and more precise imaging of individual micro structural features. Chemical analysis of large intermetallic particles present at boundaries of elongated grains was performed by energydispersive X-ray spectroscopy (EDS). The atomic percentage of indi vidual chemical elements was compared with literature. Electron backscatter diffraction (EBSD) was used for analysis of grain orientation and individual grains were determined by 15� grain misorientation. Focused ion beam (FIB) technique was applied for fatigue crack initia tion mechanism analysis including observations of local microstructural changes and formation of surface relief and early cracks. Lamellas pre pared by FIB were extracted from the cross-section of the fatigue tested specimens in the vicinity of the fatigue crack initiation sites in the di rection perpendicular to the loading axis. Electron channeling contrast imaging (ECCI) technique was adopted to reveal the sub-grains structure and fine precipitates distribution within the primary material grains. A standard procedure consisting of grinding and etching was used for the preparation of specimens for the dislocation structure analysis of the material by transmission electron microscopy (TEM) using JEOL JEM2100F microscope. The microstructure of AW 7075 alloy after heat treatment consisted of grains of the solid solution of Zn in Al with a large number of inter metallic phase particles, Fig. 1a. Elongation of grains in the direction of extrusion was a characteristic feature, Fig. 1a. The Al2CuFe, Al2CuMgZn, and Al7Cu2Fe intermetallic phase (constituent) particles were mostly observed at grain boundaries of the elongated grains. Due to the heat treatment, small sub-grains in the large elongated primary grains, Fig. 1b, and a large number of fine particles precipitated in the grains during the aging, Fig. 1c, were created. A pronounced grain elongation was found in the extrusion direction, whereas the individual grains are oriented randomly, Fig. 1d. The deeper analysis confirmed that the subgrains in large elongated primary grains were created during the extrusion. Observation of conventionally prepared foil by TEM revealed low dislocation density and pinning of dislocations by small precipitates, Fig. 1e–f. No specific dislocation arrangement was found by TEM. Typically, the T651 heat treatment results in creation of GP zones and precipitation of η’ phase in the AW 7075 alloy [17–19]. The ultimate tensile strength, σUTS, of the investigated AW 7075 – T6511 aluminum alloy is 631 MPa, elongation is 4.9 % and the hardness is 175 HV10 [33]. Dynamic elastic modulus is 71 GPa and the density is 2800 kg∙m 3. High-frequency fatigue tests (20 kHz) were performed using piezo electric ultrasonic system made by Lasur®. Fatigue tests were performed at symmetrical cyclic loading in tension-compression with stress ratio R ¼ -1. Dry and clean pressurized air was used for specimen cooling in order to prevent changes in the microstructure due to specimen heating. The specific geometry of the specimen following the requirements of the ultrasonic system with a diameter of 4 mm is shown in Fig. 2. The precise length of the specimen was adjusted to reach the resonance frequency of the system during the ultra-high cycle fatigue tests. The specimens and loading axis was in the direction of the material extrusion. Low-frequency tests were performed on Zwick/Roell Amsler HC25 servohydraulic testing machine at symmetrical tension-compression cyclic loading at cycle asymmetry ratio R ¼ -1 with the nominal fre quency of 5 Hz. The tests were performed in laboratory air and at temperature 23 � 1 � C. The same specimen geometry was used for both loading frequencies. The middle part of the machined specimens was ground and polished (grinding by SiC papers and polishing by 1 μm diamond paste with ethanol used as a lubricant) and finished by chemical-mechanical pol ishing by OP-S suspension with the aim to observe and describe the
2. Experimental material and procedures AW 7075 – T6511 aluminum alloy provided in the form of extruded bars with 15 mm in diameter and 1.5 m in length was used as the experimental material. Solution heat treatment and stress-relieving by stretching and artificial aging with minor straightening after aging were applied on experimental material (heat treatment marked as T6511). Tescan LYRA 3 XMU FEG/SEM x FIB scanning electron microscope (SEM) was used for the metallographic analysis of the material micro structure, fractographic analysis and investigation of the fatigue crack initiation. Specimens for metallographic analysis were prepared by 2
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Fig. 1. Typical microstructure of AW 7075 – T6511 extruded aluminum alloy.
