- Email: [email protected]
Tensile and indentation creep behavior of Mg–5% Sn and Mg–5% Sn–2% Di alloys
Materials Science and Engineering A 464 (2007) 124–128
Tensile and indentation creep behavior of Mg–5% Sn and Mg–5% Sn–2% Di alloys Hongmei Liu, Yungui Chen ∗ , Yongbai Tang, Shanghai Wei, Gao Niu School of Materials Science and Engineering, Sichuan University, Chengdu 610065, China Received 4 August 2006; received in revised form 24 January 2007; accepted 19 February 2007
Abstract The tensile properties and the indentation creep resistance of as-cast Mg–5 wt% Sn and Mg–5 wt% Sn–2 wt% Di alloys were studied in this paper. It has been shown that the tensile properties of Mg–5% Sn–2% Di alloy are comparative with that of AE42 alloy at room temperature and at 150 ◦ C and 175 ◦ C. The indentation creep experiments were conducted at 150 ◦ C and 175 ◦ C, suggesting that Mg–5% Sn–2% Di alloy has significantly better indentation creep resistance than AE42. The microstructure of Mg–5% Sn alloy is characterized by the presence of thermally stable Mg2 Sn particles mainly along grain boundaries, and addition of 2 wt% Di to this alloy results in the appearance of new Sn–Di phase, which further significantly enhance the strength and creep resistance. © 2007 Published by Elsevier B.V. Keywords: Mg–Sn alloy; Didymium; Tensile; Indentation creep; Microstructure
1. Introduction The interest in Mg–Sn based alloys started in the early 1930s [1,2]. Recently after 2000 there has been a renewed global interest [3–6] in these alloys which are believed to have potential for applications at elevated temperatures. The intermetallic phase Mg2 Sn in Mg–Sn alloys has a much higher melting point (770 ◦ C) than that of the Mg17 Al12 phase (462 ◦ C) in Mg–Al alloys. Mg–Sn based alloys are therefore likely to be more creep resistant at elevated temperatures than Mg–Al based alloys. Recent studies on the influence of alloying additions such as Ca, Si on the microstructure and the tensile properties and creep resistance of Mg–Sn alloys indicated clearly the improvement of their creep properties [7,8]. Rare earth (RE) elements are important alloying elements to magnesium alloys, which can improve both the room temperature and the elevated temperature properties of magnesium alloys, especially the latter [9]. But about the creep behavior Mg–Sn–RE lacks systemic and ingoing research at present. Indentation creep test is a convenient and efficient creep testing way. Masami Fujiwara et al thinks one of the great advantages of this way is that different creep information can be obtained from a limited supply of material [10]. Our previous
∗
Corresponding author. Tel.: +86 28 85405670; fax: +86 28 85407335. E-mail address: [email protected] (Y. Chen).
0921-5093/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.msea.2007.02.061
works have shown that the Mg–5% Sn and Mg–5% Sn–2% Di alloys offer good comprehensive properties [11,12]. This paper focus on the tensile and indentation creep behavior of as-cast Mg–5 wt% Sn–2 wt% Di alloys. 2. Experiment Three alloys are prepared with the nominal compositions of Mg–5 wt% Sn and Mg–5 wt% Sn–2 wt% Di, Mg–4 wt% Al–2 wt% Di (AE42). Pure magnesium (99.95 wt%) and pure tin (99.98 wt%) were melted in a magnesium oxide crucible under the protection of a RJ-2 flux. Didymium was introduced in the form of a Mg–10 wt% Di (neodymium: praseodymium = 3:1) master alloy at 770 ◦ C. The melt was stirred to assist the dissolution of the master alloy. It was then held at 750 ◦ C for about 30 min, and finally cast into a copper mould that was preheated up to 250 ◦ C. The cavity dimension of the copper is 20 mm × 110 mm × 140 mm. The specimens were cut into slices with an electrical discharge wire-cutting at the middle of each sample. The gage dimension of each specimen is 18.0 mm × 3.5 mm × 2.0 mm. Six samples of each alloy were employed to obtain one set of mechanical data. The tensile tests were conducted at room temperature 20 ◦ C, 150 ◦ C and 175 ◦ C and with a crosshead speed of 2 mm/min on an electro-universal testing machine (Instron5569).
