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The effect of Zr on the microstructure and tensile properties of hot-extruded Al–Mg2Si composite
Materials and Design 36 (2012) 323–330
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
The effect of Zr on the microstructure and tensile properties of hot-extruded Al–Mg2Si composite A. Bahrami a, A. Razaghian a,⇑, M. Emamy b, R. Khorshidi c a
Imam Khomeini International University, Qazvin, Iran School of Metallurgy and Materials, University of Tehran, 11365-4563 Tehran, Iran c Department of Materials, Faculty of Engineering, Semnan University, Semnan, Iran b
a r t i c l e
i n f o
Article history: Received 17 September 2011 Accepted 20 November 2011 Available online 26 November 2011 Keywords: A. Metal matrix composite C. Extrusion G. Fractography
a b s t r a c t This work was carried out to investigate the effect of different amounts of Zr on the microstructure and tensile properties of homogenized and hot extruded Al-15% Mg2Si composite using optical microscopy and scanning electron microscopy (SEM). The results showed that Zr addition has no significant effect on the morphology of both primary and eutectic Mg2Si phase in as-cast condition. But, applying homogenizing and extrusion processes changed the morphology of Mg2Si phases from irregular to a more spherical shape. Further results demonstrated that the average size of primary Mg2Si decreases with the addition of Zr up to 0.1% from 56 lm to 24 lm in hot-extruded condition. As the mount of Zr increased up to 0.1 wt.%, ultimate tensile strength (UTS) and elongation values were also increased from 160 MPa and 3.2% to 292 MPa and 9.5%, respectively. Fracture surface examinations revealed a transition from brittle fracture mode in as-cast composite to ductile fracture in hot-extruded Zr-modified specimens. This can be attributed to the changes in size and morphology of Mg2Si intermetallic and porosity content. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction With the enlargement of modern industry, the production of light materials with high strength is required. Light weight materials are most appropriate for the structure construction of mass transport vehicles for the aim of decreasing emission gases and increasing fuel efficiency [1,2]. Aluminum metal matrix composites as a class of advanced engineering materials have been developed for high-performance applications because of their low density, excellent castability and good wear resistance [3–8]. Particulate metal matrix composites (PMMCs) have attracted attention due to their lower costs production and inherent isotropic properties [9,10]. Among the variety techniques for PMMCs production, in situ processes exhibit thermodynamically stable systems, good particle wetting and even distribution of the reinforcing phase in comparison to their ex-situ process counterparts [11,12]. This technique has a significant potential for simplicity of production because it simply forms itself during solidification [7]. In recent years, in situ Al–Mg2Si composites have been introduced as a new group of PMMCs which contain hard particles of Mg2Si in Al-matrix [13,14]. Mg2Si is a hard intermetallic compound with a high melting point (1085 °C). Its low density and low coefficient of thermal expansion coupled with a reasonably high elastic
⇑ Corresponding author. Tel.: +98 281 3780021; fax: +98 281 3780073. E-mail address: [email protected] (A. Razaghian). 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.11.045
modulus make it a good choice as a reinforcing agent [15]. Zhang et al. showed that in terms of properties and solidification behavior, great similarities exist between Mg2Si and Si, and between Al–Mg2Si and Al–Si systems [3]. Fig. 1 shows the equilibrium phase diagram of Al–Mg2Si [3]. It can be seen that during solidification of Al-15% Mg2Si alloy, Mg2Si phase is formed as the primary phase. Then, a-Al and secondary Mg2Si co-solidify from the liquid alloy in the narrow ternary phase area. According to Eq. (1), this pseudo-eutectic reaction is completed at 583.5 °C [3,15]:
L ! L1 þ Mg2 SiP ! Mg2 SiP þ ða Al þ Mg2 SiÞE
ð1Þ
where E is a Eutectic, P is a Primary, and L1 is a Liquid in two phase region. Nevertheless, in normal as-cast Al–Mg2Si composites, the primary Mg2Si phases are usually coarse which lead to the reduced mechanical properties. Therefore, Mg2Si crystals must be modified to ensure adequate mechanical strength and ductility of the composite [6–9]. Several works have been focused on the modification of the structure with the addition of various alloying elements such as pure Na [1], K2TiF6 [4], Ce [10], Sr [14], Cu [15], Y [16], Ti [17] and additional quantities of silicon [18] to the liquid alloy. However, the improper control of some process parameters of this technique can lead to deleterious defects in cast composites such as porosity and non-uniform distribution of the particles in the matrix.
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Fig. 1. Equilibrium phase diagram of Al–Mg2Si pseudo-binary section [3].
Therefore, thermomechanical procedures such as extrusion, rolling or forging seem to be efficient processes for decreasing the porosity and obtaining a more uniform particle distribution in such composites [19–22]. Hot extrusion, as a conventional thermomechanical process, is used widely in wrought aluminum alloys and its composites. This technique is an effective method to reduce casting defects because of refinement of grains and reinforcing phases. The properties of the MMCs are significantly sensitive to the type of the reinforcement and the fabrication processing of the composite after initial production stage [23]. It has been reported that the average size of the reinforcing particles reduces during extrusion due to particle fragmentation [24]. Furthermore, by increasing extrusion ratio, UTS and elongation values of the composite are also increased [25]. However, the presence of the reinforcing particles increases the applied force for extrusion which may result in formation of particle breakage and microporosity [24]. This aim of this research was to study the microstructure and tensile properties of hot-extruded in situ Al–Mg2Si composite containing different amounts of Zr.
Fig. 3. SEM back-scattered image of the as-cast Al-15% Mg2Si composite microstructure.
