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Microstructure and mechanical properties of Al–Si–Fe–X alloys
Materials and Design 107 (2016) 491–502
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Microstructure and mechanical properties of Al–Si–Fe–X alloys Andrea Školáková a,⁎, Pavel Novák a, Dalibor Vojtěch a, Tomáš František Kubatík b a b
University of Chemistry and Technology, Prague, Department of Metals and Corrosion Engineering, Technická 5, 166 28 Prague 6, Czech Republic Institute of Plasma Physics AS CR, v.v.i., Za Slovankou 1782/3, 182 00 Prague 8, Czech Republic
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Article history: Received 23 February 2016 Received in revised form 30 May 2016 Accepted 16 June 2016 Available online 18 June 2016 Keywords: Aluminium alloys Rapid solidification Melt spinning Spark plasma sintering Thermal stability
a b s t r a c t One of the current problems concerning the re-use of aluminium alloys is the recycling of aluminium alloys with increased content of iron. One of the recent trends is the processing by powder metallurgy, producing completely new grades of aluminium alloys. The aim of the present work was to test the properties of Al–Si–Fe alloy, which can be manufactured from the high-irony scrap, as a possible material for elevated temperature applications. Since chromium and nickel accompany the iron in the scrap when stainless steels are admixed, the effect of chromium and nickel on thermal stability and mechanical properties at room and elevated temperature was also studied. Al–Si–Fe–X alloys were rapidly solidified by using melt spinning technique. The rapidly solidified ribbons were pulverized by planetary ball mill and the obtained powders were used for compaction by means of Spark Plasma Sintering. It was found that nickel improves the thermal stability as well as the mechanical properties at the elevated temperature. On the other hand, the effect of chromium on the mechanical properties at elevated temperature was almost detrimental. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Al–Si alloys are desired for use in severe conditions, which require high wear-resistance and thermal stability at elevated temperature [1]. Rapid solidification allows the refinement of microstructure [2] and its significant modification and extended solid solubility of constituting elements. Furthermore, it is possible to produce metastable phases by cooling of the melts by rates about 104− 106 K·s−1 by rapid solidification [3]. So, the cooling rate plays an important role in controlling the morphology of primary silicon, eutectic silicon and other phases [4]. These effects lead to better mechanical properties, because new metastable phases are produced during this process. The rapid solidification powder metallurgy technology allows manufacture of powders, foils or ribbons [5,6,7]. Most transition metals exhibit a low solubility and diffusivity in solid Al. The low diffusivity is useful in stabilization of the microstructure at elevated temperature, slowing down the recrystallization processes [8]. Many of them also form thermally stable phases, which pin the grain boundaries and prevent grain growth. Combining rapid solidification with the addition of alloying elements could result in the structure refinement and excellent mechanical properties at elevated temperatures. These alloys can be used at temperatures up to about 250 °C and allow providing weight reduction and high efficiency in automotive and aerospace industry [9,10,11]. The addition of transition metals significantly improved the hardness ⁎ Corresponding author. E-mail addresses: [email protected] (A. Školáková), [email protected] (T.F. Kubatík).
http://dx.doi.org/10.1016/j.matdes.2016.06.069 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
and compressive strength of rapidly solidified Al–Si–Fe alloys [10]. Thus, new aluminium alloys with transition metals made by rapid solidification processing are promising materials for applications in automotive and aerospace industries. This present study is concerned with the effect of Cr and Ni additions on microstructure and thermal stability of the Al–Si–Fe base alloys processed using rapid solidification technique (melt spinning) and rapid consolidation by Spark Plasma Sintering (SPS) process. Iron, as one of the main constituents of the tested alloys, is usually taken as an impurity in aluminium alloys, because it forms hard and brittle Al–Fe–Si phases. A majority of aluminium waste is contaminated by certain level of iron. All wrought aluminium alloys always contain at least minor iron amount. Growing concentration of iron is caused by an increasing use of recycled aluminium, where iron comes for example from the iron-containing die casting alloys, from austenitic stainless steel parts which cannot be magnetically separated from aluminium. In rapidly solidified alloys, iron increases the thermal stability due to precipitation of intermetallic compounds with iron [5,10,12]. Formation of intermetallic phases during solidification depends on the levels of Fe and Si concentration and cooling rate [13,14]. Nickel in aluminium alloys is known to improve the thermal stability due to low diffusivity in aluminium. Therefore it is added to the Al–Si based alloys for the manufacture of the pistons of internal combustion engines. Chromium is a relatively cheap alloying element and has one of the lowest diffusion coefficients in aluminium. However, its solubility in aluminium is extremely low [15]. Cr modifies the morphology of the iron-containing intermetallic compound and refines the primary silicon particles [10].