H. Liu et al. / Materials Science and Engineering A 464 (2007) 124–128
125
The indentation creep tests were carried out on a Brinell tester at 150 ◦ C and 175 ◦ C (±1 ◦ C) with a constant load of 30 kg. The creep specimens were cut by electric spark machining from the bottom of the ingots with a size of 30 mm × 15 mm × 14 mm. It kept 10 min when the temperature reaches the setting point before loading. A global quenching steel presser (Ø10 mm) was pressed into the test surface of hot materials, and oil was used as the protected medium during the creep test. Each average datum point was repeated three times at least. The selection of holding time for each datum point was 30 min, 60 min, 120 min, 240 min 480 min and 600 min, respectively. The size of indentation diameter was measured by instrumental microscope after the creep tests were completed and the samples cooled down. The microstructure and fracture analysis were carried out with the help of optical microscope (OLYMPUS-BH-2), scanning electron microscope (SEM) observations and energy dispersion spectrometry (EDS) (JEOL JSM-5910LV) analyses. 3. Results 3.1. Tensile properties Tensile properties of the alloys at various temperatures are shown in Table 1. At room temperature, the tensile properties of Mg–5% Sn–2% Di are higher than those of Mg–5% Sn and slightly less than those of AE42. With the increase of testing temperature, there is a decrease in strength for both alloys as expected. However, the decreasing degree of the Mg–5% Sn–2% Di alloy is the smallest in strength, while that of AE42 is the largest, indicating the better thermal stability of Mg–5% Sn and Mg–5% Sn–2% Di alloys. Fig. 1 shows the SEM tensile fracture micrographs of the Mg–5% Sn and Mg–5% Sn–2% Di at 175 ◦ C, which indicates the addition Di to Mg–5% Sn alloy resulting in a combination of ductile dimpling and cleavage.
Fig. 1. SEM tensile fracture micrographs of Mg–5% Sn (a) and Mg–5% Sn–2% Di (b) at 175 ◦ C.
expressed as creep rate. That is
3.2. Indentation creep behavior
d = d(t)
3.2.1. The indentation creep rate The relation between indentation diameter and holding time can be expressed as Eq. (1):
According to Eq. (2), the creep rate of each alloy can be calculated at the same temperature. The indentation creep rates are shown in Fig. 2. Eq. (3) is obtained by data fitting from Fig. 2, as follows:
d = d(t)
d = at b−1
(1)
where d is the diameter of indentation. With the aim of expressing the creep rate of the investigated alloys, the change rate of indentation diameter with the increase of holding time is
(2)
(3)
where a and b are constants, which are material-related constants, reflecting the creep resistance of materials. The bigger values of a and b are, the poorer creep resistance is. From Fig. 2
Table 1 Tensile properties of Mg–5% Sn, Mg–5% Sn–2% Di and AE42 alloys at various temperatures Alloys
Mg–5% Sn Mg–5% Sn–2% Di AE42
150 ◦ C
RT
175 ◦ C
σ b dropping ratio (%)
σ b (MPa)
δ (%)
σ b (MPa)
δ (%)
σ b (MPa)
δ (%)
150 ◦ C
175 ◦ C
122 155 165
7.8 8.4 10.1
99 132 126
11.5 17.8 21.2
81 118 104
13.6 19.3 23.4
18.9 14.8 23.6
33.6 23.9 37.0
126
H. Liu et al. / Materials Science and Engineering A 464 (2007) 124–128
Fig. 3. Logarithm of indentation rate vs. logarithm of indentation diameter at 150 ◦ C and 175 ◦ C. Fig. 2. Indentation creep rates at 150 ◦ C and 175 ◦ C.