2. Experimental procedure Industrially pure Al (99.8%), Mg (99.9%) and Si (99.5%) metals were used to prepare Al-5.5 wt.% Si-9.7 wt.% Mg alloy primary ingots of Al-15% Mg2Si composite. All materials were preheated in an electrical resistance furnace using a 10 kg graphite crucible. The MMC ingots were remelted within a small SiC crucible (with 1 kg capacity) in a resistance furnace in order to prepare alloys with 0, 0.05, 0.1, 0.3 and 0.5 wt.% Zr. When the temperature reached 750 °C, Al-5Zr master alloy was added to the molten composite. Degassing was conducted by using dry tablets containing C2Cl6 (0.3 wt.% of the molten material) for about 3 min. After cleaning off the slag, alloys with different compositions were poured into a cast iron mold preheated to 200 °C (Fig. 2a). After casting, the composite billets were cut and machined into 28 mm in length and 29 mm in diameter in order to fit into the
Fig. 2. The schematic of cast iron mold and (b) sketch map of tensile samples.
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Fig. 4. The comparison of optical microscopy images of the Al-15 wt.% Mg2Si composite (a) before and (b) after hot-extrusion at various Zr contents: (1) 0 wt.%, (2) 0.05 wt.%, (3) 0.1 wt.%, (4) 0.3 wt.% and (5) 0.5 wt.%.
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extrusion container. All the billets were exposed to homogenizing treatment in an electrical furnace at 520 °C for 4 h and subsequently slow cooling in furnace. Then, these billets were hot extruded by using a hydraulic press at a ram speed of 1 mm/s with the extrusion ratio of 18:1 at 450 °C. Extrusion process was carried out applying graphite based oil between samples and dies. According to ASTM E8-04 small size, round tensile samples were machined along the extrusion direction [26]. Tensile tests were carried out in a computerized testing machine (Zwick/Roell Z100) at a strain rate of 0.1 mm/min at room temperature. The sketch of a tensile test specimen is seen in Fig. 2b. Metallographic specimens were also prepared by polishing and etching in hydrofluoric acid solution (5% HF). In order to reveal the morphology of primary Mg2Si and eutectic Mg2Si phase, some specimens were deeply etched in mixture of 15% NaOH + 0.5% HF water solution. Microstructural parameters were determined using an optical microscope equipped with an image analysis system (Clemex Vision Pro. Ver. 3.5.025). The microstructural characteristics and fracture surfaces of the specimens were also examined by SEM performed in a VegaÓTescan SEM. Fig. 5. The size and volume fraction of primary Mg2Si particles as a function of Zr additions after hot-extrusion process.
Fig. 6. Three-dimensional morphologies of: (a) primary Mg2Si in as-cast, (b) primary Mg2Si in hot-extruded. (c) eutectic Mg2Si in as-cast and (d) eutectic Mg2Si in hotextruded Al-15% Mg2Si-0.3% Zr composite.
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3. Results and discussion 3.1. Microstructural studies A typical SEM micrograph of the as-cast Al-15% Mg2Si composite is shown in Fig. 3. It can be seen that the microstructure consists of coarse dark particles of primary Mg2Si and the bright phase a-Al in the matrix of Al–Mg2Si. From Fig. 3, it is clearly seen that the morphology of the primary Mg2Si particles in as-cast composite is irregular. Zhang et al. have reported [3] similar irregular morphology of primary Mg2Si in unmodified Al–Mg2Si composite. The eutectic structure in the composite matrix is also shown in higher magnification (Fig. 3). The optical microstructures of the Al-15% Mg2Si composite with different Zr contents (before and after hot-extrusion) are shown in Fig. 4a and b, respectively. As shown in Fig. 4a(1–5), the primary Mg2Si size decreases with the addition of Zr content up to 0.1%, but its size increases with more added Zr (Fig. 4a(4 and 5). However, Fig. 4 clearly shows that Zr addition has no influence on changing the morphology of primary Mg2Si particles. As shown in Fig. 4a1, there are hallow hopper crystals of the large primary Mg2Si crystals, however, with the addition of 0.1% Zr (Fig. 4a3), the hopper-like Mg2Si crystals reduces noticeably. It has been confirmed that the typical hopper-like morphology is found only for large primary silicon in Al–Si alloys [6]. From Fig. 4b(1–5), it can be seen that after homogenizing treatment and hot-extrusion processes, the morphology of primary Mg2Si particle drastically alters from irregular to a more round shape and its size also decreases considerably. The variation of particle size and volume fraction of primary Mg2Si particles in hot-extruded condition as a function of Zr amounts is shown in Fig. 5.
Fig. 8. XRD pattern of the hot-extruded Al-15% Mg2Si-0.5% Zr composite.
It is clear from this figure that the addition of 0.1% Zr reduces the size of the primary Mg2Si particles more than 55%, while its volume fraction almost remains constant. It has been reported that extrusion process of Al–Mg2Si MMC has a potential effect in reduction of Mg2Si particle size [25]. It is expected that extrusion process leads to the break up of the reinforcing hard particles and thereby a uniform distribution of the particles in the matrix. Fig. 6 clearly reveals the secondary electron images of threedimensional morphologies of the primary and pseudoeutectic Mg2Si particles. As seen in Fig. 6a, the morphology of primary Mg2Si is faceted in the as-cast Al-15% Mg2Si composite but its morphology is changed to a more round shape (Fig. 6b) in the homogenized and hot-extruded Al-15% Mg2Si-0.3% Zr composite.
Fig. 7. (a) SEM micrograph, (b) EDX point analysis and (c) EDX elemental map Zr of the hot-extruded Al-15% Mg2Si-0.5% Zr composite.
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Fig. 9. Tensile properties of the hot-extruded Al-15% Mg2Si as a function of added Zr: (a) ultimate tensile strength and (b) elongation (%).
