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The strain amplitude controlled fatigue behavior of pure copper with ultra large grain size
Materials Science & Engineering A 559 (2013) 170–177
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
The strain amplitude controlled fatigue behavior of pure copper with ultra large grain size H.L. Huang a, S.W. Mao a,n, D. Gan b, N.J. Ho b a b
Department of Mechanical Engineering, R.O.C. Military Academy, Kaohsiung, Taiwan, ROC Department of Materials and Optoelectronic Science, National Sun Yat-san University, Kaohsiung, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 June 2012 Received in revised form 6 August 2012 Accepted 13 August 2012 Available online 19 August 2012
The dislocation structural evolution in polycrystalline copper at constant strain amplitude during low cycle fatigue is well understood. However, the dislocation structural development at variable strain, which changes from high to low strain amplitude, has seldom been reported. Scanning electron microscope (SEM) back-scattering electron imaging was used in this study. Dislocation structure fatigue research was conducted on many copper materials in the literature. Hence, OFHC with an ultralarge grain was used in this work. The fatigue is completed using the Instron 8801 hydraulic fatigue test machine. After fatigue was instituted the microstructures were investigated using BEI of SEM and TEM. The results show that; (1) the S–N curve at reduced strain amplitude reveals softening after hardening at the initiation stage and up to fracture. No plateau area and secondary hardening occurred. (2) A special microstructure with mis-orientation cells embedded in a vein structure was observed in the ultra-large grain. (3) With small strain amplitude from high–low strain amplitude in the ultra-grain specimen, the fracture microstructure includes mis-oriented cells embedded in a loop patch structure. Under this load the micro-cracks are initiated at a small band with a mis-oriented cell structure. & 2012 Elsevier B.V. All rights reserved.
Keywords: Variable strain amplitude Dislocation structure Re-evolution Ultra-large grain
1. Introduction The copper normal dislocation evolution and microstructure characteristics in fatigued have been systematic investigated in the past decade [1–8]. It is well known that the dislocation development in low cycle fatigue for copper is a sequence of loop patches, a vein structure, persistent slip bands (PSBs), walls, cells and mis-oriented cells [9–13]. Based on the reported literature, the dislocation microstructure evolution is a step by step process from loop patches to cells at low plastic strain amplitude [14–16]. The dislocation structure easily forms dislocation cells at high plastic strain amplitude because high plastic strain amplitude induces a multiple slip system reaction [14,17]. The literature reports point out that this dislocation development is fast at the grain and twin boundaries owing to the strain localization [18–21]. At the same time the dislocation evolution in large size grain is more complete than that in small size grain [22–24]. The microstructure in front of the crack tip has seldom been reported in the literature. The dislocation structure under high crack propagation rate for copper is cell based [25,26]. In the last decade Huang et al. [27] indicated that regardless what the crack propagation rate is, the dislocation structure in front of the crack tips is cell
n
Corresponding author. Tel.: þ886 7 7466641; fax: þ886 7 7104697. E-mail address: [email protected] (S.W. Mao).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.08.067
based. It is remarkable that the crack propagation rate is retardate when the load condition changes from high to low. According to plasticity theory it is the plastic zone size that induces compressive stress. Note that the plastic strain energy is stored in dislocation structures in the microstructure during fatigue [14]. The dislocation morphology is variable when the load condition is changed during fatigue. Laird [28] pointed out that the dislocation structure changes from cells to loop patches when the load amplitude varies from high to low, however no experimental result has verified this. Huang et al.’s [29] result pointed out that the dislocation structure reevolution evolves from mis-orientation cells into loop patches for polycrystalline copper when the strain amplitude changes from high to low during fatigue. However, Huang’s result used a transmission electron microscope (TEM) to investigate the microstructure development. The disadvantage in using TEM is that the region of observation is limited. The region of observation is narrowly confined by the TEM sample preparation. The Zauter Foundation [30] reported that the microstructure can be examined using a backscatter electron image (BEI) of the scanning electron microscope view (SEM). The advantage of BEI is the sample preparation is easy and the observation region is broad. Huang et al [31,32] successfully observed the dislocation structure of copper and interstitial free (IF) steel regardless of the strain amplitude using this technique. The dislocation structure evolution in ultra-large grain (average grain size of about 600 mm) in pure copper is very seldom reported in the literature. Hence, to realize the dislocation structure variation
H.L. Huang et al. / Materials Science & Engineering A 559 (2013) 170–177
in ultra-large grain subjected to high–low strain amplitude using the BEI of SEM technical advantage, polycrystalline OFHC with ultragrain in low cycle fatigue under reduced strain amplitude was completed in this work. The microstructures of after fatigue specimens were observed using BEI of SEM and TEM.
2. Experimental A polycrystalline OFHC rod (99.95%) was used for this study. The specimens were annealed at 800 1C for 4 h in a 10 5 Torr vacuum and then cooled in the furnace. The grain sizes of the specimens were about 650–700 mm, as shown in Fig. 1. The specimen preparation followed the ASTM E647 instruction for hour glass. Low cycle fatigue was completed using a computerized Instron 8801 hydraulic testing machine at R¼ 1 (strain ratio, R¼ emin/emax) at a frequency of about 1 Hz. At first, the low cycle fatigue is controlled at 0.3% (0.2%) total strain amplitude during fatigue. Under this condition, the specimen is deformed to 4000 (8000) cycles the strain amplitude is decreased 0.3% (0.2%) to 0.2% (0.1%). After reducing the strain amplitude, the low cycle fatigue is continued to a cycle which is design by this research. The specimen was removed from the fatigue machine. The experimental load conditions are shown in Table 1.
