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The effect of Fe, Mn and Sr on the microstructure and tensile properties of A356–10% SiC composite
Materials Science and Engineering A 527 (2010) 3733–3740
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The effect of Fe, Mn and Sr on the microstructure and tensile properties of A356–10% SiC composite K. Abedi, M. Emamy ∗ School of Metallurgy and Materials, University of Tehran, North Kargar St., Tehran 11365-4563, Iran
a r t i c l e
i n f o
Article history: Received 9 October 2009 Received in revised form 26 January 2010 Accepted 17 March 2010
Keywords: A356–10% SiC Fe intermetallics T6 heat treatment Tensile properties Microstructure Modifiers
a b s t r a c t This study investigates the formation of Fe containing intermetallic compounds on the microstructure and tensile properties of A356–10% SiC composite in the Mn- and Sr-modified conditions. The composite ingots were made by stir casting process and iron was added to the remelted composites at different concentrations varied from 0.5 to 2%. For 2 wt.% Fe, different levels of Mn were added to identify the optimum Mn:Fe ratio for eliminating harmful -phase and obtaining microstructure with well distributed and fine intermetallics. 300 ppm Sr was added to the Mn-modified composite to investigate the effect of Sr and Mn on the microstructure and tensile properties of A356–10% SiC. T6 heat treatment was applied for all specimens. It was found that the addition of Mn changes the morphology of Fe-rich intermetallics from plate-like - to ␣-phase. The tensile test results demonstrated that the addition of iron considerably reduces both UTS and elongation values while, modification by Mn and Sr improves tensile properties of the composite with 1 and 2 wt.% Fe. Fractographic examination of iron containing composites also showed that large -Fe intermetallics are responsible for fracture behavior in unmodified condition. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Aluminum metal matrix composite (MMC) is being considered as a new material for its light weight, high strength, high specific modulus and good wear resistance properties. The type, size, morphology, distribution and relative quantity of the reinforcing materials are considered as some important controlling parameters which may influence the composite properties. However, MMCs have lower ductility and fracture toughness in comparison with unreinforced alloys [1–3]. Microstructural parameters such as grain size, dendrite arm spacing (DAS) and the morphology of silicon and intermetallics also play an important role in determining the mechanical properties of Al–Si alloys. Modification of the eutectic Si crystals in Al–Si alloys is achieved by rapid solidification, chemical modification and thermal modification. Several works have shown that the coarse and large Si needles can be changed into a fine well-rounded phase after T6 heat treatment [4–6]. In Al–Si cast alloys one important impurity is Fe. Since Fe has a very low solid solubility in Al (max. 0.05%), almost all Fe in Al alloys are present in the form of second phases, usually Al–Fe or Al–Fe–Si intermetallics. Fe is the most pervasive impurity in cast Al alloys due to its central role in the formation of extensive thin platelets of intermetallic compound -Al5 FeSi. This compound
∗ Corresponding author. Tel.: +98 21 82084083; fax: +98 21 82084083. E-mail address: [email protected] (M. Emamy). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.03.063
appears as needles in 2D metallographic observations. Fe containing intermetallics are known to be detrimental to the mechanical properties, because these compounds are generally thought to be brittle [6]. In the Al–Si–Fe system there are five main Ferich phases: Al3 Fe, ␣-Al8 Fe2 Si (possibly ␣-Al12 Fe3 Si2 ), -Al5 FeSi, ␦-Al4 FeSi2 and ␥-Al3 FeSi [2,3]. The ␥-Al3 FeSi phase occurs in highFe and high-Si alloys [7,8]. When the quantity of iron present in Al–Si alloys is greater than a certain value (0.4–0.5 wt.%), a significant increase in porosity is to be observed. It was suggested that the formation of coarse -Al5 FeSi intermetallic platelets restricted liquid metal-feeding during the casting process. Moreover, the Al5 FeSi intermetallics were found to be highly active sites for pore nucleation [6,9,10]. Effort has been made to control the precipitation, growth and morphology of plate-like -Al5 FeSi intermetallic phases by several investigators [11,12]. When Fe is present in an excess of specified levels, various methods have been advocated to reduce its harmful influence. These methods can be categorized into three types. The most direct method is the reduction or removal of Fe from Al melts, mainly by precipitation and sedimentation processing, i.e. the heat treatment of the liquid metal [13,14]. Since iron is inevitable and cannot be economically removed from the molten aluminum, strategies have to be developed to neutralize its negative effects. The conventional method is to add some chemical “correctors” to change the morphology from platelet -Al5 FeSi (brittle form) to globular or script forms (less brittle forms). The globular or script Fe-rich phases have been thought not to lead to brittleness [15]. Alternative methods involve various thermal
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Table 1 Chemical composition (wt.%) of A356 aluminum alloy. Si
Mg
Cu
Mn
Fe
Ni
Zn
Cr
Al
7.53
0.35
0.02
0.08
0.16
0.07
0.14
0.008
Bal.
