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Effects of rare earth elements addition on microstructures, tensile properties and fractography of A357 alloy
Materials Science & Engineering A 597 (2014) 237–244
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effects of rare earth elements addition on microstructures, tensile properties and fractography of A357 alloy Wenming Jiang a,b,n, Zitian Fan a, Yucheng Dai a, Chi Li a a b
State Key Lab of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China School of Mechanical & Electrical Engineering, Wuhan Institute of Technology, Wuhan 430073, PR China
ar t ic l e i nf o
a b s t r a c t
Article history: Received 10 December 2013 Received in revised form 1 January 2014 Accepted 3 January 2014 Available online 10 January 2014
The effects of rare earth (RE) containing Ce and La elements addition on the microstructures characteristics, tensile properties and fracture behavior of A357 alloy under as-cast and T6 conditions were systematically investigated in this study. Obtained results showed that the addition of RE obviously reduced the sizes of the α-Al primary phase and eutectic silicon particles as well as SDAS value and improved the morphology of eutectic silicon particles. The optimum level of added RE content were 0.2 wt%, and the aspect ratio of eutectic silicon particles of the A357 modified alloy under as-cast and T6 conditions decreased 142% and 174%, respectively, compared with the unmodified alloy. In addition, the addition of RE greatly improved the tensile properties of A357 alloy as result of the significant improvement in microstructure, especially in elongation under T6 condition. The fracture surfaces of the A357 unmodified alloy tensile samples showed a clear brittle fracture nature, and its fracture path passed through the eutectic silicon particles and displayed a transgranular fracture mode, leading to poorer ductility. The fracture path of the A357 modified alloys passed through the eutectic phase along the grain boundaries of the α-Al primary phase, and the fracture generated by dimple rupture with cracked eutectic silicon particles, and it showed an intergranular fracture mode, resulting in superior ductility. & 2014 Elsevier B.V. All rights reserved.
Keywords: A357 aluminum alloy Rare earth Microstructure characteristics Tensile properties Fractography
1. Introduction Currently, the aluminum–silicon (Al–Si) cast alloys are extensively used in the aerospace and automotive industries because of its many advantages including excellent castability, corrosion resistance, high strength to weight ratio as well as low coefficient of thermal expansion, etc [1–3]. The mechanical properties of aluminum–silicon cast alloys mainly depend on the chemical composition and microstructure, etc [4,5]. And the microstructure of the Al–Si cast alloys mainly consists of α-Al primary phase, Al–Si eutectic, intermetallics and other precipitate phases. The morphology and size of α-Al primary phase and Al–Si eutectic have a significant effect on mechanical properties of the Al–Si cast alloys. The tensile properties of the Al–Si cast alloys, especially in the ductility, are mainly controlled by the dendrites cell size of α-Al primary phase. Moreover, the eutectic silicon particles also play an important role in the fracture behavior and tensile ductility of Al– Si cast alloys [6]. Generally, in the normal casting condition, α-Al primary phases grow coarse dendrites, and the eutectic silicon n Corresponding author at: Huazhong University of Science and Technology, State Key Lab of Materials Processing and Die & Mould Technology, Luoyu, Wuhan 430074, PR China. Tel.: þ 86 27 87540094; fax: þ 86 27 87558252. E-mail address: [email protected] (W. Jiang).
0921-5093/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2014.01.009
particles exhibit coarse acicular needles. As a result, the mechanical properties of Al–Si cast alloys are remarkably weaken. Therefore, there are many methods to improve the morphology and size of α-Al primary phase and eutectic silicon particles, such as chemical modification [7–9], outfield modification [10,11] as well as thermal modification [12–14], etc. The chemical modification is regarded as an economical and effective modification method for the improvement of the morphology and size of α-Al primary phase and eutectic silicon particles. Generally, many chemical elements are known to be used in the chemical modification method, such as Ti, B, Sr, Na, Sb and rare earth (RE) elements including Ce, La, Yb, Eu and Sc [15– 18], etc. However, many chemical elements are only responsible for the modification of α-Al primary phase from coarse dendrites to fine grains, such as Ti, B, etc, or for the modification of eutectic silicon particles from coarse acicular needles to fine fibrous structure, such as Sr, Na, Sb, etc. The RE elements containing Ce and La not only can fine α-Al primary phase, but also can fine eutectic silicon particles [1,19]. Furthermore, the hydrogen content in the aluminum molten metal can also be decreased. Nevertheless, few literatures systematically reported that the effects of RE addition containing Ce and La elements on the microstructures characteristics, tensile properties and fracture behavior of A357 alloy, especially in investigations of the quantitative metallography of microstructure including α-Al primary
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Table 1 The nominal chemical composition of experimental alloys (wt%). Alloy Alloy Alloy Alloy Alloy
1 (unmodified) 2 3 4
Si
Mg
Ti
Fe
RE
Al
6.75 6.75 6.75 6.75
0.63 0.63 0.63 0.63
0.14 0.14 0.14 0.14
0.084 0.084 0.084 0.084
0 0.1 0.2 0.3
Bal. Bal. Bal. Bal.
tensile specimens, which were prepared according to the ASTM E8 standard. The solution treatment was firstly performed at 540 1C for 8 h, and then quenched into hot water at 80 1C. The aging treatment was then carried out at 165 1C for 6 h, and the samples were finally cooled in air. The tensile tests were carried out by using the AG-IC100KN universal testing machine at a crosshead speed of 1.5 mm/min. The hardness of A357 alloy was measured using a HB-3000 hardness test machine. The fractured surfaces of tensile samples were observed and analyzed using the JSM-7600F scanning electron microscope (SEM) equipped with facilities for energy dispersive spectroscopy (EDS). Phases analyses were performed using a X-ray diffraction (XRD) analysis with Cu Kα radiation.
