Creep properties and creep microstructure evolution of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy

Materials Science & Engineering A 684 (2017) 90–100

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Creep properties and creep microstructure evolution of Mg-2.49Nd1.82Gd-0.19Zn-0.4Zr alloy ⁎

MARK

⁎

Wenya Hana, Guangyu Yanga, , Lei Xiaoa, Jiehua Lib, , Wanqi Jiea a b

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, No. 127 Youyi Western Road, Xi’an 710072, PR China Chair of Casting Research, Montanuniversität Leoben, Franz-Josef-Straße 18, Leoben 8700, Austria

A R T I C L E I N F O

A BS T RAC T

Keywords: Mg alloys Creep TEM Grain boundary sliding Dislocation climbing

The creep properties and creep microstructure evolution of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy were investigated as a function of creep stresses and creep temperatures. It was found that the creep strain and the creep rate increases but the creep life decreases with increasing creep temperatures and creep stresses. At a defined creep stress, the creep strain increases with increasing creep temperatures. At a defined creep temperature, the creep strain increases with increasing creep stresses. α(Mg), Mg12Nd phase and Mg5Gd phase are present in the microstructure after creep. Furthermore, the Mg12Nd phase appears to increase with increasing the creep stress, creep temperature and creep time, indicating that the creep stress, creep temperature and creep time may have a significant effect on the formation of the equilibrium Mg12Nd phase during the creep. At the temperature range less than 225 ℃, the calculated Qc value is about 74–98 kJ/mol, which is very close to the grain boundary diffusion activation energy (80 kJ/mol−1), indicating that the grain boundary sliding may be a dominant factor affecting creep properties. While, at the temperature range higher than 225 ℃, the calculated Qc value is about 227–289 kJ/mol, which is much higher than the self - diffusion activation energy (135 kJ/mol−1), indicating that the dislocation climbing may be a dominant factor affecting creep properties. The fracture was observed to occur along the grain boundaries. The initiation of the crack is from a trigeminal grain boundary, indicating that the Mg12Nd phase at the grain boundary has a significant effect on the initiation of the crack.

1. Introduction

In order to further improve the creep properties in Mg-Al based alloys, three different methodologies have been used, which are mainly focused on the grain boundary sliding and the motion of lattice dislocations within the primary α-Mg grains. Firstly, other micro-alloy elements (i.e. Sr, Ca, Si, La, Gd, Sn, Mn) were added to form other intermetallic phases (i.e. Al4Sr, Al4Ca, Mg2Sn, Mg2Si) with a higher thermal stability and thereby reduce or replace the formation of the Mg17Al12 phase [6–43]. Secondly, more Zn and less Al was added to form the Mg32(Al, Zn)49 phase, instead of the Mg17Al12 phase [44,45]. Thirdly, Mg-Zn based alloys [46–54], Mg-Sn based alloys [55–67], MgCa based alloys [68,69] or Mg-RE (rare element, e.g. Nd, Gd, Y) based alloys [70–95] were developed. In particular, the addition of rare element (e.g. Nd, Gd, Y) into Mg alloy is well-accepted to be promising to improve the alloy performance at elevated temperatures. In authors' research, Mg–Nd–Zn–Zr (wt%) based alloys with Gd and / or Y addition were developed, which show a superior performance at elevated temperatures (i.e. 200–300 °C) and are very promising for further wider application [96,97]. Their precipitation behavior during the early stage of ageing was also investigated. However, there is still a

Magnesium alloys, as the lightest metal structural materials in industrial application, have been attracted more and more attention in aerospace and automotive industries [1–5]. However, conventional Mg casting alloys have been based on the Mg-Al system with additions of Zn or Mn, e.g. AZ91 alloy (Mg-9.0Al-1.0Zn, wt%), AZ31 alloy (Mg3.0Al-1.0Zn, wt%), AM60 alloy (Mg-6.0Al-0.6Mn, wt%). These alloys have good castability, mechanical properties at the room temperature and corrosion resistance. However, the application of these alloys has been limited to specific components that operate at temperatures below 150 °C due to the rapid degradation of the mechanical properties at elevated temperatures, especially the creep resistance, which is caused by the presence of Mg17Al12 phase [6]. Indeed, increasing Al concentrations has been reported to decrease the creep properties in Mg-Al based alloys. New creep-resistant Mg casting alloys are, therefore, required for the application at elevated temperatures, such as transmission cases (temperatures up to ~175 °C), engine blocks (~250 °C) and pistons (~300 °C) [3–5]. ⁎

Corresponding authors. E-mail addresses: [email protected] (G. Yang), [email protected] (J. Li).

http://dx.doi.org/10.1016/j.msea.2016.12.055 Received 7 November 2016; Received in revised form 9 December 2016; Accepted 10 December 2016 Available online 11 December 2016 0921-5093/ © 2016 Elsevier B.V. All rights reserved.

