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Two directional microstructure and effects of nanoscale dispersed Si particles on microhardness and tensile properties of AlSi7Mg melt-spun alloy
Journal of Alloys and Compounds 618 (2015) 609–614
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Two directional microstructure and effects of nanoscale dispersed Si particles on microhardness and tensile properties of AlSi7Mg melt-spun alloy Xixi Dong a,b,⇑, Liangju He a,c, Guangbao Mi a, Peijie Li a,b,d a
National Center of Novel Materials for International Research, Tsinghua University, Beijing 100084, China Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China School of Aerospace, Tsinghua University, Beijing 100084, China d State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China b c
a r t i c l e
i n f o
Article history: Received 11 June 2014 Received in revised form 27 July 2014 Accepted 5 August 2014 Available online 9 September 2014 Keywords: Melt-spun Al–Si alloy Surface microstructure Cross-sectional microstructure Si particle Mechanical property
a b s t r a c t The two directional microstructure and multiple mechanical properties of the AlSi7Mg ribbon produced by melt-spun were investigated by optical microscopy (OM), field emission gun scanning electron microscope (FEGSEM), X-ray diffraction (XRD), microhardness and tensile tests. Both the surface and cross-sectional microstructure of the melt-spun ribbon were characterized in detail to give a clear and integrated description of the microstructure. Two kinds of nanoscale Si particles were observed, i.e., small Si particles ranging from 13 to 50 nm and large Si particles ranging from 50 nm to several hundreds of nanometers with clear size boundary were dispersed both in the interior and boundary of fine a-Al. XRD results revealed supersaturated solution of Si in Al matrix to be 0.62 at.%. The ultimate tensile strength, yield strength, and hardness of the ribbon were 1.53, 1.75 and 1.56 times higher than that of the conventional cast ingot separately. The breaking elongation of the ribbon was 1.73% with intergranular fracture feature. The effects of nanoscale dispersed Si particles on the significant improvement of both hardness and tensile properties of the AlSi7Mg melt-spun ribbon were discussed in detail. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Rapid solidification process shows a marked enhancement of mechanical properties over the conventionally processed alloys [1,2]. Furthermore, it is possible to produce metastable materials such as quasi-crystals, nano-crystals and amorphous alloys by cooling metallic melts at cooling rates exceeding 104 K s 1 [3–5]. Rapid solidification is particularly attractive for aluminum alloys because the limited equilibrium solid solubility of some alloying elements in the aluminum lattice can be extended during the rapid solidification process [6]. Amongst various rapid solidification methods, besides gas atomization [2,5] and spray deposition [7], melt-spun has been extensively used. Large size aluminum alloy melt-spun ribbons with a width as large as 300 mm can be produced continuously by wider copper roller and melt-spun orifice
⇑ Corresponding author at: National Center of Novel Materials for International Research, Tsinghua University, Beijing 100084, China. Tel.: +86 10 62773639; fax: +86 10 62788074. E-mail address: [email protected] (X. Dong). http://dx.doi.org/10.1016/j.jallcom.2014.08.228 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
like the Fe-base amorphous melt-spun ribbons that has been industrially used, and the surface quality of the melt-spun ribbons can be controlled by the adjustment of technological parameters. Finally, the aluminum alloy melt-spun ribbons with good surface quality can serve as high strength light structural and protective material by multi-layer superposition. Due to the good cast ability, high strength, excellent corrosion resistance and low coefficient of thermal expansion properties of Al–Si alloys and their wide applications in aerospace, automotive and electronic fields [8], much work has been done to study the microstructure and mechanical properties of this family of aluminum alloy melt-spun ribbons [9–17]. The microstructure of the melt-spun Al–Si alloy ribbons was composed of the surface and the cross-sectional microstructure. Former studies mainly focused on the surface microstructure [9,10,13,14] and Si particles with a size of 20–70 nm were reported sporadically in the Al matrix of the Al–12 wt.%Si, A357, Al12.9SiFe and Al20Si5Fe melt-spun ribbons [6,9,11,12]. However, the characterization and investigation of the morphology and size of these nanoscale Si particles was really insufficient and ambiguous. In
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addition, the cross-sectional microstructure was investigated mainly in the form of optical observation [11–13] and scarce detail micro information about it is known. Thus, the deep study of both the surface and the cross-sectional microstructure to give a clear and integrated description of the microstructure especially the nanoscale Si particles of the Al–Si alloy melt-spun ribbons is imperious. Besides, the microhardness of the Al–Si alloy ribbons was studied in almost all of the published papers to represent the mechanical property, while tensile tests were seldom conducted [13,18] and tensile properties of most Al–Si alloy melt-spun ribbons are unknown. What is more, the enhancement of mechanical properties of the Al–Si melt-spun ribbons was mainly explained by the supersaturated solution and grain refinement effects of a-Al, while seldom did researchers relate the improvement of the mechanical properties with the microstructure of Si particles. In this paper, the commonly used AlSi7Mg alloy was chosen and its melt-spun ribbons were prepared, both the surface and the cross-sectional microstructure of the AlSi7Mg ribbon were characterized, the tensile properties of the ribbon were investigated and compared with the conventional cast ingot, and the effects of nanoscale dispersed Si particles on the improvement of both hardness and tensile properties for the AlSi7Mg melt-spun ribbon were discussed in detail. 