it can be seen that the creep rate of Mg–5% Sn is higher than that of AE42, and Mg–5% Sn–2% Di is lower than AE42 on the same holding time at 150 ◦ C. The change tendency of the creep rate at 175 ◦ C for the investigated alloys is the same as that at 150 ◦ C. The only difference is that the creep rates at 175 ◦ C are larger than those at 150 ◦ C. 3.2.2. The nominal stress exponent (n) When an indenter is pressed into the surface of a hot material, it penetrates the material first because of yielding and then by creeping [10]. The creep rate curves suggest that when the holding time beyond 240 min, the indentation creep changes slightly, so the average indentation creep from 240 to 600 min can be taken as pseudo-steady state indentation creep process. In order to understand the detailed creep mechanism of the alloys, nominal stress exponents (n) were calculated. The indentation creep n is given by Eq. (4) [10]: 1 ∂ ln ds n= 1− (4) 2 ∂ ln d where n is the creep nominal stress exponent, ds the pseudosteady state indentation creep rate, and d the indentation diameter. n could give useful information for the creep controlling mechanisms. By plotting logarithmically the pseudo-steady state indentation creep rate ds versus the indentation diameter d, we can
calculate the nominal stress exponent n of these alloys at 150 ◦ C and at 175 ◦ C, which are shown in Fig. 3 and Table 2. These values can be used to infer the dominant creep mechanisms at the present test conditions. The stress exponent n of 2–4 is generally reported for grain boundary sliding, and n of 4–6 is for dislocation climb [13], while n > 6 showing intermediate phase particles strengthening controlled creep [14]. If the intermediate phase on the grain boundary or within matrix block grain boundary sliding, dislocation motion and lattice self-diffusion, the value of n will be higher. For the Mg–5% Sn and AE42 alloys, n are 4.24 and 4.53 at 150 ◦ C, 4.01 and 4.03 at 175 ◦ C, respectively, which suggests the validity of dislocation climb mechanism. For Mg–5% Sn–2% Di, n is 6.99 at 150 ◦ C and 6.88 at 175 ◦ C, respectively, indicating a strengthening controlled creep mechanism of intermediate phase particles. 4. Discussion The results of the tensile tests and the indentation creep tests indicate Mg–5% Sn has good creep resistance, which is mainly Table 2 Values of creep nominal stress exponent Alloy
Mg–5% Sn AE42 Mg–5% Sn–2% Di
Nominal stress exponent (n) 150 ◦ C
175 ◦ C
4.24 4.52 6.99
4.01 4.03 6.88
H. Liu et al. / Materials Science and Engineering A 464 (2007) 124–128
127
Fig. 5. The optical micrographs of solid-solution treated Mg–5% Sn (a) and Mg–5% Sn–2% Di alloys (b) at 520 ◦ C for 72 h.
Fig. 4. SEM micrographs of as-cast Mg–5% Sn (a), Mg–5% Sn–2% Di (b) and AE42 (c) alloys.
attributed to the presence of thermally stable Mg2 Sn particles in Mg–5% Sn alloy as shown in Fig. 4(a). The melting point of Mg2 Sn phase is 771.5 ◦ C in Mg–Sn alloys, while that of Mg17 Al12 in Mg–Al alloy is 465 ◦ C, so the addition of Sn in Mg alloys is possibly more effective to improve the mechanical properties of Mg alloys in comparison with the addition of Al in Mg alloys at elevated temperature.
Mg–5% Sn–2% Di alloy exhibits better tensile properties and much better indentation creep resistance than Mg–5% Sn and AE42 alloy at 150 ◦ C and 175 ◦ C (AE42 was considered as an excellent creep resistant magnesium alloy at this temperature range [15]). The cause of Mg–5% Sn–2% Di alloy surpassing Mg–5% Sn in creep resistance is the addition of Di into Mg–5% Sn alloy resulting in the formation of the feather-shaped Sn–Di phase, as shown in Fig. 4(b), to which mainly distributes in the grain boundary areas except a few in matrix and EDS analysis [12] suggests that the atomic ratio of Sn and Di is about 3:2. After the as-cast Mg-5% Sn and Mg–5% Sn–2% Di alloys are treated at 520 ◦ C for 72 h, the Mg2 Sn phase dissolves in the ␣-Mg matrix but Sn–Di phase does not appear to change in Fig. 5, which indicates that the feather-shaped Sn–Di phase is thermally more stable than Mg2 Sn phase. DSC analysis [12] indicates that the Mg2 Sn phase dissolves into the ␣-Mg phase when the temperature approaches to 480 ◦ C, while the dissolving temperature of the Sn–Di phase is 556.5 ◦ C, which confirms that the Sn–Di phase has better thermal stability. This is important for further improving the creep resistance of Mg–5% Sn alloys.
