MgZnCa composites fabricated by stir casting

Materials Science and Engineering A 534 (2012) 60–67

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Microstructure and mechanical properties of SiCp/Mg Zn Ca composites fabricated by stir casting X.J. Wang ∗ , K.B. Nie, X.J. Sa, X.S. Hu, K. Wu, M.Y. Zheng National Defense Science and Technology Key Lab for Space Materials Behavior and Evaluation, Harbin Institute of Technology, Harbin 150001, PR China

a r t i c l e

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Article history: Received 18 September 2011 Received in revised form 15 November 2011 Accepted 17 November 2011 Available online 8 December 2011 Keywords: Mg Zn Ca Composites Stir casting SiC

a b s t r a c t SiCp/Mg Zn Ca composites with three volume fractions (5%, 10% and 15%) were successfully fabricated by stir casting. The particle distributions were fairly uniform in all composites, although most particles were segregated along grain boundaries in the micro-scale level. Interfacial reaction products were not observed at the interface between SiCp and matrix. SiCp affected the size and shape of the second phases as well as their distribution in the matrix. In the 5% composite, many large second phases were observed at the particle surfaces. As the particle content was increased up to 10%, large second phases were hardly observed at the SiCp surfaces. Moreover, the second phases mainly existed in the form of fine flake in the 10% and 15% composite, unlike the alloy and 5% composite in which the second phases were large. The effect of SiCp on the second phases significantly influenced the mechanical properties of the composites. Compared with the matrix alloy, 15% composites had good room temperature and high temperature mechanical properties. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Mg matrix composites can overcome low strength and low elastic modulus of Mg alloys, so they are very attractive in the lightweight fields, such as aerospace and automotive fields [1–3]. Among manufacturing methods available for Mg matrix composites, stir casting is generally accepted as an adaptable and economical route, due to its simplicity, flexibility, low processing cost and high production rate [4]. Therefore, Mg matrix composites fabricated by stir casting are widely studied in recent years [1–3]. For Mg matrix composites fabricated by stir casting, a variety of reinforcements have been used, such as SiC [2], graphite [5], and aluminium borate whiskers [6]. However, used matrix alloys are almost monotonous. Researchers mainly employed AZ91 alloy as matrix due to its good castability [1–3,5,6]. As we know, both matrix and reinforcement can significantly affect the properties of Mg matrix composites. Therefore, it is necessary and interesting to study the novel Mg alloys matrix composites. AZ91 is one of most common and commercial Mg alloys which contain Al as a major alloying element. Al improves the castability and mechanical properties at room temperature. However, at temperatures above 120 ◦ C, Mg Al based alloys like AZ91 become very soft due to the presence of the Mg17 Al12 phase (gammaphase) [7,8]. Addition of Ca and Zn elements can reduce the amount

∗ Corresponding author. Tel.: +86 451 86402291; fax: +86 451 86413922. E-mail addresses: [email protected], [email protected] (X.J. Wang). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.11.040

of gamma-phase by replacing either some or all the Al. Ca can form with Mg the stable intermetallic compound Mg2 Ca [8,9]. Zn together with Mg may form various Mg Zn intermetallic compounds. Finally, Zn and Ca together with Mg may form the stable intermetallic compound Ca2 Mg6 Zn3 [8–10]. Therefore, Mg Zn Ca alloys indicate high hardness and good creep resistance. Adding ceramic particles to Mg Zn Ca alloys may fabricate Mg matrix composites with good high temperature properties. In addition, compared with the Mg-RE (rare earth) based alloys, the Mg Zn Ca system is more promising for developing low-cost creep resistant magnesium alloys [11]. Ca has a beneficial effect on oxidation resistance, which enables improved handling of alloy melts [11]. Therefore, Mg Zn Ca matrix composites fabricated by stir casting are very cost-effective as well as good room temperature and high temperature mechanical properties. However, to date, there is no relative report on them. Thus, this article was aimed to investigate microstructure and mechanical properties of SiC particle reinforced Mg Zn Ca composites (SiCp/Mg Zn Ca) fabricated by stir casting. 2. Experiments 10 ␮m SiC particles (SiCp) were selected as the reinforcement. The composition of Mg Zn Ca alloy is Mg 4.5 wt.%Zn 1.2 wt.%Ca 0.055%Mn. The composites with three volume fractions (5%, 10% and 15%) were fabricated. The Mg Zn Ca alloy was molten at 750 ◦ C and cooled to 625 ◦ C at which the matrix alloy is in the semi-solid condition. SiCp