2. Experimental procedures The feedstock of AlSi7Mg alloy was produced by direct-chill casting with a composition given in Table 1, and Fig. 1(b) shows the optical microscopy of the microstructure of the conventional cast AlSi7Mg alloy ingot. Then the feedstock of AlSi7Mg alloy was melt-spun to ribbons using single roller melt-spun under a vacuum of 3.3E–3 Pa, as shown in Fig. 1(a). The copper wheel had a diameter of 22 cm and the rotating speed was 32.8 ms 1. The resultant ribbons were about 5.8 mm wide and 50 lm thick. The melt-spun AlSi7Mg alloy ribbons were characterized by Olympus-BX51M optical microscopy, FEGSEM and XRD. FEGSEM investigations were carried out using JSM-7001F at a voltage of 20 kV. The XRD measurements were carried out in a Rigaku H2500 diffractometer using Cu Ka1 radiation at 40 kV and 20 mA in the 2h range from 25° to 85°. For the cross-sectional observation, cold mounted specimen was prepared using standard metallographic techniques. For the surface investigations, the melt-spun ribbon was first pasted to a smooth aluminum block by the conductive adhesive, and then it was ground on 5000 grind and polished on 0.5 lm diamond lap wheels. Finally, the polished surface and cross-sectional specimens were etched in a 0.5 vol.% HF solution for detailed OM and SEM observation. The measurement of Vickers microhardness was made using a FM-800 microhardness tester with an applied load of 10 g for 10 s, and eight measurements were performed with the average reported as the microhardness. The tensile tests of the melt-spun ribbons and the conventional cast ingot were conducted on the Zwick/ Z005 and Zwick/Z020 tensile test machine at room temperature respectively with a constant speed of 2.0 mm/min. Multiple reproducible tensile tests were conducted for both the melt-spun ribbons and the ingot, and the effective tensile test results were reported as the tensile properties.
3. Results and discussion 3.1. XRD studies Fig. 2 shows the XRD patterns taken from the wheel side and free surface of the melt-spun AlSi7Mg alloy ribbon, and the XRD patterns are present in the peaks of a-Al and Si phase. The peaks of a-Al are strong while that of the Si phase are very weak, which indicates that the rapid solidification results in an increase in the solubility of Si in the Al matrix. In addition, the measured Al (1 1 1), (2 0 0), (2 2 0) and (3 1 1) peak shift yields a lattice parameter of 0.4047121 nm for the a-Al. Using the linear relationship between the lattice parameter and the atomic fraction of Si given by Bendjik et al. [19], a solid solubility of Si in Al matrix of about Table 1 Chemical compositions of the AlSi7Mg alloy investigated (wt.%). Si
Mg
Ti
Fe
Mn
Cu
Ni
Zn
Al
6.52
0.42
0.14
0.11
0.05
0.05
0.04
0.02
Balance
0.62 at.% can be obtained. Considering that the solid solubility of Si in Al at room temperature is about 0.048 at.% [20], the present solid solubility is much high. 3.2. Two directional microstructure 3.2.1. Surface microstructure As described in the experimental procedures (see Section 2), the wheel side surface of the 50 lm thick AlSi7Mg melt-spun ribbon was first pasted to a smooth aluminum block by the conductive adhesive, and then ground and polished to the surface that was 23 lm away from the wheel side for surface microstructure observation, as shown in Fig. 3(b). Fig. 3(a) is the optical micrograph of the polished surface for observation, and zone I is the chosen zone in the polished surface for detailed OM and SEM observation. The optical micrograph of the surface microstructure of zone I is shown in Fig. 3(c). It is found that discrete Si particles are dispersed both in the boundary and interior of a-Al, and there is no large plate-like Si phase that is usually distributed in the a-Al grain boundary of the conventional cast ingot (see Fig. 7(a)). The size of the a-Al grain is fine and smaller than 3 lm, as indicated by the arrow in Fig. 3(c), which is significantly smaller than that of the conventional cast ingot (see Fig. 1(b)). Fig. 3(d) and (e) shows the SEM micrographs of the surface microstructure of zone I. Two kinds of nanoscale Si particles were observed in the Al matrix. The small Si particles are smaller than 50 nm and as small as 14 nm, while the large Si particles are larger than 50 nm and as large as 271 nm, and there is clear size boundary between the small and the large Si particles. The dispersion of the nanoscale Si particles ranging from 14 to 271 nm in the melt-spun ribbon is significantly different from the large micron scale plate-like Si phase in the conventional cast ingot. 3.2.2. Cross-sectional microstructure The optical micrograph of the cross-sectional microstructure of the AlSi7Mg melt-spun ribbon is shown in Fig. 4(a), and fine a-Al grains are formed due to the high cooling rate of the melt-spun process. Fig. 4(b) shows the SEM micrograph of the three observation zones A, B and C along the cross-section of the ribbon. The center of zones A and C is near the wheel side and free surface of the ribbon respectively, while that of zone B is in the center of the crosssection. The SEM micrographs of the microstructure of zones A, B and C are shown in Fig. 4(c)–(h). Fine Si particles are found to be dispersed in the Al matrix, and the size of Si particles increases gradually from the wheel side to the free surface due to the decrease of cooling rate, as shown in Fig. 4(c), (e) and (g). From the magnification of Fig. 4(c), (e) and (g), small and large Si particles with clear size boundary are found to be dispersed both in the interior and boundary of fine a-Al, the small Si particles are smaller than 50 nm and as small as 13 nm, while the large Si particles are larger than 50 nm and as large as 755 nm, which is consistent with the above mentioned surface microstructure, as shown in Fig. 4(d), (f) and (h). It should be mentioned that small Si particles ranging from 13 to 43 nm are always present in zones A, B and C and irrelevant with cooling rates, which could be originated from the retention and development of small nanoscale microinhomogeneity in Al–Si melts that was reported by CalvoDahlborg et al. [21,22]. However, large Si particles ranging from 50 nm to several hundreds of nanometers increase gradually from zone A to C with the decrease of cooling rate, which could be related to the limited growth of Si particles during the rapid solidification process. When compared the cross-sectional microstructure with the above mentioned surface microstructure in Section 3.2.1, it can be concluded that small Si particles ranging from 13 to 50 nm
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Fig. 1. Schematic of the melt-spun process [18] (a) and the optical micrograph of the conventional cast AlSi7Mg ingot (b).
