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Indentation creep behavior of AE42 and Ca-containing AE41 alloys
Materials Letters 61 (2007) 1015 – 1019 www.elsevier.com/locate/matlet
Indentation creep behavior of AE42 and Ca-containing AE41 alloys Huang Deming, Chen Yungui ⁎, Tang Yongbai, Liu Hongmei, Niu Gao School of Materials Science and Engineering, Sichuan University, Chengdu 610065, China Received 8 November 2005; accepted 15 June 2006 Available online 13 July 2006
Abstract The indentation creep behavior of AE42 and 0.4–1.2 wt.% Ca-containing AE41 alloys was studied. The microstructure was analyzed by an optical microscope, XRD and SEM equipped EDS before and after indentation creep. The results indicate that the creep resistance of AE41 alloys is improved with the addition of Ca. The indentation creep resistance of Ca-containing AE41 alloys is better than that of AE42 at 150 °C and 175 °C. The microstructure analysis shows that the Al11Nd3 phase in AE42 is prone to decompose above 150 °C, which deteriorates its creep resistance behavior. Ca-containing AE41 alloys have good indentation creep resistance because of the formation of heat-resistant phase Al2Ca in the alloys. © 2006 Elsevier B.V. All rights reserved. Keywords: Magnesium alloy; Calcium; Rare earth; Indentation creep; Microstructure
1. Introduction In recent years, magnesium alloys are increasingly used in the automotive industry because of their low density and good damping property, etc. But their poor elevated temperature properties, especially creep resistance, limit their wider application, which are attributed to low melting point that is in the range from 470 °C to 650 °C related to their compositions. Druschitz et al. [1] considered AE42 alloy to offer the best combination of properties after several typical kinds of heat-resistance magnesium alloys were examined. Unfortunately, AE42 alloy is expensive due to the high content of rare earth elements. Moreover, its castability is poor. After a more in depth study, Moreno et al. [2] and Powell et al. [3] thought that the creep resistance of AE42 would decrease above 150 °C. The reason was that the Al11RE3 intermetallic phase in AE42 alloy was prone to decompose and resulted in the formation of Mg17Al12, which would be responsible for the deterioration of creep resistance at elevated temperature. For Mg– Al based alloys, the effective means to improve their heat resistance is alloying by adding some other elements and decreasing aluminum content simultaneously to restrain the occurrence of
⁎ Corresponding author. E-mail address: [email protected] (H. Deming). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.06.064
Mg17Al12 compound [4]. In view of this, our study will be based on AE41 alloy. A little amount of Ca was added into AE41 alloy in order to improve its properties at elevated temperature, and their indentation creep behaviors will be discussed in the paper. 2. Experimental procedure Four kinds of alloys, AE42 and three Ca-containing AE41 alloys (AEX04, AEX08 and AEX12), are prepared with nominal compositions listed in Table 1. The addition of calcium was performed by adding a master alloy Mg–30 wt.% Ca. Rare earth elements are added in the form of a master alloy Mg–10 wt.% RE (didymium consists of 75 wt.% Nd and 25 wt.% Pr). Melting of the investigated alloys was conducted in an electric resistance furnace with a mild steel crucible and protected by a selfmade flux as the protection medium. The melt was held at 750 °C for 30 min and then poured into a permanent mould at 720 °C. The mould temperature is 250 °C. The size of the cast samples is 140 mm in length, 20 mm in width and 110 mm in height. The indentation creep tests [5–8] were carried out on a Brinell tester at 150 °C and 175 °C. The temperature fluctuation is ±1 °C. The load is 294 N. The diameter of the global indenter is 10 mm. The creep specimens with the size of 30 mm × 15 mm × 14 mm were cut by electric spark machining from the bottom of the ingots. Oil was used as the protective medium during the creep
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Table 1 Chemical composition of the investigated alloys Alloy no.