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To observe the dislocation morphologies the specimens that completed the low cycle fatigue testing were cut into 0.6 mm thick slices along the cross section. The slices were ground to a thickness of 0.1–0.5 mm using abrasive paper and then punched into disks 3 mm in diameter using a Gating puncher. The 3 mm disks were twin-jet polished using Struer D2 solution at 10 V and 10 1C to prepare copper foil for the electron microscope. The difference in specimens for the BEI of SEM and TEM is the etching time. The BEI of SEM specimens received short etching time to avoid penetration and finish the surface polish. The TEM specimen received longer etching time to penetration. The aim was to create a thin region on the specimen to provide a region for TEM observation. After specimen preparation, a Philip Qutan 200 SEM and Philip 200CM TEM were employed to investigate the microstructures in the variable load low cycle fatigue specimens.
3. Results After low cycle fatigue under variable load conditions, listed in Table 1, the stress vs. fatigue number of cycles for the 0.3% strain amplitude is shown in Fig. 2. The S–N curves for the 0.2% and 0.1% strain amplitude were similar to those for the 0.3% strain
90
Stress (Mpa)
80 70 A:0.3% fatigue to fracture 60
A
B:0.3% fatigue to 4000 cycles drop to 0.2% up to fracture
50
C:0.3% fatigue to 4000 cycles drop to 0.1% up to fracture
40 B 30
Fig. 1. The OFHC grain size of pure copper annealed in 10 5 Torr vacuum for 4 hours. The average grain size is about 600–800 mm.
1
10
100 1000 Number of cycles (N)
10000
C 100000
Fig. 2. The diagram reveals stress vs. number of fatigue cycle (S–N) curves at 0.3% constant strain amplitude and variable strain amplitude.
Table 1 The fatigue test data at different strain amplitudes. Sample
Maximum strain amplitude (%)
Reduce maximum strain amplitude (%)
Reduce Number of cycles after after cycles reducing maximum strain
Final cycles SEM
TEM
A B C D E F G H I J K L M N O P Q R
0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.2 0.2 0.1
No No 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 No No 0.1 0.1 0.1 0.1 No
– – 4000 4000 4000 4000 4000 4000 4000 4000 4000 – – 8000 8000 8000 8000 –
4000 12,700 5000 7000 9000 14,000 25,973 117,000 5000 9000 24,000 8000 39,000 9000 13,000 28,000 117,000 118,000
Yes Yes Yes Yes Yes Yes Yes No No No No Yes Yes No No No Yes Yes
– – 1000 3000 5000 10,000 21,973 – 1000 5000 20,000 – – 1000 5000 20,000 109,000 –
Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes
Fatigue
Yes
Yes Yes
Yes
Yes
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amplitude. These results exhibit that regardless the strain amplitude, the S–N curves show initiated hardening, followed by softening until fatigue fracture. No plateau area is shown in the S–N curves. After the load was reduced the S–N curves reveal softening continues until fracture regardless of the strain amplitude. Fig. 3 shows the dislocation structure is cell or mis-oriented cell at fatigue fracture at 0.3% strain amplitude. Similarly, the 0.2% and 0.1% amplitude also exhibited cells and mis-oriented cells. However, to identify the dislocation structures observed by BEI of SEM in advance, a sample with the same fatigue condition was examined by TEM. The result is shown in Fig. 4, which is the same
Fig. 5. The mis-orientation cells dislocation structure examined using BEI of SEM in specimen B (0.3% strain amplitude fatigue to 4 103 cycles).
Fig. 6. The mis-orientation cells structure examined using BEI of SEM in specimen L (0.2% strain amplitude fatigue to 8 103 cycles).
Fig. 3. The dislocation structure at fracture is a mis-oriented cells structure in (a) 0.3% strain amplitude (specimen A), (b) 0.2% strain amplitude (specimen M) (0.3% strain amplitude fatigue to 3 103 cycles) reveals loop patches and vein structure embedded in two parallel walls
as Fig. 3. Figs. 5 and 6 reveal that the dislocation structure at 4 103 cycles on 0.3% strain amplitude and 8 103 cycles on 0.2% strain amplitude. Based on Figs. 5 and 6 the dislocation structures are shown to have a mis-orientation cell and cell structure. Fig. 7 reveals that the dislocation structure fatigue to 1 103, 3 103, 5 103, and 1 104 cycles after the 0.3% strain amplitude fatigue to 4 103 cycles reduced to 0.2% strain amplitude. Fig. 8 shows the dislocation structure fatigue to 1 103, 5 103and 2 104 cycles after the 0.3% strain amplitude fatigue to 4 103 cycles dropped to 0.1% strain amplitude. Fig. 9 shows the microstructure fatigue to 1 103, 5 103, and 2 104 cycles after the 0.2% strain amplitude fatigue to 8 103 cycles reduced to 0.1% strain amplitude. Figs. 6–8 show the dislocation structures from misorientation cells, cells, walls, loop patches re-evolution regardless of the load condition reduction.
4. Discussions
Fig. 4. The mis-orientation cells structure examined by TEM in specimen A (0.3% strain amplitude fatigue to fracture).
Fig. 2 shows the stress vs. number of fatigue cycles for 0.3% strain amplitude. The S–N curves show initiated hardening and then softening until fatigue fracture, with no plateau area shown in the S–N curve. Similarly, the S–N curves for 0.2% and 0.1% strain amplitude are the same as the 0.3% strain amplitude. These results vary in comparison to that for a single crystal [33]. The saturation stress for ultra-large grain is larger than that for large grain [22–24]. This is consistent with the larger grain with higher fatigue saturation stress. At the same time, the S–N
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Fig. 7. The dislocation structure examined using BEI of SEM at 0.3% dropped to 0.2% strain amplitude after 0.3% fatigue to 4 103 cycles (a) 1 103 cycles (specimen C), (b) 3 103 cycles (specimen D), (c) 5 103 cycles (specimen E), (d) 1 104 cycles (specimen F).