Table 1 Tensile properties of the experimental composites. Composites Hot-extruded Hot-extruded Hot-extruded Hot-extruded Hot-extruded
Al–15% Al–15% Al–15% Al–15% Al–15%
Mg2Si Mg2Si-0.05% Zr Mg2Si-0.1% Zr Mg2Si-0.3% Zr Mg2Si-0.5% Zr
UTS (MPa)
El. (%)
Q (MPa)
160.3 236.5 289.3 212.0 180.7
3.2 6.6 8.3 5.2 4.5
236.0 359.4 427.2 319.4 278.7
The three-dimensional morphologies of the eutectic Mg2Si in as-cast and hot-extruded Al-15% Mg2Si-0.3% Zr composites are compared in Fig. 6c–d. A plate-like structure is observed at the cell boundaries in as-cast composite, as shown in Fig. 6c. It can be clearly seen that the eutectic Mg2Si morphology in the matrix structure is mainly plates. It is important to note that after Zr addition and extrusion, the morphology of eutectic Mg2Si phase alters from plate-like to rod-like and eutectic network is broken partially. Interesting results in morphology of eutectic Mg2Si are achieved in the hot-extruded Al–Mg2Si-0.3% Zr composite, as shown in Fig. 6d. Therefore, hot extrusion can contribute to more refinement and even distribution of Mg2Si eutectics in the matrix structure. Fig. 7 illustrates SEM micrographs and EDS analysis of the new phase in hot extruded Al-15% Mg2Si-0.5% Zr MMC. This figure shows the presence of a new Zr-contaning compound in addition to primary Mg2Si particles. Fig. 8 also shows the XRD patterns of Al-15% Mg2Si-0.5% Zr MMC specimens. The revealed phases of the Zr added MMC consists of a-Al, Mg2Si and Al3Zr intermetallic. It is noticeable that by increasing Zr addition more than 0.1%, a large amount of Al3Zr intermetallic with its irregular morphology appears in the microstructure (Fig. 4a4, a5, b4, and b5). It is well known that there are two types of modification mechanisms: (1) heterogeneous nucleation, i.e., the modifier elements or their containing compounds act as the heterogeneous nucleus; (2) poisoning effect, modifying elements absorb the forehead of growth and restrict the crystal growth [6]. It is expected from the results of the current research that the positive role of Zr can be attributed to the first mechanism. Because, Al3Zr intermetallic has similar lattice to Mg2Si [16] and seems to be served as an effective heterogeneous nuclei for Mg2Si refinement. On the other hand, it is shown that the volume fraction of primary Mg2Si does not change significantly with the addition of Zr (Fig. 5). At a constant
Mg2Si volume fraction, a decrease in particle size indicates an increase in the number of particles. This implies an increase in the number of nuclei for the primary Mg2Si particles during early stages of solidification process. Therefore, it can be concluded that Al3Zr enhances the nucleation of primary Mg2Si phase in Al melt. 3.2. Tensile properties The UTS and elongation values of the extruded specimens as a function of added Zr are shown in Fig. 9. It can be seen from Fig. 9 that Zr addition up to 0.1%, in extruded condition, increases the UTS and elongation values of the composite. These results are in agreement with microstructural observations. It is evident from Fig. 9 that extrusion has a significant influence on tensile properties of the Zr-containing composite. It can be contributed to the modification and refining of both the primary and the eutectic Mg2Si. It is important to note that the combination of UTS and elongation values is a reliable parameter in quantifying tensile properties for engineering applications. Therefore, to quantify the overall tensile properties of the composite, the quality index (Q.I.) was used in this work. Q.I. is explained as a semilogarithmic plot of UTS versus the elongation to fracture and it is expressed as follows [27]:
Q:I: ¼ UTSðMPaÞ þ 150 logð%EÞ
ð2Þ
The quality index Q.I. evaluated for each composite is also listed in Table 1. As shown in Table 1, the highest quality index is achieved in hot-extruded Al-15% Mg2Si-0.1% Zr (427.2 MPa) which reveals that optimum Zr level for improving tensile properties. 3.3. Fracture characteristic Fig. 10 shows the fractured surfaces of the Al-15% Mg2Si composite in as-cast and hot-extruded conditions with different Zr contents. Fig. 10a shows the fracture surface of the as-cast specimen which reveals a coarse irregular morphology of the Mg2Si crystals in the composite. Therefore, stress concentrations may easily occur in the matrix near the sharp tips of the Mg2Si crystals. This can result in nucleating microcracks at the sharp tips of the primary Mg2Si particles and then propagating along the interfaces between matrix and Mg2Si crystals or eutectic cells. The fracture planes of
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Fig. 10. Fracture surfaces of: (a) as-cast Al-15% Mg2Si composite and (b, c and d) hot-extruded Al-15% Mg2Si composites containing 0.05 wt.%, 0.1 wt.% and 0.3 wt.% Zr concentrations, respectively.