Fig. 1. Shape and dimensions of tensile specimens (unit: mm).
3. Results and discussion phase, secondary dendrite arm spacing (SDAS) and eutectic silicon particles as well as the correlation of microstructure with mechanical properties and fracture behavior of A357 alloy. In this study, the mixed RE containing Ce and La elements were added into A357 Al–Si alloy. The aim of present work at systematically investigated the effects of mixed RE elements addition on the microstructures characteristics including the morphology and size of α-Al primary phase and eutectic silicon phase as well as SDAS, tensile properties and fracture behavior of A357 Al–Si alloy under as-cast and T6 conditions. Moreover, the correlation of microstructure with mechanical properties and fracture behavior of A357 alloy were also studied.
2. Experimental procedures The chemical composition of A357 Al–Si alloy used in this study is shown in Table 1, named Alloy 1. The RE containing 65 wt% Ce and 35 wt% La elements was added as Al-10%RE master alloy. Firstly, the A357 aluminum ingot was placed inside a graphite crucible and melted at 730 1C by using the electrical resistance furnace. The Al-10%RE master alloys with different additions (0.1, 0.2 and 0.3) were then added into the molten melt at 740 1C, named Alloy 2, Alloy 3 and Alloy 4, respectively. Subsequently, the molten metal was refined using argon gas for 15 min using a rotary graphite degasser when the temperature reached 750 1C, and the slag was then skimmed. When the melting temperature was 720 1C, the molten metal was finally poured into a metal mould, which was preheated for 250 1C. The metallographic samples were etched using 0.5% hydrofluoric acid solution after polishing. Microstructures were observed using the OLYMPUS-MG3 metallographic microscope. The secondary dendrite arm spacing (SDAS), average length of silicon particles as well as average width of silicon particles were measured by using the ImageTool metallographic analysis software. The measurement was done on 50 different areas of each microstructure in order to minimize the errors. The aspect ratio of silicon particles was calculated according to the ratio of the average length of silicon particles to the average width of silicon particles. The grain size of the α-Al primary phase was defined according to the following equation [20]: pffiffiffiffiffiffiffiffi D ¼ 2 A=π ð1Þ where A is the average area of the α-Al primary phase, which was measured using the ImageTool software. The as-cast and T6 heat treatment specimens were subjected to investigate in this study. Fig. 1 depicts the shape and dimensions of
3.1. Microstructural characterization Fig. 2 shows the optical microstructures of A357 alloys with different additions of RE under as-cast and T6 heat treatment conditions, and the optical microstructures of the eutectic zone are also exhibited in order to demonstrate a substantial microstructure difference in the morphology and size of eutectic silicon particles. In the microstructures of the A357 unmodified alloys under ascast and T6 conditions, it is evident that the coarse dendrite are observed, as shown in Fig. 2(a and e). Meanwhile, it can also be seen from Fig. 2(a and e) that the plate-like silicon particles are showed. In contrast, the microstructures are significantly improved when the A357 alloys are modified with the addition of RE, and the coarse dendrites are fined and the plate-like silicon particles become fine fibrous structure. Because the spheroidization efficiency of eutectic silicon particles mainly depends on the initial size of eutectic silicon particles [1], the eutectic silicon particles of the A357 modified alloys are obviously spheroidized and homogeneously distributed in the grain boundary after T6 heat treatment, as shown in Fig. 2(f–h). Nevertheless, when the RE addition is 0.1 wt%, some the plate-like silicon particles can also be seen in microstructure. With the addition of Re up to 0.2 wt%, the morphology and size of α-Al primary phase and eutectic silicon particles are further improved, and the α-Al primary phases display a finer structure, and the eutectic silicon particles exhibit a granular and globular structure, as shown in Fig. 2(g). With the further increase of addition of RE content, the morphology and size of α-Al primary phase and eutectic silicon particles are deteriorated, and some eutectic silicon particles with acicular needles morphology are observed, as shown in Fig. 2(h). Table 2 depicts the quantitative metallography results of microstructure features including α-Al primary phase, eutectic silicon particles as well as SDAS of A357 alloys with different RE additions. According to the results obtained from the microstructural parameters in Table 2, it is clear that the addition of RE remarkably reduces the size of α-Al primary phase and SDAS value. Meanwhile, the average length, average width and aspect ratio of eutectic silicon particles are also greatly decreased compared to the unmodified condition, and the morphology of eutectic silicon particles looks more round. With the 0.2 wt% addition of RE, the reductions under as-cast and T6 heat treatment conditions in the size of α-Al primary phase and SDAS value are 48.1%, 162.2%, and 26.6%, 84.4%, respectively, compared to the unmodified condition. Furthermore, the aspect ratio of eutectic silicon particles of the A357 modified alloy under as-cast and T6 conditions decrease 142% and 174%, respectively, compared with the unmodified alloy.
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Fig. 2. Optical microstructures of A357 alloys with different additions of RE: (a and e) unmodified, Alloy 1; (b and f) 0.1 wt% RE, Alloy 2; (c and g) 0.2 wt% RE, Alloy 3; (d and h) 0.3 wt% RE, Alloy 4; (a–d) As-cast condition and (e–h) T6 heat treatment condition.
It can be noted that the T6 heat treatment has a profound effect on the spheroidization of eutectic silicon, and it is consist with the previous studies [21–23].