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Fig. 1. Creep curves of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy as a function of testing temperatures (175 °C, 200 °C, 225 °C, 250 °C, 275 °C) in the stress range of (a) 70 MPa, (b) 80 MPa, (c) 90 MPa, and (d) 100 MPa, respectively. At a defined stress, three different temperatures have chosen. At a high stress, three low temperatures have chosen (i.e. At 70 MPa, 225 °C, 250 °C, 275 °C have chosen. At 80 MPa and at 90 MPa, 200 °C, 225 °C, 250 °C have chosen. At 100 MPa, 175 °C, 200 °C, 225 °C have chosen.).

creep temperatures (175–275 °C) were performed using a sample with 25 mm in length and 5 mm in diameter. The microstructural evolution after creep testing was characterized by using Olympus PM-G3 type optical microscope (OM), scanning electron microscopy (SEM, JEOL JSM-5800), transmission electron microscopy (TEM, Tecnai 30F operated at 300 kV) and XRD (X Pert MPDPRO). The sample for TEM investigation were mechanically ground, polished and dimpled to about 30 µm in thickness, and then ion-beam milled using a Gatan Precision Ion Polishing System (PIPS, Gatan model 691). A preparation temperature (about −10 °C) was kept constant by using a cold stage during ion beam polishing.

lacking of a detailed investigation on the creep properties and creep microstructure evolution. In this paper, the creep properties and creep microstructure evolution of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy were investigated as a function of creep stresses and creep temperatures. Meanwhile, the creep mechanism is also discussed.

2. Experimental procedures The alloy ingots with nominal compositions of Mg-2.49Nd-1.82Gd0.19Zn-0.4Zr (wt%, used throughout the paper unless noted) were melted from pure Mg (99.98), Nd, Zn, Mg-28 Gd master alloy and Mg33 Zr master alloy in an electrical-resistance furnace under the protection of anti-oxidizing flux. The melting alloys were homogenized at 780 °C for 20 min followed by casting into a sand mould at 760 °C. The composition of the alloys was determined through inductively coupled plasma atomic emission spectrum (ICP-AES) apparatus. Solution treatment was performed at 515 °C for 18 h, followed by quenching into warm water (85 °C). Ageing treatment was performed at 205 °C for 18 h, followed by cooling in air. Creep testing as a function of creep stresses (50–110 MPa) and

3. Results 3.1. Creep properties Fig. 1 shows creep curves of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy as a function of testing temperatures (175 °C, 200 °C, 225 °C, 250 °C, 275 °C) in the stress range of 70 MPa (Fig. 1a), 80 MPa (Fig. 1b), 90 MPa (Fig. 1c), and 100 MPa (Fig. 1d), respectively. At a defined stress, three different temperatures have chosen. At a high stress, three

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Fig. 2. Creep curves of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy as a function of stress (50 MPa, 60 MPa, 70 MPa, 80 MPa, 90 MPa, 100 MPa, and 110 MPa) in the testing temperature range of (a) 200 °C, (b) 225 °C, (c) 250 °C, (d) 275 °C, respectively. At a defined testing temperature, four different stresses have chosen, except at 275 °C, where three different stresses have chosen. At a high testing temperature, low stresses have chosen (i.e. At 200 °C, 80 MPa, 90 MPa, 100 MPa, 110 MPa have chosen. At 225 °C, 70 MPa, 80 MPa, 90 MPa, 100 MPa have chosen. At 250 °C, 60 MPa, 70 MPa, 80 MPa, 90 MPa have chosen. At 275 °C, 50 MPa, 60 MPa, 70 MPa, 80 MPa have chosen.). Table 1 Creep properties of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy as a function of temperatures and stresses, respectively. Temperature (℃)

Stress (MPa)

75

200

225

250

275

ε(̇ s−1)

Creep life (h)

Strain (%) (at 100 h)