Fig. 2. Geometry of the specimen for fatigue tests.
fatigue crack initiation. 3. Results
Fig. 3. S–N curves for AW7075 – T6511 aluminum alloy tested under low and high frequency.
3.1. S–N curves
low loading frequency and the highest stress amplitude (5 Hz, 350 MPa), the multiple fatigue crack initiation was observed, Fig. 4a. In all the other cases, only one fatigue crack initiation site was observed. The very rough fracture surface was characteristic for all the broken specimens, Figs. 4a–c and g-i. This can be explained by the elongated microstructure (texture) of the extruded material. After the primary crack growth in the plane almost perpendicular to the loading axis a high slope of the fracture surface was a typical feature. Two different locations of the fatigue crack initiation were found. The fatigue crack initiation was observed either on the specimen surface (Figs. 4e, f, k, and l) or in its nearest vicinity (Figs. 4d and j). The maximum depth of crack initiation site with respect to the surface was about 10 μm. This was observed, however, only in the case of the low cycle fatigue, Nf < 105 cycles (for both the loading frequencies). When the fatigue life was above 105 cycles, only surface fatigue crack initiation
Fig. 3 shows that the lifetime at 20 kHz is generally higher than that at 5 Hz. The difference increases with decreasing stress amplitude. Different slope of the S–N curves fitted to the experimental points is obvious. Fatigue endurance limit defined on the basis of 1 � 107 cycles for specimens tested at 5 Hz makes 220 MPa, while the fatigue endur ance limit determined for 20 kHz for the same number of cycles is 275 MPa. The specimen tested at the stress amplitude of 220 MPa at 20 kHz reached more than 1 � 109 cycles. Fatigue endurance limit based on 1 � 1010 cycles for specimens tested at 20 kHz is 200 MPa. 3.2. Fractographic observation The same appearance of the fatigue fracture surface was character istic for all the broken specimens, regardless of the loading frequency and the reached number of cycles to the fracture. Only in the case of the 3
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Fig. 4. Fatigue fracture surfaces and fatigue crack initiation places of AW 7075 – T6511 extruded aluminum alloy tested at 5 Hz and 20 kHz.
took place (Figs. 4e, f, k and l). The detailed inspection of the loaded specimens did not reveal any signs of the localization of the cyclic plastic deformation in a form of slip markings on the polished surface. This was observed in all the cases regardless of the loading frequency and the applied stress amplitude.
corresponding cuts, it follows that the fatigue crack initiation site for the specimen tested at 350 MPa at 5Hz was located approximately 5 μm below the surface, Fig. 5a. At lower stress amplitudes applied at 5 Hz, Fig. 5b and c, crack initiation site was at the specimen surface. A similar observation was performed on the specimens tested at 20 kHz. Subsurface fatigue crack initiation, approximately 5 μm below the surface, was observed when the stress amplitude of 350 MPa was applied, Fig. 6a. Surface fatigue crack initiation was observed at lower stress amplitudes at 20 kHz, Figs. 6b and c. EBSD analysis performed at the sections in the vicinity of the fatigue fracture initiation sites revealed the random grain orientation for indi vidual specimens, Figs. 5d–f and Figs. 6d–f. Large grains with no internal sub-grains seem to be preferable sites for the fatigue crack initiation. BSE and ECCI analysis did not show any influence of the cyclic
3.3. Fracture profile and adjacent microstructure With the aim to investigate the mechanism of the fatigue crack initiation and changes of microstructure in the nearest vicinity of the fatigue cracks, advanced SEM techniques EBSD and ECCI were adopted. Cuts of the broken specimens parallel to the loading axis through the fatigue crack initiation sites are shown in Figs. 5 and 6. From a combi nation of fracture surface observation by SEM and observation of 4
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Fig. 5. Characterization of the microstructure evolution in the crack initiation place vicinity for specimens tested at 5 Hz.