increases gradually from wheel side to free surface of the ribbon, which constitutes the microstructure of the AlSi7Mg melt-spun ribbon. 3.3. Mechanical properties The mechanical properties of melt-spun ribbons can be determined by the microhardness test and tensile test. Microhardness test is the easiest and most straight forward technique. It should be mentioned that tensile properties of melt-spun ribbons are dependent on the surface quality, so ribbons with good surface quality were chosen and multiple reproducible tensile tests were conducted.
Fig. 2. XRD patterns taken from the wheel side and free surface of the AlSi7Mg melt-spun ribbon.
and large Si particles ranging from 50 to several hundreds of nanometers with clear size boundary are dispersed both in the interior and boundary of fine a-Al, and the size of large Si particles
3.3.1. Microhardness The Vickers microhardness values of melt-spun and those of conventional cast AlSi7Mg alloy are summarized in Table 2. The microhardness of the AlSi7Mg melt-spun ribbons is approximately 1.56 times higher than that of the original ingot. The microhardness 114.8 ± 4.1 kg mm 2 obtained here for the melt-spun ribbon is somewhat lower than the values reported for the melt-spun
Fig. 3. Optical micrographs of the surface position (a) and thickness directional position (b) for observation and OM (c) and SEM (d and e) micrographs of the surface microstructure of the observation zone I in the AlSi7Mg melt-spun ribbon.
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Fig. 4. Optical micrograph (a) of the cross-sectional microstructure of the AlSi7Mg melt-spun ribbon and SEM micrographs of (b) three observation zones A (g and h), B (e and f) and C (c and d) along the cross-section of the ribbon.
Table 2 Hardness values of the melt-spun ribbon compared with the conventional cast ingot. Alloy
Status
Hardness (Vickers) (kg mm
2
AlSi7Mg
Melt-spun Ingot
115 73.8
118.7 68.5
112.3 78.2
)
AlSi9Mg (136.5 kg mm 2) ribbon [13] because of relatively lower content of Si. The high microhardness of the melt-spun alloy compared with its ingot counterpart can be partly attributed to the supersaturated solid solution and the grain refinement strengthening (see Fig. 3(c)) effects of a-Al. The atomic radius difference between two elements in solid solution provides a strain field, which interacts with dislocations and results in solution strengthening. This solid solution strengthening mechanism is also supported by the above
Average 110.6 73.4
121.6 78.2
116.9 65.3
109.7 68.9
113.2 81.3
114.8 ± 4.1 73.5 ± 5.6
mentioned XRD analyses with a solid solubility of Si in Al matrix of about 0.62 at.%. In addition, we think that the nanoscale dispersed Si particles also contribute to the improvement of the microhardness, which was seldom discussed in detail by former studies about Al–Si alloy melt-spun ribbons because of deficient characterization of nanoscale Si particles. It was found that the microhardness of the Si phase was as high as 813.3 kg mm 2 during the microhardness test of the AlSi7Mg ingot, which is significantly higher than that of a-Al. Considering that the microhardness test indentation of the
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AlSi7Mg melt-spun ribbon is in the order of a dozen micrometers (see Fig. 5(a)), which covers hundreds of nanoscale dispersed Si particles with high microhardness that is eight or more times higher than that of the supersaturated solid solution Al matrix, as shown in Fig. 5(b). Thus the diamond indenter will suffer great impediment from hundreds of nanoscale dispersed Si particles during the press process, which results in the improvement of the microhardness. Besides, the nanoscale dispersed Si particles in the Al matrix will cause the dispersion strengthening effect of the AlSi7Mg melt-spun ribbon further, which also contributes to the enhancement of the microhardness. 3.3.2. Tensile properties The tensile properties of the melt-spun AlSi7Mg alloy and the conventional cast ingot are shown in Fig. 6. The ultimate tensile strength (UTS) and yield strength (YS) of the AlSi7Mg melt-spun ribbon are 270 and 220 MPa respectively, which are significantly higher than the 177 MPa (UTS) and 126 MPa (YS) of the conventional cast ingot, although the casting defects and test errors of the melt-spun ribbons with thin thickness (50 lm) could lead to the decrease of the test values. It should be mentioned that the UTS obtained here for the AlSi7Mg ribbon (270 MPa) is similar to the reported AlSi9Mg ribbon (274 MPa) [13], while the breaking elongation obtained here for the AlSi7Mg melt-spun ribbon is slightly lower than these reported by Rios et al. [13] and Chen et al. [23] for AlSi9Mg (2.9%) and Al1.5Fe (2.3%) melt-spun ribbons. The breaking elongation of the AlSi7Mg ribbon is 1.73%, which is lower than that of the conventional cast ingot (3.1%). The nanoscale dispersed Si particles in the AlSi7 Mg ribbon can average the deformation stress in a-Al and prevent stress concentration. As is well known, the stress concentration results in the crack of the large plate-like Si phase in the conventional cast ingot, which releases the deformation stress in a-Al and serves as nucleation sites of cracks. So the deformation of the AlSi7Mg