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Influence of aging on twin boundary strengthening in magnesium alloys
Author’s Accepted Manuscript Influence of Aging on Twin Strengthening in Magnesium Alloys
boundary
Jianwei Teng, Xiaojuan Gong, Yunping Li, Yan Nie www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(17)31717-3 https://doi.org/10.1016/j.msea.2017.12.110 MSA35952
To appear in: Materials Science & Engineering A Received date: 26 September 2017 Revised date: 29 December 2017 Accepted date: 30 December 2017 Cite this article as: Jianwei Teng, Xiaojuan Gong, Yunping Li and Yan Nie, Influence of Aging on Twin boundary Strengthening in Magnesium Alloys, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2017.12.110 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of Aging on Twin boundary Strengthening in Magnesium Alloys Jianwei Teng1, Xiaojuan Gong2, Yunping Li2*, Yan Nie3 1
School of Materials Science and Engineering, Central South University, Changsha,
China 2
State Key Lab for Powder Metallurgy, Central South University, Changsha, China
3
Yuanmeng Precision Technology (Shenzhen) Institute, Shenzhen, China
Correspondence author: Y. Li, [email protected]
Abstract
In order to investigate the effect of aging on twin boundary (TB) strengthening in magnesium alloys, the cyclic compressions along two orthogonal directions of samples with and without intermediate aging are carried out. The results show that the grains in all Mg alloys AZ31, AZ61 and AZ91 are significantly refined by twin boundaries (TBs). The TB strengthening is strongly dependent on both alloying element content and aging. Although grain refinement by TB formation occurs in AZ31, it exhibits slight strengthening effect even after aging. In contrast, significant TB strengthening after aging is observed in Mg alloys AZ61 and AZ91 with high contents of alloying elements. Keywords: Magnesium alloy; Grain refinement; Twinning; Aging
1. Introduction 1
Magnesium alloys have advantages of low density, high specific strength and damping capacity [1,2], which make the alloys attractive in structural and functional applications in transportation industries to save fuel consumption [3]. However, magnesium alloys exhibit low strength and poor formability at room temperature due to the hexagonal close packed (HCP) crystal structure. Only two basal slip systems are available at room temperature and the five independent slip systems could not be activated concurrently to satisfy the Von Mises criteria [4] for homogenous plastic deformation. For this reason, twinning represents one of the most important plastic deformation modes in magnesium alloy at room temperature [3,5]. Previous results suggest that microstructure of magnesium alloys can be refined by TB formation [6] or the fragmentation of double twin bands [7], giving rise to enhanced mechanical properties [8,9]. In addition, there is a strong pinning effect of both the solute elements and the precipitates on TB motion especially after aging, leading to more significant improvement in mechanical property [10-12]. For example, the yield strength of Mg-Zn-Gd alloy can be improved after aging due to the periodical segregation of Gd and Zn in TB [10]. And the TB in Mg alloy AZ91 can also be stabilized during aging due to the presence of segregation and precipitate in TB, resulting in enhanced yield strength of alloys [13]. Although there are several researches focused on the effect of aging on TB strengthening in Mg alloys, there is still a lack of systematic research on the effect of aging on TB strengthening in different magnesium alloys. In this study, we investigated systematically the effects of aging on TB refinement and the 2
corresponding strengthening effect in the Mg alloys AZ31, AZ61 and AZ91.
2. Experimental procedure
The
cast
Mg
alloys
AZ31
(Mg-2.82Al-0.98Zn-0.25Mn,
mass%),
AZ61
(Mg-6.55Al-0.85Zn-0.29Mn, mass%) and AZ91 (Mg-8.68Al-0.52Zn-0.32Mn, mass%) were used in this study. Cubic sample with L20×W15 mm2 and 30 mm in height were cut from the as-cast alloys by using an electrical discharge machine (EDM). All samples were homogenized at 420 ℃ for 17 h followed by an immediate air cooling to room temperature. Compression tests were carried out at room temperature along the H30 mm direction to an engineering strain of 4% under a compression rate of 0.5 mm/min using the electronic universal testing machine (EUTM). This was followed by an aging at 250 ℃ for 20 min and then cooling down to room temperature in air immediately. After that, compression along L20 mm direction on same sample under similar condition was carried out followed by another aging at 250 ℃ for 20 min and cooling. Fig. 1 shows the schematic diagram of cyclic compression-aging process of Mg alloys. To shed light on how aging affects grain refinement by TB, another experiment on compression under the same condition without the intermediate aging was conducted. In order to investigate the effect of aging on mechanical property in three alloys, aging treatment at 250 ℃ for 20 min followed by air cooling to room temperature immediately was carried out on the samples without compression. Each cyclic experiment process was repeated for 10 times.
3
The specimen for metallographic observation were polished after mechanical grinding with SiC abrasive paper and etched in a solution of 5 g picric acid, 70 ml ethyl alcohol, 10 ml acetic acid and 10 ml distilled water. After that, the microstructures of all Mg alloys with and without aging were characterized by examining the centres of the samples using Horizontal Optical Microscope (HOM) along the H30 mm direction. The hardness of samples was measured by using THV-10 Vickers Hardness Tester with a load of 10 N for 10 s at room temperature for 10 times. Tensile tests were carried out using EUTM with strain rates of 0.5 mm/min at room temperature with dimensions of tensile samples as shown in Fig. 2.
3. Results
3.1 Microstructure
Fig. 3 shows the microstructures of three alloys after solution treatment (ST) at 420 ℃ for 17 h, indicating similar texture of the twin-free microstructure and a comparable mean grain size about 250 μm in all alloys after the homogenization at same conditions. The microstructural evolution of Mg alloys AZ31, AZ61 and AZ91 during the cyclic compression-aging process is shown in Fig. 4. From Fig. 4, twin number increases continuously with the progress of cyclic compression-aging in all Mg alloys. In addition, in alloy with higher content of alloying elements the twins become thinner and the twin number is obviously larger (Fig. 4). A comparison on the microstructure of the three alloys after cyclic compression (cyclic number 3) with and without aging is given in Fig. 5. In the sample after cyclic compression without the 4
intermediate aging, similar behavior is observed as shown in Fig. 5 (d-f). However, the twins are much fewer and the twin density has no difference in all Mg samples compared to that with aging (a to c), suggesting the significant influence of aging on twin density.
3.2 Grain size variation during the cyclic test
By taking into account the TB, the mean grain sizes of Mg alloys AZ31, AZ61 and AZ91 after various compression-aging cycles are calculated. The mean grain sizes of Mg alloys AZ31, AZ61 and AZ91 are 142 μm, 75 μm and 63 μm, respectively after the 1st cycle. Comparing to that of Mg alloys AZ31 (258 μm), AZ61 (232 μm) and AZ91 (245 μm) after ST, the microstructure of Mg alloys is refined significantly due to TB formation. The results are also plotted in Fig. 6, where the variation of grain size with the progress of cyclic compression-aging is more clearly observed in these alloys. Noted that the mean grain size does not decrease further in both AZ61 and AZ91 after 5 compression-aging cycles. In contrast, AZ31 alloy exhibits no obvious change until 8 cycles. In addition, grain size of AZ31 is larger than that of Mg alloys AZ61 and AZ91 after compression with aging throughout the process, as shown in Fig. 6. For the compressed sample without aging, the mean grain size of all alloys demonstrates a decremented behavior along the cyclic number, and no obvious difference is observed among them (Fig. 7). In addition, the mean grain size for all Mg alloys reaches a constant value after the cyclic number more than 5. Compared to the mean grain size in compression with aging, the grain sizes of AZ31(108 μm),
5
AZ61(102 μm) and AZ91(101 μm) after 10th compression-nonaging cycle is much larger, as shown in Fig. 8. As a result, aging treatment plays a strong role in grain refinement by TB in all Mg alloys AZ31, AZ61 and AZ91.
