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Effects of erbium modification on the microstructure and mechanical properties of A356 aluminum alloys
Materials Science & Engineering A 626 (2015) 102–107
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effects of erbium modification on the microstructure and mechanical properties of A356 aluminum alloys Z.M. Shi n, Q. Wang, G. Zhao, R.Y. Zhang School of Materials Science and Engineering, Inner Mongolia University of Technology, 010051 Hohhot, China
art ic l e i nf o
a b s t r a c t
Article history: Received 17 November 2014 Received in revised form 15 December 2014 Accepted 16 December 2014 Available online 24 December 2014
The effects of erbium (Er) modification on the microstructure and mechanical properties of A356 aluminum alloys were investigated using optical microscope, X-ray diffraction, scanning electronic microscope and mechanical testing. Experimental results show that additions of Er refined the α-Al grains and eutectic Si phases in its as-cast state; the addition of 0.3 wt% of Er has the best effects on them. The Fe-containing Al3Er phases were introduced by the modifications; by a T6 treatment, the eutectic Si phases were further sphereodized; the large Al3Er and β-Al5FeSi phases were changed into fine particles and short rods; which enhanced the hardness of the alloys. The highest strength and elongation were obtained for the 0.3 wt% of Er-modified and T6-treated A356 alloy. & 2015 Published by Elsevier B.V.
Keywords: Mechanical characterization Microanalysis X-ray diffraction Aluminum alloys Casting Grain refinement
1. Introduction The A356 aluminum alloy (Al–7Si–0.35Mg) has been widely used to manufacture assemblies in the automotive, military and aerospace industries. To improve the mechanical properties of the alloy, grain refinement and Si modification techniques are used. It has been widely reported that the additions of Al–Ti, Al–Ti–B and Al–Ti–C master alloys in the melts can refine the primary α-Al grains as they can precipitate the TiAl3, TiB2 and TiC fine particles, which act as the heterogeneous nuclei to promote the α-Al grains crystallizing [1–4]. The Al–Sr master alloy was used to modify the morphology of the eutectic Si and it changes the morphology from needle/plate to fibrous/globular forms because the Sr elements enrich in front of the growing Si phases to inhibit their preferential oriented growth [2–8]. Rare earth (RE) elements possess special physicochemical activity; their refinement and modification effects have been found in Al–Si–Mg alloys. The addition of La up to 1.0 wt% decreased the eutectic temperature and yielded Al–Ti–La–Mg and Al–Si–La intermetallics in the A356 alloy. The addition of 1.0 wt% La resulted in a full modification; however, the tensile strength was not improved [9]. The addition of Ce also decreased
n
Corresponding author. E-mail address: [email protected] (Z.M. Shi).
http://dx.doi.org/10.1016/j.msea.2014.12.062 0921-5093/& 2015 Published by Elsevier B.V.
the eutectic temperature; however, there is no direct relationship among the silicon morphology, modification efficiency and eutectic temperature. A Ce addition of less than 1.0 wt% did not modify the eutectic silicon morphology and yielded Ce–23Al–22Si and Al– 17Ce–12Ti–2Si–2Mg intermetallics; the tensile strength was not enhanced, whereas the elongation was increased with an increase of the Ce content up to 0.6 wt% [10]. On the contrary, 0.1–1.0 wt% of Ce-rich mischmetal modification coarsened the α-Al grains because the Al11RE3, Al3RE and other RE-containing compounds cannot act as heterogeneous nucleating sites for the primary a-Al grains nucleating. A minor amount of Ce-rich mischmetal (o0.2 wt%) resulted in partial modification and additions of more than 0.3 wt% yielded the full modification; however, the tensile strength and ductility were decreased because of the coarsened aAl grains and the RE-containing compounds [11]. The size of α-Al grains was increased by the Y addition due to the precipitated Al3Y compounds, and the Sr–Y complex modification yielded a refinement effect. Moreover, the eutectic Si was changed into either a granular or flaky shape in the as-cast state and most of them were further transformed into the smaller particles by the T6 heat treatment [12]. The Sc addition refined the primary α-Al grains because the precipitated Al3Sc can act as heterogeneous nucleation sites; however, it has a much weaker effect on the eutectic silicon morphology [13]. Adding 0.5 wt% of Al–5Ti–0.25C–2RE master alloy has a satisfactory refining effect; however, further increasing the content did not significantly change the effect [14].
Z.M. Shi et al. / Materials Science & Engineering A 626 (2015) 102–107
As is well known, the RE elements can be divided into light and heavy series; the physicochemical properties between them are different to some extent. Most previous research explored the modification effect of the light RE elements as mentioned above. Therefore, the present work is to investigate the effects of heavy RE element of erbium (Er) on the microstructure and mechanical properties of the A356 aluminum alloy in as-cast and T6 states and to uncover the modification mechanism.
