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Indentation creep-fatigue test on aluminum alloy 2A12
Materials Science and Engineering A 527 (2010) 4519–4522
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Indentation creep-fatigue test on aluminum alloy 2A12 Bin Zhao ∗ , Baoxing Xu, Zhufeng Yue Department of Engineering Mechanics, Northwestern Polytechnical University, Xi’an 710072, PR China
a r t i c l e
i n f o
Article history: Received 22 September 2009 Received in revised form 27 February 2010 Accepted 3 March 2010
Keywords: Indentation test Creep-fatigue Creep Deformation mechanism Aluminum alloy 2A12
a b s t r a c t Generally, the creep-fatigue behavior of materials is obtained by tensile/compression tests. In the present study, we explore the possibility of investigating the creep-fatigue behavior of aluminum alloy 2A12 at 200 ◦ C by using indentation creep-fatigue test with a flat cylindrical indenter. The experimental results show that the evolution of indentation depth with the number of cycles experiences two stages: a short transient stage and then a steady state with an approximate constant indentation depth propagation rate. The micro-observations on craters undergoing creep-fatigue and pure creep loadings indicate that the fatigue loading accelerates the evolution of damage and leads to the nucleation of cracks around the crater. These findings are useful to probe the creep-fatigue properties of materials with the indentation test. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Indentation creep is well acknowledged as an alternative approach of obtaining the creep properties of materials due to its non-destructive, fast and high accurate merits [1–5]. It has also been extended to determine the mechanical properties of materials and deformation mechanisms at high temperature. For example, Dorner et al. measured the creep stress exponent and activation energy of Ti–Al alloy with the indentation creep method at 750 ◦ C, and found that they agreed well with those from conventional tensile creep tests [6]. Schuh et al. extracted the activation volume of Pt single crystal from indentation creep data, and clarified the deformation mechanism at onset of plasticity [7]. Recently, we carried out the indentation creep test of aluminum alloy 2A12 at 200 ◦ C and found a good agreement on the creep stress exponent with that from tensile creep tests [8]. However, most of engineering alloys which served at high temperatures have to suffer from the non-steady loading. For example, the aircraft engine will undergo a variety of loading conditions at taking-off, in flight and landing, and thus a high frequency vibration loading is superimposed to the engine, resulting in the acceleration of the engine failure [9]. Therefore, the high-temperature creep deformation and failure behaviors of these materials should include the effect of fatigue loading. Liking the definition of indentation creep, a cyclic load applied on a specimen via an indenter is called indentation fatigue technique [10]. The early indentation fatigue test was carried out on
∗ Corresponding author. Tel.: +86 29 88431002; fax: +86 29 88431002. E-mail address: [email protected] (B. Zhao). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.03.013
-tin single crystals by Li and Chu [10]. They found that the indentation depth could increase with the number of cycles. The similar results were also reported in indentation fatigue test and numerical analysis of polycrystalline copper [11–13]. Based on the similarity of stress singularity at the crack tip and indenter edge, a quantitative relationship of indentation fatigue depth propagation and fatigue crack propagation were built [14]. The indentation depth propagation rate at the steady state was explicitly written as a function of stress intensity factors, which is usually employed in the fatigue crack propagation. However, to the best knowledge of authors, little work is done to investigate the influence of fatigue loading on indentation creep deformation, which is critical to the determination of creep properties of materials with the indentation creep test. Inspired by the conventional tensile creep-fatigue test at high temperature, a creep-fatigue loading spectrum is designed in the present study and applied on the surface of specimen via a flat cylindrical indenter. The emphasis will be put on the indentation response under creep-fatigue loading, and effect of fatigue loading on indentation creep deformation. The optical microscope is used to analyze the microstructures around the crater surface as well as its cross-section. The indentation test with a pure creep loading is also performed to provide comparisons.