almost all coarse Mg2Si particles exhibit clear cleavage characteristic creating a rapid fracture deriving from their intrinsic brittleness and precracked structure. As a consequence, the mechanical properties are deteriorated by coarse Mg2Si crystals. Fig. 10b–d illustrates fracture surfaces of the hot extruded specimens with different amounts of Zr. It is clearly observed from these figures that by increasing Zr addition, the fine dimple areas increase whereas the number of decohered particles is reduced. It is well established that the decreasing of particle size and increasing fine dimples leads to enhanced elongation values [28]. Consequently, it seems that the typical ductile fracture mechanism occurs in the hot-extruded composites containing Zr. It has been reported that the fracture behavior of the ductile metallic alloys containing particles depends on the strength of the matrix/particle interface [29]. When the matrix is strong enough with high work hardening rate, the load is transferred to primary Mg2Si particles and fracture takes place. After hot-extrusion of Zr-modified composites, the Mg2Si eutectic morphology alters to the rod-like. Beside, the work hardening rate of the matrix in hot-extruded specimen considerably increases and stress transfers to the particles gradually which leads to increasing cracked particles (Fig. 10c). As expected,
extrusion process leads to the increased particle breakage and formation of fragmented tiny particles and more uniformity distribution of Mg2Si particles [24]. Both Zr addition and extrusion process have been resulted in the roundness of Mg2Si. Hence nucleation sites for cracking were reduced which led to the improved tensile properties. According to Griffith’s theory, a particle breaks when its fracture stress exceeds the Griffith criterion given by:
rc ¼ kc d0:5
ð3Þ
where kc is the fracture toughness of the particles and d is the diameter of the particle. Thus the refined structure has a higher fracture stress. Hence, higher strength values are expected in hot-extruded samples containing Zr. Casting defects seem to play a significant role in the reduction of tensile properties. It is difficult to fully eliminate the presence of defects, such as, shrinkage porosity, gas porosity and oxide films in castings [9]. It is important to note that the cast MMCs basically suffer from lack of ductility. It is clear that in all specimens, heat treatment has led to an improvement in tensile ductility of the composite. The ductility improvement of the MMC by solutionizing treatment has been reported by Malekan et al.
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[30]. As Fig. 7 shows, further enhancement in ductility is obtained by extrusion process. This could be attributed to the reduction of porosity content and effective refining of Mg2Si phases. As mentioned above, it can be concluded that eliminating of sharp corners in both the primary and the eutectic Mg2Si particles by applying hot-extrusion, has led to a reduction in both the stress concentration areas and crack initiation sites. The effects arising from hot-extrusion of Zr-modified composites leads to transfer the fracture behavior from brittle, with cleavage facets, to ductile, with fine dimples. 4. Conclusions The effect of Zr on microstructural and mechanical properties of the hot-extruded Al-15% Mg2Si composite was investigated. The following conclusions can be drawn: 1. The morphology of primary Mg2Si changed from irregular to spherical shape and its average particle size decreased from 56 lm to 24 lm after hot-extrusion of cast Al-15% Mg2Si-0.1% Zr composite. 2. By applying hot-extrusion on 0.1% Zr-modified composite, the morphology of the eutectic Mg2Si phase altered from plate-like to rod-like. 3. Tensile tests of the hot-extruded Al-15% Mg2Si-0.1% Zr revealed optimum conditions for improving both UTS and elongation values. 4. The fracture surface examinations of the MMC revealed a brittle mode of failure in cast composite, however applying modification by Zr and extrusion process increased fine dimples and reduced the number of decohered particles.
Acknowledgments The authors gratefully acknowledge Imam Khomeini International University and University of Tehran for lab facilities and Iran National Science Foundation for financial support of this work. References [1] Emamy M, Khorshidi R, Honarbakhsh Raouf A. The influence of pure Na on the microstructure and tensile properties of Al–Mg2Si metal matrix composite. Mater Sci Eng A 2011;528:4337–42. [2] Kaneko S, Murakami K, Sakai T. Effect of the extrusion conditions on microstructure evolution of the extruded Al–Mg–Si–Cu alloy rods. Mater Sci Eng A 2009;500:8–15. [3] Zhang J, Fan Z, Wang YQ, Zhou BL. Microstructural development of Al-15 wt.% Mg2Si in situ composite with mischmetal addition. J Mater Sci Eng A 2000;281:104–12. [4] Zhao YG, Qin QD, Zhao YQ, Liang YH, Jiang QC. In situ Mg2Si/Al–Si composite modified by K2TiF6. J Mater Lett 2004;58:2192–4. [5] Xu CL, Wang HY, Yang YF, Jiang QC. Effect of Al–P–Ti–TiC–Nd2O3 modifier on the microstructure and mechanical properties of hypereutectic Al–20 wt.% Si alloy. J Mater Sci Eng A 2007;452–453:341–6.
[6] Qin QD, Zhao YG, Zhou W, Cong PJ. Effect of phosphorus on microstructure and growth manner of primary Mg2Si crystal in Mg2Si/Al composite. J Mater Sci Eng A 2007;447:186–91. [7] Khorshidi R, Honarbakhsh Raouf A, Emamy M, Campbell J. The study of Li effect on the microstructure and tensile properties of cast Al–Mg2Si metal matrix composite. J Alloys Compd 2011;509:9026–33. [8] Ghorbani MR, Emamy M, Nemati N. Microstructural and mechanical characterization of Al-15% Mg2Si composite containing chromium. J Mater Des 2011;32:4262–9. [9] Hadian R, Emamy M, Varahram N, Nemati N. The effect of Li on the tensile properties of cast Al–Mg2Si metal matrix composite. J Mater Sci Eng A 2008;490:250–7. [10] Zhou YG, Qin QD, Zhou W, Liang YH. Microstructure of the Ce-modified in situ Mg2Si/Al–Si–Cu composite. J Alloys Compd 2005;389:L1–4. [11] Tjong SC, Ma ZY. Microstructural and mechanical characteristics of in situ metal matrix