melt-spun ribbon happens in a high level of deformation stress, and cracks are hard to form during the tensile process, which leads to the rapid increase of the deformation stress with deformation without a clear yield point, as reported by Rios et al. [13] and Chen et al. [23]. Therefore, fracture happens instantaneously when the deformation stress in the AlSi7Mg ribbon increases rapidly to UTS. Thus the breaking elongation of the AlSi7Mg ribbon is lower than that of the conventional cast ingot since there is no obvious yield process during the tensile process. The higher values of UTS and YS of the melt-spun AlSi7Mg ribbons compared with the conventional cast ingot could be partly related to the supersaturated solution strengthening effect of the alloying elements especially Si in the Al-matrix and the grain refinement effects of a-Al (see Fig. 3(c)). In addition, we think that the nanoscale dispersed Si particles induced by the rapid solidification process also contribute to the improvement of UTS and YS, as seldom discussed in detail by former studies about Al–Si alloy
Fig. 6. Tensile properties of the AlSi7Mg melt-spun ribbon at room temperature compared with the conventional cast ingot.
melt-spun ribbons due to the deficient characterization of nanoscale Si particles and the insufficient comparison of tensile fractures between conventional cast ingot and the melt-spun ribbon. The eutectic Si phase of the conventional cast AlSi7Mg ingot is large plate-like and distributes in the grain boundary of a-Al, as shown in Fig. 7(a). The interface between large plate-like eutectic Si phase and a-Al is easy to induce stress concentration, which results in the crack of the large plate-like eutectic Si phase, as indicated by the solid arrows in Fig. 7(c). The crack of large plate-like eutectic Si phase in the grain boundary releases the deformation and stress in a-Al and stimulates the happening of deformation, which leads to the low YS of the ingot. Then, the cracked Si phase provides nucleation sites for the cavity and induces the formation of cracks across the a-Al grain [24–26], as indicated by the dotted arrows in Fig. 7(c). Finally, the development of cracks results in the fracture of ingot, which is the origin of the low UTS for the ingot. Significantly different from the large plate-like eutectic Si phase in the ingot, nanoscale Si particles were found to be dispersed in the Al matrix of the AlSi7Mg melt-spun ribbon, as shown in Fig. 7(d). The deformation stress in fine a-Al (see Fig. 3(c)) during the tensile process can be averaged to the nanoscale dispersed Si particles without crack, which means that the microstructure of fine a-Al and nanoscale dispersed Si particles can suffer higher deformation stress. Besides, the transfer of deformation and stress from one a-Al grain to another is prevented, which leads to the happening of plastic deformation under higher stress. Thus the YS of the melt-spun AlSi7Mg ribbon is increased by 1.75 times (220 MPa) comparing with the ingot (126 MPa). Because of the lack of cracked Si particles to provides nucleation sites for the cavity, cracks are difficult to be formed across the a-Al grain, and intergranular fracture happens, as indicated by the intergranular cracked fine a-Al grain in Fig. 7(e) and the intergranular fracture cells in Fig. 7(f), which results in the improvement of UTS by 1.53 times (270 MPa) comparing with the ingot (177 MPa).
Fig. 5. Hardness test indentation (a) of the AlSi7Mg melt-spun ribbon and nanoscale dispersed Si particles in the schematic area of hardness test indentation (b).
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Fig. 7. Microstructure of Si phase for conventional cast (a) and melt-spun (d) AlSi7Mg alloy and tensile fracture morphology of AlSi7Mg ingot (b and c) compared with that of melt-spun ribbon (e and f).
4. Conclusions (1) Different from former studies about the Al–Si alloy meltspun ribbons, both the surface and gradient cross-sectional microstructure were characterized in detail to give a clear and integrated description of the melt-spun AlSi7Mg ribbon, and a extended solid solubility of Si in Al matrix to be 0.62 at.% was found. (2) Two kinds of nanoscale Si particles were observed and reported, i.e., small Si particles ranging from 13 to 50 nm and large Si particles ranging from 50 nm to several hundreds of nanometers with clear size boundary were found to be dispersed both in the interior and boundary of fine a-Al, and the size of large Si particles increased gradually from wheel side to free surface of the ribbon, which constituted the microstructure of the AlSi7Mg melt-spun ribbon. (3) Tensile properties of the AlSi7Mg melt-spun ribbon were investigated, and the ultimate tensile strength, yield strength and microhardness were found to be 1.53, 1.75 and 1.56 times higher than that of the conventional cast ingot respectively. The effects of nanoscale dispersed Si particles on the significant improvement of both hardness and tensile properties for the AlSi7Mg melt-spun ribbon were provided in detail. (4) The breaking elongation of the AlSi7Mg melt-spun ribbon was 1.73%, and granular cells in the fracture surface of the ribbon suggested an intergranular fracture due to the rapid solidification process.
