Effects of Si addition on microstructure and mechanical properties of Mg–8Gd–4Y–Nd–Zr alloy

Materials and Design 43 (2013) 74–79

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Effects of Si addition on microstructure and mechanical properties of Mg–8Gd–4Y–Nd–Zr alloy Xinming Zhang, Jilong Hu ⇑, Lingying Ye, Yunlai Deng, Changping Tang, Liu Yang, Zhaoyang Liu School of Materials Science and Engineering, Central South University, Changsha 410083, PR China The Key Laboratory of Nonferrous Metal Materials Science and Engineering, Ministry of Education, Changsha 410083, PR China

a r t i c l e

i n f o

Article history: Received 13 April 2012 Accepted 14 June 2012 Available online 28 June 2012 Keywords: Non-ferrous metals and alloys Young’s modulus Strength Microstructure

a b s t r a c t Effects of Si addition (1.0 wt.%) on microstructure and mechanical properties of Mg–8Gd–4Y–Nd–Zr alloy have been investigated using scanning electron microscopy (SEM) equipped with energy dispersive spectrum (EDS), X-ray diffraction (XRD), hardness measurements and tensile testing. The results indicated that the addition of Si led to the formation of Mg2Si and (RE + Si)-rich particles, which enhanced the Young’s modulus of the alloy by 7 GPa while decreased the yield strength and ultimate strength by 10 MPa and 31 MPa, respectively. The tensile properties of the Mg–8Gd–4Y–Nd–Zr–Si alloy are as follows: Young’s modulus E = 51 GPa, yield strength r0.2 = 347 MPa, ultimate strength rb = 392 MPa and elongation d = 2.7%. The increase in Young’s modulus was attributed to the formation of particles with high Young’s modulus, while the decrease in strength was ascribed to the decrease in volume fraction of metastable b0 precipitates caused by the consumption of rare earth atoms due to the formation of the rare earth containing particles. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction High performance magnesium-rare earth (Mg–RE) alloys have been recognized as promising light structural materials for the aerospace and racing automotive industries, especially the newly developed Mg–Gd–Y alloys exhibit higher specific strength at both room and elevated temperatures. However, the low elastic modulus restricts the extensive application of this alloy [1]. Recently, it has been shown that Mg alloys containing Mg2Si particles have high potential because Mg2Si exhibits a reasonably high Young’s modulus (120 GPa), high melting point (1085 °C), high hardness (4.5  109 N m2), low density (1.99  103 kg m3) and low co-efficient of thermal expansion (7.5  106 K1) [2–4]. According to the Mg–Si phase diagram [5], the maximum solid solubility of Si in magnesium is only 0.003 at.%, extra Si atoms react with magnesium and are precipitated as an intermetallic compound of Mg2Si. And the formation of an ‘‘in situ’’ composite (Mg–Mg2Si) results in strong bonding between Mg2Si and the matrix interface [6]. Traditionally, many of the studies have been focused on the effect of Si on the microstructure, creep properties, ageing and corrosion behavior of AZ91 magnesium alloy [7–10] and corrosion behavior of Mg–Zn–Mn alloys [11–13]. However, limited research has been

⇑ Corresponding author at: School of Materials Science and Engineering, Central South University, Changsha 410083, PR China. Tel./fax: +86 0731 8883 0265. E-mail addresses: [email protected], [email protected] (J. Hu). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.06.022

carried out on the effect of Si on the microstructure and mechanical properties of Mg–RE alloys. In the present paper, effects of Si addition on microstructures and mechanical properties of Mg–8Gd–4Y–Nd–Zr alloy were investigated. The reaction feasibility of Si with Mg–8Gd–4Y–Nd– Zr alloy during melt on the base of thermodynamic analysis was discussed. The objective of this work is to produce Mg–8Gd–4Y– Nd–Zr–Si alloys owning good combination of high Young’s modulus, high strength and heat resistance.