128
H. Liu et al. / Materials Science and Engineering A 464 (2007) 124–128
The only difference in composition between AE42 and Mg–5% Sn–2% Di is 4%Al in the former and 5% Sn in the latter, while Mg–5% Sn–2% Di has much better creep resistance than that of AE42. A primary reason is the presence of thermally stable Mg2 Sn particles in Mg–5% Sn–2% Di alloy while Mg17 Al12 in AE42 as in Fig. 4(c). Another main cause is that the size of the Sn–Di phase is finer than that of Al11 Di3 phase, and Al11 Di3 phase only distribute on the grain boundary, but a fraction of Sn–Di phase within matrix (Fig. 4). The thermally stable Mg2 Sn and Sn–Di particles along grain boundaries hinder both grain boundary migrating and sliding during high-temperature exposure. The presence of fine Sn–Di phase particles within matrix also contributes to the improvement of creep resistance of Mg–5% Sn–2% Di alloy by impeding the dislocation movement. Therefore, its value of stress exponent is much higher than that depended by dislocation climb controlling creep. The third factor may be Al11 Di3 phase is instable, part of which it will decompose at high temperature during creep test. Powell et al. [15] believed that Al11 RE3 decomposed at high temperature, and formed Al2 RE and Mg17 Al12 phases at last. That is, Al11 Nd3 → 3Al2 Nd +5Al. The decomposed Al would react with Mg, and form Mg17 Al12 finally. The phase Mg17 Al12 tends soft at high temperature, which results in the deterioration of the creep resistance of the AE42 alloy. 5. Conclusions (1) The tensile properties of Mg–5% Sn–2% Di are better than those of Mg–5% Sn and slightly less than those of AE42 at room temperature. At elevated temperatures the tensile properties of Mg–5% Sn–2% Di are the best among the investigated alloys. (2) The indentation creep experiments suggest that Mg–5 wt% Sn–2 wt% Di alloy has significantly better indentation creep resistance than AE42. The calculated values of nominal stress exponent show that the Mg–5% Sn and AE42 are controlled by dislocation climb controlled creep, and
Mg–5 wt% Sn–2 wt% Di alloy experiences a intermediate phase particles-controlled creep mechanism at the present test conditions. (3) Mg–5 wt% Sn alloy contains thermally stable Mg2 Sn particles, and addition of 2 wt% Di to this alloy results in the appearance of Sn–Di phase. The presence of these particles in Mg–5% Sn–2% Di alloy is responsible for its improved tensile and creep properties, better than those of AE42 alloy. Acknowledgements This research was supported under a grant from the Science & Technology Bureau of Sichuan Province of China. The authors thank the Analysis and Testing Center of Sichuan University for providing the necessary testing instruments. Constructive comments and suggestions from the reviewer are acknowledged. References [1] [2] [3] [4] [5] [6]
[7]
[8] [9] [10] [11] [12] [13] [14] [15]
G. Derge, A.R. Kommel, R.F. Mehl, Trans. AIME 124 (1937) 367. J. Van Der, Planken, J. Mater. Sci. 4 (1969) 927–929. N. Hort, Y. Huang, T.A. Leil, et al., Adv. Eng. Mater. 8 (2006) 359–364. C.L. Mendis, C.J. Bettles, M.A. Gibson, et al., Mater. Sci. Eng. A 435–436 (2006) 163–171. T.T. Sasaki, K. Oh-ishi, T. Ohkubo, K. Honoa, Scripta Mater. 55 (2006) 251–254. S. Cohen, G.R. Goren-Muginstein, S. Avraham, in: A.L Alan (Ed.), Magnesium Technology, TMS The Minerals, Metals & Materials Society, 2004, pp. 301–305. A.L. Bowles, H. Dieringa, C. Blawert, et al., in: A.L Alan (Ed.), Magnesium Technology, TMS The Minerals, Metals & Materials Society, 2004, pp. 307–310. D.H. Kang, S.S. Park, N.J. Kim, Mater. Sci. Eng. A 413–414 (2005) 555–560. Z. Lu, Magnesium Alloys, Chemical Industry Press, Beijing, 2004. M. Fujiwara, M. Otsuka, Mater. Sci. Eng. A 319–321 (2001) 929–933. H. Liu, Y. Chen, Y. Tang, et al., J. Alloys Compd., in press. H. Liu, Y. Chen, Y. Tang, et al., Mater. Sci. Eng. A 437 (2006) 348–355. A.A. Luo, Int. Mater. Rev. 49 (2004) 13–30. G.C. Davies, D.R.H. Jones, Scripta Mater. 25 (1996) 523–528. B.R. Powell, V. Rezhets, M.P. Balogh, et al., JOM 54 (2004) 34.