Chemical composition (wt.%) Al
RE
Ca
Mg
AE42 AEX04 AEX08 AEX12
4 4 4 4
2 1 1 1
0 0.4 0.8 1.2
Bal. Bal. Bal. Bal.
test and was kept for 10 min until the temperature reaches the setting point before loading. For each alloy, six data points would be collected. Each average datum point was repeated three times at least. The selection of holding time for each data point was 30 s, 5 min, 30 min, 60 min, 120 min and 240 min, respectively. The size of the indentation diameter was measured by an instrumental microscope after the creep tests were completed and the samples cooled down. The samples were split along the indentation diameter by electric spark machining in order to observe the microstructure after creep. The microstructure analysis was carried out with the help of an optical microscope and SEM equipped EDS before and after creep. The phases of the investigated alloys were analyzed by X-ray diffractometer operating at 40 kV and 40 mA using Cu-Kα radiation. 3. Results and discussion 3.1. Indentation creep of the investigated alloys 3.1.1. The relation between indentation diameter and holding time The results of the creep test are expressed as the relation between diameter and holding time. They are shown in Fig. 1. These curves are similar to that of tensile creep. The mathematical model of these curves is: d ¼ at n
ð1Þ
Where d is the diameter of indentation, t is the holding time, α and n are constant. From the analysis of the creep test results, the values of α are between 1920 and 2060 at 150 °C and are in the range from 1970 to 2060 at 175 °C. For the investigated alloys, the value of n is among 0.020–0.023 and they are almost equal. So the creep resistance of the investigated alloys can be evaluated in terms of the value of α. The bigger the value of α is, the poorer the creep resistance is. A conclusion could be drawn from the formula of each curve that the creep resistance of AEX04 alloy is similar to
that of AE42, and the properties of AEX08 and AEX12 are better than that of AE42 alloy at 150 °C. The change of creep resistance at 175 °C among the investigated alloys are the same as that at 150 °C. The difference is that the creep resistance of AEX04 is better than that of AE42 at 175 °C. Therefore, it can be seen that calcium can improve the creep resistance of AE41 alloy at elevated temperature. 3.1.2. The relation between creep rate and holding time In order to denote the creep rate of the investigated alloys, the change rate of the indentation diameter with the increase of holding time is expressed as creep rate. That is: d V¼ lim
DtY0
Dd Dt
According to the above mentioned mathematical method, the mathematical expression of creep rate can be deduced from formula (1). That is: d V¼ ant n−1
ð2Þ
According to formula (2), the creep rate of each alloy can be calculated at the same temperature. The results are shown in Fig. 2. From the mathematical expression of the creep rates of the investigated alloys in Fig. 2, it can be seen that the creep rate of AEX08 and AEX12 is less than that of AE42 on the same holding time at 150 °C and 175 °C. So it can be concluded that the creep resistance of AEX alloys is better than that of AE42. Comparing the creep rates of each alloy on the same holding time at different temperatures, it can be found that the creep rates of 150 °C are less than those of 175 °C. Creep rates of the investigated alloys present the same tendency at 150 °C and 175 °C. The order of the creep rate from large to small is AE42, AEX04, AEX08 and AEX12. 3.1.3. The relation between indentation creep rates and load In the experiment, the load is constant. Because the indenter is spherical, and the diameter of indentation is variable with the change of the load time, and the stress on the unit surface of indentation constantly decreased with the increase of the indentation diameter. The relation between creep rate and stress of each datum point is shown in Fig. 3, where k is the slope of the curves, the abscissa denoted the stress P (here p is predigested as 4F / (πd2), F = 294 N), and the ordinate denoted the logarithm of the creep rates (that is Lnd′ = Ln(αntn−1)). Fig. 3 shows that the results are linear. The effect of stress on creep rates of AE42 and AEX04 alloy is almost alike at 150 °C. But for AEX08 and AEX12, the effect is less. The effect of stress on creep rates
Fig. 1. The indentation creep curves of magnesium alloys at 150 °C and 175 °C.
H. Deming et al. / Materials Letters 61 (2007) 1015–1019
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Fig. 2. The indentation creep rates of magnesium alloys at 150 °C and 175 °C.