Fig. 8. The dislocation structure examined using BEI of SEM at 0.3% dropped to 0.1% strain amplitude after 0.3% fatigue to 4 103 cycles (a) 1 103 cycles (specimen I), (b) high magnification of (a), (c) 5 103 cycles (specimen J), (d) 2 104 cycles (specimen K).
curve shows no secondary hardening effect. This result is different from that for small grain sizes under the same strain amplitude [15,34,35]. For 0.3% strain amplitude the specimen fatigue at 4 103 cycles and with the strain amplitude reduced to 0.2%. The microstructure is a mis-oriented cells structure. Based on Fig. 7 the dislocation structure in (a), (b), (c) and (d) is a sequence
that exhibits partial mis-oriented cells, loop patches, a mixture of walls, cells and mis-oriented cells and mis-orientation cell dislocation (specimen on C, D, E and F). This is evidence for dislocation re-development. The dislocation structure shown in Figs. 5 and 7(a) shows a slight difference, but the microstructure varying from Fig. 7(a) to (b) is conspicuous. In other words, the
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Fig. 9. The dislocation structure examined using BEI of SEM at 0.2% dropped to 0.1% strain amplitude after 0.2% fatigue to 8 103 cycles (a) 1 103 cycles (specimen N), (b) high magnification of (a), (c) 5 103 cycles (specimen O), (d) 2 104 cycles (specimen P).
Fig. 10. shows the high magnification of Fig. 7(c).
dislocation re-evolves from mis-orientation cells into a loop patch structure. This phenomenon is clearer following the fatigue cycles after reducing the strain amplitude, (specimen E). In Fig. 7(c) the dislocation structure shows apparent mis-orientation cells, cells, walls and mis-orientation cells apart from the grain boundary (Fig. 10 is, Fig. 7(c) high magnification). This result is easily observed in Fig. 9. According to the dislocation development and strain localization at the grain boundary [18–21], the dislocation structure region near the grain boundary is faster than the intra-grain and distant from the grain boundary. It reveals that the dislocation structure re-evolution reaches a balance between the microstructure and applied load and then forward to dislocation positive revolution. Using fatigue progression the dislocation evolves into mis-orientation (Figs. 7(d) and 10) when the specimen fatigue fractures after a reduction in the strain amplitude from 0.3% to 0.2%
Specimens I, J and K (Fig. 8) were fatigued to 1 103, 5 103 and 2 104 cycles after reducing the strain amplitude from 0.3% to 0.1% with fatigue to 4 103 cycles at 0.3% strain amplitude. Because the reduced strain amplitude from 0.3% to 0.1% is larger than that from 0.3% to 0.2%, the dislocation structure in specimen I (Fig. 8(a) and (b)) changed at fatigue 1 103 cycles. The dislocation structure changed into a positive evolution in specimen J (Fig. 8(c)) faster than in specimen E (Fig. 6(c)). The dislocation structure redevelopment occurred whether the reduced strain amplitude was large or small. The difference between Figs. 7 and 8 is the speed of re-evolution, balance attainment and the low energy dislocation structural development. Compared with Figs. 7 and 9 the dislocation structure in Fig. 7 is faster than that in Fig. 9 regardless of the dislocation reevolution (Figs. 7(b) and 9(c)) or positive development (Figs. 7(d) and 9(d)). This is because the plasticity strain accumulation in 0.3% reduction to 0.2% is greater than the 0.2% drop to 0.1% in spite of the magnitude of the reduced strain amplitude being equal. The dislocation structure mis-orientation cells are dominant at 0.2% strain amplitude at fatigue to 8 103 cycles (Fig. 6), but the dislocation structures are exhibited in the misorientation cells embedded in a veined structure shown in Figs. 9(a) and (b). Although Figs. 9(a) and (b) show fatigue to 1 103 cycles after reducing the strain amplitude from 0.2% to 0.1%, based on dislocation development or re-development, the mis-oriented cells in Fig. 9(a) and (b) are extended for the dislocation development or re-evolution from mis-orientation cells to cells. Fig. 9(b) shows the side of the mis-orientation cell strip revealing cells and short walls. The microstructure in Fig. 9(a) belongs to re-evolution status. This result is consistent with the load condition. In other words, the microstructure in Fig. 9(a) is created before reducing the strain amplitude. Based on the dislocation evolution, the dislocation structure should be misorientation cells embedded in cells or walls and not as shown in Fig. 9(a). To explain this microstructure, Fig. 2 is mentioned again.
H.L. Huang et al. / Materials Science & Engineering A 559 (2013) 170–177
The S–N curve reveals hardening at the initial fatigue stage followed by softening until fracture. No secondary hardening appears. Below the dislocation structure [36–38] a loop patches and veined structure is the hardening phase and the PSBs is the softening phase. The average grain size is ultra-large in this study. Ultra large grain provides the slip bands a long slip distance. At the same time, the ultra-grain fatigue saturation stress is higher resulting in a large slip band with the same operating direction. The S–N curve shows softening and induces the slip band (PSBs) to not have enough extenuation, creating multiple slip systems
175
(MPSBs). The dislocation evolution in the slip band passes for a long time during fatigue inducing strain localization that accelerates the microstructure development. Therefore, the misorientation cells form in a strip slip band (Fig. 9(a)). At the same time, the microstructures show in that the loop patch structure is embedded within a vein structure (Fig. 11(a)) and a vast region occupied by a loop patch structure near the grain boundary (mark A in Fig. 11(a)). It is well known that the dislocation evolution is a sequence from loop patches, vein structure, PSBs, walls and cells and the dislocation development is fast at the grain boundary due to strain localization [18–21]. The dislocation re-evolution should
Fig. 13. The dislocation structure at fracture is mis-orientation cells and cells structure at 0.1% strain amplitude (specimen R).