The mechanism of RE modification on the α-Al primary phase can be explained by follows. The eutectic reaction of RE occurs at 637–642 1C, and the reaction equation is presented in the
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Table 2 Quantitative metallography results for the microstructures of A357 aluminum alloy with different RE additions. Sample
Alloy
as-cast
Alloy Alloy Alloy Alloy Alloy Alloy Alloy Alloy
T6
1 (unmodified) 2 3 4 1 (unmodified) 2 3 4
Grain size of α-Al primary phase (μm)
SDAS (μm)
Average length of silicon particles (μm)
Average width of silicon particles (μm)
Aspect ratio of silicon particles
118.82 7 7.1 100.05 7 4.1 80.22 7 2.8 97.81 7 3.5 117.89 7 6.8 110.277 4.7 93.157 3.9 106.107 4.2
37.65 7 2.8 23.16 7 2.0 14.36 7 1.1 23.78 7 2.0 38.78 7 2.5 31.97 7 2.9 21.03 7 2.0 25.83 7 2.8
39.517 3.6 24.777 3.1 14.86 7 1.4 19.95 7 2.0 23.717 2.8 17.067 1.7 10.707 1.2 15.09 7 1.9
3.79 7 0.7 3.17 0.2 3.42 7 0.3 3.667 0.4 3.79 7 0.7 4.517 0.8 4.65 7 0.7 4.677 0.8
10.4 7 1.8 8.0 7 1.4 4.3 7 0.5 5.5 7 0.6 6.3 7 0.9 3.8 7 0.3 2.3 7 0.2 3.2 7 0.4
Fig. 3. SEM microstructure and EDS analysis of the A357 modified alloy with 0.2 wt% RE: (a) morphology and (b) EDS of the intermetallic compound containing Al, Si and RE.
following: L-ðα AlÞ þ Al4 ðLa; CeÞ
ð2Þ
The precipitation phases of α-Al due to the eutectic reaction supply a large number of nucleation cores for the α-Al primary phases. Meanwhile, the intermetallic compounds containing Al and Re have a very high melting point, above 1200 1C, and the lattices constant of the intermetallic compounds are similar with α-Al. The EDS analysis and X-ray diffraction patterns of the A357 modified alloy also confirm the reaction products of above eutectic reaction, and the intermetallic compounds containing Al and RE are observed, as shown in Figs. 3 and 4, especially in higher addition content of RE. Consequently, the intermetallic compounds can also supply some nucleation cores for the α-Al primary phases. As a result, the grain size of α-Al primary phase and SDAS are greatly decreased because of the large number of nucleation cores in the Al liquid. It is well known that the Sr also has a remarkable effect on the morphology and size of the eutectic silicon particles [1]. However, the Sr cannot modify the size and morphology of the α-Al primary phase as well as SDAS. The mechanism of RE modification on the eutectic silicon phases is might attributed to the concentration of RE elements on the surface of eutectic silicon particles, and it is similar to the mechanism of Sr modification on the eutectic silicon phases because of their adjacently diagonal relationship in the Periodic Table of the elements, and the widely accepted recent theory is the impurity-induced twinning theory [24]. In general, the Ce and La elements hardly dissolve in the α-Al or eutectic silicon phases because the atomic radius of the Ce and La elements are much larger than that of α-Al and silicon. As a result, the Ce and La elements mainly concentrate on the surface of eutectic silicon phases, and the growths of eutectic silicon phases are then
restricted. Thereby, the morphology of the eutectic silicon phases presents fine fibrous structure. The modified silicon fibers contain more twins than the unmodified silicon particles and have a rough microfaceted surface [25]. Therefore, the silicon fibers are crystallographically very imperfect, and each surface imperfection is a potential site to occur branching. In this case, the silicon fibers in the modified eutectic are prone to bend, curve and split to give rise to a further finer structure. As a consequence, the morphology and size of α-Al primary phase and eutectic silicon particles are evidently improved with the addition of RE.
3.2. Tensile properties Table 3 presents the tensile properties including tensile strength, yield strength, elongation and hardness of A357 alloys with different additions of RE under as-cast and T6 conditions. It is evident that the tensile strength, yield strength, elongation and hardness of A357 modified alloys in both as-cast condition and T6 condition show a significant improvement compared to the unmodified alloy. With the addition of 0.2% RE, the tensile strength, yield strength, elongation and hardness of the A357 modified alloy reach 228.1 MPa, 160.2 MPa, 2.97% and 80 HB in ascast condition, 349.1 MPa, 312.2 MPa, 3.89% and 120 HB in T6 condition, respectively. They are respectively 15.4%, 4.5%, 25.9% and 6.3% higher than those of the unmodified alloy under as-cast condition, and 11.2%, 8.4%, 34.2% and 8.3% higher than that of the unmodified alloy under T6 condition, in particularly, elongation, and it is agreement with the previous study [23]. The advantages of morphology and size of the α-Al primary phase and eutectic silicon particles as well as SDAS are responsible for the improvement of tensile properties of the A357 modified alloys [22,26].