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preheated to 600 ◦ C were quickly added into the semi-solid alloy, and the melt was stirred at 1000 rpm. The melt rapidly reheated to 750 ◦ C was then poured into a preheated steel mould (450 ◦ C) and allowed to solidify under a 100 MPa pressure to obtain ingots without porosity. For the reason of comparison, Mg Zn Ca alloy was also cast in the same condition of the composites. Microstructural examination was carried out by optical microscopy (OM), scanning electron microscope (SEM) equipped with energy dispersive spectrometer (EDS), transmission electron microscope (TEM) and high-resolution TEM (HRTEM). For OM, SEM and TEM, specimens were cut parallel to extrusion direction by electrodischarge. The specimens for OM were ground, polished and etched in acetic picral [5 ml acetic acid + 6 g picric acid + 10 ml H2 O + 100 ml ethanol (95%)]. Specimens for TEM were prepared by grinding-polishing the sample to produce a foil of 50 ␮m thickness followed by punching 3 mm diameter disks. The disks were ion beam thinned. Specimens for SEM were ground and polished with great care in order to avoid causing particle damage in this stage. The tensile tests of the SiCp/Mg Zn Ca composites and alloys were conducted on an Instron1186 universal testing machine at a constant cross-head speed of 0.5 mm/min. The shape and size of the tensile test samples were shown in Fig. 1. The high temperature tensile tests were conducted on an Instron5569 testing machine. The tensile property data (yield strength, ultimate tensile strength,

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Fig. 1. Shape and size of tensile specimens (unit: mm).

elastic modulus and elongation) were based on the average of 3–5 tests. 3. Results and discussion 3.1. Microstructure of composites Fig. 2 shows SEM microstructure of as-cast alloy and composites. For all three composites, particle macro-clusters were not observed, and particle distributions were fairly uniform. However, most particles were segregated along grain boundaries in the micro-scale level. This is very normal for metal matrix composites fabricated by stir casting. This was caused by the “push” effect of solidification front [4]. During the solidification of the composites produced by stir casting, most particles were pushed to the solidification front

Fig. 2. SEM images for alloy and composites with different particle contents. (a) Alloy, (b) 5 vol.%, (c) 10 vol.%, and (d) 15 vol.%.

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Fig. 3. High magnification SEM images for the composites. (a) 5 vol.%, (b) 10 vol.%, and (c) 15 vol.%.

while primary magnesium grains grew, so these particles were segregated in the intergranular regions during the impingement with other growing grains [4]. As shown in Fig. 2(a), a lot of large second phases distributed along the grain boundaries in the Mg Zn Ca alloy. It has been demonstrated that the second phases are mainly Ca2 Mg6 Zn3 in Mg Zn Ca alloys. As has been mentioned above, most particle also distributed along the grain boundaries in the composites. Therefore, particles affected the precipitation of the second phases of matrix in the composites, which can be observed in high magnification SEM of Fig. 3. In the 5% composite, the size and morphology of the second phases were not evidently changed, but many large second phases were observed at the particle surfaces. In SEM images, both particles and large second phases were white. Although they can be distinguished according to their different sizes and shapes, linear and surface SEM-EDS are more accurate and effective. According to the linear SEM-EDS results in Fig. 4, many particles were coated by large second phases. This may be very bad to the mechanical properties of the composite because both SiC particle and second phases are very brittle. Based on this, addition of SiC particles may impair the mechanical properties of matrix. However, it is different in the composites with higher particle contents. As shown in Fig. 3(b), as the particle content was increased up to 10%, large second phases were hardly observed at the SiCp surfaces. This can be further confirmed in the surface SEM-EDS of Fig. 5. Moreover, the second phases mainly existed in the form of fine flake in the 10% and 15% composite, unlike the alloy and 5% composite in

which the second phases were large. Therefore, the size and shape of the second phases changed in the composites with higher particle contents. Finally, from Figs. 2–5, it seems that addition of SiCp reduced the content of second phases. To sum up, SiCp affected the size and shape of the second phases as well as their distribution in the matrix. Moreover, SiCp significantly refined the grains of matrix, as shown in Fig. 6. The average grain size was about 100 ␮m in the alloy. It was about 50 ␮m in the 5% composite. The grains were respectively reduced to 25 and 20 ␮m in 10% and 15% composites.