2. Materials and methods A356 aluminum alloy ingot (7.1 wt% Si, 0.45 wt% Mg, 0.12 wt% Fe) and master alloy (Al–10 wt% Er) were used as raw materials. The A356 ingot (20 kg) was put into an induction furnace to melt at 760 1C. Different amounts of the master alloys were added to form five groups of samples with different Er contents (0, 0.1, 0.2, 0.3 and 0.4 wt%). To eliminate gases and inclusions in the melts, An N2 gas was injected into bottom of the furnace over 5 min by means of a graphite pipe with a honeycomb end to generate small bubbles. When the temperature decreased to 730 1C, the melts was stewed for 5 min and then poured into a preheated steel mold (250 1C, wall thickness 20 mm) through a pre-heated porous ceramic filter with a thickness of 20 mm and the pore size of 1 mm. The size of the formed ingots is 30 (thickness) 120 (height) 150 (width) mm. The ingots were cooled to room temperature in the mold. The samples were then treated with a standard T6 schedule (soaking at 535 1C for 4 h, quenched in water) (100 1C, soaking at 200 1C for 4 h and cooled in air). The ingots were cut along the longitudinal symmetric plane by an electric spark cutting method and machined into standard tensile bars with a working diameter of 10 mm. The microstructure was observed with a metallurgical microscope (OLYMPUS-GX51), and the average granularity and the average aspect ratio of the Si phases were statistically calculated by quantitative analysis software against 20 fields. The morphology of the phases was detected by a scanning electronic microscope (SEM, S3400-N, Hitachi) with an energy dispersive spectrometer (EDS, 7021-H, HORIBA). The surfaces of the samples used for the observation were polished and etched by an HF (5 wt %)-alcohol solution. The phase composition was analyzed by an Xray diffractometer (XRD, D/MAX-2500/PC, PIGAKV) using copper Ka radiation at 40 kV, 200 mA and at a scanning speed of 11/min. The mechanical properties were examined using a material test machine (Jinan SHT4605) with a loading speed of 1.0 mm/min. The hardness (HV) was tested by a hardness tester (HVS-30Z\LCD, Huayin). Five samples for each group were used to measure the tensile properties and hardness.
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3.1.2. T6 treatment By a T6 treatment, the plate-like eutectic Si phases were broken up and parts of the short rods were produced in the unmodified alloy (Fig. 2a). However, this is not sufficient for the refinement. For the Er-modified alloys, the Si phases were changed into short rods and very fine particles, which uniformly distributed in the matrix. The 0.3 wt% Er modification had the best effects on the Si phases (Fig. 2d); the average size of the particles is 5 μm approximately. When the content arrived at 0.4 wt%, the particle size was in turn increased. Table 1 shows the variations of the size of the phases with the modifications by a statistically analysis. It can be observed that the Er modifications greatly refined the microstructure and the T6 treatment had a significant effect on it. The best roundness of the Si phases was obtained for the 0.2–0.4 wt% Er-modified and T6treated alloys. The 0.3 wt% Er addition yielded the finest α-Al grains and Si particles. 3.2. Phase compositions The Al3Er phases were introduced by the modification (Fig. 3), which were mainly distributed in the forms of needles and bulks in the as-cast state (Fig. 4a). Through the T6 treatment, large parts of the Al3Er phases were decomposed into fine particles and short rods (Fig. 4b), which uniformly dispersed in the matrix. The EDS results show that Er cannot dissolve in the α-Al and Si phases in the as-cast and T6 states (Fig. 5). Moreover, the Fe atoms dissolved in the Al3Er phases (Fig. 5c, f). Comparing Fig. 4a and b, it can be found that the large and needle-like β-Al5FeSi phase [15] at Point G in the as-cast alloy was changed into the fine particles (Point H) by the modification and the T6 treatment. However, the ironcontaining intermetallic phases in the unmodified Al–Si alloys are generally unaffected by heat treatments [16]. 3.3. Mechanical properties The variations of the tensile strength, elongation and hardness with the Er content are shown in Figs. 6–8. In the as-cast state, the hardness increased and the elongation decreased with an increase of the Er content. The tensile strength presented a slightly higher value (195.3 MPa) when 0.3 wt% Er was added. Through the T6 treatment, the hardness increased slowly; the tensile strength and elongation presented a trend of first increasing and then decreasing with an increase of the Er content. With 0.3 wt% Er addition, the tensile strength and elongation were 311.2 MPa and 8.6%, respectively. However, they were greatly decreased when the Er content was increased to 0.4 wt%.