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The development of recycling technologies has increased for the high-irony scrap of Al–Si alloys. One of the possible methods of processing this waste is powder metallurgy using rapid solidification methods and SPS. SPS process is a modern method of sintering, which allows consolidation of various materials by the combination of uni-axial pressing and passage of high electric current [16]. This method utilizes rapid heating and lower sintering temperatures than in conventional sintering techniques [17]. The compact specimen is obtained in a very short time. The advantages are low cost, minimal grain growth and maintaining fine-grained structure [18]. In this work, the production of Al–Si–Fe, Al–Si–Fe–Cr and Al–Si–Fe– Ni alloys by powder metallurgy of rapidly solidified particles was tested. Hardness of melt-spun ribbons and compact alloys, obtained by cryogenic milling and subsequent Spark Plasma Sintering (SPS) process, was tested. Vickers hardness was measured and differential thermal analysis was carried out in order to determine mechanical properties and thermal stability, respectively.
performed on a Setaram DSC 131 instrument under argon atmosphere in the temperature range of 25–600 °C. 3. Results and discussion 3.1. Microstructure of the cast alloys Cast alloys are composed of eutectic structure α-Al and Si in which the coarse intermetallic phases Al5FeSi (AlSi12Fe7 alloy and AlSi12Fe4Cr3
2. Experimental Cast AlSi12Fe7, AlSi12Fe4Ni3 and AlSi12Fe4Cr3 alloys were prepared by melting in an electric resistance furnace. Commercial master alloys (AlSi50, AlFe10, AlNi20, AlCr5 — in wt.%) and commercial purity elements (Al, Si, Fe, Cr) were used as components for the alloy preparation. Ribbons were produced by the melt spinning process by means of casting of molten alloy onto the rotating copper wheel (CuCr1Zr0.1) with a diameter of 300 mm rotating at 30 m/s. Melting was completed under argon protective atmosphere with temperature of the melt 950 °C. The rapidly solidified ribbons were pulverized by planetary ball mill Retch PM 100 CM in a stainless steel container (ball-to-powder ratio of 15:1) for 10 min at 400 ppm. At the beginning of the milling, liquid nitrogen was added to the container. The obtained powders, with the size of the particles at less than 50 μm, were consolidated to 20 mm diameter cylindrical samples by the means of SPS. SPS was carried out in the Institute of Plasma Physics AS CR using Thermal Technology SPS 10-4 device by the pressure of 80 MPa for 10 min at 500 °C with the heating rate of 100 K/min. The samples were ground using P80–P4000 sandpapers with SiC abrasive particles and polished using D2 diamond paste. Then the samples were etched by Kroll's reagent (5 ml HNO3, 10 ml HF, 85 ml H2O). Microstructure of all the samples was investigated by the optical microscope (Olympus PME3) with Axio Vision 4.8 software, as well as by the scanning electron microscope (SEM–TESCAN VEGA 3 LMU). Phase composition was determined by X-ray diffraction (PANanalyticalX'Pert Pro, CuKα radiation). The grain size of aluminium-based matrix was determined by Scherrer Calculator in X'Pert High Score Plus software. Vickers microhardness was measured by Carl Zeiss Neophot 2 metallographic light microscope equipped with Hanneman hardness testing unit with a load of 5 g (HV 0.005) on the melt spun ribbons in longitudinal and cross section, while the hardness of the cast and compacted samples was investigated by the Vickers method with the load of 5 kg (HV 5) due to the heterogeneity of the samples. The thermal stability of the melt spun ribbons and compact alloys was evaluated as the change in hardness (HV 0.005) during the shortterm or long-term annealing performed at 100–500 °C for 1 h, respectively at 300 and 400 °C for 400 h in air. Hardness was measured every 100 h. Mechanical properties were measured on compact samples by means of the universal testing machine LabTest 5.250 SP1-VM. The device is equipped with a laser extensometer for the accurate measurement of deformation of the sample. Tests were performed at room temperature and at 300 and 400 °C on samples prepared by SPS. Differential scanning calorimetry (DSC) was performed on the rapidly solidified ribbons. The temperature program was set to a linear increase of temperature at a heating rate of 10 K/min. DSC analyses were
Fig. 1. Light micrographs of a) AlSi12Fe7, b) AlSi12Fe4Ni3 and c) AlSi12Fe4Cr3.
