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Low-cycle Fatigue Behaviors of an As-extruded Mg–12%Gd–3%Y–0.5%Zr Alloy
Available online at SciVerse ScienceDirect
J. Mater. Sci. Technol., 2013, 29(8), 775e780
Low-cycle Fatigue Behaviors of an As-extruded Mge12%Gde3%Ye0.5%Zr Alloy S.M. Yin1)*, S.X. Li2) 1) Shenyang University of Chemical Technology, Shenyang 110142, China 2) Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China [Manuscript received April 11, 2012, in revised form July 2, 2012, Available online 19 April 2013]
Cyclic deformation and fatigue behaviors of Mge12%Gde3%Ye0.5%Zr (wt%, GW123K) alloy were investigated at room temperature under axial cyclic loading in strain controlled condition. It is shown that conventional extruded GW123K alloy maintained cyclic stability at strain amplitudes ranging from 2 103 to 102. The pronounced symmetric hysteresis loops were also observed during cyclic loading. Fracture surface observations indicated that fatigue cracks mainly initiated at large Gd-riched phase or at inclusion clusters at surface or subsurface, and grain boundary (GB) and slip bands (SBs) are also preferential sites for micro-crack incubation. KEY WORDS: Magnesium alloy; Low-cycle fatigue; Cyclic stabilization; Fatigue crack; Fractography
1. Introduction High performance magnesium alloys containing yttrium or heavy rare earth (HRE) elements have attracted increasing interest in recent years for potential applications in the aerospace, aircraft and automotive industries because of their high specific strength and good thermal stability[1e7]. It has been reported[2,4] that the recently developed MgeGdeY alloys exhibit higher specific strength at both room and elevated temperatures and better creep resistance than conventional Al and Mg alloys, including WE54 (Mge5Ye2Nde2HRE, wt%), whose hightemperature strength is at the top of existing commercial magnesium alloys. As a new series of structural materials for applications to load-bearing components, knowledge about fatigue is necessary for the structural design. However, the studies on fatigue behaviors of magnesium alloy are still incomplete, covering only specific topics for limited type of Mg alloys. Most of previous studies are about the fatigue behaviors of MgeAleZn[8e12] and MgeAleMn[13e16] alloys, and few report about the low-cycle fatigue behavior of the wrought MgeGdeYeZr alloys was found. Moreover, most researches have mainly conducted high-cycle fatigue or stress * Corresponding author. Ph.D.; Tel.: þ86 24 89381051; E-mail address: [email protected] (S.M. Yin). 1005-0302/$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. http://dx.doi.org/10.1016/j.jmst.2013.04.011
controlled fatigue experiments[10e13,17]. Researches on lowcycle fatigue properties under strain controlled conditions are still lacking. Suggested in the report[17], further information is required on cyclic stressestrain relation for a better understanding on the fatigue behaviors of magnesium alloys. In the present study, fatigue behavior of an extruded Mge12% Gde3%Ye0.5%Zr (wt%, GW123K) alloy was examined under axial cyclic loading by strain controlled condition. The fatigue damage mechanism as well as the characteristic fracture morphology was also investigated by scanning electron microscopy (SEM) in detail. 2. Experimental The material used in the present study was extruded GW123K rod of a diameter of 20 mm. It was extruded in two passes at temperature of 673 K with extrusion ratio of 25:1. The conventional mechanical properties of the material are shown in Table 1. The material was characterized with higher strength and lower elongation especially in tension. Specimens for fatigue test with gauge section of 6 mm 5.5 mm 12 mm were cut along the extruded bar. Samples for fatigue tests and for texture measurement as well as for metallographic observation were treated by mechanical polishing with silicon carbide abrasive papers, followed by electro-polishing by using a solution comprised of 40 ml HClO4 and 360 ml ethanol at temperature of 273 K in order to eliminate wear tracks as well as residual stress of the surface layer. Specimens for optical microscopy (OM) were etched in acetic picral (10 ml acetic acid þ 4.2 g picric
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Table 1 Tensile and compressive properties of GW123K Loading mode
s0.2 (MPa)
sb (MPa)
d (%)
E (GPa)
Tension Compression
248 244
294 545
2.5 16
46.4