Acknowledgements The work was supported by the National Basic Research Program of China (2013CB632203).
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Two directional microstructure and effects of nanoscale dispersed Si particles on microhardness and tensile properties of AlSi7Mg melt-spun alloy Xixi Dong a,b,⇑, Liangju He a,c, Guangbao Mi a, Peijie Li a,b,d a
National Center of Novel Materials for International Research, Tsinghua University, Beijing 100084, China Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China School of Aerospace, Tsinghua University, Beijing 100084, China d State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China b c
a r t i c l e
i n f o
Article history: Received 11 June 2014 Received in revised form 27 July 2014 Accepted 5 August 2014 Available online 9 September 2014 Keywords: Melt-spun Al–Si alloy Surface microstructure Cross-sectional microstructure Si particle Mechanical property
a b s t r a c t The two directional microstructure and multiple mechanical properties of the AlSi7Mg ribbon produced by melt-spun were investigated by optical microscopy (OM), field emission gun scanning electron microscope (FEGSEM), X-ray diffraction (XRD), microhardness and tensile tests. Both the surface and cross-sectional microstructure of the melt-spun ribbon were characterized in detail to give a clear and integrated description of the microstructure. Two kinds of nanoscale Si particles were observed, i.e., small Si particles ranging from 13 to 50 nm and large Si particles ranging from 50 nm to several hundreds of nanometers with clear size boundary were dispersed both in the interior and boundary of fine a-Al. XRD results revealed supersaturated solution of Si in Al matrix to be 0.62 at.%. The ultimate tensile strength, yield strength, and hardness of the ribbon were 1.53, 1.75 and 1.56 times higher than that of the conventional cast ingot separately. The breaking elongation of the ribbon was 1.73% with intergranular fracture feature. The effects of nanoscale dispersed Si particles on the significant improvement of both hardness and tensile properties of the AlSi7Mg melt-spun ribbon were discussed in detail. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Rapid solidification process shows a marked enhancement of mechanical properties over the conventionally processed alloys [1,2]. Furthermore, it is possible to produce metastable materials such as quasi-crystals, nano-crystals and amorphous alloys by cooling metallic melts at cooling rates exceeding 104 K s 1 [3–5]. Rapid solidification is particularly attractive for aluminum alloys because the limited equilibrium solid solubility of some alloying elements in the aluminum lattice can be extended during the rapid solidification process [6]. Amongst various rapid solidification methods, besides gas atomization [2,5] and spray deposition [7], melt-spun has been extensively used. Large size aluminum alloy melt-spun ribbons with a width as large as 300 mm can be produced continuously by wider copper roller and melt-spun orifice
⇑ Corresponding author at: National Center of Novel Materials for International Research, Tsinghua University, Beijing 100084, China. Tel.: +86 10 62773639; fax: +86 10 62788074. E-mail address: [email protected] (X. Dong). http://dx.doi.org/10.1016/j.jallcom.2014.08.228 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
like the Fe-base amorphous melt-spun ribbons that has been industrially used, and the surface quality of the melt-spun ribbons can be controlled by the adjustment of technological parameters. Finally, the aluminum alloy melt-spun ribbons with good surface quality can serve as high strength light structural and protective material by multi-layer superposition. Due to the good cast ability, high strength, excellent corrosion resistance and low coefficient of thermal expansion properties of Al–Si alloys and their wide applications in aerospace, automotive and electronic fields [8], much work has been done to study the microstructure and mechanical properties of this family of aluminum alloy melt-spun ribbons [9–17]. The microstructure of the melt-spun Al–Si alloy ribbons was composed of the surface and the cross-sectional microstructure. Former studies mainly focused on the surface microstructure [9,10,13,14] and Si particles with a size of 20–70 nm were reported sporadically in the Al matrix of the Al–12 wt.%Si, A357, Al12.9SiFe and Al20Si5Fe melt-spun ribbons [6,9,11,12]. However, the characterization and investigation of the morphology and size of these nanoscale Si particles was really insufficient and ambiguous. In
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addition, the cross-sectional microstructure was investigated mainly in the form of optical observation [11–13] and scarce detail micro information about it is known. Thus, the deep study of both the surface and the cross-sectional microstructure to give a clear and integrated description of the microstructure especially the nanoscale Si particles of the Al–Si alloy melt-spun ribbons is imperious. Besides, the microhardness of the Al–Si alloy ribbons was studied in almost all of the published papers to represent the mechanical property, while tensile tests were seldom conducted [13,18] and tensile properties of most Al–Si alloy melt-spun ribbons are unknown. What is more, the enhancement of mechanical properties of the Al–Si melt-spun ribbons was mainly explained by the supersaturated solution and grain refinement effects of a-Al, while seldom did researchers relate the improvement of the mechanical properties with the microstructure of Si particles. In this paper, the commonly used AlSi7Mg alloy was chosen and its melt-spun ribbons were prepared, both the surface and the cross-sectional microstructure of the AlSi7Mg ribbon were characterized, the tensile properties of the ribbon were investigated and compared with the conventional cast ingot, and the effects of nanoscale dispersed Si particles on the improvement of both hardness and tensile properties for the AlSi7Mg melt-spun ribbon were discussed in detail. 