composites. J Mater Sci Eng R 2000;29:49–113. [12] Yue NL, Lu L, Lai MO. Application of thermodynamic calculation in the in-situ process of Al/TiB2. J Compos Struct 1999;47:691–4. [13] Qin QD, Zhao YG, Liu C, Cong PJ, Zhou W, Liang YH. Effect of holding temperature on semisolid microstructure of Mg2Si/Al composite. J Alloys Compd 2006;416:143–7. [14] Qin QD, Zhao YG, Liu C, Cong PJ, Zhou W. Strontium modification and formation of cubic primary Mg2Si crystals in Mg2Si/Al composite. J Alloys Compd 2007;454:142–6. [15] Emamy M, Nemati N, Heidarzadeh A. The influence of Cu rich intermetallic phases on the microstructure, hardness and tensile properties of Al-15% Mg2Si composite. J Mater Sci Eng A 2010;527:2998–3004. [16] Emamy M, Jafari Nodooshan HR, Malekan A. The microstructure, hardness and tensile properties of Al-15% Mg2Si in situ composite with yttrium addition. J Mater Des 2011;32:4559–66. [17] Li C, Liu X, Zhang G. Heterogeneous nucleating role of TiB2 or AlP/TiB2 coupled compounds on primary Mg2Si in Al–Mg–Si alloys. J Mater Sci Eng A 2008;497:432–7. [18] Li C, Wu Y, Li H, Liu X. Microstructural formation in hypereutectic Al–Mg2Si with extra Si. J Alloys Compd 2009;477:212–6. [19] Borrego A, Fernandez R, Cristina MDC, Ibanez J, Gonzalez-Doncel G. Influence of extrusion temperature on the microstructure and the texture of 6061Al15 vol.% SiCw PM composites. J Compos Sci Technol 2002;62:732–42. [20] Them LM, Gupta M, Cheng M. Effect of reinforced volume fraction on the evolution of reinforced size during the extrusion of Al–SiC composites. J Mater Sci Eng A 2002;326:355–63. [21] Hong SH, Chung KH, Lee CH. Effects of hot extrusion parameters on the tensile properties and microstructures of SiCw-2124 A1 composites. J Mater Sci Eng A 1996;206:225–32. [22] Tekman C, Ozdemir I, Cocen U, Onel K. The mechanical response of Al–Si–Mg/ SiCp composite: influence of porosity. J Mater Sci Eng A 2003:1–7. [23] Cöcen Ü, Önel K. Ductility and strength of extruded SiCp/aluminium-alloy composites. Compos Sci Technol 2002;62:275–82. [24] Rahmani Fard R, Akhlaghi F. Effect of extrusion temperature on the microstructure and porosity of A356-SiCp composites. J Mater Process Technol 2007;187–188:433–6. [25] Bahrami A, Razaghian A, Emamy M, Jafari HR, Mousavi GS. Microstructure and tensile properties of Al-15wt.% Mg2Si composite after hot extrusion and heat treatment. J Key Mater Eng 2011;471–472:1171–6. [26] ASTM E8-04. Standard test methods for tension testing of metallic materials. Annual Book of ASTM Standards, vol. 03.01; 2004. [27] Wang ER, Hui XD, Wang SS, Zhao YF, Chen GL. Improved mechanical properties in cast Al–Si alloys by combined alloying of Fe and Cu. J Mater Sci Eng A 2010;527:7878–84. [28] Chawla N, Shen YL. Mechanical behavior of particle reinforced metal matrix composites. J Adv Eng Mater 2001;3:357–70. [29] Babout L, Brechet Y, Maire E, Fougeres R. On the competition between particle fracture and particle decohesion in metal matrix composites. J Acta Mater 2004;52:4517–25. [30] Malekan A, Emamy M, Rassizadehghani J, Emami AR. The effect of solution temperature on the microstructure and tensile properties of Al-15% Mg2Si composite. J Mater Des 2011;32:2701–9.
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
The effect of Zr on the microstructure and tensile properties of hot-extruded Al–Mg2Si composite A. Bahrami a, A. Razaghian a,⇑, M. Emamy b, R. Khorshidi c a
Imam Khomeini International University, Qazvin, Iran School of Metallurgy and Materials, University of Tehran, 11365-4563 Tehran, Iran c Department of Materials, Faculty of Engineering, Semnan University, Semnan, Iran b
a r t i c l e
i n f o
Article history: Received 17 September 2011 Accepted 20 November 2011 Available online 26 November 2011 Keywords: A. Metal matrix composite C. Extrusion G. Fractography
a b s t r a c t This work was carried out to investigate the effect of different amounts of Zr on the microstructure and tensile properties of homogenized and hot extruded Al-15% Mg2Si composite using optical microscopy and scanning electron microscopy (SEM). The results showed that Zr addition has no significant effect on the morphology of both primary and eutectic Mg2Si phase in as-cast condition. But, applying homogenizing and extrusion processes changed the morphology of Mg2Si phases from irregular to a more spherical shape. Further results demonstrated that the average size of primary Mg2Si decreases with the addition of Zr up to 0.1% from 56 lm to 24 lm in hot-extruded condition. As the mount of Zr increased up to 0.1 wt.%, ultimate tensile strength (UTS) and elongation values were also increased from 160 MPa and 3.2% to 292 MPa and 9.5%, respectively. Fracture surface examinations revealed a transition from brittle fracture mode in as-cast composite to ductile fracture in hot-extruded Zr-modified specimens. This can be attributed to the changes in size and morphology of Mg2Si intermetallic and porosity content. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction With the enlargement of modern industry, the production of light materials with high strength is required. Light weight materials are most appropriate for the structure construction of mass transport vehicles for the aim of decreasing emission gases and increasing fuel efficiency [1,2]. Aluminum metal matrix composites as a class of advanced engineering materials have been developed for high-performance applications because of their low density, excellent castability and good wear resistance [3–8]. Particulate metal matrix composites (PMMCs) have attracted attention due to their lower costs production and inherent isotropic properties [9,10]. Among the variety techniques for PMMCs production, in situ processes exhibit thermodynamically stable systems, good particle wetting and even distribution of the reinforcing phase in comparison to their ex-situ process counterparts [11,12]. This technique has a significant potential for simplicity of production because it simply forms itself during solidification [7]. In recent years, in situ Al–Mg2Si composites have been introduced as a new group of PMMCs which contain hard particles of Mg2Si in Al-matrix [13,14]. Mg2Si is a hard intermetallic compound with a high melting point (1085 °C). Its low density and low coefficient of thermal expansion coupled with a reasonably high elastic