treatments, including melt superheating, rapid solidification and solution heat treatment of castings [4,7,9]. To convert -Fe platelets into ␣-Fe script form, Co, Cr, Mn, Mo and Ni are sometimes added. Their addition has also been reported to improve strength at high temperature. Mn has been widely used because, in addition to its significantly lower cost and better availability, -Al5 FeSi tends to be suppressed and ␣-phase is formed in its place [4,7,11,15]. Three different morphologies of ␣-phase intermetallics can form: (1) Chinese script (Al15 (Fe,Mn)3 Si2 ); (2) polyhedral and (3) star-like crystals, which are a coarser version of the ␣-phase with the same stoichiometry [15]. When  plates are present in the microstructure, tensile properties, especially elongation [16–18] are reduced as a result of their stress raising potential and their apparently brittle behavior as evidenced by cracking at low stresses [19,20]. Another modifier that was shown to be effective in controlling -phase is Sr. Sr additions have been found to convert the -phase to the more compact ␣-phase. Several investigators have found that 150–300 ppm of Sr was sufficient to neutralize the detrimental effects of the Fe-rich intermetallics and makes the Chinese script a dominant intermetallic phase in some Al–Si alloys [21–25]. With extensive data available on the modification of Fe-rich intermetallic phases in monolithic Al–Si alloys, less attention has been made to reduce the harmful effect of Fe on the cast MMCs. This study was undertaken to examine the microstructure and tensile properties of heat-treated Fe containing A356–10% SiC composite before and after modification by Mn and Sr. 2. Experimental Table 1 shows the chemical composition of the matrix alloy (A356 Al alloy). Silicon carbide particles with an average size of 40 m were used as reinforcing material into the matrix alloy. Stirring process was employed to produce A356–10% SiC Al MMCs ingots [26]. The supplied composite ingots were cut into small pieces, dried and remelted in 2-kg capacity graphite crucible by means of an electrical resistance furnace. The melting temperature was kept at 760 ± 5 ◦ C in all castings. Fe, Mn and Sr were added as Al–20% Fe, Al–20% Mn and Al–10% Sr master alloys. 13 melts were studied with different compositions (Table 2). Fe was added to the MMC at levels varying from 0.5 to 2 wt.%. To study the effect of Mn and find the best Mn:Fe ratio, five different Mn:Fe Table 2 Nominal composition, Mn/Fe ratio of A356/10% SiC composite and its code. Material codes
F0 F5 F10 F15 F20 F20M8 F20M10 F20M12 F20M14 F20M16 F10M5 F10M5S F20M10S
Additions Fe (wt.%)
Mn (wt.%)
Sr (wt.%)
Mn/Fe
0.2 0.5 1 1.5 2 2 2 2 2 2 1 1 2
0.1 – – – – 0.8 1 1.2 1.4 1.6 0.5 0.5 1
– – – – – – – – – – – 0.03 0.03
– – – – – 0.4 0.5 0.6 0.7 0.8 0.5 0.5 0.5
Fig. 1. Schematic drawing of (a) tensile test casting mould and (b) tensile test specimen.
ratios were used (0.4, 0.5, 0.6, 0.7 and 0.8) at a constant Fe concentration (2 wt.%). Sr was selected at 300 ppm level according to previous studies [4,12,15]. For example, Samuel et al. have found that 250–350 ppm Sr was sufficient to neutralize the detrimental effects of the Fe-rich intermetallics in some Al–Si alloys, appearing to lead to the fragmentation of -phase [23–25]. When the melt reached 760 ◦ C, it was thoroughly skimmed. The Fe addition was then made and the melt was stirred gently for 1 min. After about 3 min, a further addition of Mn or Sr was carried out. After complete dissolution and homogenization of all master alloys, the surface oxides were again skimmed prior to pouring the molten metal into a ductile iron mould preheated to 250 ◦ C. The iron mould and tensile test specimens were made according to ASTM B 108-03a and ASTM B557M-02a standard respectively (Fig. 1). Six specimens (three castings) were prepared for each selected composition. T6 heat treatment was applied to all specimens. Solution treatment was carried out at 530 ◦ C for 12 h and then rapidly quenched in warm water at 70 ◦ C before aging at 155 ◦ C for 4 h [27]. Microstructural studies were made on polished sample surfaces which were selected from the gauge length portion of the test bars (6 mm diameter). Normal grinding and polishing technique was used to reveal the structure. All samples of the composite listed in Table 2 were prepared for image analysis. Quantitative data on microstructure were determined using an optical microscope equipped with an image analysis system (Clemex Vision Pro. ver.3.5.025). For each sample, 50 fields at 400 times magnification were evaluated. The results of digital image analysis are presented in Table 3. Further microstructural studies of the specimens were examined by scanning electron microscopy (SEM) performed in a CamScan MV2300 equipped with the energy dispersive X-ray analysis (EDX) accessory. Tensile test were carried out in a computerized testing machine (Zwick/Roell Z100) at room temperature to obtain yield strength (YS), ultimate tensile strength (UTS) and percent elongation (%El.) at a strain rate of 1 mm/min. The fracture surfaces of tensile test specimens were also examined by similar SEM.
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Table 3 Tensile properties of examined cast composite specimens. Items codes
F0 F5 F10 F15 F20 F10M5 F20M10 F10M5S F20M10S
UTS (MPa)
YS (MPa)
El. (%)
Max.
Min.
Ave.
S.D.
Max.
Min.
Ave.
S.D.
Max.
Min.
Ave.
S.D.