2. Experimental materials and procedures Aluminum alloy 2A12, which shows an obvious creep deformation at 200 ◦ C [8], is employed in the present study. It is cut into cylindrical specimens with diameter of 25 mm and height of 20 mm. All specimens are polished to produce parallel flat surfaces in order
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Fig. 1. Indentation creep-fatigue testing fixture.
to minimize the effect of surface rough. The flat cylindrical indenter (the diameter is 1.51 mm) is manufactured with a high-speed steel material by using the same technique, and a special attention is also paid to the surface of indenter which will contact with the surface of specimen in test, and its creep deformation can be neglected in comparison to that of aluminum alloy 2A12 at 200 ◦ C [8]. The CSS2910 high-temperature creep machine, which is coupled with a flat cylindrical indentation testing fixture, is employed to perform the indentation creep-fatigue test. The material of indentation testing fixture is GH4169 and will not creep at 200 ◦ C [8]. Fig. 1 illustrates the indentation creep-fatigue testing system. The load spectrum is a typical trapezium creep-fatigue wave form, shown in Fig. 2. The Pmax and Pmin are the maximum and minimum indentation fatigue loads, respectively. Both of the maximum loading and minimum loading stages are 5 min, and the loading and unloading rates are 1 kN/min. The testing temperature is 200 ± 1 ◦ C. During the indentation test, the indentation depth and fatigue cycles can be recorded continuously by a computer through the linear variable differential transformers (LVDTs). Indentation creep test on aluminum alloy 2A12 has been performed in this testing system, and its reliability has been validated from the good agreement of creep stress exponent obtained from the indentation and tensile test. The details can be found elsewhere [8].
elastic recovery due to the periodic loading, and its magnitude is expected to depend on the time of creep stage, the mean fatigue load as well as the loading frequency. Moreover, two stages are observed in Fig. 3(a). The first stage is the transient stage that is caused by the initial sudden loading, and then a steady stage with an approximate constant indentation depth rate is followed after the initial adjustment. These characters are also similar with these
3. Results and discussion Fig. 3(a) shows the typical evolution of the indentation depth with applied fatigue cycles. The maximum load Pmax and minimum load Pmin are 1400 N and 800 N, respectively. It is observed that the indentation depth increases with the number of cycles, suggesting that the indenter will sink into the specimen continuously with the elapse of time. The width band in curve approximately reflects the
Fig. 2. Schematic of the indentation creep-fatigue loading spectrum.
Fig. 3. The evolution of the indentation depth (h) with the number of cycles (N) and creep time of aluminium alloy 2A12 at 200 ◦ C under (a) creep-fatigue load (P) of 800–1400 N, and (b) pure creep load of 1100 N, respectively.
B. Zhao et al. / Materials Science and Engineering A 527 (2010) 4519–4522
Fig. 4. Effect of the maximum creep-fatigue load (Pmax ) on the indentation depthcycles curve of aluminum alloy 2A12 at 200 ◦ C with a constant Pmin of 500 N.