⇑ Corresponding author. Tel.: +98 281 3780021; fax: +98 281 3780073. E-mail address: [email protected] (A. Razaghian). 0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.11.045
modulus make it a good choice as a reinforcing agent [15]. Zhang et al. showed that in terms of properties and solidification behavior, great similarities exist between Mg2Si and Si, and between Al–Mg2Si and Al–Si systems [3]. Fig. 1 shows the equilibrium phase diagram of Al–Mg2Si [3]. It can be seen that during solidification of Al-15% Mg2Si alloy, Mg2Si phase is formed as the primary phase. Then, a-Al and secondary Mg2Si co-solidify from the liquid alloy in the narrow ternary phase area. According to Eq. (1), this pseudo-eutectic reaction is completed at 583.5 °C [3,15]:
L ! L1 þ Mg2 SiP ! Mg2 SiP þ ða Al þ Mg2 SiÞE
ð1Þ
where E is a Eutectic, P is a Primary, and L1 is a Liquid in two phase region. Nevertheless, in normal as-cast Al–Mg2Si composites, the primary Mg2Si phases are usually coarse which lead to the reduced mechanical properties. Therefore, Mg2Si crystals must be modified to ensure adequate mechanical strength and ductility of the composite [6–9]. Several works have been focused on the modification of the structure with the addition of various alloying elements such as pure Na [1], K2TiF6 [4], Ce [10], Sr [14], Cu [15], Y [16], Ti [17] and additional quantities of silicon [18] to the liquid alloy. However, the improper control of some process parameters of this technique can lead to deleterious defects in cast composites such as porosity and non-uniform distribution of the particles in the matrix.
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Fig. 1. Equilibrium phase diagram of Al–Mg2Si pseudo-binary section [3].
Therefore, thermomechanical procedures such as extrusion, rolling or forging seem to be efficient processes for decreasing the porosity and obtaining a more uniform particle distribution in such composites [19–22]. Hot extrusion, as a conventional thermomechanical process, is used widely in wrought aluminum alloys and its composites. This technique is an effective method to reduce casting defects because of refinement of grains and reinforcing phases. The properties of the MMCs are significantly sensitive to the type of the reinforcement and the fabrication processing of the composite after initial production stage [23]. It has been reported that the average size of the reinforcing particles reduces during extrusion due to particle fragmentation [24]. Furthermore, by increasing extrusion ratio, UTS and elongation values of the composite are also increased [25]. However, the presence of the reinforcing particles increases the applied force for extrusion which may result in formation of particle breakage and microporosity [24]. This aim of this research was to study the microstructure and tensile properties of hot-extruded in situ Al–Mg2Si composite containing different amounts of Zr.
Fig. 3. SEM back-scattered image of the as-cast Al-15% Mg2Si composite microstructure.
2. Experimental procedure Industrially pure Al (99.8%), Mg (99.9%) and Si (99.5%) metals were used to prepare Al-5.5 wt.% Si-9.7 wt.% Mg alloy primary ingots of Al-15% Mg2Si composite. All materials were preheated in an electrical resistance furnace using a 10 kg graphite crucible. The MMC ingots were remelted within a small SiC crucible (with 1 kg capacity) in a resistance furnace in order to prepare alloys with 0, 0.05, 0.1, 0.3 and 0.5 wt.% Zr. When the temperature reached 750 °C, Al-5Zr master alloy was added to the molten composite. Degassing was conducted by using dry tablets containing C2Cl6 (0.3 wt.% of the molten material) for about 3 min. After cleaning off the slag, alloys with different compositions were poured into a cast iron mold preheated to 200 °C (Fig. 2a). After casting, the composite billets were cut and machined into 28 mm in length and 29 mm in diameter in order to fit into the
Fig. 2. The schematic of cast iron mold and (b) sketch map of tensile samples.
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Fig. 4. The comparison of optical microscopy images of the Al-15 wt.% Mg2Si composite (a) before and (b) after hot-extrusion at various Zr contents: (1) 0 wt.%, (2) 0.05 wt.%, (3) 0.1 wt.%, (4) 0.3 wt.% and (5) 0.5 wt.%.
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extrusion container. All the billets were exposed to homogenizing treatment in an electrical furnace at 520 °C for 4 h and subsequently slow cooling in furnace. Then, these billets were hot extruded by using a hydraulic press at a ram speed of 1 mm/s with the extrusion ratio of 18:1 at 450 °C. Extrusion process was carried out applying graphite based oil between samples and dies. According to ASTM E8-04 small size, round tensile samples were machined along the extrusion direction [26]. Tensile tests were carried out in a computerized testing machine (Zwick/Roell Z100) at a strain rate of 0.1 mm/min at room temperature. The sketch of a tensile test specimen is seen in Fig. 2b. Metallographic specimens were also prepared by polishing and etching in hydrofluoric acid solution (5% HF). In order to reveal the morphology of primary Mg2Si and eutectic Mg2Si phase, some specimens were deeply etched in mixture of 15% NaOH + 0.5% HF water solution. Microstructural parameters were determined using an optical microscope equipped with an image analysis system (Clemex Vision Pro. Ver. 3.5.025). The microstructural characteristics and fracture surfaces of the specimens were also examined by SEM performed in a VegaÓTescan SEM. Fig. 5. The size and volume fraction of primary Mg2Si particles as a function of Zr additions after hot-extrusion process.
Fig. 6. Three-dimensional morphologies of: (a) primary Mg2Si in as-cast, (b) primary Mg2Si in hot-extruded. (c) eutectic Mg2Si in as-cast and (d) eutectic Mg2Si in hotextruded Al-15% Mg2Si-0.3% Zr composite.