Fig. 11. The dislocation structure examined using BEI of SEM at 0.2% dropped to 0.1% strain amplitude 1 103 cycles after 0.2% strain amplitude fatigue to 8 103 cycles (a) loop patches embedded in veins structure (specimen N), (b) high magnification of (a).
Fig. 12. The loop patches embedded in mis-orientation cells structure examined using BEI of SEM at 0.2% dropped to 0.1% strain amplitude up to fracture after 0.2% fatigue to 8 103 cycles.
Fig. 14. The microstructure examined using BEI of SEM at 0.2% dropped to 0.1% strain amplitude up to fracture after 0.2% fatigue to 8 103 cycles (a) micro-cracks in low magnification (b) micro-cracks initiated at mis-orientation cells bands and surrounding the loop patches and walls structure.
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H.L. Huang et al. / Materials Science & Engineering A 559 (2013) 170–177
be in accordance with this rule. The loop patch structure area nearby the grain boundary exhibits residual wall debris in out off grain boundary (mark B in Fig. 11(b)). The loop patch structure approaches re-development from the vein structure. Fig. 9(c) reveals the loop patch structure and extrusion at the grain boundary under fatigue to 5 103 cycles after reducing the strain amplitude from 0.2% to 0.1%. The extrusion or intrusion results in slip band operation and accompanies the PSBs dislocation structure [39]. Therefore, this is apparent evidence for dislocation re-evolution in this load condition. Fig. 9(c) reveals the PSBs dislocation structure, which elucidates the dislocation toward positive development. Fig. 12 shows that the dislocation structure is alternated with loop patches and mis-orientation cells. As a result of the fatigue to fracture, the dislocation structure is a cell structure (Fig. 13, 0.1% strain amplitude, specimen R). The dislocation structure evolution after reducing the strain amplitude from 0.2% to 0.1% is limited by a small region. This is because it is affected by the previous dislocation structure and the reduced strain amplitude also induces retardation in the plasticity strain accumulation. Therefore, strain localization is easily produced in ultra-large grain. The dislocation structures shown in Fig. 12 are reasonable. The fatigue up to fracture occurs after reducing the strain amplitude from 0.2% to 0.1% under 8 103 cycles at 0.2% strain amplitude. The microstructures are revealed in Fig. 14. It shows micro-cracks in the grain (Fig. 14(a)). To identify the micro-cracks and microstructure in Fig. 14(a) (specimen Q), Fig. 14(b) shows high magnification. The micro-cracks embedded in the band are very clear and the microstructure in the band has a mixed walls and loop patches dislocation structure. Comparing Figs. 12 and 14, the difference is mis-orientation cells in Fig. 14. The micro-cracks are created by mis-orientation cells along the slip band direction. The cell structure with micro-cracks can still be
observed (Fig. 14(b), label in C). A similar structure (Fig. 15) was observed at fatigue to 5 103 cycles after reducing the strain amplitude from 0.3% to 0.2% under 4 103 cycles at 0.3% strain amplitude. The alternating slabs at 3–5 degrees miss angle are present.
5. Conclusions This study investigated OFHC with the ultra-grain variable fatigue test. The results reveal that the dislocation evolves from loop patches, veins, PSBs, walls, cells and then mis-orientation at the plastic strain accumulation during fatigue. The fatigue fracture that accompanies dislocation is a low energy cell structure regardless of the strain amplitude under constant strain amplitude. At the same time, the dislocation structures reveal re-evolution when the strain amplitude is subjected to a high–low variable. Special dislocation morphology was observed in ultra-larger grain OFHC. The conclusions are summarized as follows; 1. The S–N curves exhibit hardening at the first stage and then continue softening until fracture. No plateau and secondary hardening occur. This is due to the larger grain with the larger saturation stress which regulates the high saturation stress inducing a large band in the same direction. 2. In spite of the reduced strain amplitude range being the same, fatigue with higher initial strain amplitude occurs with fast dislocation structure re-development and re-attains balance between the microstructure and applied load. 3. Because these specimens have ultra-large grain and higher fatigue saturation stress, once the secondary slip system is triggered slip systems are created. At the same time, decreasing the strain amplitude induces insufficient plasticity strain energy to balance the microstructures created before the strain amplitude was reduced. Therefore, the large slip systems cannot extend but remain operating during fatigue progression. The results induce the dislocation structure evolving into mis-orientation cells in slip bands. A microstructure with a mis-orientation cell structure and bands with embedded veins is formed. 4. Compared with the constant small strain amplitude and high–low strain amplitude in the ultra-grain specimen, the fracture microstructure is different despite the final strain amplitude being identical. This is because the dislocation structure is affected by the previous microstructure formed by high strain amplitude. It is well known that dislocation structure re-evolution easily triggers positive development after re-attaining new balance between the microstructure and applied load. The low strain amplitude is insufficient to broaden the slip bands As a result the microstructure exhibits mis-oriented cells embedded in a loop patch structure. Under this load (with small strain amplitude in the final stage at variable strain amplitude fatigue) micro-cracks are initiated at a small band with mis-oriented cells.
Acknowledgments The authors would like to acknowledge the financial support of National Science Council of ROC through Contract NSC99-2221E-145-002. References
Fig. 15. The microstructure of (a) and (b) examined using BEI of SEM at 0.3% dropped to 0.2% strain amplitude at 4 103 cycles after 0.2% fatigue to 5 103 cycles. It reveals miss angle is 3–5 degrees between (a) and (b).