2. Experimental procedures The feedstock of AlSi7Mg alloy was produced by direct-chill casting with a composition given in Table 1, and Fig. 1(b) shows the optical microscopy of the microstructure of the conventional cast AlSi7Mg alloy ingot. Then the feedstock of AlSi7Mg alloy was melt-spun to ribbons using single roller melt-spun under a vacuum of 3.3E–3 Pa, as shown in Fig. 1(a). The copper wheel had a diameter of 22 cm and the rotating speed was 32.8 ms 1. The resultant ribbons were about 5.8 mm wide and 50 lm thick. The melt-spun AlSi7Mg alloy ribbons were characterized by Olympus-BX51M optical microscopy, FEGSEM and XRD. FEGSEM investigations were carried out using JSM-7001F at a voltage of 20 kV. The XRD measurements were carried out in a Rigaku H2500 diffractometer using Cu Ka1 radiation at 40 kV and 20 mA in the 2h range from 25° to 85°. For the cross-sectional observation, cold mounted specimen was prepared using standard metallographic techniques. For the surface investigations, the melt-spun ribbon was first pasted to a smooth aluminum block by the conductive adhesive, and then it was ground on 5000 grind and polished on 0.5 lm diamond lap wheels. Finally, the polished surface and cross-sectional specimens were etched in a 0.5 vol.% HF solution for detailed OM and SEM observation. The measurement of Vickers microhardness was made using a FM-800 microhardness tester with an applied load of 10 g for 10 s, and eight measurements were performed with the average reported as the microhardness. The tensile tests of the melt-spun ribbons and the conventional cast ingot were conducted on the Zwick/ Z005 and Zwick/Z020 tensile test machine at room temperature respectively with a constant speed of 2.0 mm/min. Multiple reproducible tensile tests were conducted for both the melt-spun ribbons and the ingot, and the effective tensile test results were reported as the tensile properties.
3. Results and discussion 3.1. XRD studies Fig. 2 shows the XRD patterns taken from the wheel side and free surface of the melt-spun AlSi7Mg alloy ribbon, and the XRD patterns are present in the peaks of a-Al and Si phase. The peaks of a-Al are strong while that of the Si phase are very weak, which indicates that the rapid solidification results in an increase in the solubility of Si in the Al matrix. In addition, the measured Al (1 1 1), (2 0 0), (2 2 0) and (3 1 1) peak shift yields a lattice parameter of 0.4047121 nm for the a-Al. Using the linear relationship between the lattice parameter and the atomic fraction of Si given by Bendjik et al. [19], a solid solubility of Si in Al matrix of about Table 1 Chemical compositions of the AlSi7Mg alloy investigated (wt.%). Si
Mg
Ti
Fe
Mn
Cu
Ni
Zn
Al
6.52
0.42
0.14
0.11
0.05
0.05
0.04
0.02
Balance
0.62 at.% can be obtained. Considering that the solid solubility of Si in Al at room temperature is about 0.048 at.% [20], the present solid solubility is much high. 3.2. Two directional microstructure 3.2.1. Surface microstructure As described in the experimental procedures (see Section 2), the wheel side surface of the 50 lm thick AlSi7Mg melt-spun ribbon was first pasted to a smooth aluminum block by the conductive adhesive, and then ground and polished to the surface that was 23 lm away from the wheel side for surface microstructure observation, as shown in Fig. 3(b). Fig. 3(a) is the optical micrograph of the polished surface for observation, and zone I is the chosen zone in the polished surface for detailed OM and SEM observation. The optical micrograph of the surface microstructure of zone I is shown in Fig. 3(c). It is found that discrete Si particles are dispersed both in the boundary and interior of a-Al, and there is no large plate-like Si phase that is usually distributed in the a-Al grain boundary of the conventional cast ingot (see Fig. 7(a)). The size of the a-Al grain is fine and smaller than 3 lm, as indicated by the arrow in Fig. 3(c), which is significantly smaller than that of the conventional cast ingot (see Fig. 1(b)). Fig. 3(d) and (e) shows the SEM micrographs of the surface microstructure of zone I. Two kinds of nanoscale Si particles were observed in the Al matrix. The small Si particles are smaller than 50 nm and as small as 14 nm, while the large Si particles are larger than 50 nm and as large as 271 nm, and there is clear size boundary between the small and the large Si particles. The dispersion of the nanoscale Si particles ranging from 14 to 271 nm in the melt-spun ribbon is significantly different from the large micron scale plate-like Si phase in the conventional cast ingot. 3.2.2. Cross-sectional microstructure The optical micrograph of the cross-sectional microstructure of the AlSi7Mg melt-spun ribbon is shown in Fig. 4(a), and fine a-Al grains are formed due to the high cooling rate of the melt-spun process. Fig. 4(b) shows the SEM micrograph of the three observation zones A, B and C along the cross-section of the ribbon. The center of zones A and C is near the wheel side and free surface of the ribbon respectively, while that of zone B is in the center of the crosssection. The SEM micrographs of the microstructure of zones A, B and C are shown in Fig. 4(c)–(h). Fine Si particles are found to be dispersed in the Al matrix, and the size of Si particles increases gradually from the wheel side to the free surface due to the decrease of cooling rate, as shown in Fig. 4(c), (e) and (g). From the magnification of Fig. 4(c), (e) and (g), small and large Si particles with clear size boundary are found to be dispersed both in the interior and boundary of fine a-Al, the small Si particles are smaller than 50 nm and as small as 13 nm, while the large Si particles are larger than 50 nm and as large as 755 nm, which is consistent with the above mentioned surface microstructure, as shown in Fig. 4(d), (f) and (h). It should be mentioned that small Si particles ranging from 13 to 43 nm are always present in zones A, B and C and irrelevant with cooling rates, which could be originated from the retention and development of small nanoscale microinhomogeneity in Al–Si melts that was reported by CalvoDahlborg et al. [21,22]. However, large Si particles ranging from 50 nm to several hundreds of nanometers increase gradually from zone A to C with the decrease of cooling rate, which could be related to the limited growth of Si particles during the rapid solidification process. When compared the cross-sectional microstructure with the above mentioned surface microstructure in Section 3.2.1, it can be concluded that small Si particles ranging from 13 to 50 nm
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Fig. 1. Schematic of the melt-spun process [18] (a) and the optical micrograph of the conventional cast AlSi7Mg ingot (b).
