Effects of Sm on the grain refinement, microstructures and mechanical properties of AZ31 magnesium alloy

Effects of Sm on the grain refinement, microstructures and mechanical properties of AZ31 magnesium alloy

Materials Science & Engineering A 620 (2014) 89–96 Contents lists available at ScienceDirect Materials Science & Engineering A journal homepage: www...

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Materials Science & Engineering A 620 (2014) 89–96

Contents lists available at ScienceDirect

Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea

Effects of Sm on the grain refinement, microstructures and mechanical properties of AZ31 magnesium alloy Ming Sun a,c, Xiaoyu Hu a, Liming Peng a,b,n, Penghuai Fu a, Yinghong Peng c a

National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China c School of Mechanical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China b

art ic l e i nf o

a b s t r a c t

Article history: Received 26 May 2014 Received in revised form 1 September 2014 Accepted 27 September 2014 Available online 5 October 2014

The effects of samarium (Sm) on the grain refinement, microstructures and mechanical properties of as-cast AZ31 (Mg–3Al–1Zn–0.3Mn) magnesium (Mg) alloy have been investigated. Very serious grain coarsening happens when Sm content is between 0.16% and 1.18%. This is due to both the reactions between Al and Sm which reduce the constitutional undercooling effect and the lack of Al2Sm heterogeneous nuclei. However, excellent grain refinement effect is achieved at Sm content above 2.17%, which is because the in-situ formed Al2Sm particles significantly promote heterogeneous nucleation. The main phases in AZ31–xSm alloys include α-Mg, β-Mg17Al12, Al11Sm3 and Al2Sm. The Mg17Al12 phase is gradually suppressed by the increase in Sm content, and the Al2Sm is present at a higher Sm content. Because of grain refinement strengthening and secondary phase strengthening effects, the room temperature tensile property of AZ31–3.13Sm alloy has the optimal value of YS78.7MPa–UTS216.7MPa-EL13.6%. & 2014 Elsevier B.V. All rights reserved.

Keywords: Magnesium alloy Samarium (Sm) Grain refinement Microstructure Mechanical property

1. Introduction Mg alloy for structural component use in the automotive and aerospace industries is attractive due to high specific strength and low density [1]. Among the wide range of Mg alloys, Mg–Al system alloy takes a dominating position of consumption because it is easily cast and exhibits moderate strength. Grain refinement is a very important method to improve the microstructural uniformity, mechanical properties and corrosion resistance of cast Mg components [2]. Since Mg alloy has a relatively poor formability and limited ductility at room temperature due to hexagonal close-packed (HCP) crystal structure [3], a fine initial grain size is extremely necessary in order to improve the post-casting (such as extrusion and rolling) formability [2]. However, it has long been realized that grain refinement of Mg–Al alloy is still a major issue, although lots of refinement approaches have been trialed over the last decades, such as Solute Element Additions, Superheating, Elfinal Method, Carbonbased Inoculation, Native Grain Refinement, Foreign Nucleating Particles, and Adding Minor Elements Fe, Mn or Ti [2,4]. n Corresponding author at: National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China. Tel.: þ 86 21 54742911; fax: þ86 21 34202794. E-mail address: [email protected] (L. Peng).

http://dx.doi.org/10.1016/j.msea.2014.09.106 0921-5093/& 2014 Elsevier B.V. All rights reserved.