Fig. 4. Linear SEM-EDS for the 5% composites (corresponding to Fig. 2(a)).

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Fig. 5. Surface SEM-EDS for 15% composite. (a) SEM images, the other figures are the distribution of elements marked in their top left-hand corners.

OM observation also confirmed that most particles were segregated along grain boundaries in the micro-scale level. The grain refinement was mainly caused by particles’ obstacle to grain growth. During their solidification, particles were mainly pushed to the intergranular regions, so particles limited the movement of primary Mg grain boundaries and then retarded the grain growth [12]. Fig. 7 shows the typical interface morphologies of the composites. Interfacial reaction products were not observed. HRTEM observations indicated that SiCp have a good bond with Mg Zn Ca matrix, as shown in Fig. 7(b). In addition, a fine small second phase was observed at the interface, and it was identified to be Ca2 Mg6 Zn3 by TEM diffraction pattern. This fine precipitation at interface can improve the bond strength between SiCp and matrix.

3.2. Mechanical properties of composites Fig. 8 shows the room temperature mechanical properties of the SiCp/Mg Zn Ca composites. Yield strengths (YS) and ultimate tensile strength (UTS) of composites increased with the increase of particle contents. YS of all the composites were higher than YS of the alloy. This is very normal according to microstructure of the composites. As shown in Fig. 6, the average grain sizes decreased with the increase of particle contents. The contribution to YS of this refinement can be evaluated using the Hall–Petch relation, which relates YS enhancement to grain size d [13]: GR ≈ ˇD−1/2 YS

(1)

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Fig. 6. OM for the alloy and composites. (a) 0%, (b) 5%, (c) 10%, and (d) 15%.

The value of ˇ depends on a number of factors, but is typically √ GR for 5%, 10% and around 0.1 MPa m [13]. According to this, YS 15% composites are 14 MPa, 20 MPa and 22 MPa, respectively. As shown in Fig. 8, compared with matrix alloy, the  YS for 5%, 10% and 15% composites were about 25 MPa, 56 MPa and 88 MPa. This indicates that other strengthening mechanisms also played a role. Besides grain refinement, there are Orowan strengthening, load transfer and dislocation strengthening for metal matrix composites. It is widely known that Orowan and dispersion strengthening is not significant for micro-particle reinforced metal matrix composites [13–16]. For load transfer, shear lag model is usually considered. For equiaxial particles, an increase in the yield stress LT T may be given by [16]: caused by load transfer YS LT = m 0.5f YS

(2)

 m is the yield stress of the matrix, f is the volume fraction of LT for 5%, 10% and 15% the particles. According to this, the YS composites are about 2 MPa, 4 MPa and 6 MPa, respectively. This indicated the load transfer effect was not evident in the composites under study. Trojanova et al. also found the load transfer effect was insignificant in particle reinforced Mg Li composite [16]. What’s more, the model is based on the simplifying assumption of uniform matrix deformation and therefore, yields a very simplified expression for stiffness and strength contribution. In this study, most particles were segregated along grain boundaries in the microscale level. This kind particle distribution further reduced the load transfer effect. Thus, the load transfer effect was very insignificant

in SiCp/Mg Zn Ca composites. Dislocation strengthening can play an important effect [13–15]. SiC particles can also induce multidirectional thermal stress at the particle/matrix interfaces due to the large differences of coefficient of thermal expansion between matrix and SiCp. Therefore, the high dislocation density is higher in the matrix of composites. Miller and Humphreys, assuming cubedshaped particles, have predicted an increase in dislocation density given by  = 12