252 233 180 188 164 202 195 219 218
222 182 147 153 132 175 169 181 186
234 213 168 174 152 187 188 197 205
12 18 14 16 17 10 13 14 13
185 181 165 163 154 129 157 152 158
165 157 134 135 135 105 121 127 119
168 172 150 145 140 118 134 140 135
9 9 7 8 8 6 8 6 7
1.5 1.2 1 0.9 0.9 2.68 1.7 3.6 3.2
1.1 0.7 0.6 0.6 0.55 2.18 1.2 3.1 2.9
1.25 1.1 0.8 0.7 0.7 2.34 1.5 3.4 2.7
0.17 0.12 0.18 0.16 0.22 0.35 0.2 0.25 0.16
3. Results and discussion 3.1. Microstructural characterization 3.1.1. Effect of Fe Figs. 2 and 3 show both SEM and optical micrographs of A356–10% SiC composite with different Fe contents. The basic composite (F0) had a fairy low level of Fe at 0.2%, but contained no plate-like -phase intermetallics. Platelets of -Fe appeared and increased as Fe levels were raised. After applying T6 heat treatment on the MMC specimens, slight modification of eutectic silicon was seen in the microstructure. In this condition, the -Fe needles were observed more clearly, as seen in Fig. 3 [6,15,28]. Fig. 4 shows the quantitative data of -phase obtained from microstructural examinations. As shown in Figs. 3 and 4a, at low Fe level (i.e. 0.5% Fe), -platelets are small in quantity and size. With the addition of Fe from 1 to 2 wt.%, the number of -Fe intermetallics increased appreciably. It is expected that the majority of -phase precipitates prior to the eutectic Si at higher Fe contents as primary phase [12]. The variations in average length and thickness of -phase as a function of Fe content are seen in Fig. 4b and c. Similar finding has been reported by some investigators indicating that with increasing iron content, the size of -phase increases appreciably [10,29,30]. In current investigation, no significant change was seen in variation of -phase thickness with Fe content (Fig. 4c). Due to the sharp edges of  needles, a severe stress concentration is introduced to the alloy’s matrix contributing to the brittleness of the material. The amount of particle cracking and propagation of cracks in these brit-
tle phases depend on whether these phases are formed as needles or as fine, round and small particles. It is interesting to note that similar to monolithic Al–Si alloys, Fe addition not only encourages the nucleation of -phase during solidification, but also shifts the precipitation sequence of the -phase toward a higher temperature; therefore it becomes extremely coarse [6,12]. 3.1.2. Effect of Mn Fig. 5 shows the microstructures of specimens with 2% Fe with different Mn:Fe ratios (0.4, 0.5, 0.6, 0.7 and 0.8). Different morphologies of Fe-rich intermetallics are seen with the addition of Mn. In previous researches, Mn was used as a positive modifier to suppress the formation of the coarse primary -phase and promote the formation of less harmful ␣-phase in aluminum alloys [4,12,15]. Fig. 6 shows the area fraction of different kinds of frequently ␣phase (polyhedral, star-like and Chinese script) in specimens with various Mn:Fe ratios. Fe-rich needles are still likely formed even if their size and amounts are remarkably reduced. The increase in Mn concentrations converts the intermetallics from one particular -phase morphology to another to some extent, but note that total area fraction of Fe-rich intermetallics remains approximately constant up to Mn:Fe = 0.6, then sharply increases at higher Mn:Fe ratios (Fig. 6). The formation of ␣-Chinese script instead of platelet phase depended strongly on the Mn:Fe ratio [12,15]. With Mn:Fe = 0.5, most of the Fe intermetallics precipitated as fine ␣Chinese script morphology which is less harmful to the mechanical properties of the material, although few Fe intermetallics precipi-
Fig. 2. Microstructures of as-cast A356/10% SiC with: (a) 1% Fe and (b) 2% Fe.
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Fig. 3. Microstructures of A356/10% SiC with various Fe contents after T6 heat treatment: (a) 0.5% Fe, (b) 1% Fe, (c) 1.5% Fe, (d) 2% Fe.
tated as Star-like and polyhedral morphology. So, according to the microstructural study of specimens with different Mn concentrations, Mn:Fe ratio of 0.5 was selected for tensile testing. 3.1.3. Effect of Sr In this research the effects of 0.03% Sr on the microstructure and tensile properties of the composite (with 1 and 2% Fe and Mn:Fe = 0.5) were studied. The typical microstructures of the Sr added specimens with 1 and 2 wt.% Fe are shown in Fig. 7. Fig. 8 shows the effect of Mn and Sr on the surface area fraction of different morphologies of Fe-rich intermetallics in A356–10% SiC composites containing 1 and 2% Fe. One important advantage of Sr addition to Al–Si alloy is usual refining of eutectic Si crystals [10,15]. Further modification of silicon crystals is expected after heat treatment [31]. In the current investigation, it was observed that specimens with both Mn and Sr addition have the lowest -phase surface area fraction, highly rich in fine Chinese script and modified silicon eutectic, as seen in Figs. 7 and 8. It has been reported that Sr addition promotes -phase refining by different mechanisms, including fragmentation, dissolution and decomposition of the -platelets [32]. No evidence was found to show that the addition of 0.03% Sr may neutralize the positive effect of Mn in reducing the harmful -phase [33]. 3.2. Tensile properties and fracture characteristics
Fig. 4. -phase characteristics versus Fe volume fraction in A356/10% SiC composite: (a) surface fraction, (b) average length, (c) average thickness.
Fig. 9 shows typical stress–strain curves of the modified and non-modified MMCs. The ultimate tensile strength (UTS), yield strength (YS) and percent elongation (%El.) values of the composite are shown in Table 3, Figs. 10 and 11. Fig. 10 shows the plots of UTS and elongation values of heat-treated composite as a function of Fe content. It is seen that the increase of Fe content resulted in decreasing both UTS and elongation values. This reduction is essentially due to the contribution of large and brittle -phase in material which causes reduction in ductility [10,15]. It is important to note
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Fig. 5. Microstructures of A356/10% SiC–2% Fe with various Mn/Fe ratios: (a) 0.025, (b) 0.4, (c) 0.5, (d) 0.6, (e) 0.7 and (f) 0.8.
that such composite is considered as a brittle material and its fracture is mainly controlled by interfacial characteristic of SiC particles and the matrix alloy [34]. Since the number of SiC particles and their size are fixed, the fracture depends mostly on the size and number of -platelets in view of the fact that at high Fe levels (≥1 wt.%) much lower energy is required to break these MMCs. The results from tensile testing of Sr and Mn modified samples after T6 heat treatment are shown in Fig. 11 and Table 3. No considerable change is seen in UTS and YS results of Mn and Sr modified MMC specimens but interesting results were obtained from elon-
gation values (Fig. 11c). Among the modifiers, the effects of Mn have been established in the literature. Several reports have shown that at the Mn:Fe ratio of 0.5, the -phase plates are converted to ␣-Chinese script[15,35], but overall volume fractions of intermetallics are usually increased [18]. The comparison of elongation values (Fig. 11) reveals that simultaneous addition of Mn and Sr is the most effective procedure to enhance elongation. The observed improvement in tensile properties of Fe containing A356–10% SiC can be due to the well-known advantages of modifying effect of Sr. Sr is not only effective in modifying eutectic Si
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Fig. 6. Fe-rich intermetallics surface fraction in A356/10% SiC–2% Fe with various Mn/Fe ratios.