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Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
The strain amplitude controlled fatigue behavior of pure copper with ultra large grain size H.L. Huang a, S.W. Mao a,n, D. Gan b, N.J. Ho b a b
Department of Mechanical Engineering, R.O.C. Military Academy, Kaohsiung, Taiwan, ROC Department of Materials and Optoelectronic Science, National Sun Yat-san University, Kaohsiung, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 June 2012 Received in revised form 6 August 2012 Accepted 13 August 2012 Available online 19 August 2012
The dislocation structural evolution in polycrystalline copper at constant strain amplitude during low cycle fatigue is well understood. However, the dislocation structural development at variable strain, which changes from high to low strain amplitude, has seldom been reported. Scanning electron microscope (SEM) back-scattering electron imaging was used in this study. Dislocation structure fatigue research was conducted on many copper materials in the literature. Hence, OFHC with an ultralarge grain was used in this work. The fatigue is completed using the Instron 8801 hydraulic fatigue test machine. After fatigue was instituted the microstructures were investigated using BEI of SEM and TEM. The results show that; (1) the S–N curve at reduced strain amplitude reveals softening after hardening at the initiation stage and up to fracture. No plateau area and secondary hardening occurred. (2) A special microstructure with mis-orientation cells embedded in a vein structure was observed in the ultra-large grain. (3) With small strain amplitude from high–low strain amplitude in the ultra-grain specimen, the fracture microstructure includes mis-oriented cells embedded in a loop patch structure. Under this load the micro-cracks are initiated at a small band with a mis-oriented cell structure. & 2012 Elsevier B.V. All rights reserved.
Keywords: Variable strain amplitude Dislocation structure Re-evolution Ultra-large grain
1. Introduction The copper normal dislocation evolution and microstructure characteristics in fatigued have been systematic investigated in the past decade [1–8]. It is well known that the dislocation development in low cycle fatigue for copper is a sequence of loop patches, a vein structure, persistent slip bands (PSBs), walls, cells and mis-oriented cells [9–13]. Based on the reported literature, the dislocation microstructure evolution is a step by step process from loop patches to cells at low plastic strain amplitude [14–16]. The dislocation structure easily forms dislocation cells at high plastic strain amplitude because high plastic strain amplitude induces a multiple slip system reaction [14,17]. The literature reports point out that this dislocation development is fast at the grain and twin boundaries owing to the strain localization [18–21]. At the same time the dislocation evolution in large size grain is more complete than that in small size grain [22–24]. The microstructure in front of the crack tip has seldom been reported in the literature. The dislocation structure under high crack propagation rate for copper is cell based [25,26]. In the last decade Huang et al. [27] indicated that regardless what the crack propagation rate is, the dislocation structure in front of the crack tips is cell
n
Corresponding author. Tel.: þ886 7 7466641; fax: þ886 7 7104697. E-mail address: [email protected] (S.W. Mao).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.08.067
based. It is remarkable that the crack propagation rate is retardate when the load condition changes from high to low. According to plasticity theory it is the plastic zone size that induces compressive stress. Note that the plastic strain energy is stored in dislocation structures in the microstructure during fatigue [14]. The dislocation morphology is variable when the load condition is changed during fatigue. Laird [28] pointed out that the dislocation structure changes from cells to loop patches when the load amplitude varies from high to low, however no experimental result has verified this. Huang et al.’s [29] result pointed out that the dislocation structure reevolution evolves from mis-orientation cells into loop patches for polycrystalline copper when the strain amplitude changes from high to low during fatigue. However, Huang’s result used a transmission electron microscope (TEM) to investigate the microstructure development. The disadvantage in using TEM is that the region of observation is limited. The region of observation is narrowly confined by the TEM sample preparation. The Zauter Foundation [30] reported that the microstructure can be examined using a backscatter electron image (BEI) of the scanning electron microscope view (SEM). The advantage of BEI is the sample preparation is easy and the observation region is broad. Huang et al [31,32] successfully observed the dislocation structure of copper and interstitial free (IF) steel regardless of the strain amplitude using this technique. The dislocation structure evolution in ultra-large grain (average grain size of about 600 mm) in pure copper is very seldom reported in the literature. Hence, to realize the dislocation structure variation
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in ultra-large grain subjected to high–low strain amplitude using the BEI of SEM technical advantage, polycrystalline OFHC with ultragrain in low cycle fatigue under reduced strain amplitude was completed in this work. The microstructures of after fatigue specimens were observed using BEI of SEM and TEM.
2. Experimental A polycrystalline OFHC rod (99.95%) was used for this study. The specimens were annealed at 800 1C for 4 h in a 10 5 Torr vacuum and then cooled in the furnace. The grain sizes of the specimens were about 650–700 mm, as shown in Fig. 1. The specimen preparation followed the ASTM E647 instruction for hour glass. Low cycle fatigue was completed using a computerized Instron 8801 hydraulic testing machine at R¼ 1 (strain ratio, R¼ emin/emax) at a frequency of about 1 Hz. At first, the low cycle fatigue is controlled at 0.3% (0.2%) total strain amplitude during fatigue. Under this condition, the specimen is deformed to 4000 (8000) cycles the strain amplitude is decreased 0.3% (0.2%) to 0.2% (0.1%). After reducing the strain amplitude, the low cycle fatigue is continued to a cycle which is design by this research. The specimen was removed from the fatigue machine. The experimental load conditions are shown in Table 1.