of indentation under pure creep loading in Fig. 3(b). The indentation creep load in Fig. 3(b) is equal to the average value of creep-fatigue load of Fig. 3(a), and is 1100 N. However, compared with the indentation creep-fatigue test, more time is required before the arrival of the steady state of indentation depth propagation (i.e. the transition stage) for pure indentation creep (Fig. 3(b)). Fig. 4 shows the effect of the maximum load Pmax on the evolution of indentation depth under the minimum load Pmin of 500 N. It can be seen that a higher Pmax results in a larger indentation depth at the same number of cycles. A larger steady indentation depth
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rate is also induced for a higher Pmax , which is similar with that under pure creep or fatigue loading [8,11,13]. Besides, the width of the curve is wider for a higher Pmax , which to some extent indicates a larger elastic recovery. In order to understand the deformation mechanism of indentation creep-fatigue, the optical microscope is used to obtain the microstructures around the crater surface and its cross-section. Fig. 5(a) shows the surface morphology around crater that is obtained under loading of 800–1400 N and 38 cycles (∼7 h) (its curve of indentation depth—the number of cycles is shown in Fig. 3(a)). First, the pile-up around indentation is observed. Due to the constant contact area during the sinking of flat-end punch indenter, the gap between the indenter and specimen is enlarged, and increases with the number of cycles. Second, a radial crack is observed near the rim of crater, and is expected to propagate along the radial direction. The pervious finite element analysis [12] and scanning electron microscope (SEM) [13] on the residual indentation which underwent the fatigue loading showed that the crack originated from the rim of the contact zone, and propagated toward the far from the center of crater. Similarly, under present fatiguecreep loading, the resultant crack is also expected to nucleate at the edge of contact. The first character is also observed in indentation test with pure creep loading, and is shown in Fig. 5(b) (its indentation depth—creep time curve is shown in Fig. 3(b)). However, with the same testing time (∼7 h) and mean load (1100 N) as these of indentation creep-fatigue, no cracks are observed in Fig. 5(b). The difference in microstructures indicates that the fatigue loading accelerates the damage of materials and nucleation of cracks, which shows a similar behavior with the influence of the conventional fatigue loading on the creep deformation [9]. Fig. 5(c) shows the microscope images of the indentation crosssection upon creep-fatigue loadings. The difference in size of the indentation cross-section at the indentation top and bottom profile
Fig. 5. Microscope observations of indentation in the aluminum alloy 2A12 under creep-fatigue load ((a) and (c)) and pure creep load ((b) and (d)) at 200 ◦ C. The loading range in indentation creep-fatigue test is 800–1400 N, and the cyclic number is 38 (∼7 h). The creep load is 1100 N in the indentation creep test with the creep time of 7 h.
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further shows the existence of gap between the indenter and specimen. Besides, the gap widens with the sinking of the flat indenter, which agrees well with the surface morphology in Fig. 5(a). The extension of gap also implies an increase in the magnitude of pile-up with the number of cycles due to the constant volume of materials. The obvious flow slip pattern with light and dark lines, which originates from the corners of indentation owing to the stress concentration, is also observed in Fig. 5(c), and the slip lines seem to coalesce beneath the indentation, and the junction far from the bottom of indentation is expected to be associated with the hydrostatic stress. The pattern in front of indentation is expected to the activation of multi-slip systems although the detailed mechanism is still fuzzy here. The similar feature is also found beneath the indentation under pure creep loading in Fig. 5(d). However, a rougher morphology in indentation cross-section exists with the creep-fatigue loading (Fig. 5(c)), suggesting that a more serious deformation occurs due to the effect of fatigue loading, and even the resultant nucleation of crack in the indentation surface (Fig. 5(a)). The comparison of microstructures reveals that deformation mechanism of indentation creep-fatigue is expected to the activation of the slip systems, and the fatigue loading accelerates the nucleation of the damage and cracks. Generally, the indentation depth evolution can be understood with an effective stress model, and the evolution of effective stress with periodic fatigue cycles corresponds to the dynamic process of applied load and internal stress [13,15,16]. At the onset of the loading, the applied load is far more than the internal stress, and several cycles are needed to balance them, and thus a transient stage of indentation depth propagation with a decrease of indentation depth propagation rate is observed. At the stage of steady state, an approximate constant effective stress arrives due to the balance between the applied stress and internal stress, and leads to a constant rate of indentation depth propagation. Under pure creep loading, the evolution of the effective stress decreases monotonously with creep time due to a constant applied stress until the balance arrives. Under creep-fatigue loadings, however, the fatigue loading destroys the monotonous variation of effective stress, and more time is needed to balance the internal stress and applied dynamic loading (fatigue loading), resulting in a more dissipation in the number of cycles before the arrival of steady state (Figs. 3 and 4). Besides, the steady effective stress is also the result of dynamic balance between internal stress and fatigue load, to some extent, resulting in the periodic change of indentation depth (Figs. 3 and 4). The “dynamic steady effective stress” is expected to accelerate the deformation of materials, which causes a more obvious flow lines beneath indentation (Fig. 5(c)), and even the nucleation of crack at the indentation surface (Fig. 5(a)). In addition, although the mean loading amplitude of indentation creep-fatigue is equal to the pure indentation creep load, the maximum amplitude of creep-fatigue loading is higher than the creep loading, and is expected to lead to the slip deformation between the indenter and surface of specimen because their friction cannot be avoided completely during the test. Note that the initial rough contact surface is another reason of intriguing the nucleation of the rough surface during the test. Further, the periodically dynamic variation of creep-fatigue loading is expected to further promote the slip deformation and thus enhances the magnitude of roughness. Therefore a more serious roughness is observed in the contact zone in Fig. 5(c).