loading on the evolution of the microstructure in terms of grain or subgrain coarsening, regardless of the loading frequency, Figs. 5g–i and Figs. 6d–i.
mechanism of the fatigue crack initiation at substantially different fre quencies. No signs of the localization of the plastic deformation in terms of the slip markings, development of surface relief or primary interme tallic particle cracking were observed (Figs. 4b, c, h, and i) even in the case when the fatigue cracks initiated just at the specimen surface. This holds for both the low and high frequency and for all the stress ampli tudes used. The absence of the slip markings can be attributed to a large amount of the small precipitates present in the small sub-grains that were created during production and heat treatment of the material. As re ported in Refs. [20,26,27], the interaction of dislocations with the strengthening precipitates depends on their character in respect to the matrix (coherent, semicoherent or incoherent), their size and distribu tion in the matrix. In the case of the examined AW 7075 – T6511, the metastable semicoherent η’precipitate particles were created in the microstructure due to the heat treatment applied, [17–19]. These pre cipitates act as obstacles for the dislocation motion. As a result, homo geneous stress distribution in the material occur and no specific dislocation arrangement as a result of the cyclic loading can be observed by TEM. The high density of obstacles for dislocation movement pre vents formation of long slip bands, which create intrusions and extru sions on the surface. The interactions between dislocations and precipitates were observed by TEM. Locally, the dislocations pinning on precipitates was observed, and no precipitates were cut by dislocations, regardless of the loading frequency and the stress applied. This excludes applying the fatigue crack initiation models based on the development of surface relief in the form of intrusions and extrusions. Due to the homogeneous distribution of the stress in the material and absence of localization of the cyclic plastic deformation, grain boundaries, or large intermetallic particles remains as the only possible places for the fatigue crack initiation. This mechanism can be quite complicated, because it depends also on the particles distribution and orientation, grain
3.4. Dislocation structure TEM foils extracted from the broken specimens near the fatigue crack initiation sites were examined with the aim to describe and compare dislocation structures after cycling at 5 Hz and 20 kHz. Dislocation structures shown in Fig. 7 do not exhibit any signs of localization of the cyclic plasticity in terms of 3D dislocation arrangement (cells, ladderlike structure etc.), regardless of the testing frequency. Locally, dislo cations pinning on precipitates was observed. The effect of frequency is only reflected in the dislocation density and dislocation re-arrangement. Specimens tested at 20 kHz, Figs. 7d–f, seems to exhibit higher dislo cation density when compared to the specimens tested at 5 Hz, Figs. 7a–c. 4. Discussion The surface quality and the surface state generally strongly influence the fatigue lifetime, especially in the high and ultra-high cycle fatigue region. In this work, the fatigue endurance limit of 250 MPa for 108 cycles for 20 kHz loading frequency was determined, Fig. 3. The speci mens used in this study was carefully polished. The value 250 MPa is substantially above 170 MPa reported for the identical material in Ref. [33], where the gauge length of the specimens was only conven tionally machined. The comparison of both results proves the high sensitivity of AW 7075-T6511 on the surface quality in the high and ultra-high cycle fatigue region. Carefully polished specimens with the identical geometry were used for the fatigue testing at both the frequencies with the aim to explore the 5
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Fig. 6. Characterization of the microstructure evolution in the crack initiation place vicinity for specimens tested at 20 kHz.
Fig. 7. Dislocation structure of cycled specimens analyzed in the crack initiation place vicinity for specimens tested at 5 Hz and 20 kHz, TEM.