Tensile and indentation creep behavior of Mg–5% Sn and Mg–5% Sn–2% Di alloys Hongmei Liu, Yungui Chen ∗ , Yongbai Tang, Shanghai Wei, Gao Niu School of Materials Science and Engineering, Sichuan University, Chengdu 610065, China Received 4 August 2006; received in revised form 24 January 2007; accepted 19 February 2007
Abstract The tensile properties and the indentation creep resistance of as-cast Mg–5 wt% Sn and Mg–5 wt% Sn–2 wt% Di alloys were studied in this paper. It has been shown that the tensile properties of Mg–5% Sn–2% Di alloy are comparative with that of AE42 alloy at room temperature and at 150 ◦ C and 175 ◦ C. The indentation creep experiments were conducted at 150 ◦ C and 175 ◦ C, suggesting that Mg–5% Sn–2% Di alloy has significantly better indentation creep resistance than AE42. The microstructure of Mg–5% Sn alloy is characterized by the presence of thermally stable Mg2 Sn particles mainly along grain boundaries, and addition of 2 wt% Di to this alloy results in the appearance of new Sn–Di phase, which further significantly enhance the strength and creep resistance. © 2007 Published by Elsevier B.V. Keywords: Mg–Sn alloy; Didymium; Tensile; Indentation creep; Microstructure
1. Introduction The interest in Mg–Sn based alloys started in the early 1930s [1,2]. Recently after 2000 there has been a renewed global interest [3–6] in these alloys which are believed to have potential for applications at elevated temperatures. The intermetallic phase Mg2 Sn in Mg–Sn alloys has a much higher melting point (770 ◦ C) than that of the Mg17 Al12 phase (462 ◦ C) in Mg–Al alloys. Mg–Sn based alloys are therefore likely to be more creep resistant at elevated temperatures than Mg–Al based alloys. Recent studies on the influence of alloying additions such as Ca, Si on the microstructure and the tensile properties and creep resistance of Mg–Sn alloys indicated clearly the improvement of their creep properties [7,8]. Rare earth (RE) elements are important alloying elements to magnesium alloys, which can improve both the room temperature and the elevated temperature properties of magnesium alloys, especially the latter [9]. But about the creep behavior Mg–Sn–RE lacks systemic and ingoing research at present. Indentation creep test is a convenient and efficient creep testing way. Masami Fujiwara et al thinks one of the great advantages of this way is that different creep information can be obtained from a limited supply of material [10]. Our previous
∗
Corresponding author. Tel.: +86 28 85405670; fax: +86 28 85407335. E-mail address: [email protected] (Y. Chen).
0921-5093/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.msea.2007.02.061
works have shown that the Mg–5% Sn and Mg–5% Sn–2% Di alloys offer good comprehensive properties [11,12]. This paper focus on the tensile and indentation creep behavior of as-cast Mg–5 wt% Sn–2 wt% Di alloys. 2. Experiment Three alloys are prepared with the nominal compositions of Mg–5 wt% Sn and Mg–5 wt% Sn–2 wt% Di, Mg–4 wt% Al–2 wt% Di (AE42). Pure magnesium (99.95 wt%) and pure tin (99.98 wt%) were melted in a magnesium oxide crucible under the protection of a RJ-2 flux. Didymium was introduced in the form of a Mg–10 wt% Di (neodymium: praseodymium = 3:1) master alloy at 770 ◦ C. The melt was stirred to assist the dissolution of the master alloy. It was then held at 750 ◦ C for about 30 min, and finally cast into a copper mould that was preheated up to 250 ◦ C. The cavity dimension of the copper is 20 mm × 110 mm × 140 mm. The specimens were cut into slices with an electrical discharge wire-cutting at the middle of each sample. The gage dimension of each specimen is 18.0 mm × 3.5 mm × 2.0 mm. Six samples of each alloy were employed to obtain one set of mechanical data. The tensile tests were conducted at room temperature 20 ◦ C, 150 ◦ C and 175 ◦ C and with a crosshead speed of 2 mm/min on an electro-universal testing machine (Instron5569).