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3.3. Fractography Fig. 5 exhibits the SEM fractographs of A357 alloys tensile samples with different additions of RE under as-cast and T6 heat treatment conditions. As can be seen, the fracture surfaces of the A357 unmodified alloy tensile samples in both as-cast and T6 conditions show a clear brittle fracture nature, and the quasicleavage feature can be observed, as shown in Fig. 5(a and e), leading to lower elongation value. After T6 heat treatment, few dimples exhibit in the fracture surfaces of the A357 unmodified alloy tensile samples. It is noted that the addition of RE obviously increases the number of dimples, and the fracture surfaces of the A357 modified alloy tensile samples with the addition of 0.1 wt% RE display a mixed quasi-cleavage and dimple morphology. With the RE addition increasing, the number of dimples sharply increase. When the addition of RE reach 0.2 wt%, the SEM fractographs of the A357 modified alloy tensile tested samples exhibit an obvious morphology of dimple fracture, and the dimples are very deep and distributed uniformly with high density, as shown in Fig. 5(c and g), resulting in a significant improvement of elongation. In addition, the fracture surface of the T6 heat treated alloy also indicates a much more ductile failure mode compared to that of the as-cast alloy. With the addition of RE up to 0.3 wt%, the SEM fractographs give rise to deterioration. The EDS spectra in Fig. 6(b), presenting the presence of alloying elements in the intermetallic compounds, implies the existence of the intermetallic compounds containing Al and RE in the fracture surface of the modified alloy. The intermetallic compounds
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containing Al and RE are prone to act as crack initiators and weaken the mechanical properties of the A357 modified alloy [4,23]. Fig. 7 shows the optical micrographs of side views of fractured A357 alloys tensile tested samples with and without modification. It can be evidently seen from Fig. 7(a) that many elongated eutectic silicon particles present in the microstructure of the A357 unmodified alloy. The eutectic silicon phases as a brittle phase are easily to crack and separate from Al matrix under applied loads, particular the elongated eutectic silicon particles, as they are the main sources of stress concentration [27–29]. For the A357 unmodified alloy, the elongated eutectic silicon particles are prone to rupture compared to the modified alloy with finer eutectic silicon particles, as indicated by the arrows in Fig. 7(a). On the other hand, the A357 unmodified alloy has larger SDAS value than that of the A357 modified alloy. The elongated eutectic silicon particles cluster along both cell and grain boundaries so that there is a nearly continuous wall of eutectic silicon particles around the dendrite cell. The dendrite cells behave similar to grains and strong interaction between particles and slip bands giving rise to at the cell boundaries when the plastic deformation generates. The final fracture paths then tend to pass through the eutectic silicon particles. As a consequence, the tensile sample without modification shows a transgranular fracture mode. For the A357 modified alloy, the smaller SDAS value and finer eutectic silicon particles make the grain cell boundaries more discontinuous, compared with the A357 unmodified alloy. Thereby, a stronger interaction between slip bands and plastic flow produces in the grain boundaries. The fracture of eutectic silicon particles generates in the grain boundaries, and the final fracture path tends to pass through the eutectic phase along the grain boundaries of the α-Al primary phases [27,30]. Thereby, the fracture take places mostly by dimple rupture with cracked eutectic silicon particles, and it exhibits an intergranular fracture mode, leading to superior ductility [31].
4. Conclusions In the present work, the effects of RE addition on the microstructures characteristics, tensile properties and fracture behavior of A357 alloy under as-cast and T6 conditions were investigated. Based on the experimental results obtained, the following conclusions were drawn:
Fig. 4. X-ray diffraction patterns of the A357 alloys with different additions of RE: (a) unmodified; (b) 0.1 wt% RE; (c) 0.2 wt% RE; (d) 0.3 wt% RE.
(1) The addition of RE greatly reduced the sizes of the α-Al primary phase and eutectic silicon particles as well as SDAS value. And the morphology of eutectic silicon particles was also clearly improved, especially in T6 heat treatment. When the addition of RE up to 0.2 wt%, compared with the unmodified alloy under ascast and T6 conditions, the reductions in the size of α-Al primary
Table 3 Tensile properties of the A357 alloys with different additions of RE under as-cast and T6 heat treatment conditions. Sample
Alloy
as-cast
Alloy Alloy Alloy Alloy Alloy Alloy Alloy Alloy
T6
1 (unmodified) 2 3 4 1 (unmodified) 2 3 4
Tensile strength (MPa)
Yield strength (MPa)
Elongation (%)
Hardness (HBS)
1937 9.2 208.5 7 10.5 228.17 11.6 226.0 7 10.8 3107 13.7 322.3 7 14.1 349.17 15.2 324.2 7 14.5
1537 10.6 1527 10.3 160.2 7 10.9 1537 10.2 286 7 11.5 290 7 11.7 312.2 7 12.0 287 7 11.3
2.2 7 0.2 2.75 7 0.3 2.977 0.4 2.83 7 0.3 2.56 7 0.2 3.127 0.4 3.89 7 0.6 3.36 7 0.5
75 78 78 77 80 79 79 75 110 710 115 711 120 712 116 710
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Fig. 5. SEM fractographs of A357 alloy tensile samples with different additions of RE: (a and e) unmodified, Alloy 1; (b and f) 0.1 wt% RE, Alloy 2; (c and g) 0.2 wt% RE, Alloy 3; (d and h) 0.3 wt% RE, Alloy 4; (a–d) as-cast condition and (e–h) T6 heat treatment condition.
phase and SDAS value were 48.1%, 162.2%, and 26.6%, 84.4%, respectively, and the aspect ratio of eutectic silicon particles decreased 142% and 174%, respectively.
(2) The addition of RE significantly improved the tensile properties of A357 alloy owing to the remarkable improvement in microstructure. Compared to the unmodified A357 alloy, the
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Fig. 6. SEM fractograph and EDS analysis of the intermetallic compound in the A357 modified alloy with 0.3 wt% RE: (a) fractograph and (b) EDS of the intermetallic compound.
Fig. 7. Optical micrographs of side views of fractured A357 alloy tensile samples with different additions of RE: (a) unmodified and (b) 0.2 wt% RE.
corresponding tensile strength, yield strength, elongation and hardness of the A357 modified alloy with the addition of 0.2 wt% Re increased 15.4%, 4.5%, and 6.3%, 25.9% in as-cast condition, and 11.2%, 8.4%, 34.2% and 8.3% in T6 condition, respectively. (3) The fracture path of the tensile samples of the A357 unmodified alloy passed through the eutectic silicon particles, and its fracture surfaces exhibited a clear brittle fracture nature as a transgranular fracture mode. For the A357 modified alloy, the fracture path passed through the eutectic phase along the grain boundaries of the α-Al primary phases, and the fracture took place by dimple rupture with cracked eutectic silicon particles, and finally showed an intergranular fracture mode.