90

5.32 × 10−10

> 100

0.024

100

8.96 × 10−10

> 100

0.052

110

9.42 × 10−10

> 100

0.056

120

1.02 × 10−9

> 100

0.106

80

6.47 × 10−10

> 100

0.056

90

1.06 × 10−9

> 100

0.066

100

1.41 × 10−9

> 100

0.1

110

1.54 × 10−9

> 100

0.11

60

9.08 × 10−10

> 100

0.064

70

1.24 × 10−9

> 100

0.092

80

−9

2.55 × 10

> 100

0.108

90

4.19 × 10−9

> 100

0.21

100

9.79 × 10−9

> 100

0.786

60

1.47 × 10−8

> 100

0.648

70

2.94 × 10−8

> 100

1.224

80

5.48 × 10−8

> 100

3.136

90

8.96 × 10−8

73.332

13.07

50

4.72 × 10−8

> 100

2.056

60

1.08 × 10−7

91.631

15.36

70

2.33 × 10−7

38.04

21.794

Fig. 3. XRD curves of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy after crept up to 100 h at 90 MPa as a function of temperatures of 175 °C 200 °C, 225 °C, respectively.

low temperatures have chosen (i.e. At 70 MPa, 225 °C, 250 °C, 275 °C have chosen. At 80 MPa and at 90 MPa, 200 °C, 225 °C, 250 °C have chosen. At 100 MPa, 175 °C, 200 °C, 225 °C have chosen.). Fig. 2 shows creep curves of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy as a function of stresses (50 MPa, 60 MPa, 70 MPa, 80 MPa, 90 MPa, 100 MPa, and 110 MPa) in the testing temperature range of 200 °C (Fig. 2a), 225 °C (Fig. 2b), 250 °C (Fig. 2c), 275 °C (Fig. 2d), respec-

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Fig. 4. XRD curves of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy after crept up to 100 h at 225 °C as a function of temperatures of 70 MPa, 80 MPa, 90 MPa, 100 MPa, respectively.

Fig. 5. Creep curve of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy after crept up to 270 h (fracture) at 225 °C and 90 MPa.

tively. At a defined testing temperature, four different stresses have chosen, except at 275 °C, where three different stresses have chosen. At a high testing temperature, four low stresses have chosen (i.e. At 200 °C, 80 MPa, 90 MPa, 100 MPa, 110 MPa have chosen. At 225 °C, 70 MPa, 80 MPa, 90 MPa, 100 MPa have chosen. At 250 °C, 60 MPa, 70 MPa, 80 MPa, 90 MPa have chosen. At 275 °C, 50 MPa, 60 MPa, 70 MPa have chosen.). Table 1 lists creep properties of Mg-2.49Nd1.82Gd-0.19Zn-0.4Zr alloy after creep testing up to 100 h as a function of temperatures and stresses, respectively. As shown in Figs. 1, 2 and Table 1, it is very clear that the creep strain and the creep rate increases but the creep life decreases with increasing creep temperatures and creep stresses. At a defined creep stress, the creep strain increases with increasing creep temperatures. At a defined creep temperature, the creep strain increases with increasing creep stresses.

creep. Finally, after creep up to 120 h, the creep rate increases sharply and fracture occurs, which is called tertiary creep. According to Fig. 5, three conditions (2 h, 100 h, and 270 h) are chosen for further investigation (OM, XRD, SEM and TEM). Fig. 6 shows OM image of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy after crept at 225 °C and 90 MPa for 2 h (Fig. 6a), 100 h (Fig. 6b, c) and 270 h (Fig. 6d). Fig. 6a and b are taken at the same magnification, while Fig. 6c and d are taken at the same magnification. The grain size for 2 h, 100 h and 270 h was measured to be 80.2 µm, 180.7 µm, and 193.7 µm, respectively. Compared with the grain size (69.9 µm, not shown here) prior to creep, no significant increase of the grain size was observed after creep up to 2 h, but a significant coarsening was observed after creep up to 100 h and 270 h. Fig. 7 shows XRD curves of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy after crept at 225 °C and 90 MPa for 0 h (Fig. 7a), 2 h (Fig. 7b), 100 h (Fig. 7c) and 270 h (Fig. 7d). Similar to Figs. 3 and 4, α(Mg), Mg12Nd phase and Mg5Gd phase are also present in four different creep times. Furthermore, the Mg12Nd phase appears to increase with increasing the creep time, indicating that the precipitates (i.e. β", β', β1) formed prior to the creep may be metastable and more likely to transform to the equilibrium Mg12Nd phase. Fig. 8 shows TEM bright field images (Fig. 8a, c) and corresponding SADPs (Fig. 8b, d) of the microstructure in Mg-2.49Nd-1.82Gd0.19Zn-0.4Zr alloy after crept at 225 °C and 90 MPa for 2 h. The coarse Mg5Gd phase and the Mg12Nd phase were observed along the grain boundaries, as shown in Fig. 8a,b and c,d, respectively, which is fully consistent with the XRD results (Fig. 7). Fig. 9 shows TEM bright field images (Fig. 9a–c) and corresponding SADP (Fig. 9d) of the microstructure in Mg-2.49Nd-1.82Gd-0.19Zn0.4Zr alloy after crept at 225 °C and 90 MPa for 100 h. Viewed from (0001) zone axis, the precipitates (i.e. β", β', β1) with a high number density were observed on the prism plane of the α-Mg matrix, as shown in Fig. 9a. More importantly, the second phase (more likely Mg12Nd) was observed along the grain boundaries in a regular arrangement, as shown in Fig. 9b. Furthermore, the precipitation free zone (denuded zones) was also observed along the grain boundaries, as shown in Fig. 9c, which is consistent with the previous reports [80,86]. Fig. 10 shows TEM bright field images (Fig. 10a, d) and corre-