acid þ 10 ml water þ 70 ml ethanol). The fatigue tests were performed on a servo-hydraulic testing system (Instron 8871) at ambient temperature in air under total strain amplitude control. The strain amplitudes ranging from 1.5 103 to 1.0 102 with a frequency of 0.5 Hz and S ¼ 1 (S refers to strain ratio) were applied. Subsequently, sample surface and fracture morphology were observed by scanning electronic microscopy (SEM, Quanta 600). The chemical composition at crack initiation sites was determined with energy-dispersive X-ray (EDX) analysis. 3. Results 3.1. Microstructure and texture The microstructure of this alloy is shown in Fig. 1, which consists of Mg solid solution matrix and the precipitated phase Mg5X, where X: Gd þ Y. The average grain size is about 10 mm and basically free of twin in grains before loading. Unlike most of the conventional extruded magnesium alloys that have strong ring texture[18e20], the present material has a weak {0002} texture as shown in Fig. 2. The number of grains with their basal plane parallel to ED is slightly larger than that of other orientation. 3.2. Cyclic stress response The variation of stress with the number of cycles, N, is an important feature of the low-cycle fatigue process. The cyclic stress response of a material during fatigue loading represents the locus of the stress amplitude with successive cycles, and illustrates the path by which the material achieves to its final level of stress during the total strain amplitude controlled low-cycle fatigue test. The cyclic stress response curves for the extruded GW123K alloy at various total strain amplitudes are depicted in Fig. 3. It is obvious that the cyclic stress response of the alloy is rather stable to the imposed different total strain amplitudes, particularly at low strain amplitude of 0.2%. Through close examination, slight cyclic softening can be observed at tension half cycle for all strain amplitudes tested, and slight cyclic softening
at early cycles and then slight cyclic hardening afterward can be observed at compression half cycle for all strain amplitudes tested. On the whole, over the entire range of total strain amplitudes applied in the present investigation, the variation of stress amplitude is so mildly that we could say the cyclic stabilization dominates the whole cycling period for the alloy. 3.3. Hysteresis loop The analysis of the stressestrain hysteresis loops can give a better understanding on the cyclic deformation behavior of the extruded GW123K alloy under low-cycle fatigue condition. Fig. 4 illustrates the stressestrain hysteresis loops of the extruded GW123K alloy at different total strain amplitudes, which were obtained at half-life. Compared with the hysteresis loops of extruded AZ31[8] or AZ80[21], the extruded GW123K alloy exhibits symmetric hysteresis loops and there is no signature of twinning occurring. This makes a big difference from that of extruded AZ31 or ZK60, which have pronounced asymmetric deformation behavior in tension and compression under total strain amplitude controlled cycling[9,22]. The aforementioned asymmetric deformation behavior mainly results from two factors: one is that the fraction of grains favoring tension twinning in compression is significantly higher than that of grains favoring tension twinning in tension and basal slip is in hard orientation in tension while twining is in soft orientation in compression due to the strong texture; the other is that the significant difference between the stresses needed to initiate twinning in compression and the stress needed to activate slip in tension since basal slip is in hard orientation but twinning is in soft orientation. In present study, the symmetric hysteresis loop might be due to the weak texture which results in only slightly higher fraction of the grains favored for tension twining in tension than that of the grains favoring tension twining in compression. Moreover, the weak texture favored basal slip compared with strong textured magnesium alloys[8,9,21e23]. The cyclic stressestrain response is another important material property in designing for enhanced fatigue resistance. It describes the relationship between flow stress and plastic strain amplitude under cyclic loading, and is a useful aid in understanding strain controlled cyclic deformation behavior. The information can be obtained from hysteresis loop. In present study, the fatigue strength coefficient K0 , is about 986.7 MPa, and the cyclic strain hardening exponent n’ is about 0.2247 as shown in Fig. 4(b). 3.4. Fatigue life The relation between strain amplitude and fatigue life can be described with CoffineManson and Basquin relations as shown in Fig. 5(a), and the corresponding data of wrought magnesium alloy AM50[16], AZ31[24] as well as an AleMgeSc alloy[25] were also presented which shows that the high strength magnesium alloy GW123K exhibits decent low-cycle fatigue properties as shown in Fig. 5(b). 3.5. Fracture morphology
Fig. 1 Microstructures of as-received GW123K alloy (Black particles are Gd-riched or Y-riched inter-metallic compound).