H. Liu et al. / Materials Science and Engineering A 464 (2007) 124–128
125
The indentation creep tests were carried out on a Brinell tester at 150 ◦ C and 175 ◦ C (±1 ◦ C) with a constant load of 30 kg. The creep specimens were cut by electric spark machining from the bottom of the ingots with a size of 30 mm × 15 mm × 14 mm. It kept 10 min when the temperature reaches the setting point before loading. A global quenching steel presser (Ø10 mm) was pressed into the test surface of hot materials, and oil was used as the protected medium during the creep test. Each average datum point was repeated three times at least. The selection of holding time for each datum point was 30 min, 60 min, 120 min, 240 min 480 min and 600 min, respectively. The size of indentation diameter was measured by instrumental microscope after the creep tests were completed and the samples cooled down. The microstructure and fracture analysis were carried out with the help of optical microscope (OLYMPUS-BH-2), scanning electron microscope (SEM) observations and energy dispersion spectrometry (EDS) (JEOL JSM-5910LV) analyses. 3. Results 3.1. Tensile properties Tensile properties of the alloys at various temperatures are shown in Table 1. At room temperature, the tensile properties of Mg–5% Sn–2% Di are higher than those of Mg–5% Sn and slightly less than those of AE42. With the increase of testing temperature, there is a decrease in strength for both alloys as expected. However, the decreasing degree of the Mg–5% Sn–2% Di alloy is the smallest in strength, while that of AE42 is the largest, indicating the better thermal stability of Mg–5% Sn and Mg–5% Sn–2% Di alloys. Fig. 1 shows the SEM tensile fracture micrographs of the Mg–5% Sn and Mg–5% Sn–2% Di at 175 ◦ C, which indicates the addition Di to Mg–5% Sn alloy resulting in a combination of ductile dimpling and cleavage.
Fig. 1. SEM tensile fracture micrographs of Mg–5% Sn (a) and Mg–5% Sn–2% Di (b) at 175 ◦ C.
expressed as creep rate. That is
3.2. Indentation creep behavior
d = d(t)
3.2.1. The indentation creep rate The relation between indentation diameter and holding time can be expressed as Eq. (1):
According to Eq. (2), the creep rate of each alloy can be calculated at the same temperature. The indentation creep rates are shown in Fig. 2. Eq. (3) is obtained by data fitting from Fig. 2, as follows:
d = d(t)
d = at b−1
(1)
where d is the diameter of indentation. With the aim of expressing the creep rate of the investigated alloys, the change rate of indentation diameter with the increase of holding time is
(2)
(3)
where a and b are constants, which are material-related constants, reflecting the creep resistance of materials. The bigger values of a and b are, the poorer creep resistance is. From Fig. 2
Table 1 Tensile properties of Mg–5% Sn, Mg–5% Sn–2% Di and AE42 alloys at various temperatures Alloys
Mg–5% Sn Mg–5% Sn–2% Di AE42
150 ◦ C
RT
175 ◦ C
σ b dropping ratio (%)
σ b (MPa)
δ (%)
σ b (MPa)
δ (%)
σ b (MPa)
δ (%)
150 ◦ C
175 ◦ C
122 155 165
7.8 8.4 10.1
99 132 126
11.5 17.8 21.2
81 118 104
13.6 19.3 23.4
18.9 14.8 23.6
33.6 23.9 37.0
126
H. Liu et al. / Materials Science and Engineering A 464 (2007) 124–128
Fig. 3. Logarithm of indentation rate vs. logarithm of indentation diameter at 150 ◦ C and 175 ◦ C. Fig. 2. Indentation creep rates at 150 ◦ C and 175 ◦ C.