Rare earth (RE) elements have been used to alloy Mg–Al alloy for many years. Reports on commercial Mg–Al–RE (AE) series alloys show that the mechanical property and creep resistance can be greatly improved due to the suppression of β-Mg17Al12 intermetallic compounds and the formation of highly thermal stable Al–RE phases such as Al11RE3 and Al2RE [5]. Especially the Al11RE3 intermetallic compound exerts an advantageous influence on the mechanical properties [5]. However, it is only recently known that Al2RE phase can act as a grain refiner for Mg alloy [6], which makes the grain refinement research of Mg–Al alloy promising. Qiu et al. identified Al2Y to be an effective grain refiner in a Mg–10Y alloy and further found the thermal stability of the fine grains was excellent [6,7]. Li et al. found that the grains of Mg–6Al–0.6Zn alloy were strongly refined by in-situ formed Al2Sm particles through heterogeneous nucleation [8]. Li reported that the grains of AZ31 alloy were refined by the formation of Al2Gd particles through 2% Gd addition and the rolling capability of AZ31 alloy was subsequently improved [9]. Wang reported that adding Gd into a Mg–2Al–1Zn alloy led to the formation of (Mg, Al)3Gd and Al2Gd followed by an increase in tensile properties at both room temperature and elevated temperature, but the grain refinement was not studied [10]. Dai et al. showed that Al2Gd particles that formed through the addition of 1.3% Al had a dramatic grain refinement effect (from  330 μm to  37 μm) on Mg–10Gd alloy [11]. Despite these studies on the use

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of Al–RE compounds, the alloying behaviors of rare earth Sm element in Mg–Al alloy have not been fully understood. Although AZ31 alloy is one of the successful wrought magnesium alloys, the use is to some extent limited due to its poor mechanical property. Triggered by the potential use of Al2RE refiner, as-cast AZ31–xSm alloys are investigated in this study. The present work is somewhat useful for further development of new Mg alloy.

2. Material and methods Each batch of 3 kg AZ31–xSm alloy was individually melt from raw materials of pure Mg, pure Al, pure Zn, Al–20% Mn and Mg–30% Sm master alloys in an electric furnace under the protection of mixed protective gas consisting of 99 vol%CO2 and 1 vol%SF6. After melting materials at about 730 1C, the melt was manually stirred for 5 min by a stainless steel paddle and then held for 15 min to let Al and Sm react completely and was then poured into a permanent metallic mold preheated to 200 1C (as shown in Fig. 1). The actual compositions listed in Table 1 were determined by an inductively coupled plasma atomic emission spectroscopy (ICP-AES) in Shanghai University. As-cast samples for optical microscopy (OM) observations were cut from the position “A” marked in Fig. 1. The samples were polished and then etched in an M1 etchant (29.4 g picric acid, 41 ml water, 50 ml acetic acid and 350 ml ethanol). The grain size was measured by a linear intercept method according to ASTM 112-96 using the polarized light images taken by a Zessis OM. The microstructures were further examined by a FEI Quanta 250 field emission scanning electron microscope (SEM, with EDS attached). Flat tensile samples as shown in Fig. 1 were cut from the bottom (materials here are much denser) of the cylinder ingots using an electric-spark wire-cutting machine. The gage dimensions of the tensile specimen were 48  18  1.5 mm3. Tensile test was carried out on a Zwick/Roell-20KN material test machine at a cross-head speed of 0.5 mm/min at room temperature. For every alloy, seven pieces of tensile specimen were tested and then the values are

averaged. The fractures after tensile test were also observed on SEM.

3. Results and discussion 3.1. Grain size Fig. 2 shows the grain morphologies and the average grain size plot of AZ31–xSm alloys. It can obviously be seen that the grain size increases firstly and then decreases. The grain size of base alloy AZ31 is about 462 722 μm, while small additions of Sm, i.e. 0.16–0.32% Sm, greatly coarsen the grain size. For example, the grain size of 0.16% Sm-alloy is  1438 7108 μm, which is three times bigger than that of base alloy. When Sm is increased to 1.18%, the grain size is much finer than 0.32% Sm-alloy, but is still coarser than AZ31. In contrast, when Sm content is 2.17% or 5.33%, the grain size is remarkably refined to be 173 76 μm or  10273 μm, respectively. This interestingly indicates the effect of Sm on the grain size of AZ31 alloy is non-monotonic. It is well known that both the potency of the nucleant particles and the segregating power of the solute are critical in determining the final grain size [2,3]. The grain coarsening effect at 0.16–1.18% Sm contents is possibly because of three aspects. Firstly, the chemical reactions between Al and Sm may weaken the effect of solute constitutional undercooling (ΔTc) on grain refinement. Secondly, the lack of Al2Sm particles at those lower Sm contents Table 1 The actual compositions determined by ICP-AES of AZ31–xSm alloys. Alloy