2. Experimental procedures The alloys required for this study were prepared by melting pure Mg (>99.93%), Mg-31.25% Gd (wt.%), Mg-25.48% Y (wt.%), Mg-30.15% Nd (wt.%), Mg-30.23% Zr (wt.%) master alloys and pure Si (>99.95%) in an electrical resistance furnace under the protection of Ar atmosphere. The master alloys and pure Si were added to the melt at 760 °C and stirred about 90 s at a speed of 300 rpm, then held for 20 min. Consequently, the prepared melt was poured into a preheated (250 °C) permanent low carbon steel mold (U55 mm  150 mm). Two sets of castings were made and their chemical compositions were Mg–8.0Gd–4.0Y–1.0Nd–1.0Zr (wt.%, alloy I) and Mg–8.0Gd–4.0Y–1.0Nd–1.0Zr–1.0Si (wt.%, alloy II), respectively. After solution treated at 520 °C for 12 h, samples were quenched into hot water at about 80 °C and subsequently cut into 15 mm  15 mm  2 mm pieces for ageing. The ageing treatment was carried at 215 °C in an air electric resistance furnace. The

X. Zhang et al. / Materials and Design 43 (2013) 74–79

ingots were extruded into bars at 500 °C with an extrusion ratio of 16:1. Samples for microstructure observation were initially polished using different grades of polishing papers and finally polished with 0.25 lm diamond paste. Polished samples were chemically etched in a solution of 4 vol.% nital. Microstructure observation were performed on the SEM equipped with EDS. The grain size was measured by a mean linear intercept method. XRD studies were carried out using a Rigaku D/max 2500 diffractometer (Cu Ka radiation) with a scanning angle from 10° to 80° and a scanning speed of 2°/min. The phases were identified by using the ICDD PDF2-2004 database in the Jade 6 software. Hardness tests were performed on a HV-10B type Vickers microindenter with 30 N load and 30 s holding time. The reported values of hardness in this paper were the average of nine indentations. Round tensile specimens with 6 mm gauge diameter and 30 mm gauge length were machined from the extruded bars [14]. Tensile testing was carried out on a MTS universal materials testing machine at a crosshead speed of 1 mm/min, the reported values of tensile testing in this paper were the average of three specimens.

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different. Much more block-shaped particles were detected (as shown by arrows B and C), and plenty of small dark irregular particles have also been observed (as shown by arrow D). The blockshaped particles can also be divided into two types according to the colors: hoar (arrow B) and pure white (arrow C). The EDS results indicated that the hoar particles were (RE + Si)-rich particles (Fig. 1e) while the pure white particles (Fig. 1f) were similar with that observed in alloy I. Fig. 2 shows the X-ray diffraction patterns of the as-cast and solution treated samples of alloys I and II. Compared with the results of alloy I, it is evident to note that more small diffraction peaks of Mg2Si, Gd5Si3 and YSi2 appeared in alloy II. After 520 °C/ 12 h solution treatment, the non-equilibrium eutectic phases aMg + Mg5.05Gd dissolved into the matrix, while the Mg2Si, Gd5Si3 and YSi2 particles still existed. These results agreed fairly well with the EDS results presented in Fig. 1. Thus, it is reasonable to conclude that the hoar block-shaped particles shown in Fig. 2c are the compounds of Gd5Si3 and YSi2, and the small dark irregular particles are Mg2Si. 3.2. Microstructure of the extruded bar

3. Results 3.1. Microstructures of the ingot The microstructures of the ingot after 520 °C/12 h solution treatment are shown in Fig. 1. In alloy I, non-equilibrium eutectics formed during solidification have completely dissolved, and some block-shaped particles were detected in the matrix and grain boundaries (as shown by arrow A). They were determined to be rare earth (RE)-rich particles according to the results of the EDS analysis (Fig. 1d). However, the microstructure of alloy II is quite