3.3 Mechanical property variation after cyclic test
Fig. 9 illustrates the hardness variation of Mg alloys AZ31, AZ61 and AZ91 after various compression-aging cycle. This indicates that the hardness increases slightly due to the TB strengthening in AZ31 alloy. In contrast, the hardness increases substantially in both AZ61 and AZ91 with the progress of the cyclic process. However, the incremental rate of hardness becomes lower after 5th cycle in AZ91 alloy. Fig.10
shows
the
hardness
variation
in
alloys
after
various
compression-nonaging cycles. It is observed that the increase of hardness is ignorable in all alloys AZ31, AZ61 and AZ91, implying the TB does not demonstrate strengthening in present condition, although TB refinement is obviously as indicated in Fig. 7. From the abovementioned results, it can be inferred that aging has a strong influence on mechanical properties of Mg alloy containing TB. Fig. 11 shows the stress-strain curves of Mg alloys AZ31, AZ61 and AZ91 after ST and 10th compression-aging cycles. In contrast to the negligible change in the ultimate tensile strength (UTS) in AZ31 and slight increase in AZ61, we can observe a marked increase in Mg alloy AZ91 than in the ST samples (Fig. 12(a)), which is accompanied by a slight decrease in elongation (Fig. 12(b)).
6
4. Discussion
It has been reported that the {10-12} twins were formed under compression along c-axis and there is no dynamic recrystallization during aging at 250 ℃ in Mg alloys [1,12,14]. For a given cyclic number N, twin density becomes higher in alloy with higher alloying element content (Fig. 4), since the alloying elements Al and Zn are favor of twin nucleation formed during compression [15]. During the cyclic compression-aging along the orthogonal directions, grain matrix is characterized by twin formation along two directions, leading to overlap of TBs-grain refinement [6]. TB refinement is more obvious in alloy subjected to cyclic compression-aging process. It can be seen from Fig. 4 (d), (h), (l) that the grains were subdivided by TBs into a large number of small parts after compression-aging cycles. We calculate the average area of each small part by using Image-J software. After that, we measured the mean grain size by calculating the square edge length. To clarify this in more details, we can assume that a grain is subdivided by TB into 4 parts during a single compression-aging-compression-aging cycle along the two orthogonal directions of sample. According to this, the grain is subdivided by TB into 4N parts for N compression-aging cycles and following this, the relationship between mean grain size and cyclic number can be expressed as:
𝑑2 =
𝑆 4𝑁
(1)
or 1
𝑑= 7
𝑆2 𝑁 42
(2)
where d is the mean grain size after subdivision by TB, S is the initial grain area of Mg alloys and N is the cyclic number of experiment. Since the ability of TB formation is strongly dependent on alloy content and grain size etc.., the TB refinement can be rewritten by using Eq. (2) as: 1
𝑑 = 𝑑0 +
𝑎∙𝑆 2 𝑐∙𝑁
(3)
4 2
Where d0, a and c are constants, dependent on the intrinsic characteristic of alloy. Let 1
𝑎 ∙ 𝑆 2 = 𝑏, and
𝑐 2
=𝑚, Eq. (3) can be simply expressed in: 𝑑 = 𝑑0 + 𝑏 ∙ 4−𝑚𝑁
(4)
where d is the mean grain size after TB refinements at a given cycle number N, and b, m and d0, are constants related to the characteristic of alloy. Fig. 6 shows the fitting results of grain size as a function of cyclic number during the cyclic compression-aging process. The values of d0, b, m used for fitting are listed in Table. 1(a). We can see that values of m of Mg alloys AZ31, AZ61 and AZ91 after compression-aging are 0.40, 0.82 and 1.02, respectively. From Eq. (2) and Eq. (4), m of 0.5 corresponds to grain refinement into 4 parts each cycle. By considering the values of m in these alloys, it is indicated that the ability of TB formation in AZ31 alloy is the lowest, and increases significantly in alloy with higher alloying element content. In contrast, m in Mg alloys AZ31, AZ61 and AZ91 compressed without aging is 0.38 0.33 and 0.34 (Table.1(b)), respectively. The m is almost kept to be a constant or slightly decreasing. This indicates that detwinning occurs in the pre-existing TB in alloy and TB does not play refining effect in Mg alloys if no aging was conducted. 8
Since 𝑏 ∙ 4−𝑚𝑁 is close to 0 at high value of N from Eq. (4), 𝑑0 represents the minimum grain size of alloy that can be refined by TB in present condition. From Fig. 6, d0 of Mg alloys AZ31, AZ61 and AZ91 after N compression-aging cycle is close to 31.72 µm, 19.22 µm and 19.55 µm, respectively. This is also given in Table. 1 (a), suggesting that it becomes harder for TB formation at higher N. This can be ascribed to that TB refinement in turn tends to suppress the further formation of twin in magnesium [16] as expressed by Hall-Petch relationship: 1
𝜎𝑡𝑤𝑖𝑛 = 𝜎0 + 𝑘𝑑−2
(5)
where 𝜎𝑡𝑤𝑖𝑛 is the critical resolved stress for twin formation, 𝜎0 and k are the material constants and d is grain size. We can see from Eq. (5) that with decreasing grain size, the twinning stress 𝜎𝑡𝑤𝑖𝑛 is increased, and twin becomes difficult to be formed consequently. Present results indicate that grain size refinement by TB is strongly influenced by both aging and alloying element content. It has been revealed that a large amount of Mg17Al12 appeared after aging and solute elements Al, Zn pose a stronger pinning effect on TB [10-12]. This can inhibit the occurrence of detwinning during subsequent compression and promote grain refinement. As a result, in pre-compressed Mg alloy without aging, TB is not stable and detwinning occurs easily, even though twin density increases substantially in alloy with higher Al, Zn content, as shown in Fig. 5 (d-f). It has also been reported that the strengthening effect of TB is closely related to the TB mobility. TB with lower mobility demonstrates higher strengthening effect. The stress required for twin boundary motion (TBM) is closely related to friction stress 9
𝑖𝑐
[12], which is then strongly dependent on the dislocation density around TB,