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Effects of rare earth elements addition on microstructures, tensile properties and fractography of A357 alloy Wenming Jiang a,b,n, Zitian Fan a, Yucheng Dai a, Chi Li a a b
State Key Lab of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China School of Mechanical & Electrical Engineering, Wuhan Institute of Technology, Wuhan 430073, PR China
ar t ic l e i nf o
a b s t r a c t
Article history: Received 10 December 2013 Received in revised form 1 January 2014 Accepted 3 January 2014 Available online 10 January 2014
The effects of rare earth (RE) containing Ce and La elements addition on the microstructures characteristics, tensile properties and fracture behavior of A357 alloy under as-cast and T6 conditions were systematically investigated in this study. Obtained results showed that the addition of RE obviously reduced the sizes of the α-Al primary phase and eutectic silicon particles as well as SDAS value and improved the morphology of eutectic silicon particles. The optimum level of added RE content were 0.2 wt%, and the aspect ratio of eutectic silicon particles of the A357 modified alloy under as-cast and T6 conditions decreased 142% and 174%, respectively, compared with the unmodified alloy. In addition, the addition of RE greatly improved the tensile properties of A357 alloy as result of the significant improvement in microstructure, especially in elongation under T6 condition. The fracture surfaces of the A357 unmodified alloy tensile samples showed a clear brittle fracture nature, and its fracture path passed through the eutectic silicon particles and displayed a transgranular fracture mode, leading to poorer ductility. The fracture path of the A357 modified alloys passed through the eutectic phase along the grain boundaries of the α-Al primary phase, and the fracture generated by dimple rupture with cracked eutectic silicon particles, and it showed an intergranular fracture mode, resulting in superior ductility. & 2014 Elsevier B.V. All rights reserved.
Keywords: A357 aluminum alloy Rare earth Microstructure characteristics Tensile properties Fractography
1. Introduction Currently, the aluminum–silicon (Al–Si) cast alloys are extensively used in the aerospace and automotive industries because of its many advantages including excellent castability, corrosion resistance, high strength to weight ratio as well as low coefficient of thermal expansion, etc [1–3]. The mechanical properties of aluminum–silicon cast alloys mainly depend on the chemical composition and microstructure, etc [4,5]. And the microstructure of the Al–Si cast alloys mainly consists of α-Al primary phase, Al–Si eutectic, intermetallics and other precipitate phases. The morphology and size of α-Al primary phase and Al–Si eutectic have a significant effect on mechanical properties of the Al–Si cast alloys. The tensile properties of the Al–Si cast alloys, especially in the ductility, are mainly controlled by the dendrites cell size of α-Al primary phase. Moreover, the eutectic silicon particles also play an important role in the fracture behavior and tensile ductility of Al– Si cast alloys [6]. Generally, in the normal casting condition, α-Al primary phases grow coarse dendrites, and the eutectic silicon n Corresponding author at: Huazhong University of Science and Technology, State Key Lab of Materials Processing and Die & Mould Technology, Luoyu, Wuhan 430074, PR China. Tel.: þ 86 27 87540094; fax: þ 86 27 87558252. E-mail address: [email protected] (W. Jiang).
0921-5093/$ - see front matter & 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2014.01.009
particles exhibit coarse acicular needles. As a result, the mechanical properties of Al–Si cast alloys are remarkably weaken. Therefore, there are many methods to improve the morphology and size of α-Al primary phase and eutectic silicon particles, such as chemical modification [7–9], outfield modification [10,11] as well as thermal modification [12–14], etc. The chemical modification is regarded as an economical and effective modification method for the improvement of the morphology and size of α-Al primary phase and eutectic silicon particles. Generally, many chemical elements are known to be used in the chemical modification method, such as Ti, B, Sr, Na, Sb and rare earth (RE) elements including Ce, La, Yb, Eu and Sc [15– 18], etc. However, many chemical elements are only responsible for the modification of α-Al primary phase from coarse dendrites to fine grains, such as Ti, B, etc, or for the modification of eutectic silicon particles from coarse acicular needles to fine fibrous structure, such as Sr, Na, Sb, etc. The RE elements containing Ce and La not only can fine α-Al primary phase, but also can fine eutectic silicon particles [1,19]. Furthermore, the hydrogen content in the aluminum molten metal can also be decreased. Nevertheless, few literatures systematically reported that the effects of RE addition containing Ce and La elements on the microstructures characteristics, tensile properties and fracture behavior of A357 alloy, especially in investigations of the quantitative metallography of microstructure including α-Al primary
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Table 1 The nominal chemical composition of experimental alloys (wt%). Alloy Alloy Alloy Alloy Alloy
1 (unmodified) 2 3 4
Si
Mg
Ti
Fe
RE
Al
6.75 6.75 6.75 6.75
0.63 0.63 0.63 0.63
0.14 0.14 0.14 0.14
0.084 0.084 0.084 0.084
0 0.1 0.2 0.3
Bal. Bal. Bal. Bal.
tensile specimens, which were prepared according to the ASTM E8 standard. The solution treatment was firstly performed at 540 1C for 8 h, and then quenched into hot water at 80 1C. The aging treatment was then carried out at 165 1C for 6 h, and the samples were finally cooled in air. The tensile tests were carried out by using the AG-IC100KN universal testing machine at a crosshead speed of 1.5 mm/min. The hardness of A357 alloy was measured using a HB-3000 hardness test machine. The fractured surfaces of tensile samples were observed and analyzed using the JSM-7600F scanning electron microscope (SEM) equipped with facilities for energy dispersive spectroscopy (EDS). Phases analyses were performed using a X-ray diffraction (XRD) analysis with Cu Kα radiation.
Fig. 1. Shape and dimensions of tensile specimens (unit: mm).