it can be seen that the creep rate of Mg–5% Sn is higher than that of AE42, and Mg–5% Sn–2% Di is lower than AE42 on the same holding time at 150 ◦ C. The change tendency of the creep rate at 175 ◦ C for the investigated alloys is the same as that at 150 ◦ C. The only difference is that the creep rates at 175 ◦ C are larger than those at 150 ◦ C. 3.2.2. The nominal stress exponent (n) When an indenter is pressed into the surface of a hot material, it penetrates the material first because of yielding and then by creeping [10]. The creep rate curves suggest that when the holding time beyond 240 min, the indentation creep changes slightly, so the average indentation creep from 240 to 600 min can be taken as pseudo-steady state indentation creep process. In order to understand the detailed creep mechanism of the alloys, nominal stress exponents (n) were calculated. The indentation creep n is given by Eq. (4) [10]: 1 ∂ ln ds n= 1− (4) 2 ∂ ln d where n is the creep nominal stress exponent, ds the pseudosteady state indentation creep rate, and d the indentation diameter. n could give useful information for the creep controlling mechanisms. By plotting logarithmically the pseudo-steady state indentation creep rate ds versus the indentation diameter d, we can
calculate the nominal stress exponent n of these alloys at 150 ◦ C and at 175 ◦ C, which are shown in Fig. 3 and Table 2. These values can be used to infer the dominant creep mechanisms at the present test conditions. The stress exponent n of 2–4 is generally reported for grain boundary sliding, and n of 4–6 is for dislocation climb [13], while n > 6 showing intermediate phase particles strengthening controlled creep [14]. If the intermediate phase on the grain boundary or within matrix block grain boundary sliding, dislocation motion and lattice self-diffusion, the value of n will be higher. For the Mg–5% Sn and AE42 alloys, n are 4.24 and 4.53 at 150 ◦ C, 4.01 and 4.03 at 175 ◦ C, respectively, which suggests the validity of dislocation climb mechanism. For Mg–5% Sn–2% Di, n is 6.99 at 150 ◦ C and 6.88 at 175 ◦ C, respectively, indicating a strengthening controlled creep mechanism of intermediate phase particles. 4. Discussion The results of the tensile tests and the indentation creep tests indicate Mg–5% Sn has good creep resistance, which is mainly Table 2 Values of creep nominal stress exponent Alloy
Mg–5% Sn AE42 Mg–5% Sn–2% Di
Nominal stress exponent (n) 150 ◦ C
175 ◦ C
4.24 4.52 6.99
4.01 4.03 6.88
H. Liu et al. / Materials Science and Engineering A 464 (2007) 124–128
127
Fig. 5. The optical micrographs of solid-solution treated Mg–5% Sn (a) and Mg–5% Sn–2% Di alloys (b) at 520 ◦ C for 72 h.
Fig. 4. SEM micrographs of as-cast Mg–5% Sn (a), Mg–5% Sn–2% Di (b) and AE42 (c) alloys.
attributed to the presence of thermally stable Mg2 Sn particles in Mg–5% Sn alloy as shown in Fig. 4(a). The melting point of Mg2 Sn phase is 771.5 ◦ C in Mg–Sn alloys, while that of Mg17 Al12 in Mg–Al alloy is 465 ◦ C, so the addition of Sn in Mg alloys is possibly more effective to improve the mechanical properties of Mg alloys in comparison with the addition of Al in Mg alloys at elevated temperature.
Mg–5% Sn–2% Di alloy exhibits better tensile properties and much better indentation creep resistance than Mg–5% Sn and AE42 alloy at 150 ◦ C and 175 ◦ C (AE42 was considered as an excellent creep resistant magnesium alloy at this temperature range [15]). The cause of Mg–5% Sn–2% Di alloy surpassing Mg–5% Sn in creep resistance is the addition of Di into Mg–5% Sn alloy resulting in the formation of the feather-shaped Sn–Di phase, as shown in Fig. 4(b), to which mainly distributes in the grain boundary areas except a few in matrix and EDS analysis [12] suggests that the atomic ratio of Sn and Di is about 3:2. After the as-cast Mg-5% Sn and Mg–5% Sn–2% Di alloys are treated at 520 ◦ C for 72 h, the Mg2 Sn phase dissolves in the ␣-Mg matrix but Sn–Di phase does not appear to change in Fig. 5, which indicates that the feather-shaped Sn–Di phase is thermally more stable than Mg2 Sn phase. DSC analysis [12] indicates that the Mg2 Sn phase dissolves into the ␣-Mg phase when the temperature approaches to 480 ◦ C, while the dissolving temperature of the Sn–Di phase is 556.5 ◦ C, which confirms that the Sn–Di phase has better thermal stability. This is important for further improving the creep resistance of Mg–5% Sn alloys.