4. Discussion 4.1. Effect of Er on the refinement of α-Al grains 3. Results 3.1. Microstructure 3.1.1. As-cast Fig. 1 shows the microstructure of the modified A356 alloys in their as-cast state. It can be observed that the coarse plate-like eutectic Si phases existed in the matrix for the unmodified alloy (Fig. 1a). The Er modification refined the α-Al grains and the eutectic Si phases (Fig. 1b–e). Adding 0.3 wt% Er produced the finest α-Al grains and had the best modification effect on the Si phases (Fig. 1d), which were transformed into fine particles and short rods. However, the 0.4 wt% addition in turn coarsened the αAl grains (Fig. 1e).
The A356 aluminum alloy is one of the hypoeutectic alloys. With the nucleation and growth of the primary α-Al grains, a small fraction of the Si and Mg atoms dissolve into the grains, and a large fraction of them and the other elements are enriched in front of the solids, which yield a composition supercooling and thus can refine the α-Al grains to some extent [11]. Moreover, some special compounds in the melts such as TiAl3, TiB2, TiC and Al3Sc can act as the heterogeneous nuclei to facilitate refinement of the grains [1–4,13]. Table 2 shows the physicochemical parameters of related elements and their compounds [17–19]. It can be noticed that Er has a larger atomic weight and atomic radius that those of Al and Si elements and has lower solubility in the α-Al crystals. Al3Er has
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Fig. 1. Microstructure of modified A356 alloys in as-cast state: (a) unmodified; (b) 0.1 wt%; (c) 0.2 wt%; (d) 0.3 wt%; (e) 0.4 wt%.
the same crystal structure (FCC) and a closely similar lattice constant (a ¼0.4215 nm) compared to the α-Al crystal (a ¼0.4049 nm). Moreover, the Al þAl3Er eutectics form at approximately 655 1C, which is higher than that of AlþSi (577 1C). When adding the Al–Er master alloy into the melts, the Er atoms and the very fine Al3Er particles can be precipitated [20]. The Er atoms will largely enrich in front of the α-Al grains to advance their refining. Because the atomic weight of Er is much larger than that of Al and its diffusion rate is slower, the refinement effect is stronger. Another reason is that the pre-existed fine Al3Er particles in the melts can act as the heterogeneous nuclei for the α-Al grains nucleating because the Al3Er has a same crystal structure (FCC) and a closely similar lattice constant as mentioned above. With these actions, the α-Al grains can be refined. In the present conditions, the 0.3 wt% Er addition achieved the best effect on the α-Al grain refinement. The refining effect was also found in the die-cast ADC12 Al–Si–X alloy and the pure aluminum [21,22]; the size of the precipitated Al3Er particles was 2–3 μm in diameter, which were observed in the grain boundaries to inhibit the growth of the α-Al grains. However, the Al3Er phases formed in the eutectic conversion (655 1C) cannot act as the heterogeneous nuclei to advance the α-Al grains nucleating. On the other hand, increasing the Er content (0.4 wt%) in turn reduced the refinement effect. This reduction can be ascribed to
the formation of larger Al3Er particles; which lost the heterogeneous nucleation effect. The similar phenomenon was also identified in the literature [21–26]. 4.2. Effect of Er on the eutectic Si morphology When the temperature decreases bellow 577 1C, the Al þSi eutectic cells start to precipitate in the forms of needles or plates because of the preferred orientation growth of the Si phases along their 〈112〉 directions [27,28]. Once the orientation growth is inhibited by additions of the chemical modifiers, the Si phases will turn to grow in the form of small granules. It was reported that additions of the chemical modifiers easily produce twin defects in the Si crystals, the repeated twinning makes the growth reorientate, thus, the fibers Si phases can bend, curve and split to create finer particles [27,29]. Moreover, the Si particles can grow by rapping the AlP nuclei to develop into the compact particles; however, the addition of Sr makes the AlP and the oxide bifilms deactivate as favored nucleation substrates [30,31]. Meanwhile, the Sr atoms can segregate towards the Si-rich regions and the Al2Si2Sr clusters form preferentially, which enhance the interfacial energy and poison the nucleation sites [32]. The poisoning of nucleation sites and the increase of interfacial energy yield a significant undercooling of the melts and so delays the nucleation
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Fig. 2. Microstructure of modified A356 alloys in T6 state: (a) unmodified; (b) 0.1 wt%; (c) 0.2 wt%; (d) 0.3 wt%; (e) 0.4 wt%.