3. Results and discussion phase, secondary dendrite arm spacing (SDAS) and eutectic silicon particles as well as the correlation of microstructure with mechanical properties and fracture behavior of A357 alloy. In this study, the mixed RE containing Ce and La elements were added into A357 Al–Si alloy. The aim of present work at systematically investigated the effects of mixed RE elements addition on the microstructures characteristics including the morphology and size of α-Al primary phase and eutectic silicon phase as well as SDAS, tensile properties and fracture behavior of A357 Al–Si alloy under as-cast and T6 conditions. Moreover, the correlation of microstructure with mechanical properties and fracture behavior of A357 alloy were also studied.
2. Experimental procedures The chemical composition of A357 Al–Si alloy used in this study is shown in Table 1, named Alloy 1. The RE containing 65 wt% Ce and 35 wt% La elements was added as Al-10%RE master alloy. Firstly, the A357 aluminum ingot was placed inside a graphite crucible and melted at 730 1C by using the electrical resistance furnace. The Al-10%RE master alloys with different additions (0.1, 0.2 and 0.3) were then added into the molten melt at 740 1C, named Alloy 2, Alloy 3 and Alloy 4, respectively. Subsequently, the molten metal was refined using argon gas for 15 min using a rotary graphite degasser when the temperature reached 750 1C, and the slag was then skimmed. When the melting temperature was 720 1C, the molten metal was finally poured into a metal mould, which was preheated for 250 1C. The metallographic samples were etched using 0.5% hydrofluoric acid solution after polishing. Microstructures were observed using the OLYMPUS-MG3 metallographic microscope. The secondary dendrite arm spacing (SDAS), average length of silicon particles as well as average width of silicon particles were measured by using the ImageTool metallographic analysis software. The measurement was done on 50 different areas of each microstructure in order to minimize the errors. The aspect ratio of silicon particles was calculated according to the ratio of the average length of silicon particles to the average width of silicon particles. The grain size of the α-Al primary phase was defined according to the following equation [20]: pffiffiffiffiffiffiffiffi D ¼ 2 A=π ð1Þ where A is the average area of the α-Al primary phase, which was measured using the ImageTool software. The as-cast and T6 heat treatment specimens were subjected to investigate in this study. Fig. 1 depicts the shape and dimensions of
3.1. Microstructural characterization Fig. 2 shows the optical microstructures of A357 alloys with different additions of RE under as-cast and T6 heat treatment conditions, and the optical microstructures of the eutectic zone are also exhibited in order to demonstrate a substantial microstructure difference in the morphology and size of eutectic silicon particles. In the microstructures of the A357 unmodified alloys under ascast and T6 conditions, it is evident that the coarse dendrite are observed, as shown in Fig. 2(a and e). Meanwhile, it can also be seen from Fig. 2(a and e) that the plate-like silicon particles are showed. In contrast, the microstructures are significantly improved when the A357 alloys are modified with the addition of RE, and the coarse dendrites are fined and the plate-like silicon particles become fine fibrous structure. Because the spheroidization efficiency of eutectic silicon particles mainly depends on the initial size of eutectic silicon particles [1], the eutectic silicon particles of the A357 modified alloys are obviously spheroidized and homogeneously distributed in the grain boundary after T6 heat treatment, as shown in Fig. 2(f–h). Nevertheless, when the RE addition is 0.1 wt%, some the plate-like silicon particles can also be seen in microstructure. With the addition of Re up to 0.2 wt%, the morphology and size of α-Al primary phase and eutectic silicon particles are further improved, and the α-Al primary phases display a finer structure, and the eutectic silicon particles exhibit a granular and globular structure, as shown in Fig. 2(g). With the further increase of addition of RE content, the morphology and size of α-Al primary phase and eutectic silicon particles are deteriorated, and some eutectic silicon particles with acicular needles morphology are observed, as shown in Fig. 2(h). Table 2 depicts the quantitative metallography results of microstructure features including α-Al primary phase, eutectic silicon particles as well as SDAS of A357 alloys with different RE additions. According to the results obtained from the microstructural parameters in Table 2, it is clear that the addition of RE remarkably reduces the size of α-Al primary phase and SDAS value. Meanwhile, the average length, average width and aspect ratio of eutectic silicon particles are also greatly decreased compared to the unmodified condition, and the morphology of eutectic silicon particles looks more round. With the 0.2 wt% addition of RE, the reductions under as-cast and T6 heat treatment conditions in the size of α-Al primary phase and SDAS value are 48.1%, 162.2%, and 26.6%, 84.4%, respectively, compared to the unmodified condition. Furthermore, the aspect ratio of eutectic silicon particles of the A357 modified alloy under as-cast and T6 conditions decrease 142% and 174%, respectively, compared with the unmodified alloy.
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Fig. 2. Optical microstructures of A357 alloys with different additions of RE: (a and e) unmodified, Alloy 1; (b and f) 0.1 wt% RE, Alloy 2; (c and g) 0.2 wt% RE, Alloy 3; (d and h) 0.3 wt% RE, Alloy 4; (a–d) As-cast condition and (e–h) T6 heat treatment condition.
It can be noted that the T6 heat treatment has a profound effect on the spheroidization of eutectic silicon, and it is consist with the previous studies [21–23].