˛ Tf bd

(3)

where ˛ T is the thermal misfit strain, b is the Burgers vector, d is the particle size [13]. The ˛Mg is about 26 × 10−6 , and ˛␣-Sic is about 6.6 × 10−6 . According to the fabrication process, T was about 700 ◦ C. For tensile test under room temperature, basal slip is dominate in Mg matrix, so b = 0.32 nm. Thus, the  for 5%, 10% and 15% composites are calculated to be 0.26 × 1014 , and 0.51 × 1014 and 0.77 × 1014 m−2 , respectively. The increase of YS due to thermal expansion mismatch dislocations can be predicted by [13,15,17,18] √ d YS = kGb  (4) where G is the shear modulus of the matrix, and k is a constant, approximately equal to 1.25. G = 16.5 GPa under room temperad for 5%, 10% and 15% ture [17,18]. Through calculation, the YS composites are 34 MPa, 47 MPa and 57 MPa, respectively. Based on the above calculations, the calculated results for the composites were shown in Table 1. The YS increases of the

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Fig. 7. TEM images for the interfaces. (a) Morphology, (b) HRTEM, (c) a second phase at the interfaces, (d) diffraction pattern of the second phase.

Table 1 The calculated results and experimental  YS for the composites (unit: MPa). Composites 5% 10% 15%

GR YS

LT YS

d YS

Calculated  YS

Experimental  YS

14 20 22

2 4 6

34 47 57

50 71 85

25 56 88

composites were mainly caused by grain refinement and dislocation strengthening. As far as grain size and dislocation density are concerned, the YS of composites increased with the increase of particle contents. The differences between the calculated  YS and the experimental  YS become smaller as volume fraction increases. Up to 15%, they are very close. This may be caused by the particle segregation. The particle distribution became more uniform with the increase of particle contents, as shown in Fig. 4. Eqs. (2) and (3) are based on the uniform particle distribution. Thus, the differences between the calculated results and the experimental results decrease as the particle distribution become more uniform. Although SiCp significantly improve the YS of matrix, UTS of the 5% composite was lower than UTS of the alloy, as shown in Fig. 8. However, UTS of the 15% composite was much higher than that of alloy. This is different from that in SiCp/AZ91 composites

fabricated by stir casting. For the as-cast SiCp/AZ91 composites, UTS usually is lower than that of as-cast AZ91 [3]. In addition, it was unexpected that elongation of the composites increased with the increase of particle content, which is also different from that in SiCp/AZ91 composites [2]. All these were associated with the effect of particles on the second phases. As shown in Figs. 2–4, many particles were coated by large second phases at intergranular regions in the 5% composite. Both SiCp and the second phases are very brittle, so the interface between SiCp and the large second phase cannot bear large strain. In addition, particle segregations aggravate stress concentrations at grain boundaries, which causes larger deformation strain at grain boundaries. Therefore, this kind interface resulted in lower UTS and elongation. However, as particle contents increased, the large second phases at SiCp surfaces significantly reduced and the sizes of second phases were evidently refined. This led to the increase of strength and elongation as particle contents increased. Therefore, the effect of particle on second phases significantly affected the mechanical properties of SiCp/Mg Zn Ca composites. From Fig. 8, it can be found has the 15% composites had the optimal mechanical properties under room temperature, so the 15% composite was selected to be tested at elevated temperatures. The UTS of 15% composite at different temperatures was shown in Fig. 9. For the composite, UTS at 100 ◦ C was similar to that at room

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Fig. 8. Room temperature mechanical properties of the composites. (a) Typical tensile stress–strain curves, (b) YS and UTS, (c) elastic modulus and elongation.

temperature. Even, UTS of the composite at 200 ◦ C was similar to UTS of alloy under room temperature. All these indicated that the 15% SiCp/Mg Zn Ca composite had good high temperature mechanical properties.

4. Conclusions SiCp/Mg Zn Ca composites were successfully fabricated by stir casting. The particle distributions were fairly uniform in all composites, although most particles were segregated along grain boundaries in the micro-scale level. SiCp affected the size and shape of the second phases as well as their distribution in the matrix. In the 5% composite, many large second phases were observed at the particle surfaces. As the particle content was increased up to 10%, large second phases were hardly observed at the SiCp surfaces. Moreover, the second phases mainly existed in the form of fine flake in the 10% and 15% composite. The effect of SiCp on the second phases significantly influenced the mechanical properties of the composites. Acknowledgements This work was supported by “National Natural Science Foundation of China” (Grant No. 51101043, 50801017 and 51001036) and “the Fundamental Research Funds for the Central Universities” (Grant No. HIT.NSRIF.201130). References

Fig. 9. The UTS of 15% composites at different temperatures.

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