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Fig. 2. X-ray diffraction patterns of cast alloys.
Fig. 3. Microstructure of a) AlSi12Fe7, b) AlSi12Fe4Ni3 and c) AlSi12Fe4Cr3.
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Fig. 4. Detailed SEM micrograph of rapidly solidified AlSi12Fe7 alloys.
alloy) and Al2FeSi (AlSi12Fe4Ni3 alloy) are dispersed (Figs. 1a, b, c and 2). It shows that nickel stabilizes different types of Al–Fe–Si phase than occurs in ternary Al–Fe–Si alloys.
3.2. Microstructure of melt spun ribbons Fig. 3a–c shows the optical micrographs of the microstructure for melt spun ribbons. Observation reveals that microstructure is very fine and homogeneous. This is also proved by the SEM micrograph presented in Fig. 4 showing that the particle size is in sub-micron scale. For the rapidly solidified alloy AlSi12Fe7 it can be observed that the intensity of diffraction lines of intermetallic phase Al5FeSi is lower than in as-cast state (Figs. 2, 5). Therefore it can be assumed that the quantity of this intermetallic phase decreases in comparison with the cast alloy (Fig. 2). X-ray diffraction analyses of the Al–Si–Fe alloys revealed the presence of Si, Al and Al5FeSi. This phase also replaced Al2FeSi phase for Al–Si–Fe–Ni. In the case of Al–Si–Fe–Cr alloys, the weak
Fig. 6. Hardness of a) cast alloys and b) melt spun ribbons.
diffraction lines from Al–Fe–Si intermetallics indicate that the ribbon contains very low amount of this phase. The decrease of the amount of intermetallic phases is caused by the rapid cooling and leads to supersaturation of solid solution alloying elements. Small amount of alloying elements remains to create the intermetallic phases. It can also be seen that the microstructure of the air sides is coarser than that of the wheel sides. This is due to the fact that the cooling rate on the wheel side is higher than that of the air side.
Fig. 5. X-ray diffraction patterns of the rapidly solidified ribbons.
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Fig. 7. a) DSC heating curve and b) X-ray diffraction patterns of the annealed rapidly solidified alloy AlSi12Fe7.
3.3. Hardness of alloys The addition of nickel and chromium caused increasing hardness for cast alloys (Fig. 6a). Cast alloy containing nickel was the hardest and melt spun ribbons were harder than the cast alloys. Hardness also depends on the state of alloys. The alloys prepared by rapid cooling of the melt contained very fine structure, which led to significantly higher hardness. Higher hardness is probably caused by very fine solid solution and the presence of intermetallic phases. Rapidly solidified alloy containing chromium was the hardest (Fig. 6b). This shows that the increase in hardness can be attributed to ultrafine-grained supersaturated solid solution of α-Al, which results in the internal stress in the matrix and also to fine Al5FeSi particles. In addition, the alloy may contain ultrafine coherent particles of intermetallics, which are not detectable by XRD. Al–Si–Fe alloys were much less hard than alloys with chromium and nickel. It was further found that the hardness of the cross section
is greater than the hardness of the longitudinal section. This likely indicates the presence of the texture in the rapidly solidified ribbons due to unidirectional heat dissipation. So, by evaluating the results in Fig. 6b one can clearly see that the hardness values for cross section of ribbons are higher than values for longitudinal section and increase with the addition of alloying elements. Rapid solidification has a significant effect to increase the hardness. The positive influence of the transition metals on the hardness was also noticeable. 3.4. Thermal stability of rapidly solidified ribbons 3.4.1. Thermal analysis Differential thermal analysis allows the determination of the temperature which leads to phase transformation. The decomposition of the supersaturated solid solution in rapidly solidified ribbon was
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Fig. 8. a) DSC heating curve and b) X-ray diffraction patterns of the annealed rapidly solidified alloy AlSi12Fe4Ni3.
accompanied by an exothermic reaction. Exothermic peaks indicate that there is a precipitation of intermetallic phases and silicon from the supersaturated aluminium grains. Exothermic transformation takes place at 320 °C for AlSi12Fe7 (Fig. 7a). Al5FeSi phase precipitates from the supersaturated solid solution at these temperatures (Fig. 7b). The exothermic transformations occur at 200 °C and at 300 °C for AlSi12Fe4Ni3 (Fig. 8a). Silicon precipitates from the supersaturated solid solution in this case and the phases Al5FeSi and Al2FeSi appear there (Fig. 8b). Phase transformations take place at about 200 °C and 350 °C for AlSi12Fe4Cr3 (Fig. 9a). Only silicon precipitates from the supersaturated solid solution at 200 °C and a ternary intermetallic phase is formed at 300 °C (Fig. 9b). The endothermic effect occurs at about 580 °C in all studied alloys. This effect is associated with the eutectic transformation in Al–Si system.