Fig. 3. The relations between indentation creep rates and stress at 150 °C and 175 °C.
of AE42 is more notable than Ca-containing AE41 alloys at 175 °C. For the alloys having a small amount of calcium, the stronger the creep resistance gained, the more amount of calcium added. So calcium can remarkably improve the creep resistance of AE41 alloy on heavy stress conditions. Comparing the creep rates of different alloys at 150 °C and 175 °C on the same stress conditions, it can be seen that the creep resistance of each alloy decreased when the temperature increased. But the decrease of creep resistance of Ca-containing AE41 alloys is less than that of AE42. This indicates that Ca-containing AE41 alloys have better creep resistance at higher temperature.
3.2. Microstructure analysis before and after creep 3.2.1. Microstructure change of AE42 before and after creep Fig. 4 shows the SEM micrographs of AE42 alloy before and after creep at 175 °C. As shown in Fig. 4a, there exist needle-shaped white compounds before creep. Micro-zone compositions analysis was performed on these white phases by EDS, and the results show that they contain aluminum and rare earth elements, neodymium and praseodymium. The ratio of Al, Nd and Pr atoms is 20.5:4.2:1.9. Fig. 5 is an XRD pattern of AE42 before creep, in which peaks were indexed as arising from
Fig. 4. The SEM image of AE42 alloy (a) before creep and (b) creep for 72 h.
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Fig. 5. The XRD spectra of AE42 alloy before creep and creep for 72 h.
Fig. 7. The XRD spectra of AE42 alloy before creep and creep for 72 h.
two different phases, α-Mg and Al11Nd3. It is obvious that these white needle-shaped phases are Al11Nd3, and the black is α-Mg matrix. Fig. 4b shows that white phases, shorter and slimmer, are still needle-shaped, and a small amount of granule phases emerged from it. EDS analysis indicated that the atom ratio of Al, Nd and Pr is 14.1:4.9:2.0 in the smallish needle-shaped phases and granule phases. The ratio of the above mentioned three elements is 19.8:4.5:1.7 in these slim needle-shaped phases. XRD analysis (shown in Fig. 5) exposed that the phases of AE42 after creep are α-Mg matrix, Al11Nd3 and Al2Nd. So the smallish white acicular phase and granule phase are Al2Nd, and the slim white needle-shaped phase is Al11Nd3. From the change of the microstructure of AE42 alloy before and after creep, it is clear that Al11RE3 phase is instable, part of it will decompose at high temperature during creep test. Powell et al. [3] deemed that Al11RE3 decomposed at high temperature, and formed Al2RE and Mg17Al12 phases at last. Moreno et al. [2] evidenced that this decomposability indeed existed. Therefore, it is deduced that decomposability has taken place during the high temperature creep test. That is, Al11Nd3 → 3Al2Nd + 5Al. The decomposed Al will react with Mg, and form Mg17Al12 finally. That is, 12Al + 17Mg → Mg17Al12. But Mg17Al12 was not found in our study, the reason may be that the amount of decomposed Al is small and can not be detected by XRD. Just because Al11Nd3 is instable, the phase Mg17Al12 tends to soften at high temperature, which results in the deterioration of the creep resistance of the AE42 alloy.