3.5.1. Typical fatigue crack propagation (FCP) morphology. Figs. 6 and 7 show the typical crack propagation morphology of extruded GW123K alloy. The macro-scale fracture plane is rather smooth and loading direction (LD) is normal to the fracture surface. The overall fracture image shows the crack initiation and three fatigue crack propagation regions: i.e. low FCP
S.M. Yin and S.X. Li: J. Mater. Sci. Technol., 2013, 29(8), 775e780
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Fig. 2 Pole figures of extruded GW123K alloy.
rate region (region I), regions of FCP with moderate rate (region II) and higher FCP rate region (region III). Fig. 6(b)e(d) present the high magnification SEM images of regions I, II and III, respectively. In region I, the fracture surface is characterized by profuse cleavage planes with steps. Its formation is due to the dislocation slip on preferential crystallographic plane back and forth under cyclic loading which induced micro-crack incubation along slip bands within a grain at lower maximum stress intense factor (Kmax); the typical feature of region II is rather rough fracture surface composed of many semi-cleavage planes which is due to the moderate Kmax at crack tip; there are also some shallow dimple structures observed in this region; while region III (at large Kmax) is characterized by many dimples with particles in it as shown in Fig. 6(d). It is mainly resulted from the void nucleation, growth, and coalescence, either by particle fracture or by particle matrix interface de-bonding to form the dimple rupture at large stress intensity factor. Fracture surfaces of other samples have also been investigated and it is found that the area fraction of cleavage decreases significantly with increasing strain amplitude but the morphologies of region III with different strain amplitudes are similar. It indicates that strain amplitude and Kmax have some influence on the fracture modes of the material. Since the ductility of present material is much lower than that of conventional extruded AZ31 and AM60, the edge of the dimple is thick and the depth of the dimple is shallow.
shows that large Gd-riched phases (Fig. 8(a) and (b)) or large inclusions which are composed of MgO, CaO and some sulphide (Fig. 8(c) and (d)) favoring fatigue crack incubation. It can be confirmed that large Gd-riched phase and large inclusion are main crack incubation sites and they reduce fatigue life of the material drastically compared with the samples free of large inclusions and Gd-riched phases. 3.5.3. Surface observation. Sample surface observations after fatigue test are shown in Fig. 9. High density of slip bands (SB) confined to grain inside and cracks initiated at grain boundary (GB) or at the slip bands were observed. During fatigue, slip bands formed and served as strain localized regions which resulted in the formation of extrusions and intrusions and the potential micro-crack initiation in the following cycles. These micro-cracks grew along SB and coalescence resulted in large transgranular cleavage planes which could be observed on the fracture surface. While the cracks along GB were caused by high
3.5.2. Fatigue crack initiation. The morphologies and corresponding chemical compositions of typical samples with different crack initiation mechanisms are shown in Fig. 8. It reveals that the fatigue cracks basically initiate at the surface or subsurface. The corresponding chemical compositions of the crack initiation sites were investigated with SEM-EDX. It clearly
Fig. 3 Cyclic stress response of GW123K alloy at different total strain amplitudes.
Fig. 4 (a) Hysteresis loop curves at different total strain amplitudes, (b) log (stress amplitude) vs log (plastic strain amplitude) based on the hysteresis loop curves.
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Fig. 5 (a) Strain amplitude vs number of reversal to failure curves for extruded GW123K alloy, (b) fatigue life vs applied strain amplitude for conventional light metals.
local stress at GB. These surface cracks could propagate either intergranularly or intragranularly depending on the local microstructure and local stress. 4. Discussion 4.1. Cyclic deformation behavior This material exhibits cyclic stability mainly due to three factors as follows. Firstly, micro-plastic deformation is dominated by basal slip since the material has nearly random texture;
the basal slip is preferred to be activated due to its lower critical shear stress compared with prismatic slip as well as twinning. SB can only be found within grains and no large SB crossing the sample surface can be found. The chance of interaction of slip decreases drastically, thus the dislocation hardening effect is very slight. Secondly, once the initial dislocation reaches GB and is absorbed by GB, other new dislocations will operate and slip on basal plane in the same grain will introduce another SB which parallels to the initial one. With increasing dislocation density, the subsequent basal slip will be subjected to the resistance of second phase, which results in the increase of stress for plastic
Fig. 6 SEM images of the fracture surface of sample fatigued at total strain amplitude of 0.003: (a) overall fracture surface; (b)e(d) higher magnification of crack initiation and lower FCP rate region, moderate FCP rate and higher FCP rate regions, respectively.