For a higher Pmax , the mean creep-fatigue load is higher, leading to a larger effective stress. Therefore, both of the mean indentation depth at the same number of cycles and the steady state rate of indentation depth propagation increase. Moreover, a higher loading amplitude needs more number of cycles for the arrival of the ultimately dynamic balance between the applied loading and internal stress, which is shown in Fig. 4. 4. Conclusions In the present study, indentation creep-fatigue tests are carried out on aluminum alloy 2A12 at 200 ◦ C with a flat cylindrical indenter, and the main results are summarized as follows: (1) The indenter can sink into the materials continuously with the applied fatigue cycles, and the whole process can be described with two stages: the transient stage and the steady state with an approximate constant indentation depth propagation rate. The higher maximum loading promotes the development of indentation depth, and results in a larger steady indentation depth rate. (2) Observations on the microstructures of the crater surface and its cross-section show that some materials will bulge with cycles, and develops into the pile-up. At the same time, the gap between the indenter and specimen increases with numbers of cycle due to the constant contact area. Moreover, the indentation creep-fatigue loading can lead to an obvious nucleation of cracks at the edge of crater. (3) The indentation depth propagation can be well understood through an effective stress model. Indentation creep-fatigue deformation mechanism of aluminum alloy 2A12 at 200 ◦ C is expected to the activation of the multi-slip systems, which is similar with that upon pure creep loadings, but the coupled fatigue loading accelerates the accumulation of damage and leads to nucleation of cracks with comparison of the pure indentation creep loading. Acknowledgement This work is supported by Research Fund for the Doctoral Program of Higher Education N6CJ0001. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
B.N. Lucas, W.C. Oliver, Metall. Mater. Trans. A 30 (1999) 601–610. Y.T. Cheng, C.M. Cheng, Phil. Mag. Lett. 811 (2001) 9–16. Z.F. Yue, G. Eggeler, B. Stöckhert, Comp. Mater. Sci. 21 (2001) 37–56. X. Ma, F. Yoshida, Appl. Phys. Lett. 82 (2003) 188–190. F. Yang, J.M.C. Li, Mater. Sci. Eng. A 201 (2005) 40–49. D. Dorner, K. Roller, B. Skrotzki, B. Stockhert, G. Eggeler, Mater. Sci. Eng. A 357 (2003) 346–354. C.A. Schuh, J.K. Mason, A.C. Lund, Nat. Mater. 4 (2005). Y.J. Liu, B. Zhao, B.X. Xu, Z.F. Yue, Mater. Sci. Eng. A 456 (2007) 103–108. G.A. Webster, R.A. Ainsworth, High Temperature Component Life Assessment, Chapman & Hall, London, 1994. J.C.M. Li, S.N.G. Chu, Scripta Mater. 13 (1979) 1021–1026. B. Xu, Z. Yue, J. Mater. Res. 21 (2006) 1793–1797. B. Xu, Z. Yue, J. Mater. Res. 22 (2007) 186–192. B. Xu, Z. Yue, J. Wang, Mech. Mater. 39 (2007) 1066–1080. B. Xu, Z. Yue, X. Chen, Scripta Mater. 60 (2009) 854–857. B. Xu, X. Wang, Z. Yue, J. Mater. Res. 22 (2007) 1585–1592. J.C.M. Li, Mater. Sci. Eng. A 322 (2002) 23–42.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Rapid communication
Indentation creep-fatigue test on aluminum alloy 2A12 Bin Zhao ∗ , Baoxing Xu, Zhufeng Yue Department of Engineering Mechanics, Northwestern Polytechnical University, Xi’an 710072, PR China
a r t i c l e
i n f o
Article history: Received 22 September 2009 Received in revised form 27 February 2010 Accepted 3 March 2010
Keywords: Indentation test Creep-fatigue Creep Deformation mechanism Aluminum alloy 2A12