3.2. Creep microstructure evolution Fig. 3 shows XRD curves of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy after crept up to 100 h at 90 MPa as a function of temperatures of 175 °C, 200 °C, 225 °C, respectively. α(Mg), Mg12Nd phase and Mg5Gd phase are present in three different temperatures. However, the Mg12Nd phase appears to increase with increasing the creep temperature, indicating that a higher creep temperature promotes the formation of the Mg12Nd phase. Fig. 4 shows XRD curves of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy after crept up to 100 h at 225 °C as a function of temperatures of 70 MPa, 80 MPa, 90 MPa, 100 MPa, respectively. Similar to Fig. 3, α(Mg), Mg12Nd phase and Mg5Gd phase are also present in four different creep stresses. Furthermore, the Mg12Nd phase also appears to increase with increasing the creep stress, indicating that a higher creep stress may also promote the formation of the Mg12Nd phase. However, it should be noted here that the effect of the creep stress on the formation of the Mg12Nd phase is less significant than that of the creep temperature. Fig. 5 shows creep curve of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy after crept up to 270 h (fracture) at 225 °C and 90 MPa. A typical creep curve was observed. At the early stage of creep (up to 16 h), the creep rate decreases sharply, which is called primary creep. Then, the creep rate becomes stable (about 4.19 × 10−9 /s−1), which is called second

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Fig. 6. Optical microscopy of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy after crept at 225 °C and 90 MPa for (a) 2 h, (b, c) 100 h and (d) 270 h. (a) and (b) are taken at the same magnification, (c) and (d) are taken at the same magnification.

sponding SADPs (Fig. 10b, c) of the microstructure in Mg-2.49Nd1.82Gd-0.19Zn-0.4Zr alloy after crept at 225 °C and 90 MPa for 270 h. Twinning was observed within α-Mg matrix, as shown in Fig. 10a. Fig. 10b is taken from the α-Mg matrix, while Fig. 10c is taken from the α-Mg matrix and twinning. The selected area diffraction partner (SADP) (Fig. 10c) indicates that the α-Mg matrix and twinning keep a very close orientation relationship. The precipitates (most likely equilibrium β) were also observed on the prism plane of the α-Mg matrix, as shown in Fig. 10d. The dislocation with a high number density was observed in the vicinity of the precipitates. 3.3. Creep fracture Fig. 11 shows the creep fracture surface of Mg-2.49Nd-1.82Gd0.19Zn-0.4Zr alloy after crept at 225 °C and 90 MPa for 270 h. Fig. 11a and b are taken by OM, while Fig. 11c and d are taken by SEM. As shown in Fig. 11a, the fracture occurs along the grain boundaries. The initiation of the crack is from a trigeminal grain boundary, as shown in

Fig. 7. XRD curves of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy after crept at 225 °C and 90 MPa for (a) 0 h, (b) 2 h, (c) 100 h and (d) 270 h.

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Fig. 8. TEM bright field images (a, c) and corresponding SADPs (b, d) of the microstructure in Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy after crept at 225 °C and 90 MPa for 2 h. Mg5Gd phase is shown in (a) and (b). Mg12Nd phase is shown in (c) and (d).

ε̇ = Aσ n exp (−Qc /RT )

Fig. 11b, which can be attributed to the fact that the stress concentrates at the trigeminal grain boundary during creep, once the local tensile stress exceeds the grain boundary binding force, the wedge-shaped crack at the grain boundary trigeminal point forms and the fracture occurs. As shown in Fig. 11c, the dimple topography and the second phase particles at the bottom of the dimple was observed, indicating that the Mg12Nd phase at the grain boundary has a significant effect on the initiation of the crack. Furthermore, the hole was also observed, as shown in Fig. 11d.