3.2.2. Microstructure change of Ca-containing AE41 alloys before and after creep Fig. 6 shows the microstructure of AEX08 alloy before and after creep. There is a distinct difference between AE42 and AEX08 alloys in the microstructure. Most of the white second phases in AEX08 are mainly skeleton-shaped, and the rest are a small quantity of short rodlike and granule phases. The skeleton-shaped compounds are (Mg, Al)2Ca and the short rod-like and granule phases are Al2Nd with the help of XRD and EDS analyses. Moreover, there are needle-shaped Al11Nd3, which can not be detected by XRD because of their small amount. The difference of the microstructure among AEX04, AEX08 and AEX12 is the amount of the second phase, in which the content of the Al–RE phase reduced and the amount of the (Mg,Al)2Ca phase enhanced with Ca added increasingly. It seems that there is no change in the microstructure of AEX08 before and after creep from the SEM micrograph and XRD (shown in Fig. 7) analysis, and the morphology of the (Mg,Al)2Ca phase is also unchanged after creep. Due to the similarity of their crystal structure, the (Mg,Al)2Ca compound can be hardly distinguished from the Al2Ca phase by XRD analysis [9–11]. Suzuki et al. [12] found that the (Mg,Al)2Ca compound would transform to Al2Ca phase by a shear mechanism at 300 °C, and the transformation from (Mg,Al)2Ca compound to Al2Ca phase does not require separate nucleation of the product phase but proceeds within the matrix (Mg,Al)2Ca because of the close relationship of the crystal structure, which does not cause significant changes in the microstructural
Fig. 6. The SEM image of AEX08 alloys (a) before creep and (b) creep for 72 h.
H. Deming et al. / Materials Letters 61 (2007) 1015–1019
morphology. It can be seen that the (Mg,Al)2Ca phase, different from the Al11RE3 phase, can help to stabilize the microstructure during creep at elevated temperature. When the content of RE decreases from 2 wt.% to 1 wt.% and a small amount of calcium is added constantly, the microstructure of the alloys, in which the amount of needle-shaped Al11Nd3 decreases and the content of (Mg,Al)2Ca and Al2Nd increases, will change remarkably. Because Al11Nd3 is replaced by more stable phase (Mg, Al)2Ca and Al2Nd phases, the creep resistance of Ca-containing AE41 alloys is better than that of AE42 at 150 °C and 175 °C. In addition, the amount of (Mg,Al)2Ca increases and the Al–RE phase decreases with the content of Ca increasing, which contributed to the stability of the microstructure and improves creep resistance of Ca-containing AE41 alloys.
4. Conclusions 1. The indentation creep resistance of Ca-containing AE41 alloys is better than that of AE42 at 150 °C and 175 °C, and the creep resistance of Ca-containing AE41 alloys becomes better with the increase of Ca. 2. Al11Nd3 phase is instable at elevated temperature on creep conditions. Part of it will decompose and change into Al2Nd.
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Ca-containing AE41 alloys have better creep resistance at elevated temperature because stable (Mg,Al)2Ca phases exist in their microstructure. References [1] Alan P. Druschitz, Eric R. Showalter, Joseph B. McNeill, et al., Magnesium Technology 2002 as held at the 2002 TMS Annual Meeting; Seattle, WA USA, 17–21 Feb. 2002, p. 117. [2] I.P. Moreno, T.K. Nandy, J.E. Allison, et al., Scr. Mater. 48 (2003) 1029. [3] Bob R. Powell, Vadim Rezhets, Michael P. Balogh, et al., JOM 54 (8) (1 August 2002) 34. [4] A.A. Luo, in: H.I. Kaplan, J. Hryn, B. Clow (Eds.), Magnesium Technology. The Minerals, Metals and Materials Society/AIME, 2000, p. 89. [5] A. Juhasz, P. Tasnadi, I. Kovacs, J. Mater. Sci. Lett. 5 (1986) 35–36. [6] P.M. Sargent, M.F. Ashby, Mater. Sci. Technol. 8 (1992) 594–601. [7] S.J. Sharp, M.F. Ashby, N.A. Fleck, Acta Metall. Mater. 41 (3) (1993) 685–692. [8] W.B. Li, J.L. Henshall, R.M. Hooper, et al., Acta Metall. Mater. 39 (12) (1991) 3099–3110. [9] A. Suzuki, N.D. Saddock, J.W. Jones, et al., Scr. Mater. 51 (2004) 1005. [10] P.M. Hazzledine, P. Pirouz, Scr. Metall. Mater. 28 (1993) 1277. [11] K.S. Kumar, P.M. Hazzledine, Intermetallics 112 (2004) 763. [12] A. Suzuki, N.D. Saddock, J.W. Jones, et al., Acta Mater. 53 (2005) 2823–2834.