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3. Results and discussion 3.1. Microstructural studies A typical SEM micrograph of the as-cast Al-15% Mg2Si composite is shown in Fig. 3. It can be seen that the microstructure consists of coarse dark particles of primary Mg2Si and the bright phase a-Al in the matrix of Al–Mg2Si. From Fig. 3, it is clearly seen that the morphology of the primary Mg2Si particles in as-cast composite is irregular. Zhang et al. have reported [3] similar irregular morphology of primary Mg2Si in unmodified Al–Mg2Si composite. The eutectic structure in the composite matrix is also shown in higher magnification (Fig. 3). The optical microstructures of the Al-15% Mg2Si composite with different Zr contents (before and after hot-extrusion) are shown in Fig. 4a and b, respectively. As shown in Fig. 4a(1–5), the primary Mg2Si size decreases with the addition of Zr content up to 0.1%, but its size increases with more added Zr (Fig. 4a(4 and 5). However, Fig. 4 clearly shows that Zr addition has no influence on changing the morphology of primary Mg2Si particles. As shown in Fig. 4a1, there are hallow hopper crystals of the large primary Mg2Si crystals, however, with the addition of 0.1% Zr (Fig. 4a3), the hopper-like Mg2Si crystals reduces noticeably. It has been confirmed that the typical hopper-like morphology is found only for large primary silicon in Al–Si alloys [6]. From Fig. 4b(1–5), it can be seen that after homogenizing treatment and hot-extrusion processes, the morphology of primary Mg2Si particle drastically alters from irregular to a more round shape and its size also decreases considerably. The variation of particle size and volume fraction of primary Mg2Si particles in hot-extruded condition as a function of Zr amounts is shown in Fig. 5.
Fig. 8. XRD pattern of the hot-extruded Al-15% Mg2Si-0.5% Zr composite.
It is clear from this figure that the addition of 0.1% Zr reduces the size of the primary Mg2Si particles more than 55%, while its volume fraction almost remains constant. It has been reported that extrusion process of Al–Mg2Si MMC has a potential effect in reduction of Mg2Si particle size [25]. It is expected that extrusion process leads to the break up of the reinforcing hard particles and thereby a uniform distribution of the particles in the matrix. Fig. 6 clearly reveals the secondary electron images of threedimensional morphologies of the primary and pseudoeutectic Mg2Si particles. As seen in Fig. 6a, the morphology of primary Mg2Si is faceted in the as-cast Al-15% Mg2Si composite but its morphology is changed to a more round shape (Fig. 6b) in the homogenized and hot-extruded Al-15% Mg2Si-0.3% Zr composite.
Fig. 7. (a) SEM micrograph, (b) EDX point analysis and (c) EDX elemental map Zr of the hot-extruded Al-15% Mg2Si-0.5% Zr composite.
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Fig. 9. Tensile properties of the hot-extruded Al-15% Mg2Si as a function of added Zr: (a) ultimate tensile strength and (b) elongation (%).
Table 1 Tensile properties of the experimental composites. Composites Hot-extruded Hot-extruded Hot-extruded Hot-extruded Hot-extruded
Al–15% Al–15% Al–15% Al–15% Al–15%
Mg2Si Mg2Si-0.05% Zr Mg2Si-0.1% Zr Mg2Si-0.3% Zr Mg2Si-0.5% Zr
UTS (MPa)
El. (%)
Q (MPa)
160.3 236.5 289.3 212.0 180.7
3.2 6.6 8.3 5.2 4.5
236.0 359.4 427.2 319.4 278.7
The three-dimensional morphologies of the eutectic Mg2Si in as-cast and hot-extruded Al-15% Mg2Si-0.3% Zr composites are compared in Fig. 6c–d. A plate-like structure is observed at the cell boundaries in as-cast composite, as shown in Fig. 6c. It can be clearly seen that the eutectic Mg2Si morphology in the matrix structure is mainly plates. It is important to note that after Zr addition and extrusion, the morphology of eutectic Mg2Si phase alters from plate-like to rod-like and eutectic network is broken partially. Interesting results in morphology of eutectic Mg2Si are achieved in the hot-extruded Al–Mg2Si-0.3% Zr composite, as shown in Fig. 6d. Therefore, hot extrusion can contribute to more refinement and even distribution of Mg2Si eutectics in the matrix structure. Fig. 7 illustrates SEM micrographs and EDS analysis of the new phase in hot extruded Al-15% Mg2Si-0.5% Zr MMC. This figure shows the presence of a new Zr-contaning compound in addition to primary Mg2Si particles. Fig. 8 also shows the XRD patterns of Al-15% Mg2Si-0.5% Zr MMC specimens. The revealed phases of the Zr added MMC consists of a-Al, Mg2Si and Al3Zr intermetallic. It is noticeable that by increasing Zr addition more than 0.1%, a large amount of Al3Zr intermetallic with its irregular morphology appears in the microstructure (Fig. 4a4, a5, b4, and b5). It is well known that there are two types of modification mechanisms: (1) heterogeneous nucleation, i.e., the modifier elements or their containing compounds act as the heterogeneous nucleus; (2) poisoning effect, modifying elements absorb the forehead of growth and restrict the crystal growth [6]. It is expected from the results of the current research that the positive role of Zr can be attributed to the first mechanism. Because, Al3Zr intermetallic has similar lattice to Mg2Si [16] and seems to be served as an effective heterogeneous nuclei for Mg2Si refinement. On the other hand, it is shown that the volume fraction of primary Mg2Si does not change significantly with the addition of Zr (Fig. 5). At a constant