Table 1 Statistical analysis of size of the phases. States
Er content wt%
Equivalent size of α-Al grains (μm)
Average length of Average aspect Si phases (μm) ratio of Si phases
As-cast
0 0.1 0.2 0.3 0.4
85.6 57.3 42.6 22.2 47.1
13.6 9.2 6.5 4.6 5.8
7.3 3.6 1.9 1.5 1.4
T6
0 0.1 0.2 0.3 0.4
/ / / / /
11.4 8.5 6.3 4.8 6.5
5.4 3.2 1.2 1.1 1.1 Fig. 3. XRD pattern of 0.3 wt% Er-modified A356 alloy in the as-cast state.
of eutectic phases, resulting in the refinement of the eutectic phases [33]. The similar RE elements of La and Ce additions also yielded the undercooling [9,10]. In the present conditions, even though the Al þAl3Er eutectic transformation occurs at approximately 655 1C, trace of Er atoms still remain in the melts to the Al þSi eutectic temperature
(577 1C). As the Er atom has a smaller atomic radius (0.175 nm) and a larger atomic weight (167.26) than those (0.215 nm, 87.62) of the Sr atom, it is easier to enrich in front of the eutectic cells because of its difficulty in diffusion, which greatly poisons the orientation growth of the Si phases. Moreover, the Er atom has a more physicochemical activity than that of Sr, La and Ce atoms, which can eliminate the oxide bifilms and purify the melts. With
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Fig. 4. SEM image of 0.3 wt% Er-modified A356 alloy, (a) as-cast; (b) T6.
Fig. 5. EDS analysis of 0.3 wt% Er-modified A356 alloy in as-cast and T6 states.
Fig. 6. Tensile strength of Er-modified A356 alloys.
these two reasons, the fine particles or short rods were formed. A similar phenomenon was also found in the Er-modified Al–12.6 wt% Si hypereutectic alloys, but the action mechanism was not provided [21,26].
Fig. 7. Elongation of Er-modified A356 alloys.
When soaking at the high temperature, the Si phases were further adjusted into the fine globular shape. This is because the enriched Er atoms in front of the eutectic cells increase the
Z.M. Shi et al. / Materials Science & Engineering A 626 (2015) 102–107
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The Er modification introduced the needles, bulks and rods of Al3Er phases. By the T6 treatment, they were transformed into fine particles and short rods, which uniformly distributed in the matrix. The Er addition and T6 treatment eliminated the harmful needle-like β-Al5FeSi phases in the alloys. The 0.3 wt% Er-modification and T6 treatment yielded the highest mechanical properties of the alloy.
Acknowledgment The present work is supported by the Nature Science Foundation of Inner Mongolia of China.
Fig. 8. Hardness of Er-modified A356 alloys.
Table 2 Physicochemical parameters of related elements and their compounds. Element
Al
Si
Er
Atomic number Atomic weight Atomic Radius (nm) Crystal structure Lattice constants (nm)
13 26.98 0.1432 FCC a¼ 0.4049
68 167.26 0.1734 HCP a¼ 0.3560 c ¼0.5595
Maximum solubility in Al wt% Al–Er compounds Crystal structure of Al3Er Lattice constants of Al3Er (nm) Binary eutectic cell Eutectic temperature (oC) Composition at eutectic point (wt%)
/ / / / / / /
14 28.09 0.1462 Ortho a ¼0.4737 b ¼0.4502 c ¼ 0.2550 1.65 / / / Al þ Si 577 12
0.05 Al3Er FCC a¼ 0.4215 Alþ Al3Er 655 6
Notes: HCP: hexagonal close-packed; FCC: face-centered cubic; Ortho: Orthogonal.
interfacial energy with the α-Al grains [34], which results in sphereodization of the Si particles (Oswald ripening). More Er addition (0.4 wt%) yields more enrichment in front of the eutectic cells and causes smaller Si particles to present, which are coarsened by the high-temperature soaking [35]. 4.3. Effect of Er on the mechanical properties The size, shape and content of the Si, Al3Er and β-Al5FeSi phases greatly influence the mechanical properties of the alloys. In the ascast state, even though the Er addition refined the α-Al grains and the Si phases, especially when adding 0.3 wt% Er, the brittle Al3Er and β-Al5FeSi phases presented in the forms of needles, rods and blocks, the strength and elongation were not improved. By the T6 treatment, the Si, Al3Er and β-Al5FeSi phases were changed into fine particles and short rods, which uniformly distributed in the matrix; thus, the effects of the refinement of the α-Al grains and the Si phases were represented and so the strength and elongation were largely improved, especially for the 0.3 wt% Er-modified alloy. Moreover, the uniformly dispersed hard Al3Er and βAl5FeSi phases simultaneously enhanced the hardness of the alloys. 5. Conclusions Addition of 0.3 wt% Er had the best effect on the refinement of α-Al grains and the morphology of eutectic Si phases.