The mechanism of RE modification on the α-Al primary phase can be explained by follows. The eutectic reaction of RE occurs at 637–642 1C, and the reaction equation is presented in the
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Table 2 Quantitative metallography results for the microstructures of A357 aluminum alloy with different RE additions. Sample
Alloy
as-cast
Alloy Alloy Alloy Alloy Alloy Alloy Alloy Alloy
T6
1 (unmodified) 2 3 4 1 (unmodified) 2 3 4
Grain size of α-Al primary phase (μm)
SDAS (μm)
Average length of silicon particles (μm)
Average width of silicon particles (μm)
Aspect ratio of silicon particles
118.82 7 7.1 100.05 7 4.1 80.22 7 2.8 97.81 7 3.5 117.89 7 6.8 110.277 4.7 93.157 3.9 106.107 4.2
37.65 7 2.8 23.16 7 2.0 14.36 7 1.1 23.78 7 2.0 38.78 7 2.5 31.97 7 2.9 21.03 7 2.0 25.83 7 2.8
39.517 3.6 24.777 3.1 14.86 7 1.4 19.95 7 2.0 23.717 2.8 17.067 1.7 10.707 1.2 15.09 7 1.9
3.79 7 0.7 3.17 0.2 3.42 7 0.3 3.667 0.4 3.79 7 0.7 4.517 0.8 4.65 7 0.7 4.677 0.8
10.4 7 1.8 8.0 7 1.4 4.3 7 0.5 5.5 7 0.6 6.3 7 0.9 3.8 7 0.3 2.3 7 0.2 3.2 7 0.4
Fig. 3. SEM microstructure and EDS analysis of the A357 modified alloy with 0.2 wt% RE: (a) morphology and (b) EDS of the intermetallic compound containing Al, Si and RE.
following: L-ðα AlÞ þ Al4 ðLa; CeÞ
ð2Þ
The precipitation phases of α-Al due to the eutectic reaction supply a large number of nucleation cores for the α-Al primary phases. Meanwhile, the intermetallic compounds containing Al and Re have a very high melting point, above 1200 1C, and the lattices constant of the intermetallic compounds are similar with α-Al. The EDS analysis and X-ray diffraction patterns of the A357 modified alloy also confirm the reaction products of above eutectic reaction, and the intermetallic compounds containing Al and RE are observed, as shown in Figs. 3 and 4, especially in higher addition content of RE. Consequently, the intermetallic compounds can also supply some nucleation cores for the α-Al primary phases. As a result, the grain size of α-Al primary phase and SDAS are greatly decreased because of the large number of nucleation cores in the Al liquid. It is well known that the Sr also has a remarkable effect on the morphology and size of the eutectic silicon particles [1]. However, the Sr cannot modify the size and morphology of the α-Al primary phase as well as SDAS. The mechanism of RE modification on the eutectic silicon phases is might attributed to the concentration of RE elements on the surface of eutectic silicon particles, and it is similar to the mechanism of Sr modification on the eutectic silicon phases because of their adjacently diagonal relationship in the Periodic Table of the elements, and the widely accepted recent theory is the impurity-induced twinning theory [24]. In general, the Ce and La elements hardly dissolve in the α-Al or eutectic silicon phases because the atomic radius of the Ce and La elements are much larger than that of α-Al and silicon. As a result, the Ce and La elements mainly concentrate on the surface of eutectic silicon phases, and the growths of eutectic silicon phases are then
restricted. Thereby, the morphology of the eutectic silicon phases presents fine fibrous structure. The modified silicon fibers contain more twins than the unmodified silicon particles and have a rough microfaceted surface [25]. Therefore, the silicon fibers are crystallographically very imperfect, and each surface imperfection is a potential site to occur branching. In this case, the silicon fibers in the modified eutectic are prone to bend, curve and split to give rise to a further finer structure. As a consequence, the morphology and size of α-Al primary phase and eutectic silicon particles are evidently improved with the addition of RE.
3.2. Tensile properties Table 3 presents the tensile properties including tensile strength, yield strength, elongation and hardness of A357 alloys with different additions of RE under as-cast and T6 conditions. It is evident that the tensile strength, yield strength, elongation and hardness of A357 modified alloys in both as-cast condition and T6 condition show a significant improvement compared to the unmodified alloy. With the addition of 0.2% RE, the tensile strength, yield strength, elongation and hardness of the A357 modified alloy reach 228.1 MPa, 160.2 MPa, 2.97% and 80 HB in ascast condition, 349.1 MPa, 312.2 MPa, 3.89% and 120 HB in T6 condition, respectively. They are respectively 15.4%, 4.5%, 25.9% and 6.3% higher than those of the unmodified alloy under as-cast condition, and 11.2%, 8.4%, 34.2% and 8.3% higher than that of the unmodified alloy under T6 condition, in particularly, elongation, and it is agreement with the previous study [23]. The advantages of morphology and size of the α-Al primary phase and eutectic silicon particles as well as SDAS are responsible for the improvement of tensile properties of the A357 modified alloys [22,26].