incoherent twin boundary [17], solute atoms [10], and precipitates [18]. Solute atoms and precipitates have a slight influence in pinning TB in Mg alloy AZ31 due to a lower alloying element, while this becomes more significant in alloy with higher content of Al, Zn [14]. This indicated that TB refinement is not robust in AZ31. The friction stress on account of hindrance of dislocations is decreased, since the dislocation density is decreased after aging. In addition, the transition of incoherent TB into coherent one after aging, which shows higher TBM is another reason for the friction decreased in Mg alloy AZ31 [17, 19]. As a result, detwinning can take place during subsequent compression in AZ31, and the TB strengthening is not effective. On the contrary, the solute atoms and precipitates have a strong influence on stabilizing TB in Mg alloys AZ61 and AZ91 having high alloying element content [14]. This is due to the fact that friction stress is increased substantially due to higher content of solutes atoms and precipitates, especially after aging. In this case, detwinning becomes more difficulty in AZ61 and AZ91 with higher alloying element and TB refinement becomes more significant after aging. Although TB refinement occurs in Mg alloy AZ31, no significant increase of hardness is observed after compression-aging cycle as shown in Fig. 9. This is also the case in ultimate tensile strength (UTS) as shown in Fig. 12 (a). The non-significant variation in mechanical property of AZ31 alloy should be related to the low friction stress of TB, leading to the easy occurrence of detwinning. In contrast to the slight variation of hardness in AZ31 alloy, hardness increases 10
significantly in Mg alloy AZ61 and AZ91 after cyclic compression-aging, as observed in Fig. 9. This should be ascribed to the fact that segregation of solute atoms and precipitates on TB, which are increased after aging, are the main reasons of stabilizing TB in Mg alloy AZ61 and AZ91 with a higher alloying element [14, 20-22] leading to an obvious TB refinement. TB strengthening 𝜎𝑦 can be well predicted by the Hall-Petch relationship: 1
𝜎𝑦 = 𝜎0 + 𝑘𝑑 −2
(6)
where 𝜎0 is the friction stress, 𝑑 is the grain size and k is a constant, as shown in Fig. 13. The slope of curve k in alloy subjected to compression-aging demonstrates 1
an incremental behaviour, and in an order of AZ31 (31.21 MPa ∙ 𝜇𝑚2 ), AZ61 (84.76 1
1
MPa ∙ 𝜇𝑚2 ) and AZ91 (182.56 MPa ∙ 𝜇𝑚2 ), as shown in Fig. 14. These values are much higher than that of alloy without the intermediate aging (Fig. 13 (b)), implying strong influence of alloying element and aging on TB strengthening.
5.Conclusion
In the present study, the effects of aging on twin boundary strengthening in Mg alloys AZ31, AZ61 and AZ91 was investigated. The main conclusion can be summarized as follows: 1. The {10-12} twins can form during compression to strain of 4% in our study. And TB refinement can be observed in all Mg alloys AZ31, AZ61 and AZ91 after cyclic compression with and without intermediate aging. TB refining process as a function of cyclic number can be well expressed in an exponential formula in Mg alloys.
11
2. The TBM can be stabilized after aging in Mg alloys AZ61 and AZ91, contributing to significant grain refinement. 3. TB strengthening is not obviously observed in AZ31 alloy. However, both the hardness and UTS increase obviously in Mg alloys AZ61 and AZ91, especially in AZ91, suggesting the strong influence of alloying element and aging on TB strengthening.
Acknowledgments The authors gratefully acknowledge the financial support from project of The Science Fund for Distinguished Young Scholars of Hunan Province, China (2016JJ1016), and the project of Innovation and Entrepreneur Team Introduced by Guangdong Province (201301G0105337290).
References
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[12] Y.J. Cui, Y.P. Li, Z.C. Wang, X. Ding, Y. Koizumi, H.K. Bian, L.Y. Lin, A. Chiba, Impact of solute elements on detwinning in magnesium and its alloys, Int. J. Plast. 91 (2017) 134-159. [13] R. Zeng, Precipitation hardening in AZ91 magnesium alloy, University of Birmingham, (2013) p22 [14] Y.J. Cui, Y.P. Li, Z.C. Wang, Y. Koizumi, A. Chiba, Regulating twin boundary mobility by annealing in magnesium and its alloys, Inter J Plast, 99 (2017) 1-18. [15] J.D. Robson, N. Stanford, M.R. Barnett, Effect of particles in promoting twin nucleation in a Mg-5 wt.% Zn alloy, Scr. Mater., 63 (2010) 823-826. [16] H. Somekawa, T. Mukai, Hall–Petch relation for deformation twinning in solid solution magnesium alloys, Mater. Sci. Eng. A. 561 (2013) 378-385. [17] Y.J. Cui, Y.P. Li, S.H. Sun, H. Huang, Z.C. Wang, Y. Koizzumi, A. Chiba, Enhanced damping capacity of magnesium alloys by tensile twin boundaries, Scr. Mater. 101 (2015) 8-11. [18] N. Stanford, J. Geng, Y. B. Chun, C. H. J. Davies, J. F. Nie, M. R. Barnett, Effect of plate-shaped particle distributions on the deformation behaviour of magnesium alloy AZ91 in tension and compression, Acta. Mater., 60 (2012) 218-228 [19] X.Y. Shi, Y. Liu, J. Lu, R.E.A. Williams, D.J. Li, X.Q. Zeng, A. A. Luo, Formation of a new incoherent twin boundary in a Mg-3Gd alloy, Scr. Mater., 112 (2016) 136-139. [20] M. Ghazisaeidi, L.G. Hector Jr., W.A. Curtin, Solute strengthening of twinning dislocations in Mg alloys, Acta. Mater. 80 (2014) 278-287. 14
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Captions: Fig. 1. Schematic diagram of cyclic compression-aging process. Fig. 2. Dimensions of tensile samples for the tensile test Fig. 3. Microstructure of (a) AZ31, (b) AZ61 and (c) AZ91 after solution treatment (ST) at 420 ℃ for 17 h. Fig. 4. Microstructure of (a,b,c,d) AZ31, (e,f,g,h) AZ61 and (i,j,k,l) AZ91 after compression (a,e,i) 1 cycle, (b,f,j) 3 cycles, (c,g,k) 5 cycles, (d,h,l) 10 cycles with aging at 250 ℃ for 20 min. Fig. 5. Microstructure of (a) AZ31, (b) AZ61 and (c) AZ91 after compression for 3 cycles with aging and (d) AZ31, (e) AZ61 and (f) AZ91 after compression for 3 cycles without aging. Fig. 6. Mean grain size of Mg alloys AZ31, AZ61 and AZ91 at different compression-aging cycles. Fig. 7. Mean grain size of Mg alloys AZ31, AZ61 and AZ91 at different compression cycles (no aging). Fig. 8. Mean grain size of AZ31, AZ61 and AZ91 after 10 compressions with and without aging 15
Fig. 9. Hardness variation of Mg alloys AZ31, AZ61 and AZ91 at different compression-aging cycles. Fig. 10. Hardness variation of Mg alloys AZ31, AZ61 and AZ91 at different compression cycles (no aging). Fig. 11. Stress-strain curves in samples after solution treatment and 10th compression-aging of AZ31, AZ61 and AZ91 alloys. Fig. 12. Variation on ultimate tensile strength (UTS) (a) and elongation (b) of Mg alloys AZ31, AZ61 and AZ91 in ST condition and after 10th compression-aging cycles. Fig. 13. Hardness variation of Mg alloys AZ31, AZ61 and AZ91 alloys on different grain size caused by different cycles of experiment compression with (a) and without (b) aging. Fig. 14. Variation of slope for Hall-Petch relation in the Mg alloys AZ31, AZ61 and AZ91.