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effects of erbium modification on the microstructure and mechanical properties of A356 aluminum alloys Z.M. Shi n, Q. Wang, G. Zhao, R.Y. Zhang School of Materials Science and Engineering, Inner Mongolia University of Technology, 010051 Hohhot, China
art ic l e i nf o
a b s t r a c t
Article history: Received 17 November 2014 Received in revised form 15 December 2014 Accepted 16 December 2014 Available online 24 December 2014
The effects of erbium (Er) modification on the microstructure and mechanical properties of A356 aluminum alloys were investigated using optical microscope, X-ray diffraction, scanning electronic microscope and mechanical testing. Experimental results show that additions of Er refined the α-Al grains and eutectic Si phases in its as-cast state; the addition of 0.3 wt% of Er has the best effects on them. The Fe-containing Al3Er phases were introduced by the modifications; by a T6 treatment, the eutectic Si phases were further sphereodized; the large Al3Er and β-Al5FeSi phases were changed into fine particles and short rods; which enhanced the hardness of the alloys. The highest strength and elongation were obtained for the 0.3 wt% of Er-modified and T6-treated A356 alloy. & 2015 Published by Elsevier B.V.
Keywords: Mechanical characterization Microanalysis X-ray diffraction Aluminum alloys Casting Grain refinement
1. Introduction The A356 aluminum alloy (Al–7Si–0.35Mg) has been widely used to manufacture assemblies in the automotive, military and aerospace industries. To improve the mechanical properties of the alloy, grain refinement and Si modification techniques are used. It has been widely reported that the additions of Al–Ti, Al–Ti–B and Al–Ti–C master alloys in the melts can refine the primary α-Al grains as they can precipitate the TiAl3, TiB2 and TiC fine particles, which act as the heterogeneous nuclei to promote the α-Al grains crystallizing [1–4]. The Al–Sr master alloy was used to modify the morphology of the eutectic Si and it changes the morphology from needle/plate to fibrous/globular forms because the Sr elements enrich in front of the growing Si phases to inhibit their preferential oriented growth [2–8]. Rare earth (RE) elements possess special physicochemical activity; their refinement and modification effects have been found in Al–Si–Mg alloys. The addition of La up to 1.0 wt% decreased the eutectic temperature and yielded Al–Ti–La–Mg and Al–Si–La intermetallics in the A356 alloy. The addition of 1.0 wt% La resulted in a full modification; however, the tensile strength was not improved [9]. The addition of Ce also decreased
n
Corresponding author. E-mail address: [email protected] (Z.M. Shi).
http://dx.doi.org/10.1016/j.msea.2014.12.062 0921-5093/& 2015 Published by Elsevier B.V.
the eutectic temperature; however, there is no direct relationship among the silicon morphology, modification efficiency and eutectic temperature. A Ce addition of less than 1.0 wt% did not modify the eutectic silicon morphology and yielded Ce–23Al–22Si and Al– 17Ce–12Ti–2Si–2Mg intermetallics; the tensile strength was not enhanced, whereas the elongation was increased with an increase of the Ce content up to 0.6 wt% [10]. On the contrary, 0.1–1.0 wt% of Ce-rich mischmetal modification coarsened the α-Al grains because the Al11RE3, Al3RE and other RE-containing compounds cannot act as heterogeneous nucleating sites for the primary a-Al grains nucleating. A minor amount of Ce-rich mischmetal (o0.2 wt%) resulted in partial modification and additions of more than 0.3 wt% yielded the full modification; however, the tensile strength and ductility were decreased because of the coarsened aAl grains and the RE-containing compounds [11]. The size of α-Al grains was increased by the Y addition due to the precipitated Al3Y compounds, and the Sr–Y complex modification yielded a refinement effect. Moreover, the eutectic Si was changed into either a granular or flaky shape in the as-cast state and most of them were further transformed into the smaller particles by the T6 heat treatment [12]. The Sc addition refined the primary α-Al grains because the precipitated Al3Sc can act as heterogeneous nucleation sites; however, it has a much weaker effect on the eutectic silicon morphology [13]. Adding 0.5 wt% of Al–5Ti–0.25C–2RE master alloy has a satisfactory refining effect; however, further increasing the content did not significantly change the effect [14].
Z.M. Shi et al. / Materials Science & Engineering A 626 (2015) 102–107
As is well known, the RE elements can be divided into light and heavy series; the physicochemical properties between them are different to some extent. Most previous research explored the modification effect of the light RE elements as mentioned above. Therefore, the present work is to investigate the effects of heavy RE element of erbium (Er) on the microstructure and mechanical properties of the A356 aluminum alloy in as-cast and T6 states and to uncover the modification mechanism.