3.4.2. Short term annealing Thermal stability was determined so that the rapidly solidified alloy was annealed for 1 h in the temperature range of 100–500 °C. Good thermal stability was observed for alloys which were alloyed with chromium and nickel (Fig. 10). Mechanical properties of the Al–Si–Fe alloys decrease after exceeding 300 °C. The hardness of quaternary alloy was much higher in both cases above at this temperature. Furthermore, alloy containing chromium was the hardest. XRD patterns show (Figs. 7b, 8b, 9b) that the phases Al5FeSi or Al2FeSi precipitate during annealing of all alloys. Differences in thermal stability are likely caused by the different amounts of the thermally stabilizing (slowly diffusing) transition metals, which are contained in the supersaturated solid solution of α-Al. Higher supersaturation allows precipitation of more hardening phases during thermal exposure. The
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Fig. 9. a) DSC heating curve and b) X-ray diffraction patterns of the annealed rapidly solidified alloy AlSi12Fe4Cr3.
hardness decreased after the temperature exceeds 300 °C for all alloys. It is caused by gradual coarsening particles, which cause the hardness decrease.
3.4.3. Long term annealing Rapidly solidified alloys were annealed for 400 h at temperatures of 300 and 400 °C. Hardness was measured every 100 h. Alloys, that were annealed at 300 °C, were harder than alloys annealed at 400 °C. This fact was proved in all cases (Fig. 11a–c). The hardness of alloy with nickel had been always the highest after 100 h and the hardness was constant throughout the annealing (Fig. 11b). Therefore, this alloy can be considered as thermally stable. The hardness of the alloys Al–Si–Fe increased after 400 h (Fig. 11a). Probably intermetallic phase Al5FeSi precipitated from the supersaturated solid solution (Fig. 12). However, its hardness
Fig. 10. Vickers microhardness vs. annealing temperature.
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on the hardness was noticeable. Microstructure of melt-spun ribbons was very fine and homogeneous. In the case of chromium-containing alloy, rapid solidification suppressed formation of intermetallics. The hardness decreased after the temperature exceeds 300 °C for all alloys during short-term annealing but chromium and nickel improved thermal stability. Alloys annealed at 300 °C for 400 h ware harder then alloys annealed at 400 °C. The hardness of Al–Si–Fe–Ni alloys was almost constant throughout the annealing and the hardness of alloy with chromium only decreased. Therefore, the addition of Ni to Al–Si–Fe improved the thermal stability as expected. 3.4.4. Microstructure of compact alloys The compaction of the powder obtained from rapidly solidified ribbons was done by Spark Plasma Sintering. The grains did not grow noticeably during the process (Table 1). The structure is very fine and homogeneous (Fig. 14a–c). Uniform and fine intermetallic phase is dispersed in the structure, even though a very slight coarsening of the primary intermetallics can be observed (Fig. 15). The XRD patterns show the presence of Al5FeSi phase in the structure of AlSi12Fe7 and AlSi12Fe4Cr3 alloys (Fig. 16). Very small quantities of this phase appeared in the structure of the rapidly solidified alloy with chromium (Fig. 5). This indicates that this phase formed during SPS. Alloy with nickel contained Al2FeSi phase (Fig. 16), which was identified in rapidly solidified alloy annealed for 1 h at 300 °C but not in the structure of rapidly solidified alloy before annealing. According to the XRD pattern, the needle-shaped phase distributed in the ribbons was analysed as a Al5FeSi compound (Fig. 5), and it was transformed partially to a stable Al2SiFe compound after annealing (Fig. 8b). This phase (Al2FeSi) is very fine and it causes the increase of hardness of compact alloy. Its average dimension is about 0.069 μm. There was no notable change in XRD pattern test between cast and consolidated powder. The structure is composed by supersaturated solid solution of α-Al and intermetallic phases, which are stable and very fine.