Table 1. d0, b and m of the exponential formula and R-Square of fitting curve after compression with (a) and without (b) aging.
16
load
load aging
aging load
load
...
load
load
load
load
Cyclic number:
aging
aging
1
N
Fig. 1. Schematic diagram of cyclic compression-aging process.
2 mm
1.6 mm
24.9 mm
11.5 mm
R1.7 mm
5 mm
5 mm
Fig. 2. Dimensions of tensile samples for the tensile test
17
100μm
100μm
100μm
Fig. 3. Microstructure of (a) AZ31, (b) AZ61 and (c) AZ91 after solution treatment (ST) at 420 ℃ for 17 h.
Cycles:
1
3
5
10
AZ31 100μm
100μm
100μm
100μm
100μm
100μm
100μm
100μm
100μm
100μm
100μm
100μm
AZ61
AZ91
Fig. 4. Microstructure of (a,b,c,d) AZ31, (e,f,g,h) AZ61 and (i,j,k,l) AZ91 after compression (a,e,i) 1 cycle, (b,f,j) 3 cycles, (c,g,k) 5 cycles, (d,h,l) 10 cycles with aging at 250 ℃ for 20 min.
18
AZ31
AZ61
AZ91
100μm
100μm
100μm
100μm
100μm
100μm
Grain size after compression with aging, d(m)
Fig. 5. Microstructure of (a) AZ31, (b) AZ61 and (c) AZ91 after compression for 3 cycles with aging and (d) AZ31, (e) AZ61 and (f) AZ91 after compression for 3 cycles without aging.
300 AZ31 experimental data AZ61 experimental data AZ91 experimental data
270 240
AZ31 fitting curve AZ61 fitting curve AZ91 fitting curve
210 180 150 120 90 60 30 0
0
2 4 6 8 Cyclic number of experiment, N
10
Fig. 6. Mean grain size of Mg alloys AZ31, AZ61 and AZ91 at different compression-aging cycles.
19
Grain size after compression without aging, d (m)
300 AZ31 experimental data AZ61 experimental data AZ91 experimental data
270
AZ31 fitting curve AZ61 fitting curve AZ91 fitting curve
240 210 180 150 120 90 60 0
2
4
6
8
10
Cyclic number of experiment, N
Fig. 7. Mean grain size of Mg alloys AZ31, AZ61 and AZ91 at different compression cycles (no aging).
Mean grain size ,d (m)
120
compression with aging compression without aging
100 80 60 40 20 0
AZ31
AZ61
AZ91
Fig. 8. Mean grain size of AZ31, AZ61 and AZ91 after 10th compression with and without aging
20
Hardness after compression with aging,HV
100 90
AZ91
80 70 AZ61 60 AZ31
50 40 0
2 4 6 8 Cyclic number of experiment, N
10
Hardness after compression without aging,HV
Fig. 9. Hardness variation of Mg alloys AZ31, AZ61 and AZ91 at different compression-aging cycles.
100 90 80 70 AZ91 60 AZ61 50 40
AZ31 0
2 4 6 8 Cyclic number of experiment, N
10
Fig. 10. Hardness variation of Mg alloys AZ31, AZ61 and AZ91 at different compression cycles (no aging).
21
AZ31
225
150 125 100 75 50
175 150 125 100
25 0
ST 10th
200
175
True stress, (Mpa)
True stress, (Mpa)
200
AZ61
225
ST 10th
75 50 25
0
1
2
3
4
5
6
7
0
8
0
1
True strain, (%)
3
4
5
6
7
True strain, (%)
AZ91
225
ST 10th
200
True stress, (Mpa)
2
175 150 125 100 75 50 25 0 0
1
2
3
4
5
6
7
8
True strain, (%)
Fig. 11. Stress-strain curves in samples after solution treatment and 10th compression-aging of AZ31, AZ61 and AZ91 alloys.
(a)
(b) 8
240 1%up
10 Cycles
22%up
5%up
160 120 80 40 0
7
Tensile strain, (%)
Tensile strength, UTS(MPa)
ST
200
4%down
6
ST
10 Cycles
8%down
20%down
AZ61
AZ91
5 4 3 2 1 0
AZ31
AZ61
AZ91
AZ31
Fig. 12. Variation on ultimate tensile strength (UTS) (a) and elongation (b) of Mg alloys AZ31, AZ61 and AZ91 in ST condition and after 10th compression-aging cycles.
22
8
(b) AZ91 Hardness, HV
Hardness , HV
(a) 95 90 85 80 75 70 65 60 55 50 45 40
AZ61
AZ31
0.05
0.10
d
0.15 0.20 -1/2 (m )
0.25
95 90 85 80 75 70 65 60 55 50 45 40
AZ91 AZ61 AZ31
0.06
-1/2
0.07
0.08 -1/2
d
(m
0.09 -1/2
)
Fig. 13. Hardness variation of Mg alloys AZ31, AZ61 and AZ91 alloys on different grain size caused by different cycles of experiment compression with (a) and without (b) aging.
23
0.10
220 200
1/2
Slope, K(MPad )
180 160 140 120 100 80 60 40 20 AZ31
AZ61
AZ91
Fig. 14. Variation of slope for Hall-Petch relation in the Mg alloys AZ31, AZ61 and AZ91.