crystals of the matrix alloy (A356), but also it controls the morphology of Fe intermetallics [12]. The Sr additions were reported to deactivate (or “poison”) the nucleation sites for -phase by Samuel et al., who also stated that 300 ppm was the optimum level of added Sr [23]. Paray et al. have found that 150–300 ppm of Sr makes the ␣Chinese script the dominant intermetallic phase in 6061 alloy [21]. Furthermore, Sr is known to be a surface-active element and may improve strength of interface bonding between SiC and aluminum matrix [12,14,36,38]. To investigate the fracture behavior, several specimens were selected for fractographic examination by SEM. It was found that fracture of all specimens occurred in a brittle manor. Examinations of several fractured faces of non-modified specimens with high Fe concentrations showed broken intermetallics on the fracture surfaces. The fractured faces of the A356–10% SiC–2% Fe showed the presence of -phase plates, as presented in Fig. 12. Coarse -phase intermetallics in the microstructure not only may cause initiation of cracks, but also it can encourage fast crack propagation. The fracture planes of almost all coarse -phase plates exhibit clear cleavage creating a rapid fracture driving from their intrinsic brittleness. As expected number of -platelets near shrinkage pore, with dendrites of matrix alloy, are visible in Fig. 12. Regardless of
Fig. 8. Surface area fraction of Fe-rich intermetallics in A356/10% SiC (1 and 2% Fe) with and without Mn and Sr additions.
whether the fracture is initiated in structural defects such as porosity and oxide inclusions or microstructural features such as eutectic or intermetallic phases, properties can be expected to be controlled by the defect or feature that leads to the largest stress concentration. It can be indicated that the fracture of high iron containing specimens is mainly controlled by -phase (Al5 FeSi) and porosity [10,15]. Because coarse intermetallics are seen as one of the main sources of crack initiators in the microstructure, modification by Mn to obtain fine and well distribution of intermetallics may result in altering the failure mechanism. Fig. 13 shows the fracture surface of the A356–10% SiC/2% Fe–1% Mn and the EDX analysis of ␣-phase (i.e. Chinese script with the appearance of dendritic growth) found on the fracture surface. The tensile properties of 1 and 2% Fe containing MMCs are improved by the addition of 0.5 and 1% Mn respectively, which confirms the general rule that the detrimental effect of Fe is minimized when the Mn:Fe ratio is close to 0.5. For Fe added MMCs with Mn:Fe = 0.5, most of the observed intermetallics are Chinese script which are consistent with the previous findings with monolithic alloys [4,12]. The fracture surface of the composite with 2% Fe–1% Mn–0.03% Sr is shown in Fig. 14. Fine dimples are observed in fracture surface of the Mn and Sr modified composite specimens, as seen in Fig. 14. Among these three composites, the composite with both Mn and
Fig. 7. Microstructures of A356/10% SiC–2% Fe with: (a) 1% Mn and (b) 1% Mn + 0.03% Sr, after T6 heat treatment.
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Fig. 11. Tensile properties of A356/10% SiC composite (1 and 2% Fe) with and without Mn and Sr, (a) UTS, (b) YS and (c) %El.
Fig. 9. Typical stress–strain curves of the composites with: (a) 0.5–2% Fe, (b) 1% Fe, 1% Fe + 0.5% Mn, 1% Fe + 0.5% Mn + 0.03% Sr and (c) 2% Fe, 2% Fe + 1% Mn, 2% Fe + 1% Mn + 0.03% Sr.
Fig. 10. The variations of UTS and %El. as a function of Fe content in A356/10% SiC composite.
Sr modifiers showed the best tensile strength results. The highest elongation values were achieved in Mn and Sr modified MMC specimens. It has been also reported that the addition of active alloying elements such as Sr can modify the composite material by producing a transient layer between the particles and the matrix [37]. This transient layer surrounds the particles with a structure that can be similar in both the particle and the matrix alloy and improves wettability [5]. Garcia et al. have shown that covering of particles by Sr, during solidification, can improve wettability and promotes a higher incorporation of particles in Al–SiCp cast composites [38]. When the interface is strong enough, the load is transferred to
Fig. 12. Fracture surface of A356/10% SiC–2% Fe and EDX microanalysis obtained from -phases.
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the proper tensile strength and especially higher elongation values. 4. Fracture micrographs reveal an apparently brittle fracture with large facets of -phase plate in composites with higher Fe level, whereas Mn and Sr modified specimens show fine dimples on the fractured faces. Acknowledgements The authors would like to acknowledge the University of Tehran for financial support and providing lab facilities for this research. References
Fig. 13. Fracture surface of A356/10% SiC–2% Fe + 1% Mn and EDX microanalysis obtained from ␣-phases, showing enlarged ␣-Chinese script and SiC particles.
Fig. 14. Fracture surface of A356/10% SiC–2% Fe with1% Mn and 0.03% Sr, showing dimples on the fracture surface.