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Fig. 7 SEM images of the fracture surface of sample fatigued at total strain amplitude of 0.004: (a) overall fracture surface; (b)e(d) higher magnification of crack initiation and lower FCP rate region, moderate FCP rate and higher FCP rate regions, respectively.
deformation. In addition, when basal slip terminates in some soften oriented grains, a little large stress needs to activate basal slip in other grains with orientation deviated slightly from the favorable orientation. These factors mentioned above result in the slight cyclic hardening.
Finally, the main fatigue crack initiation and growth result in the bearing section of the sample decreasing correspondingly and slightly cyclic softening. The role of the third factor can be compensated by the hardening factors and the material exhibits cyclic stability.
Fig. 8 (a) and (c) Typical crack initiation sites, (b) and (d) corresponding EDX spectra of the sites.
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Fig. 9 SEM images of fatigued surface of sample.
4.2. Fatigue life The material exhibits sound fatigue life compared with the aluminum alloy and some magnesium alloys. But it is not the intrinsic fatigue life for the material. It is known that most of fatigue life is dominated by the process of the crack incubation, especially at lower strain amplitudes or lower stress amplitudes. In the present study, the large inclusion regions (consist of MgO and some sulfides) with dimension about 100 mm 100 mm were found, and it is much larger than grain. Besides, large Gdriched phase was also observed on sample surface. These large defects serve as fatigue crack initiation sites and result in the premature failure of the samples. If these large defects can be eliminated by improved smelting process, the material will exhibit much longer fatigue life since fatigue life of the material is very sensitive to those large defects. 5. Conclusion The conventional extruded GW123K magnesium alloy exhibited cyclic stability during fatigue. The main cracks preferentially initiated at large inclusion or at Gd-riched phase at surface or subsurface. Micro-crack could also incubate at slip bands or at grain boundary. The fracture morphology of the material was greatly influenced by maximum stress intensity factor Kmax: large transgranular cleavage dominates the regions at lower Kmax especially at lower amplitudes while ductile fracture characterized by shallow dimples dominates large Kmax regions and rupture regions. Acknowledgments This work was supported by the National Natural Science Foundation of China under Grant No. 50571102. The authors are grateful to W. Gao and H.J. Yang for assistance with the EBSD measurement. REFERENCES [1] C.M. Zhang, X. Hui, Z.G. Li, G.L. Chen, Mater. Lett. 62 (2008) 1129e1131.
[2] L.L. Rokhlin, Magnesium Alloys Containing Rear Earth Metals, Taylor and Francis, London, 2003. [3] S.M. He, X.Q. Zeng, L.M. Peng, X. Gao, J.F. Nie, W.J. Ding, J. Alloy. Compd. 427 (2007) 316e323. [4] I. Anthony, S. Kamado, Y. Kojima, Mater. Trans. 42 (2001) 1206e 1211. [5] J. Wang, J. Meng, D.P. Zhang, D.X. Tang, Mater. Sci. Eng. A 456 (2007) 78e84. [6] S.M. He, X.Q. Zeng, L.M. Peng, X. Gao, J.F. Nie, W.J. Ding, J. Alloy. Compd. 421 (2006) 309e313. [7] A.A. Luo, Int. Mater. Rev. 49 (2004) 13e30. [8] S.M. Yin, H.J. Yang, S.X. Li, S.D. Wu, F. Yang, Scripta Mater. 58 (2008) 751e754. [9] S. Hasegawa, Y. Tsuchida, H. Yano, M. Matsui, Int. J. Fatig. 29 (2007) 1839e1845. [10] F. Yang, S.M. Yin, S.X. Li, Z.F. Zhang, Mater. Sci. Eng. A 491 (2008) 131e136. [11] T.S. Shih, W.S. Liu, Y.J. Chen, Mater. Sci. Eng. A 325 (2002) 152e 162. [12] M.F. Horstemeyer, N. Yang, K. Gall, D.L. McDowell, J. Fan, P.M. Gullett, Acta Mater. 52 (2004) 1327e1336. [13] Y. Lu, F. Taheri, M. Gharghouri, J. Alloy. Compd. 466 (2008) 214e227. [14] L.J. Chen, C.Y. Wang, W. Wu, Z. Liu, G.M. Stoica, L. Wu, P.K. Liaw, Metall. Mater. Trans. A 38 (2007) 2235e2241. [15] K. Gall, G. Biallas, H.J. Maier, P. Gullett, M.F. Horstemeyer, D.L. Mcdowell, Metall. Mater. Trans. A 35 (2004) 321e331. [16] S.G. Lee, G.R. Patel, A.M. Gokhale, Scripta Mater. 52 (2005) 1063e1068. [17] V.V. Ogarevic, R.I. Stephens, Annu. Rev. Mater. Sci. 20 (1990) 141e177. [18] Y.N. Wang, J.C. Huang, Acta Mater. 55 (2007) 897e905. [19] S. Kleiner, P.J. Uggowitzer, Mater. Sci. Eng. A 379 (2004) 258e 263. [20] Y. Chino, K. Kimura, M. Hakamada, M. Mabuchi, Mater. Sci. Eng. A 485 (2008) 311e317. [21] H. Zenner, F. Renner, Int. J. Fatig. 24 (2002) 1255e1260. [22] M. Suzuki, T. Kimura, J. Koike, K. Maruyama, Scripta Mater. 48 (2003) 997e1002. [23] M.R. Barnett, Z. Keshavarz, A.G. Beer, D. Atwell, Acta Mater. 52 (2004) 5093e5103. [24] S. Begum, D.L. Chen, S. Xu, A.A. Luo, Int. J. Fatig. 31 (2009) 726e735. [25] A. Vinogradov, A. Washikita, K. Kitagawa, V.I. Kopylov, Mater. Sci. Eng. A 349 (2003) 318e326.