Al

Zn

Mn

Sm

Fe

A B C D E F G

2.76 2.87 2.85 2.86 2.76 3.02 2.78

0.84 0.84 0.91 0.93 0.87 0.97 0.84

0.28 0.28 0.29 0.28 0.26 0.27 0.27

– 0.16 0.32 1.18 2.17 3.13 5.33

0.023 0.016 0.023 0.035 0.012 0.035 0.008

Fig. 1. The schematic for mold, ingot and dimension of tensile test sample.

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2000

Average grain size 1500

Average grain size, µm

1000

400 300 200 100 0 0

1

2

3

4

5

6

Sm content, wt.%

Fig. 2. The optical microstructures of AZ31–xSm alloys. (a) x¼ 0; (b) x¼0.16%; (c) x¼0.32%; (d) x¼ 1.18%; (e) x¼ 2.17%; (f) x¼3.13%; (g) x¼ 5.33%; and (h) the average grain size plot.

cannot supply sufficient heterogeneous nucleation sites. Thirdly, it is reported that the native Al–(Fe)–C–O type compounds, which are believed to be naturally introduced by the inevitable trace impurities such as Fe and C in the raw materials, can promote grain refinement in Mg–Al alloys [12,13]. The addition of Sm possibly removes or poisons the native Al–(Fe)–C–O particles. However, the third aspect needs strong evidences. In terms of the very excellent grain refinement effects that achieve at 3.13– 5.33% Sm contents, it is aware that in-situ Al2Sm particles are present at most of the grain centers. The misfit between Al2Sm and Mg is only 0.45% [6] and this effectively promotes heterogeneous nucleation. Moreover, the needle-like Al11Sm3 compound also plays a role in restraining dendrites or grains from growing rapidly in the alloys with higher Sm contents. 3.2. Phases and microstructures Fig. 3 shows XRD patterns of the AZ31–xSm alloys. It can be seen the phases in base alloy AZ31 are consisted of α-Mg matrix and β-Mg17Al12. Zn is mainly in the α-Mg solid solution, and the Mn-rich phase Al8Mn5 is undetectable in XRD pattern as Mn content is only  0.3%. When Sm is added, the higher electronegativity difference between Sm and Al element (than that between Mg and Al) favors the preferential formation of Al–Sm compounds [7]. With the gradual appearances of new phases Al2Sm and Al11Sm3, the β-Mg17Al12 phase finally vanishes when Sm reaches 1.18. Additionally, the Mg41Sm5 secondary phase that

Fig. 3. XRD patterns of AZ31–xSm alloys.

is present in Mg–Sm binary alloy does not form in this study, since most of Sm has combined with Al. Fig. 4 shows the typical SEM morphologies and EDS results of various phases in AZ31–xSm alloys. The microstructure of AZ31 is consisted of α-Mg matrix, the dispersedly distributed granular

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Fig. 4. Typical SEM morphologies and EDS results of different phases in AZ31–xSm alloys. (a) 0 Sm alloy, P1: β-Mg17Al12, P2: Al8Mn5; (b) 1.18% Sm alloy, P3: Al8Mn5; (c) 3.13% Sm alloy, P4: Al11Sm3; (d) 5.33% Sm alloy, P5: Al2Sm.