Mg2Si volume fraction, a decrease in particle size indicates an increase in the number of particles. This implies an increase in the number of nuclei for the primary Mg2Si particles during early stages of solidification process. Therefore, it can be concluded that Al3Zr enhances the nucleation of primary Mg2Si phase in Al melt. 3.2. Tensile properties The UTS and elongation values of the extruded specimens as a function of added Zr are shown in Fig. 9. It can be seen from Fig. 9 that Zr addition up to 0.1%, in extruded condition, increases the UTS and elongation values of the composite. These results are in agreement with microstructural observations. It is evident from Fig. 9 that extrusion has a significant influence on tensile properties of the Zr-containing composite. It can be contributed to the modification and refining of both the primary and the eutectic Mg2Si. It is important to note that the combination of UTS and elongation values is a reliable parameter in quantifying tensile properties for engineering applications. Therefore, to quantify the overall tensile properties of the composite, the quality index (Q.I.) was used in this work. Q.I. is explained as a semilogarithmic plot of UTS versus the elongation to fracture and it is expressed as follows [27]:
Q:I: ¼ UTSðMPaÞ þ 150 logð%EÞ
ð2Þ
The quality index Q.I. evaluated for each composite is also listed in Table 1. As shown in Table 1, the highest quality index is achieved in hot-extruded Al-15% Mg2Si-0.1% Zr (427.2 MPa) which reveals that optimum Zr level for improving tensile properties. 3.3. Fracture characteristic Fig. 10 shows the fractured surfaces of the Al-15% Mg2Si composite in as-cast and hot-extruded conditions with different Zr contents. Fig. 10a shows the fracture surface of the as-cast specimen which reveals a coarse irregular morphology of the Mg2Si crystals in the composite. Therefore, stress concentrations may easily occur in the matrix near the sharp tips of the Mg2Si crystals. This can result in nucleating microcracks at the sharp tips of the primary Mg2Si particles and then propagating along the interfaces between matrix and Mg2Si crystals or eutectic cells. The fracture planes of
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Fig. 10. Fracture surfaces of: (a) as-cast Al-15% Mg2Si composite and (b, c and d) hot-extruded Al-15% Mg2Si composites containing 0.05 wt.%, 0.1 wt.% and 0.3 wt.% Zr concentrations, respectively.
almost all coarse Mg2Si particles exhibit clear cleavage characteristic creating a rapid fracture deriving from their intrinsic brittleness and precracked structure. As a consequence, the mechanical properties are deteriorated by coarse Mg2Si crystals. Fig. 10b–d illustrates fracture surfaces of the hot extruded specimens with different amounts of Zr. It is clearly observed from these figures that by increasing Zr addition, the fine dimple areas increase whereas the number of decohered particles is reduced. It is well established that the decreasing of particle size and increasing fine dimples leads to enhanced elongation values [28]. Consequently, it seems that the typical ductile fracture mechanism occurs in the hot-extruded composites containing Zr. It has been reported that the fracture behavior of the ductile metallic alloys containing particles depends on the strength of the matrix/particle interface [29]. When the matrix is strong enough with high work hardening rate, the load is transferred to primary Mg2Si particles and fracture takes place. After hot-extrusion of Zr-modified composites, the Mg2Si eutectic morphology alters to the rod-like. Beside, the work hardening rate of the matrix in hot-extruded specimen considerably increases and stress transfers to the particles gradually which leads to increasing cracked particles (Fig. 10c). As expected,
extrusion process leads to the increased particle breakage and formation of fragmented tiny particles and more uniformity distribution of Mg2Si particles [24]. Both Zr addition and extrusion process have been resulted in the roundness of Mg2Si. Hence nucleation sites for cracking were reduced which led to the improved tensile properties. According to Griffith’s theory, a particle breaks when its fracture stress exceeds the Griffith criterion given by:
rc ¼ kc d0:5
ð3Þ
where kc is the fracture toughness of the particles and d is the diameter of the particle. Thus the refined structure has a higher fracture stress. Hence, higher strength values are expected in hot-extruded samples containing Zr. Casting defects seem to play a significant role in the reduction of tensile properties. It is difficult to fully eliminate the presence of defects, such as, shrinkage porosity, gas porosity and oxide films in castings [9]. It is important to note that the cast MMCs basically suffer from lack of ductility. It is clear that in all specimens, heat treatment has led to an improvement in tensile ductility of the composite. The ductility improvement of the MMC by solutionizing treatment has been reported by Malekan et al.
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[30]. As Fig. 7 shows, further enhancement in ductility is obtained by extrusion process. This could be attributed to the reduction of porosity content and effective refining of Mg2Si phases. As mentioned above, it can be concluded that eliminating of sharp corners in both the primary and the eutectic Mg2Si particles by applying hot-extrusion, has led to a reduction in both the stress concentration areas and crack initiation sites. The effects arising from hot-extrusion of Zr-modified composites leads to transfer the fracture behavior from brittle, with cleavage facets, to ductile, with fine dimples. 4. Conclusions The effect of Zr on microstructural and mechanical properties of the hot-extruded Al-15% Mg2Si composite was investigated. The following conclusions can be drawn: 1. The morphology of primary Mg2Si changed from irregular to spherical shape and its average particle size decreased from 56 lm to 24 lm after hot-extrusion of cast Al-15% Mg2Si-0.1% Zr composite. 2. By applying hot-extrusion on 0.1% Zr-modified composite, the morphology of the eutectic Mg2Si phase altered from plate-like to rod-like. 3. Tensile tests of the hot-extruded Al-15% Mg2Si-0.1% Zr revealed optimum conditions for improving both UTS and elongation values. 4. The fracture surface examinations of the MMC revealed a brittle mode of failure in cast composite, however applying modification by Zr and extrusion process increased fine dimples and reduced the number of decohered particles.
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