Fig. 11. Vickers microhardness (HV 0.005) of rapidly solidified alloys a) AlSi12Fe7, b) AlSi12Fe4Ni3 and c) AlSi12Fe4Cr3 vs. annealing time.
does not exceed the hardness of alloy with nickel. In contrast, the hardness of alloy containing chromium only decreased (Fig. 11c). The structure is composed of a supersaturated solution before annealing and although the precipitation of intermetallics was detected during heating, its effect is not sufficient in this case. XRD pattern shows (Fig. 12) very remarkable process. Phase Al5FeSi had precipitated in the case of AlSi12Fe7 alloys. The hardness of this alloy increased after 400 h of annealing. Alloy AlSi12Fe4Ni3, that was annealed at 300 °C, was composed of new phases Al5FeNi and Al8Fe2Si. These phases caused the increase of hardness after long-term annealing. Phase Al8Fe2Si also appeared during annealing at 400 °C, but phase Al5FeNi was replaced by phase Al5FeSi, which was identified previously (Figs. 8b, 12). The size of the primary particles of intermetallics slightly increased (Fig. 13), as compared with Fig. 4. Microstructure of the cast alloys was composed of large intermetallic phases. The coarse microstructure of the cast alloys resulted in a low hardness. The addition of nickel and chromium caused the increase of hardness of cast alloys. The positive influence of the transition metals
3.4.5. Hardness of compact alloys Hardness of compact alloys is shown in Fig. 17a. Hardness increased up to three times. It is caused by the very fine structure of these alloys. The high hardness for all compact alloys compared with their cast counterparts can be attributed to finer structure. The highest hardness belongs to the Al–Si–Fe–Ni alloys and the lowest hardness was measured for Al–Si–Fe–Cr. Results show, that the compact alloy with chromium has lower hardness in comparison with the rapidly solidified alloy with chromium (Figs. 6b, 17a). The same trend was observed for Al–Si–Fe–Ni alloys but not for Al–Si–Fe alloys. 3.5. Thermal stability of compact alloys 3.5.1. Short term annealing Compact alloys were annealed for 1 h at 100–500 °C. Hardness of compact alloys is lower in comparison with the rapidly solidified alloy in the whole temperature range. Compact alloy AlSi12Fe4Ni3 is the hardest and the hardness is constant at 100–500 °C (Fig. 17b). Alloying by nickel thus positively affects the thermal stability of the alloy. On the other hand, chromium-alloyed compact material has the lowest hardness during annealing. There can be a strong increase of hardness observed at 300 °C, attributed probably to the precipitation of Al5FeSi intermetallics, as proved by DSC and XRD in the case of rapidly solidified ribbons (Fig. 9a, b). At the temperatures higher than 300 °C, the hardness decreases. It can be concluded that the effect of chromium on thermal stability of Al–Si–Fe alloys is rather negative. Basic compact Al–Si–Fe alloys are harder than Al–Si–Fe–Cr throughout the testing interval and show almost the same trends in change of the hardness with annealing temperature.
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Fig. 12. X-ray diffraction patterns of the annealed rapidly solidified alloy for 400 h.
3.5.2. Long term annealing The alloys were also annealed for 400 h at 300 and 400 °C and the hardness was measured every 100 h. The difference between the hardness of the alloys, which were annealed at 300 and 400 °C, is small (Fig. 18a–c) in comparison with
the hardness of the melt spun ribbons, when the hardness was significantly higher for the alloys annealed at 300 °C (Fig. 11a–c). Hardness of alloy AlSi12Fe7 gradually increased for temperature 400 °C and decreased for 300 °C (Fig. 18a). This alloy is also harder than alloy with chromium (Fig. 18b). Hardness of alloy with chromium decreased only slightly and can be considered almost constant (Fig. 18b), unlike melt spun ribbons with chromium, when hardness changed very sharply (Fig. 11c). Alloy with nickel was the hardest throughout thermal exposure again (Fig. 18c). The addition of nickel increases thermal stability while the addition of chromium decreases hardness but its value is also almost constant. Compact alloys retained very fine structure with submicron size intermetallics. Phase composition was same, as in the case of cast alloys.
Table 1 Grain size vs. state of the alloys. State of alloys
Alloys
Grain size of Al matrix [μm]
Cast alloys
AlSi12Fe7 AlSi12Fe4Ni3 AlSi12Fe4Cr3 AlSi12Fe7 AlSi12Fe4Ni3 AlSi12Fe4Cr3 AlSi12Fe7 AlSi12Fe4Ni3 AlSi12Fe4Cr3
0.261 0.234 0.236 0.159 0.169 0.131 0.171 0.171 0.225
Rapidly solidified alloys
Compact alloys Fig. 13. Detailed SEM micrograph of annealed rapidly solidified AlSi12Fe7 alloys.