SiC particles and fracture occurs as soon as the threshold stress is reached. 4. Conclusion The morphology of Fe containing intermetallic compounds in heat-treated A356–10% SiC composite at different levels of Fe was investigated. Mn and Sr were used to investigate their modification effects on the structure and tensile properties of the Fe added MMCs and a number of conclusions drawn. 1. The addition of Fe causes precipitation of large -Al5 FeSi. 2. The addition of a modest level of Mn at the Mn:Fe ratio of 0.5 converts almost all -phase plates to fine ␣-Chinese script phase, leading to improved tensile strength and elongation. 3. The addition of Mn and 0.03% Sr is highly beneficial to reduce the deleterious effects of -phase, as evidenced by
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Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
The effect of Fe, Mn and Sr on the microstructure and tensile properties of A356–10% SiC composite K. Abedi, M. Emamy ∗ School of Metallurgy and Materials, University of Tehran, North Kargar St., Tehran 11365-4563, Iran
a r t i c l e
i n f o
Article history: Received 9 October 2009 Received in revised form 26 January 2010 Accepted 17 March 2010
Keywords: A356–10% SiC Fe intermetallics T6 heat treatment Tensile properties Microstructure Modifiers
a b s t r a c t This study investigates the formation of Fe containing intermetallic compounds on the microstructure and tensile properties of A356–10% SiC composite in the Mn- and Sr-modified conditions. The composite ingots were made by stir casting process and iron was added to the remelted composites at different concentrations varied from 0.5 to 2%. For 2 wt.% Fe, different levels of Mn were added to identify the optimum Mn:Fe ratio for eliminating harmful -phase and obtaining microstructure with well distributed and fine intermetallics. 300 ppm Sr was added to the Mn-modified composite to investigate the effect of Sr and Mn on the microstructure and tensile properties of A356–10% SiC. T6 heat treatment was applied for all specimens. It was found that the addition of Mn changes the morphology of Fe-rich intermetallics from plate-like - to ␣-phase. The tensile test results demonstrated that the addition of iron considerably reduces both UTS and elongation values while, modification by Mn and Sr improves tensile properties of the composite with 1 and 2 wt.% Fe. Fractographic examination of iron containing composites also showed that large -Fe intermetallics are responsible for fracture behavior in unmodified condition. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Aluminum metal matrix composite (MMC) is being considered as a new material for its light weight, high strength, high specific modulus and good wear resistance properties. The type, size, morphology, distribution and relative quantity of the reinforcing materials are considered as some important controlling parameters which may influence the composite properties. However, MMCs have lower ductility and fracture toughness in comparison with unreinforced alloys [1–3]. Microstructural parameters such as grain size, dendrite arm spacing (DAS) and the morphology of silicon and intermetallics also play an important role in determining the mechanical properties of Al–Si alloys. Modification of the eutectic Si crystals in Al–Si alloys is achieved by rapid solidification, chemical modification and thermal modification. Several works have shown that the coarse and large Si needles can be changed into a fine well-rounded phase after T6 heat treatment [4–6]. In Al–Si cast alloys one important impurity is Fe. Since Fe has a very low solid solubility in Al (max. 0.05%), almost all Fe in Al alloys are present in the form of second phases, usually Al–Fe or Al–Fe–Si intermetallics. Fe is the most pervasive impurity in cast Al alloys due to its central role in the formation of extensive thin platelets of intermetallic compound -Al5 FeSi. This compound
∗ Corresponding author. Tel.: +98 21 82084083; fax: +98 21 82084083. E-mail address: [email protected] (M. Emamy). 0921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.03.063
appears as needles in 2D metallographic observations. Fe containing intermetallics are known to be detrimental to the mechanical properties, because these compounds are generally thought to be brittle [6]. In the Al–Si–Fe system there are five main Ferich phases: Al3 Fe, ␣-Al8 Fe2 Si (possibly ␣-Al12 Fe3 Si2 ), -Al5 FeSi, ␦-Al4 FeSi2 and ␥-Al3 FeSi [2,3]. The ␥-Al3 FeSi phase occurs in highFe and high-Si alloys [7,8]. When the quantity of iron present in Al–Si alloys is greater than a certain value (0.4–0.5 wt.%), a significant increase in porosity is to be observed. It was suggested that the formation of coarse -Al5 FeSi intermetallic platelets restricted liquid metal-feeding during the casting process. Moreover, the Al5 FeSi intermetallics were found to be highly active sites for pore nucleation [6,9,10]. Effort has been made to control the precipitation, growth and morphology of plate-like -Al5 FeSi intermetallic phases by several investigators [11,12]. When Fe is present in an excess of specified levels, various methods have been advocated to reduce its harmful influence. These methods can be categorized into three types. The most direct method is the reduction or removal of Fe from Al melts, mainly by precipitation and sedimentation processing, i.e. the heat treatment of the liquid metal [13,14]. Since iron is inevitable and cannot be economically removed from the molten aluminum, strategies have to be developed to neutralize its negative effects. The conventional method is to add some chemical “correctors” to change the morphology from platelet -Al5 FeSi (brittle form) to globular or script forms (less brittle forms). The globular or script Fe-rich phases have been thought not to lead to brittleness [15]. Alternative methods involve various thermal
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Table 1 Chemical composition (wt.%) of A356 aluminum alloy. Si
Mg
Cu
Mn
Fe
Ni
Zn
Cr
Al
7.53
0.35
0.02
0.08
0.16
0.07
0.14
0.008
Bal.