2. Materials and methods A356 aluminum alloy ingot (7.1 wt% Si, 0.45 wt% Mg, 0.12 wt% Fe) and master alloy (Al–10 wt% Er) were used as raw materials. The A356 ingot (20 kg) was put into an induction furnace to melt at 760 1C. Different amounts of the master alloys were added to form five groups of samples with different Er contents (0, 0.1, 0.2, 0.3 and 0.4 wt%). To eliminate gases and inclusions in the melts, An N2 gas was injected into bottom of the furnace over 5 min by means of a graphite pipe with a honeycomb end to generate small bubbles. When the temperature decreased to 730 1C, the melts was stewed for 5 min and then poured into a preheated steel mold (250 1C, wall thickness 20 mm) through a pre-heated porous ceramic filter with a thickness of 20 mm and the pore size of 1 mm. The size of the formed ingots is 30 (thickness) 120 (height) 150 (width) mm. The ingots were cooled to room temperature in the mold. The samples were then treated with a standard T6 schedule (soaking at 535 1C for 4 h, quenched in water) (100 1C, soaking at 200 1C for 4 h and cooled in air). The ingots were cut along the longitudinal symmetric plane by an electric spark cutting method and machined into standard tensile bars with a working diameter of 10 mm. The microstructure was observed with a metallurgical microscope (OLYMPUS-GX51), and the average granularity and the average aspect ratio of the Si phases were statistically calculated by quantitative analysis software against 20 fields. The morphology of the phases was detected by a scanning electronic microscope (SEM, S3400-N, Hitachi) with an energy dispersive spectrometer (EDS, 7021-H, HORIBA). The surfaces of the samples used for the observation were polished and etched by an HF (5 wt %)-alcohol solution. The phase composition was analyzed by an Xray diffractometer (XRD, D/MAX-2500/PC, PIGAKV) using copper Ka radiation at 40 kV, 200 mA and at a scanning speed of 11/min. The mechanical properties were examined using a material test machine (Jinan SHT4605) with a loading speed of 1.0 mm/min. The hardness (HV) was tested by a hardness tester (HVS-30Z\LCD, Huayin). Five samples for each group were used to measure the tensile properties and hardness.
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3.1.2. T6 treatment By a T6 treatment, the plate-like eutectic Si phases were broken up and parts of the short rods were produced in the unmodified alloy (Fig. 2a). However, this is not sufficient for the refinement. For the Er-modified alloys, the Si phases were changed into short rods and very fine particles, which uniformly distributed in the matrix. The 0.3 wt% Er modification had the best effects on the Si phases (Fig. 2d); the average size of the particles is 5 μm approximately. When the content arrived at 0.4 wt%, the particle size was in turn increased. Table 1 shows the variations of the size of the phases with the modifications by a statistically analysis. It can be observed that the Er modifications greatly refined the microstructure and the T6 treatment had a significant effect on it. The best roundness of the Si phases was obtained for the 0.2–0.4 wt% Er-modified and T6treated alloys. The 0.3 wt% Er addition yielded the finest α-Al grains and Si particles. 3.2. Phase compositions The Al3Er phases were introduced by the modification (Fig. 3), which were mainly distributed in the forms of needles and bulks in the as-cast state (Fig. 4a). Through the T6 treatment, large parts of the Al3Er phases were decomposed into fine particles and short rods (Fig. 4b), which uniformly dispersed in the matrix. The EDS results show that Er cannot dissolve in the α-Al and Si phases in the as-cast and T6 states (Fig. 5). Moreover, the Fe atoms dissolved in the Al3Er phases (Fig. 5c, f). Comparing Fig. 4a and b, it can be found that the large and needle-like β-Al5FeSi phase [15] at Point G in the as-cast alloy was changed into the fine particles (Point H) by the modification and the T6 treatment. However, the ironcontaining intermetallic phases in the unmodified Al–Si alloys are generally unaffected by heat treatments [16]. 3.3. Mechanical properties The variations of the tensile strength, elongation and hardness with the Er content are shown in Figs. 6–8. In the as-cast state, the hardness increased and the elongation decreased with an increase of the Er content. The tensile strength presented a slightly higher value (195.3 MPa) when 0.3 wt% Er was added. Through the T6 treatment, the hardness increased slowly; the tensile strength and elongation presented a trend of first increasing and then decreasing with an increase of the Er content. With 0.3 wt% Er addition, the tensile strength and elongation were 311.2 MPa and 8.6%, respectively. However, they were greatly decreased when the Er content was increased to 0.4 wt%.