(1)

where，ε̇ is the creep rate, A is a constant related to the alloy composition and microstructure, σ is the creep stress; n is the stress exponent; R is a constant (8.314 J /(mol K)); Qc is the creep activation energy (J / mol); T is the temperature (K). Eq. (1) can be rewritten as Eq. (2):

ln ε̇ = ln A + n ln σ − Qc /(RT )

(2)

At a defined temperature，there is a linear relationship between ln ε̇ and lnσ. The slope is the stress exponent n. Therefore, the stress exponent n can be calculated to 2–3 in the temperature range from 175–200 ℃ and 4–6 in the temperature range from 225–275 ℃, as shown in Fig. 12. At a defined stress，there is also a linear relationship between ln ε̇

4. Discussion The creep rate is dependent on the temperature and stress, as shown in Figs. 1 and 2. It can be described using Eq. (1) [79,80]:

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Fig. 9. TEM bright field images (a, b, c) and corresponding SADP (d) of the microstructure in Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy after crept at 225 °C and 90 MPa for 100 h.

mechanism at a micro scale is still required.

and 1/T. The slope is −Qc / R . Therefore, the creep activation energy Qc can be calculated to 74–98 kJ/mol in the temperature range less than 225 ℃ and 227–289 kJ/mol in the temperature higher than 225 ℃, as shown in Fig. 13. It should be noted here that, at the temperature range less than 225 ℃, the calculated Qc value (74–98 kJ/mol) is very close to the grain boundary diffusion activation energy (80 kJ/mol−1), indicating that the grain boundary sliding may be a dominant factor affecting creep properties (see Fig. 10b), while at the temperature range higher than 225 ℃, the calculated Qc value (227–289 kJ/mol) is much higher than the self - diffusion activation energy (135 kJ/mol−1), indicating that the dislocation climbing may be a dominant factor affecting creep properties (see Figs. 9a, 10d). However, in most cases, both the grain boundary sliding and the dislocation climbing are responsible for the creep properties. Further investigation on the creep

5. Conclusions The creep properties and creep microstructure evolution of Mg2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy were investigated as a function of creep stresses and creep temperatures. The main conclusions can be drawn: (1) The creep strain and the creep rate increases but the creep life decreases with increasing creep temperatures and creep stresses. At a defined creep stress, the creep strain increases with increasing creep temperatures. At a defined creep temperature, the creep strain increases with increasing creep stresses.

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Fig. 10. TEM bright field images (a, d) and corresponding SADPs (b, c) of the microstructure in Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy after crept at 225 °C and 90 MPa for 270 h. (a) shows twinning within the α-Mg matrix. (b) is taken from the α-Mg matrix. (c) is taken from the α-Mg matrix and twinning. (d) shows the precipitates (i.e. β) on the prism plane of the αMg matrix.

(2) α(Mg), Mg12Nd phase and Mg5Gd phase are present. Furthermore, the Mg12Nd phase appears to increase with increasing the creep stress, creep temperature and creep time, indicating that the creep stress, creep temperature and creep time may have a significant effect on the formation of the equilibrium Mg12Nd phase. (3) At the temperature range less than 225 ℃, the calculated Qc value (74–98 kJ/mol) is very close to the grain boundary diffusion activation energy (80 kJ/mol−1), indicating that the grain boundary sliding may be a dominant factor affecting creep properties. (4) At the temperature range higher than 225 ℃, the calculated Qc value (227–289 kJ/mol) is much higher than the self - diffusion activation energy (135 kJ/mol−1), indicating that the dislocation climbing may be a dominant factor affecting creep properties.

However, in most cases, both the grain boundary sliding and the dislocation climbing are responsible for the creep properties. (5) The fracture occurs along the grain boundaries. The initiation of the crack is from a trigeminal grain boundary. The Mg12Nd phase at the grain boundary has a significant effect on the initiation of the crack. Acknowledgements Financial supports by the National Natural Science Foundation of China (No. 51227001 and 51420105005) and the Research Fund of the State Key Laboratory of Solidification Processing (NWPU), China No. 138-QP-2015, No. KP201522) are gratefully acknowledged. J.H. Li

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Fig. 11. Fracture surface of Mg-2.49Nd-1.82Gd-0.19Zn-0.4Zr alloy after crept at 225 °C and 90 MPa for 270 h. (a) and (b) are taken by optical microscopy, (c) and (d) are taken by SEM.

Fig. 12. Linear curves between lnand lnσ. The data is taken from Table 1. Fig. 13. Linear curves between lnand 1/T. The data is taken from Table 1.

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