Table 1. d0, b and m of the exponential formula and R-Square of fitting curve after compression with (a) and without (b) aging. (a)
AZ31
AZ61
AZ91
d0
31.72
19.33
19.55
b
211.93
208.13
223.58
m
0.40
0.82
1.02
R-Square
0.96
0.98
0.94
24
(b)
AZ31
AZ61
AZ91
d0
95.59
102.93
94.97
b
174.37
129.49
153.68
m
0.38
0.33
0.34
R-Square
0.88
0.99
0.97
25
boundary
Jianwei Teng, Xiaojuan Gong, Yunping Li, Yan Nie www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(17)31717-3 https://doi.org/10.1016/j.msea.2017.12.110 MSA35952
To appear in: Materials Science & Engineering A Received date: 26 September 2017 Revised date: 29 December 2017 Accepted date: 30 December 2017 Cite this article as: Jianwei Teng, Xiaojuan Gong, Yunping Li and Yan Nie, Influence of Aging on Twin boundary Strengthening in Magnesium Alloys, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2017.12.110 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of Aging on Twin boundary Strengthening in Magnesium Alloys Jianwei Teng1, Xiaojuan Gong2, Yunping Li2*, Yan Nie3 1
School of Materials Science and Engineering, Central South University, Changsha,
China 2
State Key Lab for Powder Metallurgy, Central South University, Changsha, China
3
Yuanmeng Precision Technology (Shenzhen) Institute, Shenzhen, China
Correspondence author: Y. Li, [email protected]
Abstract
In order to investigate the effect of aging on twin boundary (TB) strengthening in magnesium alloys, the cyclic compressions along two orthogonal directions of samples with and without intermediate aging are carried out. The results show that the grains in all Mg alloys AZ31, AZ61 and AZ91 are significantly refined by twin boundaries (TBs). The TB strengthening is strongly dependent on both alloying element content and aging. Although grain refinement by TB formation occurs in AZ31, it exhibits slight strengthening effect even after aging. In contrast, significant TB strengthening after aging is observed in Mg alloys AZ61 and AZ91 with high contents of alloying elements. Keywords: Magnesium alloy; Grain refinement; Twinning; Aging
1. Introduction 1
Magnesium alloys have advantages of low density, high specific strength and damping capacity [1,2], which make the alloys attractive in structural and functional applications in transportation industries to save fuel consumption [3]. However, magnesium alloys exhibit low strength and poor formability at room temperature due to the hexagonal close packed (HCP) crystal structure. Only two basal slip systems are available at room temperature and the five independent slip systems could not be activated concurrently to satisfy the Von Mises criteria [4] for homogenous plastic deformation. For this reason, twinning represents one of the most important plastic deformation modes in magnesium alloy at room temperature [3,5]. Previous results suggest that microstructure of magnesium alloys can be refined by TB formation [6] or the fragmentation of double twin bands [7], giving rise to enhanced mechanical properties [8,9]. In addition, there is a strong pinning effect of both the solute elements and the precipitates on TB motion especially after aging, leading to more significant improvement in mechanical property [10-12]. For example, the yield strength of Mg-Zn-Gd alloy can be improved after aging due to the periodical segregation of Gd and Zn in TB [10]. And the TB in Mg alloy AZ91 can also be stabilized during aging due to the presence of segregation and precipitate in TB, resulting in enhanced yield strength of alloys [13]. Although there are several researches focused on the effect of aging on TB strengthening in Mg alloys, there is still a lack of systematic research on the effect of aging on TB strengthening in different magnesium alloys. In this study, we investigated systematically the effects of aging on TB refinement and the 2
corresponding strengthening effect in the Mg alloys AZ31, AZ61 and AZ91.
2. Experimental procedure
The
cast
Mg
alloys
AZ31
(Mg-2.82Al-0.98Zn-0.25Mn,
mass%),
AZ61
(Mg-6.55Al-0.85Zn-0.29Mn, mass%) and AZ91 (Mg-8.68Al-0.52Zn-0.32Mn, mass%) were used in this study. Cubic sample with L20×W15 mm2 and 30 mm in height were cut from the as-cast alloys by using an electrical discharge machine (EDM). All samples were homogenized at 420 ℃ for 17 h followed by an immediate air cooling to room temperature. Compression tests were carried out at room temperature along the H30 mm direction to an engineering strain of 4% under a compression rate of 0.5 mm/min using the electronic universal testing machine (EUTM). This was followed by an aging at 250 ℃ for 20 min and then cooling down to room temperature in air immediately. After that, compression along L20 mm direction on same sample under similar condition was carried out followed by another aging at 250 ℃ for 20 min and cooling. Fig. 1 shows the schematic diagram of cyclic compression-aging process of Mg alloys. To shed light on how aging affects grain refinement by TB, another experiment on compression under the same condition without the intermediate aging was conducted. In order to investigate the effect of aging on mechanical property in three alloys, aging treatment at 250 ℃ for 20 min followed by air cooling to room temperature immediately was carried out on the samples without compression. Each cyclic experiment process was repeated for 10 times.
3
The specimen for metallographic observation were polished after mechanical grinding with SiC abrasive paper and etched in a solution of 5 g picric acid, 70 ml ethyl alcohol, 10 ml acetic acid and 10 ml distilled water. After that, the microstructures of all Mg alloys with and without aging were characterized by examining the centres of the samples using Horizontal Optical Microscope (HOM) along the H30 mm direction. The hardness of samples was measured by using THV-10 Vickers Hardness Tester with a load of 10 N for 10 s at room temperature for 10 times. Tensile tests were carried out using EUTM with strain rates of 0.5 mm/min at room temperature with dimensions of tensile samples as shown in Fig. 2.
3. Results
3.1 Microstructure
Fig. 3 shows the microstructures of three alloys after solution treatment (ST) at 420 ℃ for 17 h, indicating similar texture of the twin-free microstructure and a comparable mean grain size about 250 μm in all alloys after the homogenization at same conditions. The microstructural evolution of Mg alloys AZ31, AZ61 and AZ91 during the cyclic compression-aging process is shown in Fig. 4. From Fig. 4, twin number increases continuously with the progress of cyclic compression-aging in all Mg alloys. In addition, in alloy with higher content of alloying elements the twins become thinner and the twin number is obviously larger (Fig. 4). A comparison on the microstructure of the three alloys after cyclic compression (cyclic number 3) with and without aging is given in Fig. 5. In the sample after cyclic compression without the 4
intermediate aging, similar behavior is observed as shown in Fig. 5 (d-f). However, the twins are much fewer and the twin density has no difference in all Mg samples compared to that with aging (a to c), suggesting the significant influence of aging on twin density.
3.2 Grain size variation during the cyclic test
By taking into account the TB, the mean grain sizes of Mg alloys AZ31, AZ61 and AZ91 after various compression-aging cycles are calculated. The mean grain sizes of Mg alloys AZ31, AZ61 and AZ91 are 142 μm, 75 μm and 63 μm, respectively after the 1st cycle. Comparing to that of Mg alloys AZ31 (258 μm), AZ61 (232 μm) and AZ91 (245 μm) after ST, the microstructure of Mg alloys is refined significantly due to TB formation. The results are also plotted in Fig. 6, where the variation of grain size with the progress of cyclic compression-aging is more clearly observed in these alloys. Noted that the mean grain size does not decrease further in both AZ61 and AZ91 after 5 compression-aging cycles. In contrast, AZ31 alloy exhibits no obvious change until 8 cycles. In addition, grain size of AZ31 is larger than that of Mg alloys AZ61 and AZ91 after compression with aging throughout the process, as shown in Fig. 6. For the compressed sample without aging, the mean grain size of all alloys demonstrates a decremented behavior along the cyclic number, and no obvious difference is observed among them (Fig. 7). In addition, the mean grain size for all Mg alloys reaches a constant value after the cyclic number more than 5. Compared to the mean grain size in compression with aging, the grain sizes of AZ31(108 μm),
5
AZ61(102 μm) and AZ91(101 μm) after 10th compression-nonaging cycle is much larger, as shown in Fig. 8. As a result, aging treatment plays a strong role in grain refinement by TB in all Mg alloys AZ31, AZ61 and AZ91.