J. Mater. Sci. Technol., 2013, 29(8), 775e780
Low-cycle Fatigue Behaviors of an As-extruded Mge12%Gde3%Ye0.5%Zr Alloy S.M. Yin1)*, S.X. Li2) 1) Shenyang University of Chemical Technology, Shenyang 110142, China 2) Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China [Manuscript received April 11, 2012, in revised form July 2, 2012, Available online 19 April 2013]
Cyclic deformation and fatigue behaviors of Mge12%Gde3%Ye0.5%Zr (wt%, GW123K) alloy were investigated at room temperature under axial cyclic loading in strain controlled condition. It is shown that conventional extruded GW123K alloy maintained cyclic stability at strain amplitudes ranging from 2 103 to 102. The pronounced symmetric hysteresis loops were also observed during cyclic loading. Fracture surface observations indicated that fatigue cracks mainly initiated at large Gd-riched phase or at inclusion clusters at surface or subsurface, and grain boundary (GB) and slip bands (SBs) are also preferential sites for micro-crack incubation. KEY WORDS: Magnesium alloy; Low-cycle fatigue; Cyclic stabilization; Fatigue crack; Fractography
1. Introduction High performance magnesium alloys containing yttrium or heavy rare earth (HRE) elements have attracted increasing interest in recent years for potential applications in the aerospace, aircraft and automotive industries because of their high specific strength and good thermal stability[1e7]. It has been reported[2,4] that the recently developed MgeGdeY alloys exhibit higher specific strength at both room and elevated temperatures and better creep resistance than conventional Al and Mg alloys, including WE54 (Mge5Ye2Nde2HRE, wt%), whose hightemperature strength is at the top of existing commercial magnesium alloys. As a new series of structural materials for applications to load-bearing components, knowledge about fatigue is necessary for the structural design. However, the studies on fatigue behaviors of magnesium alloy are still incomplete, covering only specific topics for limited type of Mg alloys. Most of previous studies are about the fatigue behaviors of MgeAleZn[8e12] and MgeAleMn[13e16] alloys, and few report about the low-cycle fatigue behavior of the wrought MgeGdeYeZr alloys was found. Moreover, most researches have mainly conducted high-cycle fatigue or stress * Corresponding author. Ph.D.; Tel.: þ86 24 89381051; E-mail address: [email protected] (S.M. Yin). 1005-0302/$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. http://dx.doi.org/10.1016/j.jmst.2013.04.011
controlled fatigue experiments[10e13,17]. Researches on lowcycle fatigue properties under strain controlled conditions are still lacking. Suggested in the report[17], further information is required on cyclic stressestrain relation for a better understanding on the fatigue behaviors of magnesium alloys. In the present study, fatigue behavior of an extruded Mge12% Gde3%Ye0.5%Zr (wt%, GW123K) alloy was examined under axial cyclic loading by strain controlled condition. The fatigue damage mechanism as well as the characteristic fracture morphology was also investigated by scanning electron microscopy (SEM) in detail. 2. Experimental The material used in the present study was extruded GW123K rod of a diameter of 20 mm. It was extruded in two passes at temperature of 673 K with extrusion ratio of 25:1. The conventional mechanical properties of the material are shown in Table 1. The material was characterized with higher strength and lower elongation especially in tension. Specimens for fatigue test with gauge section of 6 mm 5.5 mm 12 mm were cut along the extruded bar. Samples for fatigue tests and for texture measurement as well as for metallographic observation were treated by mechanical polishing with silicon carbide abrasive papers, followed by electro-polishing by using a solution comprised of 40 ml HClO4 and 360 ml ethanol at temperature of 273 K in order to eliminate wear tracks as well as residual stress of the surface layer. Specimens for optical microscopy (OM) were etched in acetic picral (10 ml acetic acid þ 4.2 g picric