increases gradually from wheel side to free surface of the ribbon, which constitutes the microstructure of the AlSi7Mg melt-spun ribbon. 3.3. Mechanical properties The mechanical properties of melt-spun ribbons can be determined by the microhardness test and tensile test. Microhardness test is the easiest and most straight forward technique. It should be mentioned that tensile properties of melt-spun ribbons are dependent on the surface quality, so ribbons with good surface quality were chosen and multiple reproducible tensile tests were conducted.
Fig. 2. XRD patterns taken from the wheel side and free surface of the AlSi7Mg melt-spun ribbon.
and large Si particles ranging from 50 to several hundreds of nanometers with clear size boundary are dispersed both in the interior and boundary of fine a-Al, and the size of large Si particles
3.3.1. Microhardness The Vickers microhardness values of melt-spun and those of conventional cast AlSi7Mg alloy are summarized in Table 2. The microhardness of the AlSi7Mg melt-spun ribbons is approximately 1.56 times higher than that of the original ingot. The microhardness 114.8 ± 4.1 kg mm 2 obtained here for the melt-spun ribbon is somewhat lower than the values reported for the melt-spun
Fig. 3. Optical micrographs of the surface position (a) and thickness directional position (b) for observation and OM (c) and SEM (d and e) micrographs of the surface microstructure of the observation zone I in the AlSi7Mg melt-spun ribbon.
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Fig. 4. Optical micrograph (a) of the cross-sectional microstructure of the AlSi7Mg melt-spun ribbon and SEM micrographs of (b) three observation zones A (g and h), B (e and f) and C (c and d) along the cross-section of the ribbon.
Table 2 Hardness values of the melt-spun ribbon compared with the conventional cast ingot. Alloy
Status
Hardness (Vickers) (kg mm
2
AlSi7Mg
Melt-spun Ingot
115 73.8
118.7 68.5
112.3 78.2
)
AlSi9Mg (136.5 kg mm 2) ribbon [13] because of relatively lower content of Si. The high microhardness of the melt-spun alloy compared with its ingot counterpart can be partly attributed to the supersaturated solid solution and the grain refinement strengthening (see Fig. 3(c)) effects of a-Al. The atomic radius difference between two elements in solid solution provides a strain field, which interacts with dislocations and results in solution strengthening. This solid solution strengthening mechanism is also supported by the above
Average 110.6 73.4
121.6 78.2
116.9 65.3
109.7 68.9
113.2 81.3
114.8 ± 4.1 73.5 ± 5.6
mentioned XRD analyses with a solid solubility of Si in Al matrix of about 0.62 at.%. In addition, we think that the nanoscale dispersed Si particles also contribute to the improvement of the microhardness, which was seldom discussed in detail by former studies about Al–Si alloy melt-spun ribbons because of deficient characterization of nanoscale Si particles. It was found that the microhardness of the Si phase was as high as 813.3 kg mm 2 during the microhardness test of the AlSi7Mg ingot, which is significantly higher than that of a-Al. Considering that the microhardness test indentation of the
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AlSi7Mg melt-spun ribbon is in the order of a dozen micrometers (see Fig. 5(a)), which covers hundreds of nanoscale dispersed Si particles with high microhardness that is eight or more times higher than that of the supersaturated solid solution Al matrix, as shown in Fig. 5(b). Thus the diamond indenter will suffer great impediment from hundreds of nanoscale dispersed Si particles during the press process, which results in the improvement of the microhardness. Besides, the nanoscale dispersed Si particles in the Al matrix will cause the dispersion strengthening effect of the AlSi7Mg melt-spun ribbon further, which also contributes to the enhancement of the microhardness. 3.3.2. Tensile properties The tensile properties of the melt-spun AlSi7Mg alloy and the conventional cast ingot are shown in Fig. 6. The ultimate tensile strength (UTS) and yield strength (YS) of the AlSi7Mg melt-spun ribbon are 270 and 220 MPa respectively, which are significantly higher than the 177 MPa (UTS) and 126 MPa (YS) of the conventional cast ingot, although the casting defects and test errors of the melt-spun ribbons with thin thickness (50 lm) could lead to the decrease of the test values. It should be mentioned that the UTS obtained here for the AlSi7Mg ribbon (270 MPa) is similar to the reported AlSi9Mg ribbon (274 MPa) [13], while the breaking elongation obtained here for the AlSi7Mg melt-spun ribbon is slightly lower than these reported by Rios et al. [13] and Chen et al. [23] for AlSi9Mg (2.9%) and Al1.5Fe (2.3%) melt-spun ribbons. The breaking elongation of the AlSi7Mg ribbon is 1.73%, which is lower than that of the conventional cast ingot (3.1%). The nanoscale dispersed Si particles in the AlSi7 Mg ribbon can average the deformation stress in a-Al and prevent stress concentration. As is well known, the stress concentration results in the crack of the large plate-like Si phase in the conventional cast ingot, which releases the deformation stress in a-Al and serves as nucleation sites of cracks. So the deformation of the AlSi7Mg melt-spun ribbon happens in a high level of deformation stress, and cracks are hard to form during the tensile process, which leads to the rapid increase of the deformation stress with deformation without a clear yield point, as reported by Rios et al. [13] and Chen et al. [23]. Therefore, fracture happens instantaneously when the deformation stress in the AlSi7Mg ribbon increases rapidly to UTS. Thus the breaking elongation of the AlSi7Mg ribbon is lower than that of the conventional cast ingot since there is no obvious yield process during the tensile process. The higher values of UTS and YS of the melt-spun AlSi7Mg ribbons compared with the conventional cast ingot could be partly related to the supersaturated solution strengthening effect of the alloying elements especially Si in the Al-matrix and the grain refinement effects of a-Al (see Fig. 3(c)). In addition, we think that the nanoscale dispersed Si particles induced by the rapid solidification process also contribute to the improvement of UTS and YS, as seldom discussed in detail by former studies about Al–Si alloy
Fig. 6. Tensile properties of the AlSi7Mg melt-spun ribbon at room temperature compared with the conventional cast ingot.