3.3 Mechanical property variation after cyclic test
Fig. 9 illustrates the hardness variation of Mg alloys AZ31, AZ61 and AZ91 after various compression-aging cycle. This indicates that the hardness increases slightly due to the TB strengthening in AZ31 alloy. In contrast, the hardness increases substantially in both AZ61 and AZ91 with the progress of the cyclic process. However, the incremental rate of hardness becomes lower after 5th cycle in AZ91 alloy. Fig.10
shows
the
hardness
variation
in
alloys
after
various
compression-nonaging cycles. It is observed that the increase of hardness is ignorable in all alloys AZ31, AZ61 and AZ91, implying the TB does not demonstrate strengthening in present condition, although TB refinement is obviously as indicated in Fig. 7. From the abovementioned results, it can be inferred that aging has a strong influence on mechanical properties of Mg alloy containing TB. Fig. 11 shows the stress-strain curves of Mg alloys AZ31, AZ61 and AZ91 after ST and 10th compression-aging cycles. In contrast to the negligible change in the ultimate tensile strength (UTS) in AZ31 and slight increase in AZ61, we can observe a marked increase in Mg alloy AZ91 than in the ST samples (Fig. 12(a)), which is accompanied by a slight decrease in elongation (Fig. 12(b)).
6
4. Discussion
It has been reported that the {10-12} twins were formed under compression along c-axis and there is no dynamic recrystallization during aging at 250 ℃ in Mg alloys [1,12,14]. For a given cyclic number N, twin density becomes higher in alloy with higher alloying element content (Fig. 4), since the alloying elements Al and Zn are favor of twin nucleation formed during compression [15]. During the cyclic compression-aging along the orthogonal directions, grain matrix is characterized by twin formation along two directions, leading to overlap of TBs-grain refinement [6]. TB refinement is more obvious in alloy subjected to cyclic compression-aging process. It can be seen from Fig. 4 (d), (h), (l) that the grains were subdivided by TBs into a large number of small parts after compression-aging cycles. We calculate the average area of each small part by using Image-J software. After that, we measured the mean grain size by calculating the square edge length. To clarify this in more details, we can assume that a grain is subdivided by TB into 4 parts during a single compression-aging-compression-aging cycle along the two orthogonal directions of sample. According to this, the grain is subdivided by TB into 4N parts for N compression-aging cycles and following this, the relationship between mean grain size and cyclic number can be expressed as:
𝑑2 =
𝑆 4𝑁
(1)
or 1
𝑑= 7
𝑆2 𝑁 42
(2)
where d is the mean grain size after subdivision by TB, S is the initial grain area of Mg alloys and N is the cyclic number of experiment. Since the ability of TB formation is strongly dependent on alloy content and grain size etc.., the TB refinement can be rewritten by using Eq. (2) as: 1
𝑑 = 𝑑0 +
𝑎∙𝑆 2 𝑐∙𝑁
(3)
4 2
Where d0, a and c are constants, dependent on the intrinsic characteristic of alloy. Let 1
𝑎 ∙ 𝑆 2 = 𝑏, and
𝑐 2
=𝑚, Eq. (3) can be simply expressed in: 𝑑 = 𝑑0 + 𝑏 ∙ 4−𝑚𝑁
(4)
where d is the mean grain size after TB refinements at a given cycle number N, and b, m and d0, are constants related to the characteristic of alloy. Fig. 6 shows the fitting results of grain size as a function of cyclic number during the cyclic compression-aging process. The values of d0, b, m used for fitting are listed in Table. 1(a). We can see that values of m of Mg alloys AZ31, AZ61 and AZ91 after compression-aging are 0.40, 0.82 and 1.02, respectively. From Eq. (2) and Eq. (4), m of 0.5 corresponds to grain refinement into 4 parts each cycle. By considering the values of m in these alloys, it is indicated that the ability of TB formation in AZ31 alloy is the lowest, and increases significantly in alloy with higher alloying element content. In contrast, m in Mg alloys AZ31, AZ61 and AZ91 compressed without aging is 0.38 0.33 and 0.34 (Table.1(b)), respectively. The m is almost kept to be a constant or slightly decreasing. This indicates that detwinning occurs in the pre-existing TB in alloy and TB does not play refining effect in Mg alloys if no aging was conducted. 8
Since 𝑏 ∙ 4−𝑚𝑁 is close to 0 at high value of N from Eq. (4), 𝑑0 represents the minimum grain size of alloy that can be refined by TB in present condition. From Fig. 6, d0 of Mg alloys AZ31, AZ61 and AZ91 after N compression-aging cycle is close to 31.72 µm, 19.22 µm and 19.55 µm, respectively. This is also given in Table. 1 (a), suggesting that it becomes harder for TB formation at higher N. This can be ascribed to that TB refinement in turn tends to suppress the further formation of twin in magnesium [16] as expressed by Hall-Petch relationship: 1
𝜎𝑡𝑤𝑖𝑛 = 𝜎0 + 𝑘𝑑−2
(5)
where 𝜎𝑡𝑤𝑖𝑛 is the critical resolved stress for twin formation, 𝜎0 and k are the material constants and d is grain size. We can see from Eq. (5) that with decreasing grain size, the twinning stress 𝜎𝑡𝑤𝑖𝑛 is increased, and twin becomes difficult to be formed consequently. Present results indicate that grain size refinement by TB is strongly influenced by both aging and alloying element content. It has been revealed that a large amount of Mg17Al12 appeared after aging and solute elements Al, Zn pose a stronger pinning effect on TB [10-12]. This can inhibit the occurrence of detwinning during subsequent compression and promote grain refinement. As a result, in pre-compressed Mg alloy without aging, TB is not stable and detwinning occurs easily, even though twin density increases substantially in alloy with higher Al, Zn content, as shown in Fig. 5 (d-f). It has also been reported that the strengthening effect of TB is closely related to the TB mobility. TB with lower mobility demonstrates higher strengthening effect. The stress required for twin boundary motion (TBM) is closely related to friction stress 9
𝑖𝑐
[12], which is then strongly dependent on the dislocation density around TB,
incoherent twin boundary [17], solute atoms [10], and precipitates [18]. Solute atoms and precipitates have a slight influence in pinning TB in Mg alloy AZ31 due to a lower alloying element, while this becomes more significant in alloy with higher content of Al, Zn [14]. This indicated that TB refinement is not robust in AZ31. The friction stress on account of hindrance of dislocations is decreased, since the dislocation density is decreased after aging. In addition, the transition of incoherent TB into coherent one after aging, which shows higher TBM is another reason for the friction decreased in Mg alloy AZ31 [17, 19]. As a result, detwinning can take place during subsequent compression in AZ31, and the TB strengthening is not effective. On the contrary, the solute atoms and precipitates have a strong influence on stabilizing TB in Mg alloys AZ61 and AZ91 having high alloying element content [14]. This is due to the fact that friction stress is increased substantially due to higher content of solutes atoms and precipitates, especially after aging. In this case, detwinning becomes more difficulty in AZ61 and AZ91 with higher alloying element and TB refinement becomes more significant after aging. Although TB refinement occurs in Mg alloy AZ31, no significant increase of hardness is observed after compression-aging cycle as shown in Fig. 9. This is also the case in ultimate tensile strength (UTS) as shown in Fig. 12 (a). The non-significant variation in mechanical property of AZ31 alloy should be related to the low friction stress of TB, leading to the easy occurrence of detwinning. In contrast to the slight variation of hardness in AZ31 alloy, hardness increases 10
significantly in Mg alloy AZ61 and AZ91 after cyclic compression-aging, as observed in Fig. 9. This should be ascribed to the fact that segregation of solute atoms and precipitates on TB, which are increased after aging, are the main reasons of stabilizing TB in Mg alloy AZ61 and AZ91 with a higher alloying element [14, 20-22] leading to an obvious TB refinement. TB strengthening 𝜎𝑦 can be well predicted by the Hall-Petch relationship: 1
𝜎𝑦 = 𝜎0 + 𝑘𝑑 −2
(6)
where 𝜎0 is the friction stress, 𝑑 is the grain size and k is a constant, as shown in Fig. 13. The slope of curve k in alloy subjected to compression-aging demonstrates 1
an incremental behaviour, and in an order of AZ31 (31.21 MPa ∙ 𝜇𝑚2 ), AZ61 (84.76 1
1
MPa ∙ 𝜇𝑚2 ) and AZ91 (182.56 MPa ∙ 𝜇𝑚2 ), as shown in Fig. 14. These values are much higher than that of alloy without the intermediate aging (Fig. 13 (b)), implying strong influence of alloying element and aging on TB strengthening.