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Table 1 Tensile and compressive properties of GW123K Loading mode
s0.2 (MPa)
sb (MPa)
d (%)
E (GPa)
Tension Compression
248 244
294 545
2.5 16
46.4
acid þ 10 ml water þ 70 ml ethanol). The fatigue tests were performed on a servo-hydraulic testing system (Instron 8871) at ambient temperature in air under total strain amplitude control. The strain amplitudes ranging from 1.5 103 to 1.0 102 with a frequency of 0.5 Hz and S ¼ 1 (S refers to strain ratio) were applied. Subsequently, sample surface and fracture morphology were observed by scanning electronic microscopy (SEM, Quanta 600). The chemical composition at crack initiation sites was determined with energy-dispersive X-ray (EDX) analysis. 3. Results 3.1. Microstructure and texture The microstructure of this alloy is shown in Fig. 1, which consists of Mg solid solution matrix and the precipitated phase Mg5X, where X: Gd þ Y. The average grain size is about 10 mm and basically free of twin in grains before loading. Unlike most of the conventional extruded magnesium alloys that have strong ring texture[18e20], the present material has a weak {0002} texture as shown in Fig. 2. The number of grains with their basal plane parallel to ED is slightly larger than that of other orientation. 3.2. Cyclic stress response The variation of stress with the number of cycles, N, is an important feature of the low-cycle fatigue process. The cyclic stress response of a material during fatigue loading represents the locus of the stress amplitude with successive cycles, and illustrates the path by which the material achieves to its final level of stress during the total strain amplitude controlled low-cycle fatigue test. The cyclic stress response curves for the extruded GW123K alloy at various total strain amplitudes are depicted in Fig. 3. It is obvious that the cyclic stress response of the alloy is rather stable to the imposed different total strain amplitudes, particularly at low strain amplitude of 0.2%. Through close examination, slight cyclic softening can be observed at tension half cycle for all strain amplitudes tested, and slight cyclic softening
at early cycles and then slight cyclic hardening afterward can be observed at compression half cycle for all strain amplitudes tested. On the whole, over the entire range of total strain amplitudes applied in the present investigation, the variation of stress amplitude is so mildly that we could say the cyclic stabilization dominates the whole cycling period for the alloy. 3.3. Hysteresis loop The analysis of the stressestrain hysteresis loops can give a better understanding on the cyclic deformation behavior of the extruded GW123K alloy under low-cycle fatigue condition. Fig. 4 illustrates the stressestrain hysteresis loops of the extruded GW123K alloy at different total strain amplitudes, which were obtained at half-life. Compared with the hysteresis loops of extruded AZ31[8] or AZ80[21], the extruded GW123K alloy exhibits symmetric hysteresis loops and there is no signature of twinning occurring. This makes a big difference from that of extruded AZ31 or ZK60, which have pronounced asymmetric deformation behavior in tension and compression under total strain amplitude controlled cycling[9,22]. The aforementioned asymmetric deformation behavior mainly results from two factors: one is that the fraction of grains favoring tension twinning in compression is significantly higher than that of grains favoring tension twinning in tension and basal slip is in hard orientation in tension while twining is in soft orientation in compression due to the strong texture; the other is that the significant difference between the stresses needed to initiate twinning in compression and the stress needed to activate slip in tension since basal slip is in hard orientation but twinning is in soft orientation. In present study, the symmetric hysteresis loop might be due to the weak texture which results in only slightly higher fraction of the grains favored for tension twining in tension than that of the grains favoring tension twining in compression. Moreover, the weak texture favored basal slip compared with strong textured magnesium alloys[8,9,21e23]. The cyclic stressestrain response is another important material property in designing for enhanced fatigue resistance. It describes the relationship between flow stress and plastic strain amplitude under cyclic loading, and is a useful aid in understanding strain controlled cyclic deformation behavior. The information can be obtained from hysteresis loop. In present study, the fatigue strength coefficient K0 , is about 986.7 MPa, and the cyclic strain hardening exponent n’ is about 0.2247 as shown in Fig. 4(b). 