β-Mg17Al12 (see Point 1, P1) and the short-rod shaped Al8Mn5 phase (see Point 2, P2). When Sm is added, although the Al8Mn5 phase (see Point 3, P3) seems to be mixed with the needles of

Al11Sm3 phase, it can still be recognized due to much thicker rod sizes. The needle-like (or weed-like) Al11Sm3 phase is very clearly seen at any Sm content (above 0), and the morphology of Al11Sm3

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volume fraction of second phases, %

12

volume fraction 10

8

6

4

2

0 0

1

2

3

4

5

6

Sm content, wt.% Fig. 5. SEM images of AZ31–xSm alloys: (a) x ¼0; (b) x¼ 0.16%; (c) x¼ 0.32%; (d) x¼ 1.18%; (e) x ¼ 2.17%; (f) x ¼3.13%; (g) x ¼ 5.33%, and (h) volume fraction of secondary phases.

agrees well with that reported by Jianli [14]. In the 2D image, parts of the Al11Sm3 display point-shape (see Point 4, P4) as they happen to be polished from the vertical direction of needles. Compared with the relatively lower quantity and smaller size of Al2Sm particle at 2.17 Sm content, lots of polygonal Al2Sm particles (see Point 5, P5) with much bigger size (most around 5–9 μm in diameter) can be easily found when Sm content is 5.33%. Fig. 5(a)–(g) shows the SEM images of AZ31–xSm alloys. In these images, it is difficult to distinguish between the granular β-Mg17Al12 phase and similar shaped Al11Sm3 phase. However, it is confirmed by XRD results and EDS analysis that Sm addition decreases the fraction of Mg17Al12 phase, and the Mg17Al12 phase is difficult to find at Sm content is above 1.18. It is displayed in Fig. 5(h) that the volume fraction of secondary phase, i.e. the Mg17Al12 phase in base alloy while mainly the Al11Sm3 phase in Sm-containing alloys, increases from 1% of base alloy to 10% of 5.33% Sm-alloy. Moreover, the Al11Sm phase becomes more developed when Sm content reaches the range of 2.17–5.33%,

and it is even difficult to accurately define the morphology of Al11Sm3 compound since it has very irregular shape. The size distribution of possible active Al2Sm nucleation particles in both Mg–3Al–3.13Sm and Mg–3Al–5.33Sm alloy is shown in Fig. 6. It is noted that the possible active particles indicate particles those locate at the grain centers. The Al2Sm particles that act as nuclei are likely to be between 3–11 μm in size, among which the size range of 5–9 μm seems to have the largest frequency (  70%). This particle size distribution is quite similar to that of the Al2Gd in Mg–10Gd–(1–1.3)Al alloy [10] and that of Al2Y in Mg–10Y–(1–3)Al alloy [6], but has a relatively bigger size than Zr particles (usually 1–5 μm) in Zr-bearing Mg alloys [15]. The size difference between Zr and Al2RE nuclei is attributed to their own inoculation features. Zr is always “ex-situ” introduced by a Mg–Zr master alloy followed by a deliberate settling out of larger Zr particles (above 5 μm) during melting Mg alloys [15], which makes the Zr particle size in the final casting is not affected by Mg alloy chemistry and solidification conditions.

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20

3.13Sm alloy 5.33Sm alloy

220

YS UTS

200

16

YS & UTS, MPa

Relative frequency, %

EL. 180

20

15

10

160

12

140 120

8

100 80

5

Elongation, %

25

4

60 0

40 0

0 3~4

4~5

5~6

6~7

7~8

8~9

9~10 10~11

Al2 Sm partice size, µm

1

2

3

4

5

6

Sm content, wt.% Fig. 7. Room temperature tensile properties of AZ31–xSm alloys.

Fig. 6. The size distribution of possible active Al2Sm nucleation particles in AZ31–xSm alloys.