Indentation creep behavior of AE42 and Ca-containing AE41 alloys Huang Deming, Chen Yungui ⁎, Tang Yongbai, Liu Hongmei, Niu Gao School of Materials Science and Engineering, Sichuan University, Chengdu 610065, China Received 8 November 2005; accepted 15 June 2006 Available online 13 July 2006
Abstract The indentation creep behavior of AE42 and 0.4–1.2 wt.% Ca-containing AE41 alloys was studied. The microstructure was analyzed by an optical microscope, XRD and SEM equipped EDS before and after indentation creep. The results indicate that the creep resistance of AE41 alloys is improved with the addition of Ca. The indentation creep resistance of Ca-containing AE41 alloys is better than that of AE42 at 150 °C and 175 °C. The microstructure analysis shows that the Al11Nd3 phase in AE42 is prone to decompose above 150 °C, which deteriorates its creep resistance behavior. Ca-containing AE41 alloys have good indentation creep resistance because of the formation of heat-resistant phase Al2Ca in the alloys. © 2006 Elsevier B.V. All rights reserved. Keywords: Magnesium alloy; Calcium; Rare earth; Indentation creep; Microstructure
1. Introduction In recent years, magnesium alloys are increasingly used in the automotive industry because of their low density and good damping property, etc. But their poor elevated temperature properties, especially creep resistance, limit their wider application, which are attributed to low melting point that is in the range from 470 °C to 650 °C related to their compositions. Druschitz et al. [1] considered AE42 alloy to offer the best combination of properties after several typical kinds of heat-resistance magnesium alloys were examined. Unfortunately, AE42 alloy is expensive due to the high content of rare earth elements. Moreover, its castability is poor. After a more in depth study, Moreno et al. [2] and Powell et al. [3] thought that the creep resistance of AE42 would decrease above 150 °C. The reason was that the Al11RE3 intermetallic phase in AE42 alloy was prone to decompose and resulted in the formation of Mg17Al12, which would be responsible for the deterioration of creep resistance at elevated temperature. For Mg– Al based alloys, the effective means to improve their heat resistance is alloying by adding some other elements and decreasing aluminum content simultaneously to restrain the occurrence of
⁎ Corresponding author. E-mail address: [email protected] (H. Deming). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.06.064
Mg17Al12 compound [4]. In view of this, our study will be based on AE41 alloy. A little amount of Ca was added into AE41 alloy in order to improve its properties at elevated temperature, and their indentation creep behaviors will be discussed in the paper. 2. Experimental procedure Four kinds of alloys, AE42 and three Ca-containing AE41 alloys (AEX04, AEX08 and AEX12), are prepared with nominal compositions listed in Table 1. The addition of calcium was performed by adding a master alloy Mg–30 wt.% Ca. Rare earth elements are added in the form of a master alloy Mg–10 wt.% RE (didymium consists of 75 wt.% Nd and 25 wt.% Pr). Melting of the investigated alloys was conducted in an electric resistance furnace with a mild steel crucible and protected by a selfmade flux as the protection medium. The melt was held at 750 °C for 30 min and then poured into a permanent mould at 720 °C. The mould temperature is 250 °C. The size of the cast samples is 140 mm in length, 20 mm in width and 110 mm in height. The indentation creep tests [5–8] were carried out on a Brinell tester at 150 °C and 175 °C. The temperature fluctuation is ±1 °C. The load is 294 N. The diameter of the global indenter is 10 mm. The creep specimens with the size of 30 mm × 15 mm × 14 mm were cut by electric spark machining from the bottom of the ingots. Oil was used as the protective medium during the creep
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Table 1 Chemical composition of the investigated alloys Alloy no.