3.4. Fatigue life The relation between strain amplitude and fatigue life can be described with CoffineManson and Basquin relations as shown in Fig. 5(a), and the corresponding data of wrought magnesium alloy AM50[16], AZ31[24] as well as an AleMgeSc alloy[25] were also presented which shows that the high strength magnesium alloy GW123K exhibits decent low-cycle fatigue properties as shown in Fig. 5(b). 3.5. Fracture morphology
Fig. 1 Microstructures of as-received GW123K alloy (Black particles are Gd-riched or Y-riched inter-metallic compound).
3.5.1. Typical fatigue crack propagation (FCP) morphology. Figs. 6 and 7 show the typical crack propagation morphology of extruded GW123K alloy. The macro-scale fracture plane is rather smooth and loading direction (LD) is normal to the fracture surface. The overall fracture image shows the crack initiation and three fatigue crack propagation regions: i.e. low FCP
S.M. Yin and S.X. Li: J. Mater. Sci. Technol., 2013, 29(8), 775e780
777
Fig. 2 Pole figures of extruded GW123K alloy.
rate region (region I), regions of FCP with moderate rate (region II) and higher FCP rate region (region III). Fig. 6(b)e(d) present the high magnification SEM images of regions I, II and III, respectively. In region I, the fracture surface is characterized by profuse cleavage planes with steps. Its formation is due to the dislocation slip on preferential crystallographic plane back and forth under cyclic loading which induced micro-crack incubation along slip bands within a grain at lower maximum stress intense factor (Kmax); the typical feature of region II is rather rough fracture surface composed of many semi-cleavage planes which is due to the moderate Kmax at crack tip; there are also some shallow dimple structures observed in this region; while region III (at large Kmax) is characterized by many dimples with particles in it as shown in Fig. 6(d). It is mainly resulted from the void nucleation, growth, and coalescence, either by particle fracture or by particle matrix interface de-bonding to form the dimple rupture at large stress intensity factor. Fracture surfaces of other samples have also been investigated and it is found that the area fraction of cleavage decreases significantly with increasing strain amplitude but the morphologies of region III with different strain amplitudes are similar. It indicates that strain amplitude and Kmax have some influence on the fracture modes of the material. Since the ductility of present material is much lower than that of conventional extruded AZ31 and AM60, the edge of the dimple is thick and the depth of the dimple is shallow.
shows that large Gd-riched phases (Fig. 8(a) and (b)) or large inclusions which are composed of MgO, CaO and some sulphide (Fig. 8(c) and (d)) favoring fatigue crack incubation. It can be confirmed that large Gd-riched phase and large inclusion are main crack incubation sites and they reduce fatigue life of the material drastically compared with the samples free of large inclusions and Gd-riched phases. 3.5.3. Surface observation. Sample surface observations after fatigue test are shown in Fig. 9. High density of slip bands (SB) confined to grain inside and cracks initiated at grain boundary (GB) or at the slip bands were observed. During fatigue, slip bands formed and served as strain localized regions which resulted in the formation of extrusions and intrusions and the potential micro-crack initiation in the following cycles. These micro-cracks grew along SB and coalescence resulted in large transgranular cleavage planes which could be observed on the fracture surface. While the cracks along GB were caused by high
3.5.2. Fatigue crack initiation. The morphologies and corresponding chemical compositions of typical samples with different crack initiation mechanisms are shown in Fig. 8. It reveals that the fatigue cracks basically initiate at the surface or subsurface. The corresponding chemical compositions of the crack initiation sites were investigated with SEM-EDX. It clearly
Fig. 3 Cyclic stress response of GW123K alloy at different total strain amplitudes.