a b s t r a c t Generally, the creep-fatigue behavior of materials is obtained by tensile/compression tests. In the present study, we explore the possibility of investigating the creep-fatigue behavior of aluminum alloy 2A12 at 200 ◦ C by using indentation creep-fatigue test with a flat cylindrical indenter. The experimental results show that the evolution of indentation depth with the number of cycles experiences two stages: a short transient stage and then a steady state with an approximate constant indentation depth propagation rate. The micro-observations on craters undergoing creep-fatigue and pure creep loadings indicate that the fatigue loading accelerates the evolution of damage and leads to the nucleation of cracks around the crater. These findings are useful to probe the creep-fatigue properties of materials with the indentation test. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Indentation creep is well acknowledged as an alternative approach of obtaining the creep properties of materials due to its non-destructive, fast and high accurate merits [1–5]. It has also been extended to determine the mechanical properties of materials and deformation mechanisms at high temperature. For example, Dorner et al. measured the creep stress exponent and activation energy of Ti–Al alloy with the indentation creep method at 750 ◦ C, and found that they agreed well with those from conventional tensile creep tests [6]. Schuh et al. extracted the activation volume of Pt single crystal from indentation creep data, and clarified the deformation mechanism at onset of plasticity [7]. Recently, we carried out the indentation creep test of aluminum alloy 2A12 at 200 ◦ C and found a good agreement on the creep stress exponent with that from tensile creep tests [8]. However, most of engineering alloys which served at high temperatures have to suffer from the non-steady loading. For example, the aircraft engine will undergo a variety of loading conditions at taking-off, in flight and landing, and thus a high frequency vibration loading is superimposed to the engine, resulting in the acceleration of the engine failure [9]. Therefore, the high-temperature creep deformation and failure behaviors of these materials should include the effect of fatigue loading. Liking the definition of indentation creep, a cyclic load applied on a specimen via an indenter is called indentation fatigue technique [10]. The early indentation fatigue test was carried out on
∗ Corresponding author. Tel.: +86 29 88431002; fax: +86 29 88431002. E-mail address: [email protected] (B. Zhao). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.03.013
-tin single crystals by Li and Chu [10]. They found that the indentation depth could increase with the number of cycles. The similar results were also reported in indentation fatigue test and numerical analysis of polycrystalline copper [11–13]. Based on the similarity of stress singularity at the crack tip and indenter edge, a quantitative relationship of indentation fatigue depth propagation and fatigue crack propagation were built [14]. The indentation depth propagation rate at the steady state was explicitly written as a function of stress intensity factors, which is usually employed in the fatigue crack propagation. However, to the best knowledge of authors, little work is done to investigate the influence of fatigue loading on indentation creep deformation, which is critical to the determination of creep properties of materials with the indentation creep test. Inspired by the conventional tensile creep-fatigue test at high temperature, a creep-fatigue loading spectrum is designed in the present study and applied on the surface of specimen via a flat cylindrical indenter. The emphasis will be put on the indentation response under creep-fatigue loading, and effect of fatigue loading on indentation creep deformation. The optical microscope is used to analyze the microstructures around the crater surface as well as its cross-section. The indentation test with a pure creep loading is also performed to provide comparisons.