4. Discussion 4.1. Effect of Er on the refinement of α-Al grains 3. Results 3.1. Microstructure 3.1.1. As-cast Fig. 1 shows the microstructure of the modified A356 alloys in their as-cast state. It can be observed that the coarse plate-like eutectic Si phases existed in the matrix for the unmodified alloy (Fig. 1a). The Er modification refined the α-Al grains and the eutectic Si phases (Fig. 1b–e). Adding 0.3 wt% Er produced the finest α-Al grains and had the best modification effect on the Si phases (Fig. 1d), which were transformed into fine particles and short rods. However, the 0.4 wt% addition in turn coarsened the αAl grains (Fig. 1e).
The A356 aluminum alloy is one of the hypoeutectic alloys. With the nucleation and growth of the primary α-Al grains, a small fraction of the Si and Mg atoms dissolve into the grains, and a large fraction of them and the other elements are enriched in front of the solids, which yield a composition supercooling and thus can refine the α-Al grains to some extent [11]. Moreover, some special compounds in the melts such as TiAl3, TiB2, TiC and Al3Sc can act as the heterogeneous nuclei to facilitate refinement of the grains [1–4,13]. Table 2 shows the physicochemical parameters of related elements and their compounds [17–19]. It can be noticed that Er has a larger atomic weight and atomic radius that those of Al and Si elements and has lower solubility in the α-Al crystals. Al3Er has
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Fig. 1. Microstructure of modified A356 alloys in as-cast state: (a) unmodified; (b) 0.1 wt%; (c) 0.2 wt%; (d) 0.3 wt%; (e) 0.4 wt%.
the same crystal structure (FCC) and a closely similar lattice constant (a ¼0.4215 nm) compared to the α-Al crystal (a ¼0.4049 nm). Moreover, the Al þAl3Er eutectics form at approximately 655 1C, which is higher than that of AlþSi (577 1C). When adding the Al–Er master alloy into the melts, the Er atoms and the very fine Al3Er particles can be precipitated [20]. The Er atoms will largely enrich in front of the α-Al grains to advance their refining. Because the atomic weight of Er is much larger than that of Al and its diffusion rate is slower, the refinement effect is stronger. Another reason is that the pre-existed fine Al3Er particles in the melts can act as the heterogeneous nuclei for the α-Al grains nucleating because the Al3Er has a same crystal structure (FCC) and a closely similar lattice constant as mentioned above. With these actions, the α-Al grains can be refined. In the present conditions, the 0.3 wt% Er addition achieved the best effect on the α-Al grain refinement. The refining effect was also found in the die-cast ADC12 Al–Si–X alloy and the pure aluminum [21,22]; the size of the precipitated Al3Er particles was 2–3 μm in diameter, which were observed in the grain boundaries to inhibit the growth of the α-Al grains. However, the Al3Er phases formed in the eutectic conversion (655 1C) cannot act as the heterogeneous nuclei to advance the α-Al grains nucleating. On the other hand, increasing the Er content (0.4 wt%) in turn reduced the refinement effect. This reduction can be ascribed to
the formation of larger Al3Er particles; which lost the heterogeneous nucleation effect. The similar phenomenon was also identified in the literature [21–26]. 4.2. Effect of Er on the eutectic Si morphology When the temperature decreases bellow 577 1C, the Al þSi eutectic cells start to precipitate in the forms of needles or plates because of the preferred orientation growth of the Si phases along their 〈112〉 directions [27,28]. Once the orientation growth is inhibited by additions of the chemical modifiers, the Si phases will turn to grow in the form of small granules. It was reported that additions of the chemical modifiers easily produce twin defects in the Si crystals, the repeated twinning makes the growth reorientate, thus, the fibers Si phases can bend, curve and split to create finer particles [27,29]. Moreover, the Si particles can grow by rapping the AlP nuclei to develop into the compact particles; however, the addition of Sr makes the AlP and the oxide bifilms deactivate as favored nucleation substrates [30,31]. Meanwhile, the Sr atoms can segregate towards the Si-rich regions and the Al2Si2Sr clusters form preferentially, which enhance the interfacial energy and poison the nucleation sites [32]. The poisoning of nucleation sites and the increase of interfacial energy yield a significant undercooling of the melts and so delays the nucleation
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Fig. 2. Microstructure of modified A356 alloys in T6 state: (a) unmodified; (b) 0.1 wt%; (c) 0.2 wt%; (d) 0.3 wt%; (e) 0.4 wt%.