boundary orientation to the loading axis and neighbouring grains slip systems orientation. In the vast majority of cases investigated in this study, a surface fa tigue crack initiation was observed. In a small number of events, fatigue crack initiation was observed just below the surface. In these cases, it
can be assumed that the primary intermetallic phase particles could act as the stress concentrators an thus increase the probability of the fatigue crack initiation in these areas, Figs. 4d and 4j. Payne et al. [22] observed that the fatigue cracks initiated due to the cracking of the iron-based particles present in the matrix. Following cyclic loading led to the 6
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crack propagation to the matrix. Contrary to the observation of Payne et al., [22], authors in the present study did not observe any particular intermetallic phase particles debonding or particle cracking. On the other hand, also Bozek et al. in [24] proposed a model calculating the probability of the fatigue crack initiation on the Fe-rich intermetallic particles. The surface initiation of fatigue cracks is generally influenced also by the environment. In our case, the environment at low frequency loading was the laboratory air and the laboratory temperature. The temperature of the specimens tested at 20 kHz was controlled by a cooled and dried air and did not exceed on the specimen surface 30 � C. Though the environmental conditions were at both frequencies different, no influ ence of the cooling medium on the fatigue crack initiation in sense of cavitation or oxidation was observed. The macroscopic appearance of the fatigue fracture surface was the same for both the testing frequencies. After initiation and primary crack growth in a plane perpendicular to the loading axis, the macroscopic crack plane inclines to the angle of 45–60� to the specimen axis. This finding corresponds to observation published in literature [33,34]. The fracture surface relief correlates to the material microstructure elonga tion in the extrusion direction. Since no difference in the fatigue cracks initiation mechanism (i.e. surface/internal) at low and high loading frequency was observed by SEM on the fracture surfaces and specimen surfaces, TEM analysis of the microstructure as close as possible to the crack initiation site was per formed. Specimens for TEM were prepared by both, conventional and FIB procedures. Foils prepared by the conventional procedure did not show any specific dislocation arrangement indicating localization of cyclic plastic deformation, which could result in the crack initiation. TEM specimens prepared by the conventional way (etching) do not, obviously, contain areas representing the very close vicinity of the initiated crack. Contrary to this, FIB technique allows preparing sitespecific TEM foils. Therefore this technique was adopted to prepare TEM foils from the grains where fatigue cracks initiated. Observation of FIB foils similarly did not show any clear signs of localization of cyclic plasticity. Only a small difference in the dislocation arrangement and density between the specimens tested at 5 Hz and 20 kHz was observed. Finer dislocation structure and higher dislocation density were observed in the case of the specimens tested at 20 kHz when compared to the specimens tested at 5 Hz. The difference could be connected with lower probability of dislocation movement due to the higher deformation rate at 20 kHz comparing to the 5 Hz loading. Consequently, higher local stress is required for the dislocation movement in the case of higher loading rate [13]. Lower dislocation activity at higher testing frequency results in reduced creation of dislocation sources, in diminished multi plication of dislocations and consequently in reduced tendency to rearrangement of dislocation structure. This explains the observed shift of the S–N curve for high frequency loading.