2. Experimental materials and procedures Aluminum alloy 2A12, which shows an obvious creep deformation at 200 ◦ C [8], is employed in the present study. It is cut into cylindrical specimens with diameter of 25 mm and height of 20 mm. All specimens are polished to produce parallel flat surfaces in order
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B. Zhao et al. / Materials Science and Engineering A 527 (2010) 4519–4522
Fig. 1. Indentation creep-fatigue testing fixture.
to minimize the effect of surface rough. The flat cylindrical indenter (the diameter is 1.51 mm) is manufactured with a high-speed steel material by using the same technique, and a special attention is also paid to the surface of indenter which will contact with the surface of specimen in test, and its creep deformation can be neglected in comparison to that of aluminum alloy 2A12 at 200 ◦ C [8]. The CSS2910 high-temperature creep machine, which is coupled with a flat cylindrical indentation testing fixture, is employed to perform the indentation creep-fatigue test. The material of indentation testing fixture is GH4169 and will not creep at 200 ◦ C [8]. Fig. 1 illustrates the indentation creep-fatigue testing system. The load spectrum is a typical trapezium creep-fatigue wave form, shown in Fig. 2. The Pmax and Pmin are the maximum and minimum indentation fatigue loads, respectively. Both of the maximum loading and minimum loading stages are 5 min, and the loading and unloading rates are 1 kN/min. The testing temperature is 200 ± 1 ◦ C. During the indentation test, the indentation depth and fatigue cycles can be recorded continuously by a computer through the linear variable differential transformers (LVDTs). Indentation creep test on aluminum alloy 2A12 has been performed in this testing system, and its reliability has been validated from the good agreement of creep stress exponent obtained from the indentation and tensile test. The details can be found elsewhere [8].
elastic recovery due to the periodic loading, and its magnitude is expected to depend on the time of creep stage, the mean fatigue load as well as the loading frequency. Moreover, two stages are observed in Fig. 3(a). The first stage is the transient stage that is caused by the initial sudden loading, and then a steady stage with an approximate constant indentation depth rate is followed after the initial adjustment. These characters are also similar with these
3. Results and discussion Fig. 3(a) shows the typical evolution of the indentation depth with applied fatigue cycles. The maximum load Pmax and minimum load Pmin are 1400 N and 800 N, respectively. It is observed that the indentation depth increases with the number of cycles, suggesting that the indenter will sink into the specimen continuously with the elapse of time. The width band in curve approximately reflects the
Fig. 2. Schematic of the indentation creep-fatigue loading spectrum.
Fig. 3. The evolution of the indentation depth (h) with the number of cycles (N) and creep time of aluminium alloy 2A12 at 200 ◦ C under (a) creep-fatigue load (P) of 800–1400 N, and (b) pure creep load of 1100 N, respectively.
B. Zhao et al. / Materials Science and Engineering A 527 (2010) 4519–4522
Fig. 4. Effect of the maximum creep-fatigue load (Pmax ) on the indentation depthcycles curve of aluminum alloy 2A12 at 200 ◦ C with a constant Pmin of 500 N.