melt-spun ribbons due to the deficient characterization of nanoscale Si particles and the insufficient comparison of tensile fractures between conventional cast ingot and the melt-spun ribbon. The eutectic Si phase of the conventional cast AlSi7Mg ingot is large plate-like and distributes in the grain boundary of a-Al, as shown in Fig. 7(a). The interface between large plate-like eutectic Si phase and a-Al is easy to induce stress concentration, which results in the crack of the large plate-like eutectic Si phase, as indicated by the solid arrows in Fig. 7(c). The crack of large plate-like eutectic Si phase in the grain boundary releases the deformation and stress in a-Al and stimulates the happening of deformation, which leads to the low YS of the ingot. Then, the cracked Si phase provides nucleation sites for the cavity and induces the formation of cracks across the a-Al grain [24–26], as indicated by the dotted arrows in Fig. 7(c). Finally, the development of cracks results in the fracture of ingot, which is the origin of the low UTS for the ingot. Significantly different from the large plate-like eutectic Si phase in the ingot, nanoscale Si particles were found to be dispersed in the Al matrix of the AlSi7Mg melt-spun ribbon, as shown in Fig. 7(d). The deformation stress in fine a-Al (see Fig. 3(c)) during the tensile process can be averaged to the nanoscale dispersed Si particles without crack, which means that the microstructure of fine a-Al and nanoscale dispersed Si particles can suffer higher deformation stress. Besides, the transfer of deformation and stress from one a-Al grain to another is prevented, which leads to the happening of plastic deformation under higher stress. Thus the YS of the melt-spun AlSi7Mg ribbon is increased by 1.75 times (220 MPa) comparing with the ingot (126 MPa). Because of the lack of cracked Si particles to provides nucleation sites for the cavity, cracks are difficult to be formed across the a-Al grain, and intergranular fracture happens, as indicated by the intergranular cracked fine a-Al grain in Fig. 7(e) and the intergranular fracture cells in Fig. 7(f), which results in the improvement of UTS by 1.53 times (270 MPa) comparing with the ingot (177 MPa).
Fig. 5. Hardness test indentation (a) of the AlSi7Mg melt-spun ribbon and nanoscale dispersed Si particles in the schematic area of hardness test indentation (b).
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Fig. 7. Microstructure of Si phase for conventional cast (a) and melt-spun (d) AlSi7Mg alloy and tensile fracture morphology of AlSi7Mg ingot (b and c) compared with that of melt-spun ribbon (e and f).
4. Conclusions (1) Different from former studies about the Al–Si alloy meltspun ribbons, both the surface and gradient cross-sectional microstructure were characterized in detail to give a clear and integrated description of the melt-spun AlSi7Mg ribbon, and a extended solid solubility of Si in Al matrix to be 0.62 at.% was found. (2) Two kinds of nanoscale Si particles were observed and reported, i.e., small Si particles ranging from 13 to 50 nm and large Si particles ranging from 50 nm to several hundreds of nanometers with clear size boundary were found to be dispersed both in the interior and boundary of fine a-Al, and the size of large Si particles increased gradually from wheel side to free surface of the ribbon, which constituted the microstructure of the AlSi7Mg melt-spun ribbon. (3) Tensile properties of the AlSi7Mg melt-spun ribbon were investigated, and the ultimate tensile strength, yield strength and microhardness were found to be 1.53, 1.75 and 1.56 times higher than that of the conventional cast ingot respectively. The effects of nanoscale dispersed Si particles on the significant improvement of both hardness and tensile properties for the AlSi7Mg melt-spun ribbon were provided in detail. (4) The breaking elongation of the AlSi7Mg melt-spun ribbon was 1.73%, and granular cells in the fracture surface of the ribbon suggested an intergranular fracture due to the rapid solidification process.
Acknowledgements The work was supported by the National Basic Research Program of China (2013CB632203).
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