However, the Al2Sm forms “in-situ” during solidification of Mg– Al–Sm alloy, and this implies that the presence and size distribution of active Al2Sm particle is significantly dependent on the melt composition and solidification conditions. On the other hand, according to free growth theory, the undercooling for free growth (ΔTfg) is simply given by ΔTfg ¼4σ/(ΔSvd), where σ is the solid– liquid interfacial energy, ΔSv is the entropy of fusion per unit volume, d is the the nucleant particle diameter [16,17]. This equation indicates that larger particles become active nucleants at smaller undercoolings, while smaller particles remain inactive if the maximum undercooling reached in the melt does not exceed their critical ΔTfg [17]. The higher constitutional undercooling generated by soluble Zr can substantially facilitate heterogeneous nucleation of Mg on Zr particle substrates [15], which makes diameter of Zr is not so important. In contrast, the formation of Al2Sm and Al11Sm3 consumes Al and Sm, which then reduces the constitutional undercooling effect of solutes Al and Sm. The undercooling to promote Mg nucleation is weaken, and therefore nucleation prefers to happen on the bigger sized Al2Sm particles whose nucleation barriers are much lower. To understand how the cooling condition affects the formation, size and refinement efficiency of Al2Sm particle, further study needs to be undertaken. 3.3. Tensile properties and fractures The tensile properties at room temperature including yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) are plotted in Fig. 7. The property of base alloy AZ31 is 66.3– 173.9 MPa-6.3% (note that the three values correspond to YS, UTS and EL, respectively, the same hereinafter). It is clear that both of YS and UTS decreases firstly when addition of Sm is very small, i.e. 0.16% and 0.32% Sm, and then begins to recover at 1.18% Sm. The AZ31–1.18Sm alloy has a higher tensile property (56–177.2 MPa8.6%) than AZ31–0.32Sm alloy, but the value is just approximately equal to that of AZ31. Increasing Sm from 1.18% to 3.13% greatly improves the values of tensile properties to 78.7–216.7 MPa-13.6%. However, further increasing Sm content to 5.33% does not result in remarkable changes in YS and UTS except a slight decrease in elongation (from  13% to  10%). Fig. 8(a)–(d) shows typical SEM images of fracture surfaces of AZ31 alloy with 0% Sm, 0.32% Sm, 2.17% Sm and 3.13% Sm, respectively, and the corresponding enlarge views for the selected rectangular region are illustrated in Fig. 8(a0 )–(d0 ). From the low magnification images, it is seen that the facture surface of 3.13% Sm alloy is much more flat compared to that of the rest alloys. It can also be seen from the high magnification images that all

fractures mainly have cleavage planes and tearing ridges features. In addition, lots of fractured Al11Sm3 phases can be found in 2.17% Sm alloy (Fig. 8(c0 )), which is because that the secondary phase (mainly Al11Sm3 as the amount of Al2Sm is relatively less) strengthening effect plays an important role in fracturing. While Sm content is 3.13%, both of fractured Al11Sm3 and Al2Sm particles are easily found at the fracture surface in Fig. 8(d0 ). This means the improvement in tensile property of AZ31–3.13Sm alloy is certainly associated with the co-existence of Al2Sm and Al11Sm3. The change in mechanical property is closely related to the microstructure evolutions of AZ31–xSm alloy. Although the main secondary phase β-Mg17Al12 has certain strengthening effect, the mechanical property of AZ31 is low as a result of a very coarse grain size ( 462 722 μm) according to the classical Hall–Petch rule. A small addition of Sm, i.e. within 0.16–0.32%, results in a dramatical decrease in tensile property due to the serious grain coarsening (above 1400–1500 μm). The grain size of 1.18%-Sm alloy becomes smaller than that of the previous Sm level (0.32%) but still much coarser than AZ31, the mechanical property thus slightly recovers. The secondary phase Al11Sm3 with a 4 vol% fraction in this alloy also acts as a main strengthening role. When Sm is above 2.17%, especially at 3.13%, the strengthening mechanism is mostly ascribed to grain refinement strengthening and secondary phase strengthening, while the solid-solution strengthening effect of solutes is actually limited due to the formations of large amount of Al11Sm and Al2Sm compounds. The in-situ formed Al2Sm particles cause very good grain refinement effect of on AZ31 alloy, and thus the tensile properties are enhanced according to the Hall– Petch rule. Moreover, because the secondary phases Al11Sm3 can provide very effective hindrances to grain boundary sliding and dislocation motion during deformation process [8,14,18], the strength of alloy with Sm content above 2.17% is improved by Orowan mechanism. However, when Sm is 5.33%, the much higher volume fraction ( 10 vol% as seen in Fig. 5(h)) as well as bigger size of secondary phase Al11Sm3 (Fig. 5(g)) causes stress concentration and this is harmful to the tensile property. This is similar to the situation that too much Cemm (cerium-rich misch metal) addition causes coarsening of Al11RE3 and thus deteriorates mechanical property of Mg–9Al alloy [19]. Therefore, AZ31–3.13Sm alloy has the optimal tensile property with value of 78.7–216.7 MPa-13.6%, increased by 19–25–116% compared to the property of AZ31. Some types of Al–RE compounds have been known to be strengthening phases for Mg alloy. Tian et al. [20] reported that the addition of 3%Y or 2Ymmþ1Cemm greatly improved the tensile property of Mg–3Al alloy. The tensile property of the above two alloys was 59–145 MPa-7.3% and 75–201 MPa-8.2%, respectively (part of values were roughly read from the plots in this reference). Wang et al. [21] reported that the addition of 0.6Ce þ0.3Y