Fig. 3 presents the microstructure of the longitudinal and transverse direction of the extruded bars. It reveals a typical equiaxed and more refined grain structure with an average grain size of 4 lm in alloy I, which is attributed to the fully dynamic recrystallization (DRX) during the extrusion process. Since Mg alloys have relatively lower stacking fault energy (60–78 kJ mol1) [15], DRX generally predominates in deformed Mg alloys. Grain refinement by DRX in deformed Mg alloys has been widely observed, especially at elevated temperature (above 513 K) [16]. However, as shown in Fig. 3d, the microstructure of the extruded alloy II con-

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 1. SEM images of the alloys after solution treated: (a) alloy I; (b) alloy II; (c) magnified image of (b); (d and e) and (f) EDS results of point A, B and C, respectively.

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X. Zhang et al. / Materials and Design 43 (2013) 74–79

Fig. 2. XRD spectrums of the as-cast and solution treated samples: (a) as-cast, alloy I; (b) as-cast, alloy II; (c) solution treated, alloy I; and (d) solution treated, alloy II.

sists of about 40% unrecrystallized regions, which have serrated boundaries and elongated along the extrusion direction. In addition, the particles observed in Fig. 1 were crushed and also distributed along the extrusion direction (Fig. 3c).

prolonged, the hardness of alloy I increased rapidly and reached their peak after ageing 12 h. By contrast, the hardness-increasing trend of alloy II was a little gentle. The hardness of these samples reached the peak hardness of 115 HV at the expense of 20 h. Obviously, after reaching peak hardness, the hardness of both alloys decreased slightly and then became constant for a longer time. According to the results in Section 3.1, there were plenty of (RE + Si)-rich particles in alloy II, which consumed most of the rare earth atoms in the matrix and decreased the volume fraction of nano-scale precipitates. On the other hand, the strengthening effect of these coarse block-shaped particles was limited according to the well-known Orowan mechanism [17]. Thus, it is the formation of (RE + Si)-rich particles that decreased the peak hardness of alloy II. The tensile properties of the extruded alloys I and II are presented in Table 1. Apparently, the Young’s modulus increased while the ultimate strength and yield strength decreased with the addition of silicon. More specifically, the Young’s modulus of alloy II increased by 7 GPa compared with alloy I, while the yield strength and ultimate strength decreased by 10 MPa and 31 MPa, respectively.

4. Discussion

3.3. Mechanical properties

4.1. Thermodynamic analysis

The ageing hardness curves of the extruded bars aged at 215 °C are shown in Fig. 4. The peak hardness of the extruded bar of alloy II was 10 HV lower than that of alloy I. When the ageing time was

As we know, when selecting the in situ synthesized reinforce phases of the composites, it usually depends on whether the reinforce phases can automatically generate by adding elements to the

(a)

(b)

(c)

(d)

Longitudinal (extruded) direction

Transverse (extruded) direction

Fig. 3. SEM images of the extruded bars: (a and b) alloy I; (c and d) alloy II.

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X. Zhang et al. / Materials and Design 43 (2013) 74–79

C p ¼ a þ b  103 T þ c  105 T 2

ð7Þ

where DGT is the change in Gibbs free energy of the whole reaction, J/mol; DHT is the standard enthalpy change of the reaction at the temperature T, J/mol; T is the absolute temperature, K; DST is the standard entropy change of the reaction at the temperature T, J/ (mol K); m is the stoichiometric number of reaction components; Tm is the melting point of the phase, K; DHT m is the standard enthalpy change of the reaction at the melting point temperature, J/mol; Cp is the heat capacity at constant pressure, J/(mol K); a, b and c are the temperature coefficients of the material’s heat capacity. The standard Gibbs free energy change DGT for Eqs. (1–3) was theoretically calculated according to the thermodynamics date (as shown in Table 2) [19–22] and plotted in Fig. 5.

DG1T ¼



77; 237 þ 14:2T ð298:15 K 6 T 6 923 KÞ 100; 416 þ 39:3T ð923 K < T 6 1361 KÞ ½19

Fig. 4. Ageing hardness curves of the extruded bars.