treatments, including melt superheating, rapid solidification and solution heat treatment of castings [4,7,9]. To convert -Fe platelets into ␣-Fe script form, Co, Cr, Mn, Mo and Ni are sometimes added. Their addition has also been reported to improve strength at high temperature. Mn has been widely used because, in addition to its significantly lower cost and better availability, -Al5 FeSi tends to be suppressed and ␣-phase is formed in its place [4,7,11,15]. Three different morphologies of ␣-phase intermetallics can form: (1) Chinese script (Al15 (Fe,Mn)3 Si2 ); (2) polyhedral and (3) star-like crystals, which are a coarser version of the ␣-phase with the same stoichiometry [15]. When  plates are present in the microstructure, tensile properties, especially elongation [16–18] are reduced as a result of their stress raising potential and their apparently brittle behavior as evidenced by cracking at low stresses [19,20]. Another modifier that was shown to be effective in controlling -phase is Sr. Sr additions have been found to convert the -phase to the more compact ␣-phase. Several investigators have found that 150–300 ppm of Sr was sufficient to neutralize the detrimental effects of the Fe-rich intermetallics and makes the Chinese script a dominant intermetallic phase in some Al–Si alloys [21–25]. With extensive data available on the modification of Fe-rich intermetallic phases in monolithic Al–Si alloys, less attention has been made to reduce the harmful effect of Fe on the cast MMCs. This study was undertaken to examine the microstructure and tensile properties of heat-treated Fe containing A356–10% SiC composite before and after modification by Mn and Sr. 2. Experimental Table 1 shows the chemical composition of the matrix alloy (A356 Al alloy). Silicon carbide particles with an average size of 40 m were used as reinforcing material into the matrix alloy. Stirring process was employed to produce A356–10% SiC Al MMCs ingots [26]. The supplied composite ingots were cut into small pieces, dried and remelted in 2-kg capacity graphite crucible by means of an electrical resistance furnace. The melting temperature was kept at 760 ± 5 ◦ C in all castings. Fe, Mn and Sr were added as Al–20% Fe, Al–20% Mn and Al–10% Sr master alloys. 13 melts were studied with different compositions (Table 2). Fe was added to the MMC at levels varying from 0.5 to 2 wt.%. To study the effect of Mn and find the best Mn:Fe ratio, five different Mn:Fe Table 2 Nominal composition, Mn/Fe ratio of A356/10% SiC composite and its code. Material codes
F0 F5 F10 F15 F20 F20M8 F20M10 F20M12 F20M14 F20M16 F10M5 F10M5S F20M10S
Additions Fe (wt.%)
Mn (wt.%)
Sr (wt.%)
Mn/Fe
0.2 0.5 1 1.5 2 2 2 2 2 2 1 1 2
0.1 – – – – 0.8 1 1.2 1.4 1.6 0.5 0.5 1
– – – – – – – – – – – 0.03 0.03
– – – – – 0.4 0.5 0.6 0.7 0.8 0.5 0.5 0.5
Fig. 1. Schematic drawing of (a) tensile test casting mould and (b) tensile test specimen.
ratios were used (0.4, 0.5, 0.6, 0.7 and 0.8) at a constant Fe concentration (2 wt.%). Sr was selected at 300 ppm level according to previous studies [4,12,15]. For example, Samuel et al. have found that 250–350 ppm Sr was sufficient to neutralize the detrimental effects of the Fe-rich intermetallics in some Al–Si alloys, appearing to lead to the fragmentation of -phase [23–25]. When the melt reached 760 ◦ C, it was thoroughly skimmed. The Fe addition was then made and the melt was stirred gently for 1 min. After about 3 min, a further addition of Mn or Sr was carried out. After complete dissolution and homogenization of all master alloys, the surface oxides were again skimmed prior to pouring the molten metal into a ductile iron mould preheated to 250 ◦ C. The iron mould and tensile test specimens were made according to ASTM B 108-03a and ASTM B557M-02a standard respectively (Fig. 1). Six specimens (three castings) were prepared for each selected composition. T6 heat treatment was applied to all specimens. Solution treatment was carried out at 530 ◦ C for 12 h and then rapidly quenched in warm water at 70 ◦ C before aging at 155 ◦ C for 4 h [27]. Microstructural studies were made on polished sample surfaces which were selected from the gauge length portion of the test bars (6 mm diameter). Normal grinding and polishing technique was used to reveal the structure. All samples of the composite listed in Table 2 were prepared for image analysis. Quantitative data on microstructure were determined using an optical microscope equipped with an image analysis system (Clemex Vision Pro. ver.3.5.025). For each sample, 50 fields at 400 times magnification were evaluated. The results of digital image analysis are presented in Table 3. Further microstructural studies of the specimens were examined by scanning electron microscopy (SEM) performed in a CamScan MV2300 equipped with the energy dispersive X-ray analysis (EDX) accessory. Tensile test were carried out in a computerized testing machine (Zwick/Roell Z100) at room temperature to obtain yield strength (YS), ultimate tensile strength (UTS) and percent elongation (%El.) at a strain rate of 1 mm/min. The fracture surfaces of tensile test specimens were also examined by similar SEM.
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Table 3 Tensile properties of examined cast composite specimens. Items codes
F0 F5 F10 F15 F20 F10M5 F20M10 F10M5S F20M10S
UTS (MPa)
YS (MPa)
El. (%)
Max.
Min.
Ave.
S.D.
Max.
Min.
Ave.
S.D.
Max.
Min.
Ave.
S.D.