5.Conclusion
In the present study, the effects of aging on twin boundary strengthening in Mg alloys AZ31, AZ61 and AZ91 was investigated. The main conclusion can be summarized as follows: 1. The {10-12} twins can form during compression to strain of 4% in our study. And TB refinement can be observed in all Mg alloys AZ31, AZ61 and AZ91 after cyclic compression with and without intermediate aging. TB refining process as a function of cyclic number can be well expressed in an exponential formula in Mg alloys.
11
2. The TBM can be stabilized after aging in Mg alloys AZ61 and AZ91, contributing to significant grain refinement. 3. TB strengthening is not obviously observed in AZ31 alloy. However, both the hardness and UTS increase obviously in Mg alloys AZ61 and AZ91, especially in AZ91, suggesting the strong influence of alloying element and aging on TB strengthening.
Acknowledgments The authors gratefully acknowledge the financial support from project of The Science Fund for Distinguished Young Scholars of Hunan Province, China (2016JJ1016), and the project of Innovation and Entrepreneur Team Introduced by Guangdong Province (201301G0105337290).
References
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with different strain rates at 423K, Mater. Sci. Eng. A, 612 (2014) 423-430. [4] J.A. Yasi, L. G. Hector, D.R. Trinkle, First-principles data for solid-solution strengthening of magnesium: From geometry and chemistry to properties, Acta Mater., 58 (2010) 5704-5713. [5] J. C. Baird, B. Li, S.Y. Parast, S.J. Horstemeyer, L.G. Hector Jr., P.T. Wang, M.F. Horstemeyer, Localized twin bands in sheet bending of a magnesium alloy, Scr. Mater., 67 (2012) 471-474. [6] H.H. Yu, Y.C. Xin, A. Chapuis, X.X. Huang, R.L. Xin, Q. Liu, The different effects of twin boundary and grain boundary on reducing tension-compression yield asymmetry of Mg alloys, Sci. Reports, 6 (2016) 292283. [7] Y.P. Li, S. Wu, H.K. Bian, N. Tang, B. Liu, Y. Koizumi, A. Chiba, Grain refinement due to complex twin formation in rapid hot forging of magnesium alloy, Scr. Mater, 68 (2013) 171-174. [8] Y.C Xin, M.Y. Wang, Z. Zeng, M.G. Nie, Q. Liu, Strengthening and toughening of magnesium alloy by {10-12} extension twins, Scr. Mater., 66 (2012) 25-28. [9] Y. P. Li, M. Enoki, Anelastic recovery of pure magnesium quantitatively evaluated by acoustic emission, J. Mater. Res. 26 (2011) 3098-3106. [10] J. F. Nie, Y. M. Zhu, J. Z. Liu, X. Y. Fang, Periodic segregation of solute stoms in fully coherent twin boundaries, Science. 340 (2013) 957-960. [11] Y.C Xin, X.J. Zhou, H.W. Chen, J.F. Nie, H. Zhang, Y.Y. Zhang, Q. Liu, Annealing hardening in detwinning deformation of Mg-3Al- 1Zn alloy, Mater. Sci. Eng. A, 594 (2014) 287-291. 13
[12] Y.J. Cui, Y.P. Li, Z.C. Wang, X. Ding, Y. Koizumi, H.K. Bian, L.Y. Lin, A. Chiba, Impact of solute elements on detwinning in magnesium and its alloys, Int. J. Plast. 91 (2017) 134-159. [13] R. Zeng, Precipitation hardening in AZ91 magnesium alloy, University of Birmingham, (2013) p22 [14] Y.J. Cui, Y.P. Li, Z.C. Wang, Y. Koizumi, A. Chiba, Regulating twin boundary mobility by annealing in magnesium and its alloys, Inter J Plast, 99 (2017) 1-18. [15] J.D. Robson, N. Stanford, M.R. Barnett, Effect of particles in promoting twin nucleation in a Mg-5 wt.% Zn alloy, Scr. Mater., 63 (2010) 823-826. [16] H. Somekawa, T. Mukai, Hall–Petch relation for deformation twinning in solid solution magnesium alloys, Mater. Sci. Eng. A. 561 (2013) 378-385. [17] Y.J. Cui, Y.P. Li, S.H. Sun, H. Huang, Z.C. Wang, Y. Koizzumi, A. Chiba, Enhanced damping capacity of magnesium alloys by tensile twin boundaries, Scr. Mater. 101 (2015) 8-11. [18] N. Stanford, J. Geng, Y. B. Chun, C. H. J. Davies, J. F. Nie, M. R. Barnett, Effect of plate-shaped particle distributions on the deformation behaviour of magnesium alloy AZ91 in tension and compression, Acta. Mater., 60 (2012) 218-228 [19] X.Y. Shi, Y. Liu, J. Lu, R.E.A. Williams, D.J. Li, X.Q. Zeng, A. A. Luo, Formation of a new incoherent twin boundary in a Mg-3Gd alloy, Scr. Mater., 112 (2016) 136-139. [20] M. Ghazisaeidi, L.G. Hector Jr., W.A. Curtin, Solute strengthening of twinning dislocations in Mg alloys, Acta. Mater. 80 (2014) 278-287. 14
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Captions: Fig. 1. Schematic diagram of cyclic compression-aging process. Fig. 2. Dimensions of tensile samples for the tensile test Fig. 3. Microstructure of (a) AZ31, (b) AZ61 and (c) AZ91 after solution treatment (ST) at 420 ℃ for 17 h. Fig. 4. Microstructure of (a,b,c,d) AZ31, (e,f,g,h) AZ61 and (i,j,k,l) AZ91 after compression (a,e,i) 1 cycle, (b,f,j) 3 cycles, (c,g,k) 5 cycles, (d,h,l) 10 cycles with aging at 250 ℃ for 20 min. Fig. 5. Microstructure of (a) AZ31, (b) AZ61 and (c) AZ91 after compression for 3 cycles with aging and (d) AZ31, (e) AZ61 and (f) AZ91 after compression for 3 cycles without aging. Fig. 6. Mean grain size of Mg alloys AZ31, AZ61 and AZ91 at different compression-aging cycles. Fig. 7. Mean grain size of Mg alloys AZ31, AZ61 and AZ91 at different compression cycles (no aging). Fig. 8. Mean grain size of AZ31, AZ61 and AZ91 after 10 compressions with and without aging 15
Fig. 9. Hardness variation of Mg alloys AZ31, AZ61 and AZ91 at different compression-aging cycles. Fig. 10. Hardness variation of Mg alloys AZ31, AZ61 and AZ91 at different compression cycles (no aging). Fig. 11. Stress-strain curves in samples after solution treatment and 10th compression-aging of AZ31, AZ61 and AZ91 alloys. Fig. 12. Variation on ultimate tensile strength (UTS) (a) and elongation (b) of Mg alloys AZ31, AZ61 and AZ91 in ST condition and after 10th compression-aging cycles. Fig. 13. Hardness variation of Mg alloys AZ31, AZ61 and AZ91 alloys on different grain size caused by different cycles of experiment compression with (a) and without (b) aging. Fig. 14. Variation of slope for Hall-Petch relation in the Mg alloys AZ31, AZ61 and AZ91.