DG2T ¼ 76; 893 þ 13:19T ðT P 298:15 KÞ ½20 Table 1 The mechanical properties of extruded alloys I and II. Alloy

Elastic modulus/GPa

Ultimate strength/MPa

Yield strength/ MPa

Elongation/ %

I II

44 ± 1 51 ± 2

423 ± 5 392 ± 6

357 ± 6 347 ± 8

3.4 ± 0.2 2.7 ± 0.3

matrix. The criteria is whether the change in Gibbs free energy of the reaction (DGT) is negative. According to the thermodynamic principle, for the following reactions:

2Mg þ Si ¼ Mg2 Si ðDG1T Þ

Y þ 2Si ¼ YSi2

It can be clearly seen that the changes in standard state Gibbs free energies of the three reactions are all negative, which indicates that the above reactions are all favorable in Mg–Gd–Y–Nd–Zr–Si system. Furthermore, when the temperature was higher than 923 K, the decrease in Gibbs free energy of reactions (2) and (3) were higher than that for reaction (1). Therefore, we can conclude that it is much easier for the rare earth elements Gd and Y react with Si during the melting process, which is the major reason for the formation of plenty of (RE + Si)-rich particles in alloy II.

ð1Þ

ðDG2T Þ

5Gd þ 3Si ¼ Gd5 Si3

DG3T ¼ 69; 990 þ 7:13T ðT P 298:15 KÞ ½21; 22

ð2Þ

ðDG3T Þ

ð3Þ

Their Gibbs free energy can be calculated by using the following method [18]:

DGT ¼ DHT  T DST DHT ¼ DH298:15 þ þ

Z

ð4Þ

Z

hX

Tm

i

mDC p1 dT þ DHT m

298:15 T

hX

i

mDC p2 dT

ð5Þ

Tm

DST ¼ DS298:15 þ

Z

Tm

298:15

P

mDC p1 T

dT þ

DH T m þ Tm

Z

T

P

Tm

mDC p2 T

dT

ð6Þ Fig. 5. The variation in reactions’ Gibbs free energy as a function of temperature.

Table 2 The thermal parameters of part element and compound. Element and compound

Mga Mgb Sia Mg2Sia a b

Solid. Liquid.

Cp (J mol1 K1) a

b  103

c  105

22.300 32.635 23.932 73.304

10.25 – 2.469 14.979

0.431 – 4.142 8.828

DH298.15 (kJ mol1)

DS298.15 (J mol1)

Applied temperature (K)

0 DH923 = 8.786 0 79.078

32.677 – 18.828 63.806

298.15–923 923–1361 298.15–1685 298.15–1373

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Table 3 The calculated elastic constants (in GPa) of Hexagonal Gd5Si3 and YSi2 compounds. Compounds