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To observe the dislocation morphologies the specimens that completed the low cycle fatigue testing were cut into 0.6 mm thick slices along the cross section. The slices were ground to a thickness of 0.1–0.5 mm using abrasive paper and then punched into disks 3 mm in diameter using a Gating puncher. The 3 mm disks were twin-jet polished using Struer D2 solution at 10 V and 10 1C to prepare copper foil for the electron microscope. The difference in specimens for the BEI of SEM and TEM is the etching time. The BEI of SEM specimens received short etching time to avoid penetration and finish the surface polish. The TEM specimen received longer etching time to penetration. The aim was to create a thin region on the specimen to provide a region for TEM observation. After specimen preparation, a Philip Qutan 200 SEM and Philip 200CM TEM were employed to investigate the microstructures in the variable load low cycle fatigue specimens.
3. Results After low cycle fatigue under variable load conditions, listed in Table 1, the stress vs. fatigue number of cycles for the 0.3% strain amplitude is shown in Fig. 2. The S–N curves for the 0.2% and 0.1% strain amplitude were similar to those for the 0.3% strain
90
Stress (Mpa)
80 70 A:0.3% fatigue to fracture 60
A
B:0.3% fatigue to 4000 cycles drop to 0.2% up to fracture
50
C:0.3% fatigue to 4000 cycles drop to 0.1% up to fracture
40 B 30
Fig. 1. The OFHC grain size of pure copper annealed in 10 5 Torr vacuum for 4 hours. The average grain size is about 600–800 mm.
1
10
100 1000 Number of cycles (N)
10000
C 100000
Fig. 2. The diagram reveals stress vs. number of fatigue cycle (S–N) curves at 0.3% constant strain amplitude and variable strain amplitude.
Table 1 The fatigue test data at different strain amplitudes. Sample
Maximum strain amplitude (%)
Reduce maximum strain amplitude (%)
Reduce Number of cycles after after cycles reducing maximum strain
Final cycles SEM
TEM
A B C D E F G H I J K L M N O P Q R
0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.2 0.2 0.1
No No 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.1 0.1 No No 0.1 0.1 0.1 0.1 No
– – 4000 4000 4000 4000 4000 4000 4000 4000 4000 – – 8000 8000 8000 8000 –
4000 12,700 5000 7000 9000 14,000 25,973 117,000 5000 9000 24,000 8000 39,000 9000 13,000 28,000 117,000 118,000
Yes Yes Yes Yes Yes Yes Yes No No No No Yes Yes No No No Yes Yes
– – 1000 3000 5000 10,000 21,973 – 1000 5000 20,000 – – 1000 5000 20,000 109,000 –
Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes
Fatigue
Yes
Yes Yes
Yes
Yes
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amplitude. These results exhibit that regardless the strain amplitude, the S–N curves show initiated hardening, followed by softening until fatigue fracture. No plateau area is shown in the S–N curves. After the load was reduced the S–N curves reveal softening continues until fracture regardless of the strain amplitude. Fig. 3 shows the dislocation structure is cell or mis-oriented cell at fatigue fracture at 0.3% strain amplitude. Similarly, the 0.2% and 0.1% amplitude also exhibited cells and mis-oriented cells. However, to identify the dislocation structures observed by BEI of SEM in advance, a sample with the same fatigue condition was examined by TEM. The result is shown in Fig. 4, which is the same
Fig. 5. The mis-orientation cells dislocation structure examined using BEI of SEM in specimen B (0.3% strain amplitude fatigue to 4 103 cycles).
Fig. 6. The mis-orientation cells structure examined using BEI of SEM in specimen L (0.2% strain amplitude fatigue to 8 103 cycles).
Fig. 3. The dislocation structure at fracture is a mis-oriented cells structure in (a) 0.3% strain amplitude (specimen A), (b) 0.2% strain amplitude (specimen M) (0.3% strain amplitude fatigue to 3 103 cycles) reveals loop patches and vein structure embedded in two parallel walls
as Fig. 3. Figs. 5 and 6 reveal that the dislocation structure at 4 103 cycles on 0.3% strain amplitude and 8 103 cycles on 0.2% strain amplitude. Based on Figs. 5 and 6 the dislocation structures are shown to have a mis-orientation cell and cell structure. Fig. 7 reveals that the dislocation structure fatigue to 1 103, 3 103, 5 103, and 1 104 cycles after the 0.3% strain amplitude fatigue to 4 103 cycles reduced to 0.2% strain amplitude. Fig. 8 shows the dislocation structure fatigue to 1 103, 5 103and 2 104 cycles after the 0.3% strain amplitude fatigue to 4 103 cycles dropped to 0.1% strain amplitude. Fig. 9 shows the microstructure fatigue to 1 103, 5 103, and 2 104 cycles after the 0.2% strain amplitude fatigue to 8 103 cycles reduced to 0.1% strain amplitude. Figs. 6–8 show the dislocation structures from misorientation cells, cells, walls, loop patches re-evolution regardless of the load condition reduction.
4. Discussions
Fig. 4. The mis-orientation cells structure examined by TEM in specimen A (0.3% strain amplitude fatigue to fracture).
Fig. 2 shows the stress vs. number of fatigue cycles for 0.3% strain amplitude. The S–N curves show initiated hardening and then softening until fatigue fracture, with no plateau area shown in the S–N curve. Similarly, the S–N curves for 0.2% and 0.1% strain amplitude are the same as the 0.3% strain amplitude. These results vary in comparison to that for a single crystal [33]. The saturation stress for ultra-large grain is larger than that for large grain [22–24]. This is consistent with the larger grain with higher fatigue saturation stress. At the same time, the S–N
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Fig. 7. The dislocation structure examined using BEI of SEM at 0.3% dropped to 0.2% strain amplitude after 0.3% fatigue to 4 103 cycles (a) 1 103 cycles (specimen C), (b) 3 103 cycles (specimen D), (c) 5 103 cycles (specimen E), (d) 1 104 cycles (specimen F).