252 233 180 188 164 202 195 219 218
222 182 147 153 132 175 169 181 186
234 213 168 174 152 187 188 197 205
12 18 14 16 17 10 13 14 13
185 181 165 163 154 129 157 152 158
165 157 134 135 135 105 121 127 119
168 172 150 145 140 118 134 140 135
9 9 7 8 8 6 8 6 7
1.5 1.2 1 0.9 0.9 2.68 1.7 3.6 3.2
1.1 0.7 0.6 0.6 0.55 2.18 1.2 3.1 2.9
1.25 1.1 0.8 0.7 0.7 2.34 1.5 3.4 2.7
0.17 0.12 0.18 0.16 0.22 0.35 0.2 0.25 0.16
3. Results and discussion 3.1. Microstructural characterization 3.1.1. Effect of Fe Figs. 2 and 3 show both SEM and optical micrographs of A356–10% SiC composite with different Fe contents. The basic composite (F0) had a fairy low level of Fe at 0.2%, but contained no plate-like -phase intermetallics. Platelets of -Fe appeared and increased as Fe levels were raised. After applying T6 heat treatment on the MMC specimens, slight modification of eutectic silicon was seen in the microstructure. In this condition, the -Fe needles were observed more clearly, as seen in Fig. 3 [6,15,28]. Fig. 4 shows the quantitative data of -phase obtained from microstructural examinations. As shown in Figs. 3 and 4a, at low Fe level (i.e. 0.5% Fe), -platelets are small in quantity and size. With the addition of Fe from 1 to 2 wt.%, the number of -Fe intermetallics increased appreciably. It is expected that the majority of -phase precipitates prior to the eutectic Si at higher Fe contents as primary phase [12]. The variations in average length and thickness of -phase as a function of Fe content are seen in Fig. 4b and c. Similar finding has been reported by some investigators indicating that with increasing iron content, the size of -phase increases appreciably [10,29,30]. In current investigation, no significant change was seen in variation of -phase thickness with Fe content (Fig. 4c). Due to the sharp edges of  needles, a severe stress concentration is introduced to the alloy’s matrix contributing to the brittleness of the material. The amount of particle cracking and propagation of cracks in these brit-
tle phases depend on whether these phases are formed as needles or as fine, round and small particles. It is interesting to note that similar to monolithic Al–Si alloys, Fe addition not only encourages the nucleation of -phase during solidification, but also shifts the precipitation sequence of the -phase toward a higher temperature; therefore it becomes extremely coarse [6,12]. 3.1.2. Effect of Mn Fig. 5 shows the microstructures of specimens with 2% Fe with different Mn:Fe ratios (0.4, 0.5, 0.6, 0.7 and 0.8). Different morphologies of Fe-rich intermetallics are seen with the addition of Mn. In previous researches, Mn was used as a positive modifier to suppress the formation of the coarse primary -phase and promote the formation of less harmful ␣-phase in aluminum alloys [4,12,15]. Fig. 6 shows the area fraction of different kinds of frequently ␣phase (polyhedral, star-like and Chinese script) in specimens with various Mn:Fe ratios. Fe-rich needles are still likely formed even if their size and amounts are remarkably reduced. The increase in Mn concentrations converts the intermetallics from one particular -phase morphology to another to some extent, but note that total area fraction of Fe-rich intermetallics remains approximately constant up to Mn:Fe = 0.6, then sharply increases at higher Mn:Fe ratios (Fig. 6). The formation of ␣-Chinese script instead of platelet phase depended strongly on the Mn:Fe ratio [12,15]. With Mn:Fe = 0.5, most of the Fe intermetallics precipitated as fine ␣Chinese script morphology which is less harmful to the mechanical properties of the material, although few Fe intermetallics precipi-
Fig. 2. Microstructures of as-cast A356/10% SiC with: (a) 1% Fe and (b) 2% Fe.
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Fig. 3. Microstructures of A356/10% SiC with various Fe contents after T6 heat treatment: (a) 0.5% Fe, (b) 1% Fe, (c) 1.5% Fe, (d) 2% Fe.
tated as Star-like and polyhedral morphology. So, according to the microstructural study of specimens with different Mn concentrations, Mn:Fe ratio of 0.5 was selected for tensile testing. 3.1.3. Effect of Sr In this research the effects of 0.03% Sr on the microstructure and tensile properties of the composite (with 1 and 2% Fe and Mn:Fe = 0.5) were studied. The typical microstructures of the Sr added specimens with 1 and 2 wt.% Fe are shown in Fig. 7. Fig. 8 shows the effect of Mn and Sr on the surface area fraction of different morphologies of Fe-rich intermetallics in A356–10% SiC composites containing 1 and 2% Fe. One important advantage of Sr addition to Al–Si alloy is usual refining of eutectic Si crystals [10,15]. Further modification of silicon crystals is expected after heat treatment [31]. In the current investigation, it was observed that specimens with both Mn and Sr addition have the lowest -phase surface area fraction, highly rich in fine Chinese script and modified silicon eutectic, as seen in Figs. 7 and 8. It has been reported that Sr addition promotes -phase refining by different mechanisms, including fragmentation, dissolution and decomposition of the -platelets [32]. No evidence was found to show that the addition of 0.03% Sr may neutralize the positive effect of Mn in reducing the harmful -phase [33]. 3.2. Tensile properties and fracture characteristics
Fig. 4. -phase characteristics versus Fe volume fraction in A356/10% SiC composite: (a) surface fraction, (b) average length, (c) average thickness.
Fig. 9 shows typical stress–strain curves of the modified and non-modified MMCs. The ultimate tensile strength (UTS), yield strength (YS) and percent elongation (%El.) values of the composite are shown in Table 3, Figs. 10 and 11. Fig. 10 shows the plots of UTS and elongation values of heat-treated composite as a function of Fe content. It is seen that the increase of Fe content resulted in decreasing both UTS and elongation values. This reduction is essentially due to the contribution of large and brittle -phase in material which causes reduction in ductility [10,15]. It is important to note
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Fig. 5. Microstructures of A356/10% SiC–2% Fe with various Mn/Fe ratios: (a) 0.025, (b) 0.4, (c) 0.5, (d) 0.6, (e) 0.7 and (f) 0.8.
that such composite is considered as a brittle material and its fracture is mainly controlled by interfacial characteristic of SiC particles and the matrix alloy [34]. Since the number of SiC particles and their size are fixed, the fracture depends mostly on the size and number of -platelets in view of the fact that at high Fe levels (≥1 wt.%) much lower energy is required to break these MMCs. The results from tensile testing of Sr and Mn modified samples after T6 heat treatment are shown in Fig. 11 and Table 3. No considerable change is seen in UTS and YS results of Mn and Sr modified MMC specimens but interesting results were obtained from elon-
gation values (Fig. 11c). Among the modifiers, the effects of Mn have been established in the literature. Several reports have shown that at the Mn:Fe ratio of 0.5, the -phase plates are converted to ␣-Chinese script[15,35], but overall volume fractions of intermetallics are usually increased [18]. The comparison of elongation values (Fig. 11) reveals that simultaneous addition of Mn and Sr is the most effective procedure to enhance elongation. The observed improvement in tensile properties of Fe containing A356–10% SiC can be due to the well-known advantages of modifying effect of Sr. Sr is not only effective in modifying eutectic Si
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Fig. 6. Fe-rich intermetallics surface fraction in A356/10% SiC–2% Fe with various Mn/Fe ratios.