of indentation under pure creep loading in Fig. 3(b). The indentation creep load in Fig. 3(b) is equal to the average value of creep-fatigue load of Fig. 3(a), and is 1100 N. However, compared with the indentation creep-fatigue test, more time is required before the arrival of the steady state of indentation depth propagation (i.e. the transition stage) for pure indentation creep (Fig. 3(b)). Fig. 4 shows the effect of the maximum load Pmax on the evolution of indentation depth under the minimum load Pmin of 500 N. It can be seen that a higher Pmax results in a larger indentation depth at the same number of cycles. A larger steady indentation depth
4521
rate is also induced for a higher Pmax , which is similar with that under pure creep or fatigue loading [8,11,13]. Besides, the width of the curve is wider for a higher Pmax , which to some extent indicates a larger elastic recovery. In order to understand the deformation mechanism of indentation creep-fatigue, the optical microscope is used to obtain the microstructures around the crater surface and its cross-section. Fig. 5(a) shows the surface morphology around crater that is obtained under loading of 800–1400 N and 38 cycles (∼7 h) (its curve of indentation depth—the number of cycles is shown in Fig. 3(a)). First, the pile-up around indentation is observed. Due to the constant contact area during the sinking of flat-end punch indenter, the gap between the indenter and specimen is enlarged, and increases with the number of cycles. Second, a radial crack is observed near the rim of crater, and is expected to propagate along the radial direction. The pervious finite element analysis [12] and scanning electron microscope (SEM) [13] on the residual indentation which underwent the fatigue loading showed that the crack originated from the rim of the contact zone, and propagated toward the far from the center of crater. Similarly, under present fatiguecreep loading, the resultant crack is also expected to nucleate at the edge of contact. The first character is also observed in indentation test with pure creep loading, and is shown in Fig. 5(b) (its indentation depth—creep time curve is shown in Fig. 3(b)). However, with the same testing time (∼7 h) and mean load (1100 N) as these of indentation creep-fatigue, no cracks are observed in Fig. 5(b). The difference in microstructures indicates that the fatigue loading accelerates the damage of materials and nucleation of cracks, which shows a similar behavior with the influence of the conventional fatigue loading on the creep deformation [9]. Fig. 5(c) shows the microscope images of the indentation crosssection upon creep-fatigue loadings. The difference in size of the indentation cross-section at the indentation top and bottom profile
Fig. 5. Microscope observations of indentation in the aluminum alloy 2A12 under creep-fatigue load ((a) and (c)) and pure creep load ((b) and (d)) at 200 ◦ C. The loading range in indentation creep-fatigue test is 800–1400 N, and the cyclic number is 38 (∼7 h). The creep load is 1100 N in the indentation creep test with the creep time of 7 h.
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further shows the existence of gap between the indenter and specimen. Besides, the gap widens with the sinking of the flat indenter, which agrees well with the surface morphology in Fig. 5(a). The extension of gap also implies an increase in the magnitude of pile-up with the number of cycles due to the constant volume of materials. The obvious flow slip pattern with light and dark lines, which originates from the corners of indentation owing to the stress concentration, is also observed in Fig. 5(c), and the slip lines seem to coalesce beneath the indentation, and the junction far from the bottom of indentation is expected to be associated with the hydrostatic stress. The pattern in front of indentation is expected to the activation of multi-slip systems although the detailed mechanism is still fuzzy here. The similar feature is also found beneath the indentation under pure creep loading in Fig. 5(d). However, a rougher morphology in indentation cross-section exists with the creep-fatigue loading (Fig. 5(c)), suggesting that a more serious deformation occurs due to the effect of fatigue loading, and even the resultant nucleation of crack in the indentation surface (Fig. 5(a)). The comparison of microstructures reveals that deformation mechanism of indentation creep-fatigue is expected to the activation of the slip systems, and the fatigue loading accelerates the nucleation of the damage and cracks. Generally, the indentation depth evolution can be understood with an effective stress model, and the evolution of effective stress with periodic fatigue cycles corresponds to the dynamic process of applied load and internal stress [13,15,16]. At the onset of the loading, the applied load is far more than the internal stress, and several cycles are needed to balance them, and thus a transient stage of indentation depth propagation with a decrease of indentation depth propagation rate is observed. At the stage of steady state, an approximate constant effective stress arrives due to the balance between the applied stress and internal stress, and leads to a constant rate of indentation depth propagation. Under pure creep loading, the evolution of the effective stress decreases monotonously with creep time due to a constant applied stress until the balance arrives. Under creep-fatigue loadings, however, the fatigue loading destroys the monotonous variation of effective stress, and more time is needed to balance the internal stress and applied dynamic loading (fatigue loading), resulting in a more dissipation in the number of cycles before the arrival of steady state (Figs. 3 and 4). Besides, the steady effective stress is also the result of dynamic balance between internal stress and fatigue load, to some extent, resulting in the periodic change of indentation depth (Figs. 3 and 4). The “dynamic steady effective stress” is expected to accelerate the deformation of materials, which causes a more obvious flow lines beneath indentation (Fig. 5(c)), and even the nucleation of crack at the indentation surface (Fig. 5(a)). In addition, although the mean loading amplitude of indentation creep-fatigue is equal to the pure indentation creep load, the maximum amplitude of creep-fatigue loading is higher than the creep loading, and is expected to lead to the slip deformation between the indenter and surface of specimen because their friction cannot be avoided completely during the test. Note that the initial rough contact surface is another reason of intriguing the nucleation of the rough surface during the test. Further, the periodically dynamic variation of creep-fatigue loading is expected to further promote the slip deformation and thus enhances the magnitude of roughness. Therefore a more serious roughness is observed in the contact zone in Fig. 5(c).