Fig. 8. The dislocation structure examined using BEI of SEM at 0.3% dropped to 0.1% strain amplitude after 0.3% fatigue to 4 103 cycles (a) 1 103 cycles (specimen I), (b) high magnification of (a), (c) 5 103 cycles (specimen J), (d) 2 104 cycles (specimen K).
curve shows no secondary hardening effect. This result is different from that for small grain sizes under the same strain amplitude [15,34,35]. For 0.3% strain amplitude the specimen fatigue at 4 103 cycles and with the strain amplitude reduced to 0.2%. The microstructure is a mis-oriented cells structure. Based on Fig. 7 the dislocation structure in (a), (b), (c) and (d) is a sequence
that exhibits partial mis-oriented cells, loop patches, a mixture of walls, cells and mis-oriented cells and mis-orientation cell dislocation (specimen on C, D, E and F). This is evidence for dislocation re-development. The dislocation structure shown in Figs. 5 and 7(a) shows a slight difference, but the microstructure varying from Fig. 7(a) to (b) is conspicuous. In other words, the
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Fig. 9. The dislocation structure examined using BEI of SEM at 0.2% dropped to 0.1% strain amplitude after 0.2% fatigue to 8 103 cycles (a) 1 103 cycles (specimen N), (b) high magnification of (a), (c) 5 103 cycles (specimen O), (d) 2 104 cycles (specimen P).
Fig. 10. shows the high magnification of Fig. 7(c).
dislocation re-evolves from mis-orientation cells into a loop patch structure. This phenomenon is clearer following the fatigue cycles after reducing the strain amplitude, (specimen E). In Fig. 7(c) the dislocation structure shows apparent mis-orientation cells, cells, walls and mis-orientation cells apart from the grain boundary (Fig. 10 is, Fig. 7(c) high magnification). This result is easily observed in Fig. 9. According to the dislocation development and strain localization at the grain boundary [18–21], the dislocation structure region near the grain boundary is faster than the intra-grain and distant from the grain boundary. It reveals that the dislocation structure re-evolution reaches a balance between the microstructure and applied load and then forward to dislocation positive revolution. Using fatigue progression the dislocation evolves into mis-orientation (Figs. 7(d) and 10) when the specimen fatigue fractures after a reduction in the strain amplitude from 0.3% to 0.2%
Specimens I, J and K (Fig. 8) were fatigued to 1 103, 5 103 and 2 104 cycles after reducing the strain amplitude from 0.3% to 0.1% with fatigue to 4 103 cycles at 0.3% strain amplitude. Because the reduced strain amplitude from 0.3% to 0.1% is larger than that from 0.3% to 0.2%, the dislocation structure in specimen I (Fig. 8(a) and (b)) changed at fatigue 1 103 cycles. The dislocation structure changed into a positive evolution in specimen J (Fig. 8(c)) faster than in specimen E (Fig. 6(c)). The dislocation structure redevelopment occurred whether the reduced strain amplitude was large or small. The difference between Figs. 7 and 8 is the speed of re-evolution, balance attainment and the low energy dislocation structural development. Compared with Figs. 7 and 9 the dislocation structure in Fig. 7 is faster than that in Fig. 9 regardless of the dislocation reevolution (Figs. 7(b) and 9(c)) or positive development (Figs. 7(d) and 9(d)). This is because the plasticity strain accumulation in 0.3% reduction to 0.2% is greater than the 0.2% drop to 0.1% in spite of the magnitude of the reduced strain amplitude being equal. The dislocation structure mis-orientation cells are dominant at 0.2% strain amplitude at fatigue to 8 103 cycles (Fig. 6), but the dislocation structures are exhibited in the misorientation cells embedded in a veined structure shown in Figs. 9(a) and (b). Although Figs. 9(a) and (b) show fatigue to 1 103 cycles after reducing the strain amplitude from 0.2% to 0.1%, based on dislocation development or re-development, the mis-oriented cells in Fig. 9(a) and (b) are extended for the dislocation development or re-evolution from mis-orientation cells to cells. Fig. 9(b) shows the side of the mis-orientation cell strip revealing cells and short walls. The microstructure in Fig. 9(a) belongs to re-evolution status. This result is consistent with the load condition. In other words, the microstructure in Fig. 9(a) is created before reducing the strain amplitude. Based on the dislocation evolution, the dislocation structure should be misorientation cells embedded in cells or walls and not as shown in Fig. 9(a). To explain this microstructure, Fig. 2 is mentioned again.
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The S–N curve reveals hardening at the initial fatigue stage followed by softening until fracture. No secondary hardening appears. Below the dislocation structure [36–38] a loop patches and veined structure is the hardening phase and the PSBs is the softening phase. The average grain size is ultra-large in this study. Ultra large grain provides the slip bands a long slip distance. At the same time, the ultra-grain fatigue saturation stress is higher resulting in a large slip band with the same operating direction. The S–N curve shows softening and induces the slip band (PSBs) to not have enough extenuation, creating multiple slip systems
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(MPSBs). The dislocation evolution in the slip band passes for a long time during fatigue inducing strain localization that accelerates the microstructure development. Therefore, the misorientation cells form in a strip slip band (Fig. 9(a)). At the same time, the microstructures show in that the loop patch structure is embedded within a vein structure (Fig. 11(a)) and a vast region occupied by a loop patch structure near the grain boundary (mark A in Fig. 11(a)). It is well known that the dislocation evolution is a sequence from loop patches, vein structure, PSBs, walls and cells and the dislocation development is fast at the grain boundary due to strain localization [18–21]. The dislocation re-evolution should
Fig. 13. The dislocation structure at fracture is mis-orientation cells and cells structure at 0.1% strain amplitude (specimen R).