128
H. Liu et al. / Materials Science and Engineering A 464 (2007) 124–128
The only difference in composition between AE42 and Mg–5% Sn–2% Di is 4%Al in the former and 5% Sn in the latter, while Mg–5% Sn–2% Di has much better creep resistance than that of AE42. A primary reason is the presence of thermally stable Mg2 Sn particles in Mg–5% Sn–2% Di alloy while Mg17 Al12 in AE42 as in Fig. 4(c). Another main cause is that the size of the Sn–Di phase is finer than that of Al11 Di3 phase, and Al11 Di3 phase only distribute on the grain boundary, but a fraction of Sn–Di phase within matrix (Fig. 4). The thermally stable Mg2 Sn and Sn–Di particles along grain boundaries hinder both grain boundary migrating and sliding during high-temperature exposure. The presence of fine Sn–Di phase particles within matrix also contributes to the improvement of creep resistance of Mg–5% Sn–2% Di alloy by impeding the dislocation movement. Therefore, its value of stress exponent is much higher than that depended by dislocation climb controlling creep. The third factor may be Al11 Di3 phase is instable, part of which it will decompose at high temperature during creep test. Powell et al. [15] believed that Al11 RE3 decomposed at high temperature, and formed Al2 RE and Mg17 Al12 phases at last. That is, Al11 Nd3 → 3Al2 Nd +5Al. The decomposed Al would react with Mg, and form Mg17 Al12 finally. The phase Mg17 Al12 tends soft at high temperature, which results in the deterioration of the creep resistance of the AE42 alloy. 5. Conclusions (1) The tensile properties of Mg–5% Sn–2% Di are better than those of Mg–5% Sn and slightly less than those of AE42 at room temperature. At elevated temperatures the tensile properties of Mg–5% Sn–2% Di are the best among the investigated alloys. (2) The indentation creep experiments suggest that Mg–5 wt% Sn–2 wt% Di alloy has significantly better indentation creep resistance than AE42. The calculated values of nominal stress exponent show that the Mg–5% Sn and AE42 are controlled by dislocation climb controlled creep, and
Mg–5 wt% Sn–2 wt% Di alloy experiences a intermediate phase particles-controlled creep mechanism at the present test conditions. (3) Mg–5 wt% Sn alloy contains thermally stable Mg2 Sn particles, and addition of 2 wt% Di to this alloy results in the appearance of Sn–Di phase. The presence of these particles in Mg–5% Sn–2% Di alloy is responsible for its improved tensile and creep properties, better than those of AE42 alloy. Acknowledgements This research was supported under a grant from the Science & Technology Bureau of Sichuan Province of China. The authors thank the Analysis and Testing Center of Sichuan University for providing the necessary testing instruments. Constructive comments and suggestions from the reviewer are acknowledged. References [1] [2] [3] [4] [5] [6]
[7]
[8] [9] [10] [11] [12] [13] [14] [15]
G. Derge, A.R. Kommel, R.F. Mehl, Trans. AIME 124 (1937) 367. J. Van Der, Planken, J. Mater. Sci. 4 (1969) 927–929. N. Hort, Y. Huang, T.A. Leil, et al., Adv. Eng. Mater. 8 (2006) 359–364. C.L. Mendis, C.J. Bettles, M.A. Gibson, et al., Mater. Sci. Eng. A 435–436 (2006) 163–171. T.T. Sasaki, K. Oh-ishi, T. Ohkubo, K. Honoa, Scripta Mater. 55 (2006) 251–254. S. Cohen, G.R. Goren-Muginstein, S. Avraham, in: A.L Alan (Ed.), Magnesium Technology, TMS The Minerals, Metals & Materials Society, 2004, pp. 301–305. A.L. Bowles, H. Dieringa, C. Blawert, et al., in: A.L Alan (Ed.), Magnesium Technology, TMS The Minerals, Metals & Materials Society, 2004, pp. 307–310. D.H. Kang, S.S. Park, N.J. Kim, Mater. Sci. Eng. A 413–414 (2005) 555–560. Z. Lu, Magnesium Alloys, Chemical Industry Press, Beijing, 2004. M. Fujiwara, M. Otsuka, Mater. Sci. Eng. A 319–321 (2001) 929–933. H. Liu, Y. Chen, Y. Tang, et al., J. Alloys Compd., in press. H. Liu, Y. Chen, Y. Tang, et al., Mater. Sci. Eng. A 437 (2006) 348–355. A.A. Luo, Int. Mater. Rev. 49 (2004) 13–30. G.C. Davies, D.R.H. Jones, Scripta Mater. 25 (1996) 523–528. B.R. Powell, V. Rezhets, M.P. Balogh, et al., JOM 54 (2004) 34.