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Fig. 8. Typical SEM images of tensile fracture surfaces of alloys with: (a) 0 Sm, (b) 0.32% Sm, (c) 2.17% Sm, and (d) 3.13% Sm. Fig. 8(a0 )–(d0 ) on the right column show the corresponding higher magnification of the selected rectangular regions in (a)–(d).

improved the tensile property of AM50 Mg alloy from 75– 150 MPa-3.2% to 120–245 MPa-4.2% through the formation of Al2Y and Al11Ce3 phases. Son et al. [18] reported that the homogeneously distributed Al2Sm intermetallic compounds in Mg–5Al– 3Ca–2Sm alloy effectively suppressed the grain growth of recrystallized grains during hot extrusion and the mechanical property was enhanced. In this study, the distinct strengthening effect of Sm on AZ31 alloy indicates that the Al2Sm and Al11Sm3 compounds are very promising in development of new Mg alloys. In addition, Al2Sm phase has a decomposition temperature of 1500 1C, and Al11Sm3 phase is likewise reported to have better thermal stability than β-Mg17Al12 phase. Similar to the reported slower grain growth rate of the AZ31–3RE (the Ce-rich misch

metal) alloy [22], the Al2Sm and Al11Sm3 phases are likely beneficial to the improvement of grain size thermal stability as well as the elevated temperature mechanical property of Mg alloy [14,23,24], which will be studied in future.

4. Conclusions (1) Serious grain coarsening of AZ31 alloy happens at Sm content within the range of 0.16–1.18%, which is mainly due to both the reduction in constitutional undercooling effect of solute and the lack of Al2Sm heterogeneous nuclei. In contrast, excellent grain refinement effect is achieved at Sm content above 2.17%,

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because the in-situ formed Al2Sm particles significantly promote heterogeneous nucleation. Most of the Al2Sm particles at grain centers of refined alloys are between 5 and 9 μm in size. (2) Phases of AZ31–xSm alloy mainly include α-Mg, Mg17Al12, Al11Sm3 and Al2Sm, among which the presence of Al2Sm is closely related to a higher Sm content, whilst the Mg17Al12 phase is gradually suppressed by the increase in Sm content. (3) AZ31–3.13Sm alloy has the optimal room temperature tensile property, i.e. YS78.7MPa–UTS216.7MPa-EL13.6%, which is attributed to the grain refinement strengthening and secondary phase strengthening effects. Fracture surface analysis indicates that AZ31–xSm alloys have a mixed fracture features consisted of cleavage planes and tearing ridges.

Acknowledgments This work is supported by SJTU Special Funds for Science and Technology Innovation (No. 13X100030018), and National Natural Science Foundation of China (No. 51201103). References [1] Y.X. Wang, X.Q. Zeng, W.J. Ding, Scr. Mater. 54 (2006) 269–273. [2] D.H. StJohn, M.A. Easton, Q. Ma, J.A. Taylor, Metall. Mater. Trans. A 44 (2013) 2935–2949.

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