Table 1 Statistical analysis of size of the phases. States
Er content wt%
Equivalent size of α-Al grains (μm)
Average length of Average aspect Si phases (μm) ratio of Si phases
As-cast
0 0.1 0.2 0.3 0.4
85.6 57.3 42.6 22.2 47.1
13.6 9.2 6.5 4.6 5.8
7.3 3.6 1.9 1.5 1.4
T6
0 0.1 0.2 0.3 0.4
/ / / / /
11.4 8.5 6.3 4.8 6.5
5.4 3.2 1.2 1.1 1.1 Fig. 3. XRD pattern of 0.3 wt% Er-modified A356 alloy in the as-cast state.
of eutectic phases, resulting in the refinement of the eutectic phases [33]. The similar RE elements of La and Ce additions also yielded the undercooling [9,10]. In the present conditions, even though the Al þAl3Er eutectic transformation occurs at approximately 655 1C, trace of Er atoms still remain in the melts to the Al þSi eutectic temperature
(577 1C). As the Er atom has a smaller atomic radius (0.175 nm) and a larger atomic weight (167.26) than those (0.215 nm, 87.62) of the Sr atom, it is easier to enrich in front of the eutectic cells because of its difficulty in diffusion, which greatly poisons the orientation growth of the Si phases. Moreover, the Er atom has a more physicochemical activity than that of Sr, La and Ce atoms, which can eliminate the oxide bifilms and purify the melts. With
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Fig. 4. SEM image of 0.3 wt% Er-modified A356 alloy, (a) as-cast; (b) T6.
Fig. 5. EDS analysis of 0.3 wt% Er-modified A356 alloy in as-cast and T6 states.
Fig. 6. Tensile strength of Er-modified A356 alloys.
these two reasons, the fine particles or short rods were formed. A similar phenomenon was also found in the Er-modified Al–12.6 wt% Si hypereutectic alloys, but the action mechanism was not provided [21,26].
Fig. 7. Elongation of Er-modified A356 alloys.
When soaking at the high temperature, the Si phases were further adjusted into the fine globular shape. This is because the enriched Er atoms in front of the eutectic cells increase the
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The Er modification introduced the needles, bulks and rods of Al3Er phases. By the T6 treatment, they were transformed into fine particles and short rods, which uniformly distributed in the matrix. The Er addition and T6 treatment eliminated the harmful needle-like β-Al5FeSi phases in the alloys. The 0.3 wt% Er-modification and T6 treatment yielded the highest mechanical properties of the alloy.
Acknowledgment The present work is supported by the Nature Science Foundation of Inner Mongolia of China.
Fig. 8. Hardness of Er-modified A356 alloys.
Table 2 Physicochemical parameters of related elements and their compounds. Element
Al
Si
Er
Atomic number Atomic weight Atomic Radius (nm) Crystal structure Lattice constants (nm)
13 26.98 0.1432 FCC a¼ 0.4049
68 167.26 0.1734 HCP a¼ 0.3560 c ¼0.5595
Maximum solubility in Al wt% Al–Er compounds Crystal structure of Al3Er Lattice constants of Al3Er (nm) Binary eutectic cell Eutectic temperature (oC) Composition at eutectic point (wt%)
/ / / / / / /
14 28.09 0.1462 Ortho a ¼0.4737 b ¼0.4502 c ¼ 0.2550 1.65 / / / Al þ Si 577 12
0.05 Al3Er FCC a¼ 0.4215 Alþ Al3Er 655 6
Notes: HCP: hexagonal close-packed; FCC: face-centered cubic; Ortho: Orthogonal.
interfacial energy with the α-Al grains [34], which results in sphereodization of the Si particles (Oswald ripening). More Er addition (0.4 wt%) yields more enrichment in front of the eutectic cells and causes smaller Si particles to present, which are coarsened by the high-temperature soaking [35]. 4.3. Effect of Er on the mechanical properties The size, shape and content of the Si, Al3Er and β-Al5FeSi phases greatly influence the mechanical properties of the alloys. In the ascast state, even though the Er addition refined the α-Al grains and the Si phases, especially when adding 0.3 wt% Er, the brittle Al3Er and β-Al5FeSi phases presented in the forms of needles, rods and blocks, the strength and elongation were not improved. By the T6 treatment, the Si, Al3Er and β-Al5FeSi phases were changed into fine particles and short rods, which uniformly distributed in the matrix; thus, the effects of the refinement of the α-Al grains and the Si phases were represented and so the strength and elongation were largely improved, especially for the 0.3 wt% Er-modified alloy. Moreover, the uniformly dispersed hard Al3Er and βAl5FeSi phases simultaneously enhanced the hardness of the alloys. 5. Conclusions Addition of 0.3 wt% Er had the best effect on the refinement of α-Al grains and the morphology of eutectic Si phases.
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