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3.3. Fractography Fig. 5 exhibits the SEM fractographs of A357 alloys tensile samples with different additions of RE under as-cast and T6 heat treatment conditions. As can be seen, the fracture surfaces of the A357 unmodified alloy tensile samples in both as-cast and T6 conditions show a clear brittle fracture nature, and the quasicleavage feature can be observed, as shown in Fig. 5(a and e), leading to lower elongation value. After T6 heat treatment, few dimples exhibit in the fracture surfaces of the A357 unmodified alloy tensile samples. It is noted that the addition of RE obviously increases the number of dimples, and the fracture surfaces of the A357 modified alloy tensile samples with the addition of 0.1 wt% RE display a mixed quasi-cleavage and dimple morphology. With the RE addition increasing, the number of dimples sharply increase. When the addition of RE reach 0.2 wt%, the SEM fractographs of the A357 modified alloy tensile tested samples exhibit an obvious morphology of dimple fracture, and the dimples are very deep and distributed uniformly with high density, as shown in Fig. 5(c and g), resulting in a significant improvement of elongation. In addition, the fracture surface of the T6 heat treated alloy also indicates a much more ductile failure mode compared to that of the as-cast alloy. With the addition of RE up to 0.3 wt%, the SEM fractographs give rise to deterioration. The EDS spectra in Fig. 6(b), presenting the presence of alloying elements in the intermetallic compounds, implies the existence of the intermetallic compounds containing Al and RE in the fracture surface of the modified alloy. The intermetallic compounds
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containing Al and RE are prone to act as crack initiators and weaken the mechanical properties of the A357 modified alloy [4,23]. Fig. 7 shows the optical micrographs of side views of fractured A357 alloys tensile tested samples with and without modification. It can be evidently seen from Fig. 7(a) that many elongated eutectic silicon particles present in the microstructure of the A357 unmodified alloy. The eutectic silicon phases as a brittle phase are easily to crack and separate from Al matrix under applied loads, particular the elongated eutectic silicon particles, as they are the main sources of stress concentration [27–29]. For the A357 unmodified alloy, the elongated eutectic silicon particles are prone to rupture compared to the modified alloy with finer eutectic silicon particles, as indicated by the arrows in Fig. 7(a). On the other hand, the A357 unmodified alloy has larger SDAS value than that of the A357 modified alloy. The elongated eutectic silicon particles cluster along both cell and grain boundaries so that there is a nearly continuous wall of eutectic silicon particles around the dendrite cell. The dendrite cells behave similar to grains and strong interaction between particles and slip bands giving rise to at the cell boundaries when the plastic deformation generates. The final fracture paths then tend to pass through the eutectic silicon particles. As a consequence, the tensile sample without modification shows a transgranular fracture mode. For the A357 modified alloy, the smaller SDAS value and finer eutectic silicon particles make the grain cell boundaries more discontinuous, compared with the A357 unmodified alloy. Thereby, a stronger interaction between slip bands and plastic flow produces in the grain boundaries. The fracture of eutectic silicon particles generates in the grain boundaries, and the final fracture path tends to pass through the eutectic phase along the grain boundaries of the α-Al primary phases [27,30]. Thereby, the fracture take places mostly by dimple rupture with cracked eutectic silicon particles, and it exhibits an intergranular fracture mode, leading to superior ductility [31].
4. Conclusions In the present work, the effects of RE addition on the microstructures characteristics, tensile properties and fracture behavior of A357 alloy under as-cast and T6 conditions were investigated. Based on the experimental results obtained, the following conclusions were drawn:
Fig. 4. X-ray diffraction patterns of the A357 alloys with different additions of RE: (a) unmodified; (b) 0.1 wt% RE; (c) 0.2 wt% RE; (d) 0.3 wt% RE.
(1) The addition of RE greatly reduced the sizes of the α-Al primary phase and eutectic silicon particles as well as SDAS value. And the morphology of eutectic silicon particles was also clearly improved, especially in T6 heat treatment. When the addition of RE up to 0.2 wt%, compared with the unmodified alloy under ascast and T6 conditions, the reductions in the size of α-Al primary
Table 3 Tensile properties of the A357 alloys with different additions of RE under as-cast and T6 heat treatment conditions. Sample
Alloy
as-cast
Alloy Alloy Alloy Alloy Alloy Alloy Alloy Alloy
T6
1 (unmodified) 2 3 4 1 (unmodified) 2 3 4
Tensile strength (MPa)
Yield strength (MPa)
Elongation (%)
Hardness (HBS)
1937 9.2 208.5 7 10.5 228.17 11.6 226.0 7 10.8 3107 13.7 322.3 7 14.1 349.17 15.2 324.2 7 14.5
1537 10.6 1527 10.3 160.2 7 10.9 1537 10.2 286 7 11.5 290 7 11.7 312.2 7 12.0 287 7 11.3
2.2 7 0.2 2.75 7 0.3 2.977 0.4 2.83 7 0.3 2.56 7 0.2 3.127 0.4 3.89 7 0.6 3.36 7 0.5
75 78 78 77 80 79 79 75 110 710 115 711 120 712 116 710
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Fig. 5. SEM fractographs of A357 alloy tensile samples with different additions of RE: (a and e) unmodified, Alloy 1; (b and f) 0.1 wt% RE, Alloy 2; (c and g) 0.2 wt% RE, Alloy 3; (d and h) 0.3 wt% RE, Alloy 4; (a–d) as-cast condition and (e–h) T6 heat treatment condition.
phase and SDAS value were 48.1%, 162.2%, and 26.6%, 84.4%, respectively, and the aspect ratio of eutectic silicon particles decreased 142% and 174%, respectively.
(2) The addition of RE significantly improved the tensile properties of A357 alloy owing to the remarkable improvement in microstructure. Compared to the unmodified A357 alloy, the
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Fig. 6. SEM fractograph and EDS analysis of the intermetallic compound in the A357 modified alloy with 0.3 wt% RE: (a) fractograph and (b) EDS of the intermetallic compound.
Fig. 7. Optical micrographs of side views of fractured A357 alloy tensile samples with different additions of RE: (a) unmodified and (b) 0.2 wt% RE.
corresponding tensile strength, yield strength, elongation and hardness of the A357 modified alloy with the addition of 0.2 wt% Re increased 15.4%, 4.5%, and 6.3%, 25.9% in as-cast condition, and 11.2%, 8.4%, 34.2% and 8.3% in T6 condition, respectively. (3) The fracture path of the tensile samples of the A357 unmodified alloy passed through the eutectic silicon particles, and its fracture surfaces exhibited a clear brittle fracture nature as a transgranular fracture mode. For the A357 modified alloy, the fracture path passed through the eutectic phase along the grain boundaries of the α-Al primary phases, and the fracture took place by dimple rupture with cracked eutectic silicon particles, and finally showed an intergranular fracture mode.
Acknowledgments The authors gratefully acknowledge the financial support received from the National Natural Science Foundation of China (No. 51204124), the China Postdoctoral Science Foundation (No. 2012M511610) and the Analytical & Testing Center of Huazhong University of Science and Technology (HUST). References [1] O. ElSebaie, A.M. Samuel, F.H. Samuel, H.W. Doty, Mater. Sci. Eng. A 480 (2008) 342–355.
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