Chemical composition (wt.%) Al
RE
Ca
Mg
AE42 AEX04 AEX08 AEX12
4 4 4 4
2 1 1 1
0 0.4 0.8 1.2
Bal. Bal. Bal. Bal.
test and was kept for 10 min until the temperature reaches the setting point before loading. For each alloy, six data points would be collected. Each average datum point was repeated three times at least. The selection of holding time for each data point was 30 s, 5 min, 30 min, 60 min, 120 min and 240 min, respectively. The size of the indentation diameter was measured by an instrumental microscope after the creep tests were completed and the samples cooled down. The samples were split along the indentation diameter by electric spark machining in order to observe the microstructure after creep. The microstructure analysis was carried out with the help of an optical microscope and SEM equipped EDS before and after creep. The phases of the investigated alloys were analyzed by X-ray diffractometer operating at 40 kV and 40 mA using Cu-Kα radiation. 3. Results and discussion 3.1. Indentation creep of the investigated alloys 3.1.1. The relation between indentation diameter and holding time The results of the creep test are expressed as the relation between diameter and holding time. They are shown in Fig. 1. These curves are similar to that of tensile creep. The mathematical model of these curves is: d ¼ at n
ð1Þ
Where d is the diameter of indentation, t is the holding time, α and n are constant. From the analysis of the creep test results, the values of α are between 1920 and 2060 at 150 °C and are in the range from 1970 to 2060 at 175 °C. For the investigated alloys, the value of n is among 0.020–0.023 and they are almost equal. So the creep resistance of the investigated alloys can be evaluated in terms of the value of α. The bigger the value of α is, the poorer the creep resistance is. A conclusion could be drawn from the formula of each curve that the creep resistance of AEX04 alloy is similar to
that of AE42, and the properties of AEX08 and AEX12 are better than that of AE42 alloy at 150 °C. The change of creep resistance at 175 °C among the investigated alloys are the same as that at 150 °C. The difference is that the creep resistance of AEX04 is better than that of AE42 at 175 °C. Therefore, it can be seen that calcium can improve the creep resistance of AE41 alloy at elevated temperature. 3.1.2. The relation between creep rate and holding time In order to denote the creep rate of the investigated alloys, the change rate of the indentation diameter with the increase of holding time is expressed as creep rate. That is: d V¼ lim
DtY0
Dd Dt
According to the above mentioned mathematical method, the mathematical expression of creep rate can be deduced from formula (1). That is: d V¼ ant n−1
ð2Þ
According to formula (2), the creep rate of each alloy can be calculated at the same temperature. The results are shown in Fig. 2. From the mathematical expression of the creep rates of the investigated alloys in Fig. 2, it can be seen that the creep rate of AEX08 and AEX12 is less than that of AE42 on the same holding time at 150 °C and 175 °C. So it can be concluded that the creep resistance of AEX alloys is better than that of AE42. Comparing the creep rates of each alloy on the same holding time at different temperatures, it can be found that the creep rates of 150 °C are less than those of 175 °C. Creep rates of the investigated alloys present the same tendency at 150 °C and 175 °C. The order of the creep rate from large to small is AE42, AEX04, AEX08 and AEX12. 3.1.3. The relation between indentation creep rates and load In the experiment, the load is constant. Because the indenter is spherical, and the diameter of indentation is variable with the change of the load time, and the stress on the unit surface of indentation constantly decreased with the increase of the indentation diameter. The relation between creep rate and stress of each datum point is shown in Fig. 3, where k is the slope of the curves, the abscissa denoted the stress P (here p is predigested as 4F / (πd2), F = 294 N), and the ordinate denoted the logarithm of the creep rates (that is Lnd′ = Ln(αntn−1)). Fig. 3 shows that the results are linear. The effect of stress on creep rates of AE42 and AEX04 alloy is almost alike at 150 °C. But for AEX08 and AEX12, the effect is less. The effect of stress on creep rates
Fig. 1. The indentation creep curves of magnesium alloys at 150 °C and 175 °C.
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Fig. 2. The indentation creep rates of magnesium alloys at 150 °C and 175 °C.