For a higher Pmax , the mean creep-fatigue load is higher, leading to a larger effective stress. Therefore, both of the mean indentation depth at the same number of cycles and the steady state rate of indentation depth propagation increase. Moreover, a higher loading amplitude needs more number of cycles for the arrival of the ultimately dynamic balance between the applied loading and internal stress, which is shown in Fig. 4. 4. Conclusions In the present study, indentation creep-fatigue tests are carried out on aluminum alloy 2A12 at 200 ◦ C with a flat cylindrical indenter, and the main results are summarized as follows: (1) The indenter can sink into the materials continuously with the applied fatigue cycles, and the whole process can be described with two stages: the transient stage and the steady state with an approximate constant indentation depth propagation rate. The higher maximum loading promotes the development of indentation depth, and results in a larger steady indentation depth rate. (2) Observations on the microstructures of the crater surface and its cross-section show that some materials will bulge with cycles, and develops into the pile-up. At the same time, the gap between the indenter and specimen increases with numbers of cycle due to the constant contact area. Moreover, the indentation creep-fatigue loading can lead to an obvious nucleation of cracks at the edge of crater. (3) The indentation depth propagation can be well understood through an effective stress model. Indentation creep-fatigue deformation mechanism of aluminum alloy 2A12 at 200 ◦ C is expected to the activation of the multi-slip systems, which is similar with that upon pure creep loadings, but the coupled fatigue loading accelerates the accumulation of damage and leads to nucleation of cracks with comparison of the pure indentation creep loading. Acknowledgement This work is supported by Research Fund for the Doctoral Program of Higher Education N6CJ0001. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
B.N. Lucas, W.C. Oliver, Metall. Mater. Trans. A 30 (1999) 601–610. Y.T. Cheng, C.M. Cheng, Phil. Mag. Lett. 811 (2001) 9–16. Z.F. Yue, G. Eggeler, B. Stöckhert, Comp. Mater. Sci. 21 (2001) 37–56. X. Ma, F. Yoshida, Appl. Phys. Lett. 82 (2003) 188–190. F. Yang, J.M.C. Li, Mater. Sci. Eng. A 201 (2005) 40–49. D. Dorner, K. Roller, B. Skrotzki, B. Stockhert, G. Eggeler, Mater. Sci. Eng. A 357 (2003) 346–354. C.A. Schuh, J.K. Mason, A.C. Lund, Nat. Mater. 4 (2005). Y.J. Liu, B. Zhao, B.X. Xu, Z.F. Yue, Mater. Sci. Eng. A 456 (2007) 103–108. G.A. Webster, R.A. Ainsworth, High Temperature Component Life Assessment, Chapman & Hall, London, 1994. J.C.M. Li, S.N.G. Chu, Scripta Mater. 13 (1979) 1021–1026. B. Xu, Z. Yue, J. Mater. Res. 21 (2006) 1793–1797. B. Xu, Z. Yue, J. Mater. Res. 22 (2007) 186–192. B. Xu, Z. Yue, J. Wang, Mech. Mater. 39 (2007) 1066–1080. B. Xu, Z. Yue, X. Chen, Scripta Mater. 60 (2009) 854–857. B. Xu, X. Wang, Z. Yue, J. Mater. Res. 22 (2007) 1585–1592. J.C.M. Li, Mater. Sci. Eng. A 322 (2002) 23–42.