Table 1. d0, b and m of the exponential formula and R-Square of fitting curve after compression with (a) and without (b) aging.
16
load
load aging
aging load
load
...
load
load
load
load
Cyclic number:
aging
aging
1
N
Fig. 1. Schematic diagram of cyclic compression-aging process.
2 mm
1.6 mm
24.9 mm
11.5 mm
R1.7 mm
5 mm
5 mm
Fig. 2. Dimensions of tensile samples for the tensile test
17
100μm
100μm
100μm
Fig. 3. Microstructure of (a) AZ31, (b) AZ61 and (c) AZ91 after solution treatment (ST) at 420 ℃ for 17 h.
Cycles:
1
3
5
10
AZ31 100μm
100μm
100μm
100μm
100μm
100μm
100μm
100μm
100μm
100μm
100μm
100μm
AZ61
AZ91
Fig. 4. Microstructure of (a,b,c,d) AZ31, (e,f,g,h) AZ61 and (i,j,k,l) AZ91 after compression (a,e,i) 1 cycle, (b,f,j) 3 cycles, (c,g,k) 5 cycles, (d,h,l) 10 cycles with aging at 250 ℃ for 20 min.
18
AZ31
AZ61
AZ91
100μm
100μm
100μm
100μm
100μm
100μm
Grain size after compression with aging, d(m)
Fig. 5. Microstructure of (a) AZ31, (b) AZ61 and (c) AZ91 after compression for 3 cycles with aging and (d) AZ31, (e) AZ61 and (f) AZ91 after compression for 3 cycles without aging.
300 AZ31 experimental data AZ61 experimental data AZ91 experimental data
270 240
AZ31 fitting curve AZ61 fitting curve AZ91 fitting curve
210 180 150 120 90 60 30 0
0
2 4 6 8 Cyclic number of experiment, N
10
Fig. 6. Mean grain size of Mg alloys AZ31, AZ61 and AZ91 at different compression-aging cycles.
19
Grain size after compression without aging, d (m)
300 AZ31 experimental data AZ61 experimental data AZ91 experimental data
270
AZ31 fitting curve AZ61 fitting curve AZ91 fitting curve
240 210 180 150 120 90 60 0
2
4
6
8
10
Cyclic number of experiment, N
Fig. 7. Mean grain size of Mg alloys AZ31, AZ61 and AZ91 at different compression cycles (no aging).
Mean grain size ,d (m)
120
compression with aging compression without aging
100 80 60 40 20 0
AZ31
AZ61
AZ91
Fig. 8. Mean grain size of AZ31, AZ61 and AZ91 after 10th compression with and without aging
20
Hardness after compression with aging,HV
100 90
AZ91
80 70 AZ61 60 AZ31
50 40 0
2 4 6 8 Cyclic number of experiment, N
10
Hardness after compression without aging,HV
Fig. 9. Hardness variation of Mg alloys AZ31, AZ61 and AZ91 at different compression-aging cycles.
100 90 80 70 AZ91 60 AZ61 50 40
AZ31 0
2 4 6 8 Cyclic number of experiment, N
10
Fig. 10. Hardness variation of Mg alloys AZ31, AZ61 and AZ91 at different compression cycles (no aging).
21
AZ31
225
150 125 100 75 50
175 150 125 100
25 0
ST 10th
200
175
True stress, (Mpa)
True stress, (Mpa)
200
AZ61
225
ST 10th
75 50 25
0
1
2
3
4
5
6
7
0
8
0
1
True strain, (%)
3
4
5
6
7
True strain, (%)
AZ91
225
ST 10th
200
True stress, (Mpa)
2
175 150 125 100 75 50 25 0 0
1
2
3
4
5
6
7
8
True strain, (%)
Fig. 11. Stress-strain curves in samples after solution treatment and 10th compression-aging of AZ31, AZ61 and AZ91 alloys.
(a)
(b) 8
240 1%up
10 Cycles
22%up
5%up
160 120 80 40 0
7
Tensile strain, (%)
Tensile strength, UTS(MPa)
ST
200
4%down
6
ST
10 Cycles
8%down
20%down
AZ61
AZ91
5 4 3 2 1 0
AZ31
AZ61
AZ91
AZ31
Fig. 12. Variation on ultimate tensile strength (UTS) (a) and elongation (b) of Mg alloys AZ31, AZ61 and AZ91 in ST condition and after 10th compression-aging cycles.
22
8
(b) AZ91 Hardness, HV
Hardness , HV
(a) 95 90 85 80 75 70 65 60 55 50 45 40
AZ61
AZ31
0.05
0.10
d
0.15 0.20 -1/2 (m )
0.25
95 90 85 80 75 70 65 60 55 50 45 40
AZ91 AZ61 AZ31
0.06
-1/2
0.07
0.08 -1/2
d
(m
0.09 -1/2
)
Fig. 13. Hardness variation of Mg alloys AZ31, AZ61 and AZ91 alloys on different grain size caused by different cycles of experiment compression with (a) and without (b) aging.
23
0.10
220 200
1/2
Slope, K(MPad )
180 160 140 120 100 80 60 40 20 AZ31
AZ61
AZ91
Fig. 14. Variation of slope for Hall-Petch relation in the Mg alloys AZ31, AZ61 and AZ91.
Table 1. d0, b and m of the exponential formula and R-Square of fitting curve after compression with (a) and without (b) aging. (a)
AZ31
AZ61
AZ91
d0
31.72
19.33
19.55
b
211.93
208.13
223.58
m
0.40
0.82
1.02
R-Square
0.96
0.98
0.94
24
(b)
AZ31
AZ61
AZ91
d0
95.59
102.93
94.97
b
174.37
129.49
153.68
m
0.38
0.33
0.34
R-Square
0.88
0.99
0.97
25