Fig. 3. The relations between indentation creep rates and stress at 150 °C and 175 °C.
of AE42 is more notable than Ca-containing AE41 alloys at 175 °C. For the alloys having a small amount of calcium, the stronger the creep resistance gained, the more amount of calcium added. So calcium can remarkably improve the creep resistance of AE41 alloy on heavy stress conditions. Comparing the creep rates of different alloys at 150 °C and 175 °C on the same stress conditions, it can be seen that the creep resistance of each alloy decreased when the temperature increased. But the decrease of creep resistance of Ca-containing AE41 alloys is less than that of AE42. This indicates that Ca-containing AE41 alloys have better creep resistance at higher temperature.
3.2. Microstructure analysis before and after creep 3.2.1. Microstructure change of AE42 before and after creep Fig. 4 shows the SEM micrographs of AE42 alloy before and after creep at 175 °C. As shown in Fig. 4a, there exist needle-shaped white compounds before creep. Micro-zone compositions analysis was performed on these white phases by EDS, and the results show that they contain aluminum and rare earth elements, neodymium and praseodymium. The ratio of Al, Nd and Pr atoms is 20.5:4.2:1.9. Fig. 5 is an XRD pattern of AE42 before creep, in which peaks were indexed as arising from
Fig. 4. The SEM image of AE42 alloy (a) before creep and (b) creep for 72 h.
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Fig. 5. The XRD spectra of AE42 alloy before creep and creep for 72 h.
Fig. 7. The XRD spectra of AE42 alloy before creep and creep for 72 h.
two different phases, α-Mg and Al11Nd3. It is obvious that these white needle-shaped phases are Al11Nd3, and the black is α-Mg matrix. Fig. 4b shows that white phases, shorter and slimmer, are still needle-shaped, and a small amount of granule phases emerged from it. EDS analysis indicated that the atom ratio of Al, Nd and Pr is 14.1:4.9:2.0 in the smallish needle-shaped phases and granule phases. The ratio of the above mentioned three elements is 19.8:4.5:1.7 in these slim needle-shaped phases. XRD analysis (shown in Fig. 5) exposed that the phases of AE42 after creep are α-Mg matrix, Al11Nd3 and Al2Nd. So the smallish white acicular phase and granule phase are Al2Nd, and the slim white needle-shaped phase is Al11Nd3. From the change of the microstructure of AE42 alloy before and after creep, it is clear that Al11RE3 phase is instable, part of it will decompose at high temperature during creep test. Powell et al. [3] deemed that Al11RE3 decomposed at high temperature, and formed Al2RE and Mg17Al12 phases at last. Moreno et al. [2] evidenced that this decomposability indeed existed. Therefore, it is deduced that decomposability has taken place during the high temperature creep test. That is, Al11Nd3 → 3Al2Nd + 5Al. The decomposed Al will react with Mg, and form Mg17Al12 finally. That is, 12Al + 17Mg → Mg17Al12. But Mg17Al12 was not found in our study, the reason may be that the amount of decomposed Al is small and can not be detected by XRD. Just because Al11Nd3 is instable, the phase Mg17Al12 tends to soften at high temperature, which results in the deterioration of the creep resistance of the AE42 alloy.