Fig. 4 (a) Hysteresis loop curves at different total strain amplitudes, (b) log (stress amplitude) vs log (plastic strain amplitude) based on the hysteresis loop curves.
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Fig. 5 (a) Strain amplitude vs number of reversal to failure curves for extruded GW123K alloy, (b) fatigue life vs applied strain amplitude for conventional light metals.
local stress at GB. These surface cracks could propagate either intergranularly or intragranularly depending on the local microstructure and local stress. 4. Discussion 4.1. Cyclic deformation behavior This material exhibits cyclic stability mainly due to three factors as follows. Firstly, micro-plastic deformation is dominated by basal slip since the material has nearly random texture;
the basal slip is preferred to be activated due to its lower critical shear stress compared with prismatic slip as well as twinning. SB can only be found within grains and no large SB crossing the sample surface can be found. The chance of interaction of slip decreases drastically, thus the dislocation hardening effect is very slight. Secondly, once the initial dislocation reaches GB and is absorbed by GB, other new dislocations will operate and slip on basal plane in the same grain will introduce another SB which parallels to the initial one. With increasing dislocation density, the subsequent basal slip will be subjected to the resistance of second phase, which results in the increase of stress for plastic
Fig. 6 SEM images of the fracture surface of sample fatigued at total strain amplitude of 0.003: (a) overall fracture surface; (b)e(d) higher magnification of crack initiation and lower FCP rate region, moderate FCP rate and higher FCP rate regions, respectively.
S.M. Yin and S.X. Li: J. Mater. Sci. Technol., 2013, 29(8), 775e780
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Fig. 7 SEM images of the fracture surface of sample fatigued at total strain amplitude of 0.004: (a) overall fracture surface; (b)e(d) higher magnification of crack initiation and lower FCP rate region, moderate FCP rate and higher FCP rate regions, respectively.
deformation. In addition, when basal slip terminates in some soften oriented grains, a little large stress needs to activate basal slip in other grains with orientation deviated slightly from the favorable orientation. These factors mentioned above result in the slight cyclic hardening.
Finally, the main fatigue crack initiation and growth result in the bearing section of the sample decreasing correspondingly and slightly cyclic softening. The role of the third factor can be compensated by the hardening factors and the material exhibits cyclic stability.
Fig. 8 (a) and (c) Typical crack initiation sites, (b) and (d) corresponding EDX spectra of the sites.
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S.M. Yin and S.X. Li: J. Mater. Sci. Technol., 2013, 29(8), 775e780
Fig. 9 SEM images of fatigued surface of sample.
4.2. Fatigue life The material exhibits sound fatigue life compared with the aluminum alloy and some magnesium alloys. But it is not the intrinsic fatigue life for the material. It is known that most of fatigue life is dominated by the process of the crack incubation, especially at lower strain amplitudes or lower stress amplitudes. In the present study, the large inclusion regions (consist of MgO and some sulfides) with dimension about 100 mm 100 mm were found, and it is much larger than grain. Besides, large Gdriched phase was also observed on sample surface. These large defects serve as fatigue crack initiation sites and result in the premature failure of the samples. If these large defects can be eliminated by improved smelting process, the material will exhibit much longer fatigue life since fatigue life of the material is very sensitive to those large defects. 5. Conclusion The conventional extruded GW123K magnesium alloy exhibited cyclic stability during fatigue. The main cracks preferentially initiated at large inclusion or at Gd-riched phase at surface or subsurface. Micro-crack could also incubate at slip bands or at grain boundary. The fracture morphology of the material was greatly influenced by maximum stress intensity factor Kmax: large transgranular cleavage dominates the regions at lower Kmax especially at lower amplitudes while ductile fracture characterized by shallow dimples dominates large Kmax regions and rupture regions. Acknowledgments This work was supported by the National Natural Science Foundation of China under Grant No. 50571102. The authors are grateful to W. Gao and H.J. Yang for assistance with the EBSD measurement. REFERENCES [1] C.M. Zhang, X. Hui, Z.G. Li, G.L. Chen, Mater. Lett. 62 (2008) 1129e1131.
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