Fig. 11. The dislocation structure examined using BEI of SEM at 0.2% dropped to 0.1% strain amplitude 1 103 cycles after 0.2% strain amplitude fatigue to 8 103 cycles (a) loop patches embedded in veins structure (specimen N), (b) high magnification of (a).
Fig. 12. The loop patches embedded in mis-orientation cells structure examined using BEI of SEM at 0.2% dropped to 0.1% strain amplitude up to fracture after 0.2% fatigue to 8 103 cycles.
Fig. 14. The microstructure examined using BEI of SEM at 0.2% dropped to 0.1% strain amplitude up to fracture after 0.2% fatigue to 8 103 cycles (a) micro-cracks in low magnification (b) micro-cracks initiated at mis-orientation cells bands and surrounding the loop patches and walls structure.
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be in accordance with this rule. The loop patch structure area nearby the grain boundary exhibits residual wall debris in out off grain boundary (mark B in Fig. 11(b)). The loop patch structure approaches re-development from the vein structure. Fig. 9(c) reveals the loop patch structure and extrusion at the grain boundary under fatigue to 5 103 cycles after reducing the strain amplitude from 0.2% to 0.1%. The extrusion or intrusion results in slip band operation and accompanies the PSBs dislocation structure [39]. Therefore, this is apparent evidence for dislocation re-evolution in this load condition. Fig. 9(c) reveals the PSBs dislocation structure, which elucidates the dislocation toward positive development. Fig. 12 shows that the dislocation structure is alternated with loop patches and mis-orientation cells. As a result of the fatigue to fracture, the dislocation structure is a cell structure (Fig. 13, 0.1% strain amplitude, specimen R). The dislocation structure evolution after reducing the strain amplitude from 0.2% to 0.1% is limited by a small region. This is because it is affected by the previous dislocation structure and the reduced strain amplitude also induces retardation in the plasticity strain accumulation. Therefore, strain localization is easily produced in ultra-large grain. The dislocation structures shown in Fig. 12 are reasonable. The fatigue up to fracture occurs after reducing the strain amplitude from 0.2% to 0.1% under 8 103 cycles at 0.2% strain amplitude. The microstructures are revealed in Fig. 14. It shows micro-cracks in the grain (Fig. 14(a)). To identify the micro-cracks and microstructure in Fig. 14(a) (specimen Q), Fig. 14(b) shows high magnification. The micro-cracks embedded in the band are very clear and the microstructure in the band has a mixed walls and loop patches dislocation structure. Comparing Figs. 12 and 14, the difference is mis-orientation cells in Fig. 14. The micro-cracks are created by mis-orientation cells along the slip band direction. The cell structure with micro-cracks can still be
observed (Fig. 14(b), label in C). A similar structure (Fig. 15) was observed at fatigue to 5 103 cycles after reducing the strain amplitude from 0.3% to 0.2% under 4 103 cycles at 0.3% strain amplitude. The alternating slabs at 3–5 degrees miss angle are present.
5. Conclusions This study investigated OFHC with the ultra-grain variable fatigue test. The results reveal that the dislocation evolves from loop patches, veins, PSBs, walls, cells and then mis-orientation at the plastic strain accumulation during fatigue. The fatigue fracture that accompanies dislocation is a low energy cell structure regardless of the strain amplitude under constant strain amplitude. At the same time, the dislocation structures reveal re-evolution when the strain amplitude is subjected to a high–low variable. Special dislocation morphology was observed in ultra-larger grain OFHC. The conclusions are summarized as follows; 1. The S–N curves exhibit hardening at the first stage and then continue softening until fracture. No plateau and secondary hardening occur. This is due to the larger grain with the larger saturation stress which regulates the high saturation stress inducing a large band in the same direction. 2. In spite of the reduced strain amplitude range being the same, fatigue with higher initial strain amplitude occurs with fast dislocation structure re-development and re-attains balance between the microstructure and applied load. 3. Because these specimens have ultra-large grain and higher fatigue saturation stress, once the secondary slip system is triggered slip systems are created. At the same time, decreasing the strain amplitude induces insufficient plasticity strain energy to balance the microstructures created before the strain amplitude was reduced. Therefore, the large slip systems cannot extend but remain operating during fatigue progression. The results induce the dislocation structure evolving into mis-orientation cells in slip bands. A microstructure with a mis-orientation cell structure and bands with embedded veins is formed. 4. Compared with the constant small strain amplitude and high–low strain amplitude in the ultra-grain specimen, the fracture microstructure is different despite the final strain amplitude being identical. This is because the dislocation structure is affected by the previous microstructure formed by high strain amplitude. It is well known that dislocation structure re-evolution easily triggers positive development after re-attaining new balance between the microstructure and applied load. The low strain amplitude is insufficient to broaden the slip bands As a result the microstructure exhibits mis-oriented cells embedded in a loop patch structure. Under this load (with small strain amplitude in the final stage at variable strain amplitude fatigue) micro-cracks are initiated at a small band with mis-oriented cells.
Acknowledgments The authors would like to acknowledge the financial support of National Science Council of ROC through Contract NSC99-2221E-145-002. References
Fig. 15. The microstructure of (a) and (b) examined using BEI of SEM at 0.3% dropped to 0.2% strain amplitude at 4 103 cycles after 0.2% fatigue to 5 103 cycles. It reveals miss angle is 3–5 degrees between (a) and (b).
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