3.2.2. Microstructure change of Ca-containing AE41 alloys before and after creep Fig. 6 shows the microstructure of AEX08 alloy before and after creep. There is a distinct difference between AE42 and AEX08 alloys in the microstructure. Most of the white second phases in AEX08 are mainly skeleton-shaped, and the rest are a small quantity of short rodlike and granule phases. The skeleton-shaped compounds are (Mg, Al)2Ca and the short rod-like and granule phases are Al2Nd with the help of XRD and EDS analyses. Moreover, there are needle-shaped Al11Nd3, which can not be detected by XRD because of their small amount. The difference of the microstructure among AEX04, AEX08 and AEX12 is the amount of the second phase, in which the content of the Al–RE phase reduced and the amount of the (Mg,Al)2Ca phase enhanced with Ca added increasingly. It seems that there is no change in the microstructure of AEX08 before and after creep from the SEM micrograph and XRD (shown in Fig. 7) analysis, and the morphology of the (Mg,Al)2Ca phase is also unchanged after creep. Due to the similarity of their crystal structure, the (Mg,Al)2Ca compound can be hardly distinguished from the Al2Ca phase by XRD analysis [9–11]. Suzuki et al. [12] found that the (Mg,Al)2Ca compound would transform to Al2Ca phase by a shear mechanism at 300 °C, and the transformation from (Mg,Al)2Ca compound to Al2Ca phase does not require separate nucleation of the product phase but proceeds within the matrix (Mg,Al)2Ca because of the close relationship of the crystal structure, which does not cause significant changes in the microstructural
Fig. 6. The SEM image of AEX08 alloys (a) before creep and (b) creep for 72 h.
H. Deming et al. / Materials Letters 61 (2007) 1015–1019
morphology. It can be seen that the (Mg,Al)2Ca phase, different from the Al11RE3 phase, can help to stabilize the microstructure during creep at elevated temperature. When the content of RE decreases from 2 wt.% to 1 wt.% and a small amount of calcium is added constantly, the microstructure of the alloys, in which the amount of needle-shaped Al11Nd3 decreases and the content of (Mg,Al)2Ca and Al2Nd increases, will change remarkably. Because Al11Nd3 is replaced by more stable phase (Mg, Al)2Ca and Al2Nd phases, the creep resistance of Ca-containing AE41 alloys is better than that of AE42 at 150 °C and 175 °C. In addition, the amount of (Mg,Al)2Ca increases and the Al–RE phase decreases with the content of Ca increasing, which contributed to the stability of the microstructure and improves creep resistance of Ca-containing AE41 alloys.
4. Conclusions 1. The indentation creep resistance of Ca-containing AE41 alloys is better than that of AE42 at 150 °C and 175 °C, and the creep resistance of Ca-containing AE41 alloys becomes better with the increase of Ca. 2. Al11Nd3 phase is instable at elevated temperature on creep conditions. Part of it will decompose and change into Al2Nd.
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Ca-containing AE41 alloys have better creep resistance at elevated temperature because stable (Mg,Al)2Ca phases exist in their microstructure. References [1] Alan P. Druschitz, Eric R. Showalter, Joseph B. McNeill, et al., Magnesium Technology 2002 as held at the 2002 TMS Annual Meeting; Seattle, WA USA, 17–21 Feb. 2002, p. 117. [2] I.P. Moreno, T.K. Nandy, J.E. Allison, et al., Scr. Mater. 48 (2003) 1029. [3] Bob R. Powell, Vadim Rezhets, Michael P. Balogh, et al., JOM 54 (8) (1 August 2002) 34. [4] A.A. Luo, in: H.I. Kaplan, J. Hryn, B. Clow (Eds.), Magnesium Technology. The Minerals, Metals and Materials Society/AIME, 2000, p. 89. [5] A. Juhasz, P. Tasnadi, I. Kovacs, J. Mater. Sci. Lett. 5 (1986) 35–36. [6] P.M. Sargent, M.F. Ashby, Mater. Sci. Technol. 8 (1992) 594–601. [7] S.J. Sharp, M.F. Ashby, N.A. Fleck, Acta Metall. Mater. 41 (3) (1993) 685–692. [8] W.B. Li, J.L. Henshall, R.M. Hooper, et al., Acta Metall. Mater. 39 (12) (1991) 3099–3110. [9] A. Suzuki, N.D. Saddock, J.W. Jones, et al., Scr. Mater. 51 (2004) 1005. [10] P.M. Hazzledine, P. Pirouz, Scr. Metall. Mater. 28 (1993) 1277. [11] K.S. Kumar, P.M. Hazzledine, Intermetallics 112 (2004) 763. [12] A. Suzuki, N.D. Saddock, J.W. Jones, et al., Acta Mater. 53 (2005) 2823–2834.