crystals of the matrix alloy (A356), but also it controls the morphology of Fe intermetallics [12]. The Sr additions were reported to deactivate (or “poison”) the nucleation sites for -phase by Samuel et al., who also stated that 300 ppm was the optimum level of added Sr [23]. Paray et al. have found that 150–300 ppm of Sr makes the ␣Chinese script the dominant intermetallic phase in 6061 alloy [21]. Furthermore, Sr is known to be a surface-active element and may improve strength of interface bonding between SiC and aluminum matrix [12,14,36,38]. To investigate the fracture behavior, several specimens were selected for fractographic examination by SEM. It was found that fracture of all specimens occurred in a brittle manor. Examinations of several fractured faces of non-modified specimens with high Fe concentrations showed broken intermetallics on the fracture surfaces. The fractured faces of the A356–10% SiC–2% Fe showed the presence of -phase plates, as presented in Fig. 12. Coarse -phase intermetallics in the microstructure not only may cause initiation of cracks, but also it can encourage fast crack propagation. The fracture planes of almost all coarse -phase plates exhibit clear cleavage creating a rapid fracture driving from their intrinsic brittleness. As expected number of -platelets near shrinkage pore, with dendrites of matrix alloy, are visible in Fig. 12. Regardless of
Fig. 8. Surface area fraction of Fe-rich intermetallics in A356/10% SiC (1 and 2% Fe) with and without Mn and Sr additions.
whether the fracture is initiated in structural defects such as porosity and oxide inclusions or microstructural features such as eutectic or intermetallic phases, properties can be expected to be controlled by the defect or feature that leads to the largest stress concentration. It can be indicated that the fracture of high iron containing specimens is mainly controlled by -phase (Al5 FeSi) and porosity [10,15]. Because coarse intermetallics are seen as one of the main sources of crack initiators in the microstructure, modification by Mn to obtain fine and well distribution of intermetallics may result in altering the failure mechanism. Fig. 13 shows the fracture surface of the A356–10% SiC/2% Fe–1% Mn and the EDX analysis of ␣-phase (i.e. Chinese script with the appearance of dendritic growth) found on the fracture surface. The tensile properties of 1 and 2% Fe containing MMCs are improved by the addition of 0.5 and 1% Mn respectively, which confirms the general rule that the detrimental effect of Fe is minimized when the Mn:Fe ratio is close to 0.5. For Fe added MMCs with Mn:Fe = 0.5, most of the observed intermetallics are Chinese script which are consistent with the previous findings with monolithic alloys [4,12]. The fracture surface of the composite with 2% Fe–1% Mn–0.03% Sr is shown in Fig. 14. Fine dimples are observed in fracture surface of the Mn and Sr modified composite specimens, as seen in Fig. 14. Among these three composites, the composite with both Mn and
Fig. 7. Microstructures of A356/10% SiC–2% Fe with: (a) 1% Mn and (b) 1% Mn + 0.03% Sr, after T6 heat treatment.
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Fig. 11. Tensile properties of A356/10% SiC composite (1 and 2% Fe) with and without Mn and Sr, (a) UTS, (b) YS and (c) %El.
Fig. 9. Typical stress–strain curves of the composites with: (a) 0.5–2% Fe, (b) 1% Fe, 1% Fe + 0.5% Mn, 1% Fe + 0.5% Mn + 0.03% Sr and (c) 2% Fe, 2% Fe + 1% Mn, 2% Fe + 1% Mn + 0.03% Sr.
Fig. 10. The variations of UTS and %El. as a function of Fe content in A356/10% SiC composite.
Sr modifiers showed the best tensile strength results. The highest elongation values were achieved in Mn and Sr modified MMC specimens. It has been also reported that the addition of active alloying elements such as Sr can modify the composite material by producing a transient layer between the particles and the matrix [37]. This transient layer surrounds the particles with a structure that can be similar in both the particle and the matrix alloy and improves wettability [5]. Garcia et al. have shown that covering of particles by Sr, during solidification, can improve wettability and promotes a higher incorporation of particles in Al–SiCp cast composites [38]. When the interface is strong enough, the load is transferred to
Fig. 12. Fracture surface of A356/10% SiC–2% Fe and EDX microanalysis obtained from -phases.
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the proper tensile strength and especially higher elongation values. 4. Fracture micrographs reveal an apparently brittle fracture with large facets of -phase plate in composites with higher Fe level, whereas Mn and Sr modified specimens show fine dimples on the fractured faces. Acknowledgements The authors would like to acknowledge the University of Tehran for financial support and providing lab facilities for this research. References
Fig. 13. Fracture surface of A356/10% SiC–2% Fe + 1% Mn and EDX microanalysis obtained from ␣-phases, showing enlarged ␣-Chinese script and SiC particles.
Fig. 14. Fracture surface of A356/10% SiC–2% Fe with1% Mn and 0.03% Sr, showing dimples on the fracture surface.
SiC particles and fracture occurs as soon as the threshold stress is reached. 4. Conclusion The morphology of Fe containing intermetallic compounds in heat-treated A356–10% SiC composite at different levels of Fe was investigated. Mn and Sr were used to investigate their modification effects on the structure and tensile properties of the Fe added MMCs and a number of conclusions drawn. 1. The addition of Fe causes precipitation of large -Al5 FeSi. 2. The addition of a modest level of Mn at the Mn:Fe ratio of 0.5 converts almost all -phase plates to fine ␣-Chinese script phase, leading to improved tensile strength and elongation. 3. The addition of Mn and 0.03% Sr is highly beneficial to reduce the deleterious effects of -phase, as evidenced by
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