amplitude range used); particularly the grain size, particle distribution or the grain orientation. The only clear observable effect was slightly different dislocation density and dislocation arrangement in the nearest vicinity of the initiated fatigue cracks, however, no specific dislocation structure related to the crack initiation was found. Detailed TEM ob servations of the dislocation structure close to the fatigue crack initia tion sites, indicate pinning of dislocations on precipitates. Author contributions �: Conceptualization, Methodology, Validation, Investiga S. Fintova tion, Resources, Writing - Original Draft, Writing - Review & Editing, Visualization, Supervision, Project administration I. Kub� ena: Conceptualization, Methodology, Investigation, Writing Review & Editing L. Tr�sko: Conceptualization, Resources V. Horník: Investigation L. Kunz: Conceptualization, Writing - Review & Editing Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This research has been supported by the Ministry of Education, Youth and Sports of the Czech Republic under the project m-IPMinfra (CZ.02.1.01/0.0/0.0/16_013/0001823) and the equipment and the base of research infrastructure IPMinfra were used during the research ac tivities. The research has been supported also by the Science Grant Agency of the Slovak Republic under the project No. 1/0029/18. References [1] H. Hu, X. Wang, Effect of heat treatment on the in-plane anisotropy of as-rolled 7050 aluminum alloy, Metals 6 (2016) 79, https://doi.org/10.3390/met6040079. [2] H. Mughrabi, Specific features and mechanisms of fatigue in the ultrahigh-cycle regime, Int. J. Fatigue 28 (11) (2006) 1501–1508, https://doi.org/10.1016/j. ijfatigue.2005.05.018. [3] T. Nicholas, High Cycle Fatigue: A Mechanics of Materials Perspective, Elsevier Science, 2006. [4] C. Wang, A. Nikitin, A. Shanyavskiy, C. Bathias, An understanding of crack growth in VHCF from an internal inclusion in high strength steel, in: Crack Paths, Gruppo Italiano Frattura, Gaeta, Italy, 2012, pp. 367–374. [5] J.G. Antonopoulos, L.M. Brown, A.T. Winter, Vacancy dipoles in fatigued copper, Philos. Mag. 34 (4) (1976) 549–563, https://doi.org/10.1080/ 14786437608223793. [6] T. Zhai, J.W. Martin, G.A.D. Briggs, Fatigue damage in aluminium single crystals. I. On the surface containing the slip Burgers vector, Acta Metall. Mater. 43 (10) (1995) 3813–3825, https://doi.org/10.1016/0956-7151(95)90165-5. [7] J. Pol� ak, Cyclic Plasticity and Low Cycle Fatigue Life of Metals, Elsevier, 1991. [8] J. Pol� ak, J. Man, Mechanisms of extrusion and intrusion formation in fatigued crystalline materials, Mater. Sci. Eng. A 596 (2014) 15–24, https://doi.org/ 10.1016/j.msea.2013.12.005. [9] J. Pol� ak, J. Man, Fatigue crack initiation – the role of point defects, Int. J. Fatigue 65 (2014) 18–27, https://doi.org/10.1016/j.ijfatigue.2013.10.016. [10] U. Essmann, U. G€ osele, H. Mughrabi, A model of extrusions and intrusions in fatigued metals I. Point-defect production and the growth of extrusions, Philos. Mag. A 44 (1981) 405–426, https://doi.org/10.1080/01418618108239541. [11] T.H. Lin, S.R. Lin, Micromechanics theory of fatigue crack initiation applied to time-dependent fatigue, in: F.J. T (Ed.), Fatigue Mechanisms, ASTM International, 1979, pp. 707–728, https://doi.org/10.1520/STP35911S. [12] M. Papakyriacou, H. Mayer, C. Pypen, H. Plenk Jr., S. Stanzl-Tschegg, Influence of loading frequency on high cycle fatigue properties of b.c.c. and h.c.p. metals, Mater. Sci. Eng. A 308 (1–2) (2001) 143–152, https://doi.org/10.1016/S09215093(00)01978-X. [13] P.M. Anderson, J.P. Hirth, J. Lothe, Theory of Dislocations, Cambridge University Press, 2017. [14] C. Bathias, L. Drouillac, P. Le François, How and why the fatigue S–N curve does not approach a horizontal asymptote, Int. J. Fatigue 23 (Supplement 1) (2001) 143–151, https://doi.org/10.1016/S0142-1123(01)00123-2.
5. Conclusions The fatigue life of AW 7075-T6511 aluminum alloy under symmet rical tension-compression cycling at 5 and 20 kHz frequency was experimentally determined. It was found that the fatigue life at the 20 kHz frequency continuously decreases in the interval from 107to 1010 cycles to failure. The fatigue endurance limit of 220 MPa determined for 107 cycles at the loading frequency of 5 Hz increased to 275 MPa for the loading frequency of 20 kHz. The fatigue endurance limit at a high frequency based on 1 � 1010 cycles was 200 MPa. No influence of the loading frequency and stress amplitude on the fatigue crack initiation site (surface/subsurface) was observed. Subsur face fatigue crack initiation was characteristic for high stress amplitudes (350 MPa), while surface crack initiation was characteristic for stress amplitudes resulting in fatigue life higher than 105 cycles. The microstructure of the investigated alloy remained practically unchanged due to cycling at low and high frequency (in the stress 7
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