C11

C12

C13

C33

C44

C66

BR

BV

BRVH

GR

GV

GRVH

E

Gd5Si3 YSi2

163.0 198.5

127.0 70.4

6.3 39.1

980.0 167.4

40.4 39.1

18.0 64.0

127.7 93.8

176.1 95.8

151.9 94.8

30.5 52.0

97.5 56.2

64.0 54.1

168.4 136.4

4.2. Relationship between elastic modulus and phase composition It is well known that the elastic property is one of intrinsic nature of materials, and is related to the bonding force among atoms. Elastic modulus, which represents the binding force between atoms, is one of the material essential characteristics and relates to the crystal structure, alloying addition as well as phase transformation. The elastic modulus of a multiphase alloy is mainly determined by the elastic modulus and the volume fractions of its constituent phases [23,24]. It is well observed that the addition of Si element promotes the formation of some new phases (Mg2Si and (RE + Si)-rich particles) and results in the variation of the microstructure including the phase composition and amount of each phase, which is responsible for the variation of the Young’s modulus of alloy II. According to the rule of mixtures (ROM), the Young’s modulus of the composite increases with the increasing of volume fractions and the Young’s modulus of the reinforcement. In the present paper, the Mg2Si, Gd5Si3 and YSi2 phases were detected in alloy II. According to the K value method of X-ray diffraction, we quantificationally calculated the volume fractions of Mg2Si, Gd5Si3 and YSi2 phases (Fig. 2b), which were 2.3%, 1.5% and 0.9%, respectively. The Young’s modulus of Mg2Si is 120 GPa, which is much higher than magnesium alloys. However, few data of Young’s modulus for the rare earth-silicon compounds are reported. Hence, the computer program CASTEP (Cambridge Serial Total Energy Package in Material Studio modeling, Accelrys), a first-principles plane-wave pseudopotentials method based on density functional theory (DFT), was used in this work to calculate the elastic constants of Gd5Si3 and YSi2 compounds. Table 3 lists the calculated elastic constants, based on which the polycrystalline bulk modulus B, shear modulus G and Young’s modulus E were obtained using the Voigt-Reuss-Hill averaging scheme [25]. In the present calculations, the cutoff energy of atomic wave functions (PWs), Ecut, is set at 360 eV. The special points sampling integration over the Brillouin zones were employed using the Monkhorstpark method with a 4  4  4 special k-point mesh for YSi2 and 7  7  6 for Gd5Si3. The tolerance for geometry optimization is set as the difference of total energy within 5  106 eV/atom, maximum ionic Hellmann–Feynman force within 0.01 eV/Å and maximum stress within 0.02 eV/Å3. According to the calculated results, the Young’s modulus of Gd5Si3 and YSi2 compounds are 168.4 GPa and 136.4 GPa, respectively, which are also much higher than magnesium and its alloys. To sum up, the calculated Young’s modulus of alloy II by using ROM is 48.5 GPa, which is close to the experimental value. Thus, it is reasonable to conclude that the improvement in Young’s modulus of alloy II is attributed to the formation of compounds with high Young’s modulus. 4.3. The effect of Si addition on age-hardening response Due to the large solubility of Gd, Y elements in Mg matrix at high temperature and its rapid decrease with lowering temperature [26], the Mg–Gd–Y alloys show remarkable age-hardening response during isothermal ageing at low temperature, especially between 200 and 250 °C. In our previous work [27], a four-stage

Fig. 6. XRD spectrums of the peak-aged samples of the extruded bars: (a) alloy I; and (b) alloy II.

precipitation sequence in Mg–8Gd–4Y–Nd–Zr alloy has been determined: supersaturated a-Mg solid solution ? b00 (D019) ? b0 (bco) ? b1(fcc) ? b(fcc). Among the four precipitate phases, the coherent b0 phase is considered to be primary strengthening phase. And the yield strength or hardness usually peaks as the materials form fine b0 precipitates during ageing. Fig. 6 shows the X-ray diffraction patterns of the peak-aged samples of the extruded bars. According to the XRD results, it is obvious to note that plenty of b0 precipitated in 215 °C peak-ageing condition, which is related to the peak hardness. The volume fraction of b0 precipitates in alloy I and II were 13.3% and 9.6%, respectively. Therefore, it is reasonable to conclude that the number density of the b0 precipitates decreased with the addition of Si element, which finally lead to the decreasing of peak hardness and tensile strength of the extruded alloy II.

5. Conclusions A novel alloy of Mg–8Gd–4Y–Nd–Zr–Si with high Young’s modulus has been developed. The tensile properties of this alloy are as follows: Young’s modulus E = 51 GPa, yield strength r0.2 = 347 MPa, ultimate strength rb = 392 MPa, and elongation d = 2.7%. The increase in Young’s modulus was attributed to the formation of Mg2Si, Gd5Si3 and YSi2 particles with high Young’s modulus. The decrease in yield strength and ultimate strength was ascribed to formation of rare earth containing particles, which consumed plenty of rare earth atoms in the matrix and decreased the agehardening effect during ageing.

Acknowledgment The authors would like to appreciate the financial supports from The National Basic Research Program, China.

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