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Fig. 15. SEM micrograph of AlSi12Fe7 alloys.
3.5.3. Mechanical properties of compact alloys Mechanical properties were measured on compact samples and it was found that alloy with nickel has yield strength at room temperature only slightly higher (474 MPa) than alloy Al–Si–Fe (467 MPa). Yield strength dramatically decreased with increasing temperature (Fig. 19). The yield strength of alloy with chromium was the worst at laboratory temperature and at 300 °C. Moreover, yield strength of all alloys measured at 400 °C was about the same. Yield strength (measured at 400 ° C) was only slightly higher than that of pure aluminium after annealing (20 MPa) [19].
4. Conclusion
Fig. 14. Microstructure of a) AlSi12Fe7, b) AlSi12Fe4Ni3 and c) AlSi12Fe4Cr3.
Nickel alloying resulted in significantly higher hardness of the compact powder metallurgy prepared sample. Hardness of compact alloys was lower at comparison with rapidly solidified alloys during short term annealing. Nickel stabilized mechanical properties at elevated temperatures. Chromium affected the thermal stability positively, keeping the hardness constant throughout the test duration, but its effect on the mechanical properties at 300 and 400 °C was rather negative.
This study was focused on the microstructure, phase composition and thermal stability of Al–Si–Fe–X alloys prepared by melt spinning process and Spark Plasma Sintering. Microstructure of the cast alloys was composed of large intermetallic phases (Al5FeSi and Al2FeSi). Microstructure of melt-spun ribbons was very fine and homogeneous. It implies that the hardness of the ribbons was remarkably superior to that of the cast alloy. X-ray diffraction analyses of the Al–Si–Fe alloys revealed the presence of Si, Al and Al5FeSi. This phase also replaced Al2FeSi phase for Al–Si–Fe–Ni. In the case of Al–Si–Fe–Cr alloys, the weak diffraction lines from Al–Fe–Si intermetallics indicated that the ribbon contains very low amount of this phase. Compact alloys retained very fine structure. Uniform and fine intermetallic phase is dispersed in the structure. Phase composition was the same as in the case of cast alloys. The XRD patterns showed the presence of Al5FeSi phase in the structure of AlSi12Fe7 and AlSi12Fe4Cr3 alloys. Alloy with nickel contained Al2SiFe phase. The needle-shaped phase distributed in the ribbons was analysed as a Al5FeSi compound, and it was transformed partially to a stable Al2SiFe compound after annealing. This phase was very fine and it caused the increase of hardness of melt-spun ribbons. Nickel stabilized mechanical properties at elevated temperatures. Chromium affected the thermal stability positively, keeping the hardness constant throughout the test duration, but its effect on the mechanical properties at 300 and 400 °C was rather negative. Results showed that the alloys, especially the Al–Si–Fe–Ni alloys, could find possible application at elevated temperatures, e.g. in car engines. When assumed that this alloy could be manufactured from iron-rich scrap of Al–Si alloy, it can be highly beneficial for this industrial sector.
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Fig. 16. X-ray diffraction patterns of compact alloys.
Fig. 17. a) Hardness and b) microhardness of compact alloys vs. temperature.
Fig. 18. Vickers microhardness (HV 0.005) of compact alloys a) AlSi12Fe7, b) AlSi12Fe4Cr3 and c) AlSi12Fe4Ni3 vs. annealing time.
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Fig. 19. Yield stress of compact alloys.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Microstructure and mechanical properties of Al–Si–Fe–X alloys Andrea Školáková a,⁎, Pavel Novák a, Dalibor Vojtěch a, Tomáš František Kubatík b a b
University of Chemistry and Technology, Prague, Department of Metals and Corrosion Engineering, Technická 5, 166 28 Prague 6, Czech Republic Institute of Plasma Physics AS CR, v.v.i., Za Slovankou 1782/3, 182 00 Prague 8, Czech Republic
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Article history: Received 23 February 2016 Received in revised form 30 May 2016 Accepted 16 June 2016 Available online 18 June 2016 Keywords: Aluminium alloys Rapid solidification Melt spinning Spark plasma sintering Thermal stability
a b s t r a c t One of the current problems concerning the re-use of aluminium alloys is the recycling of aluminium alloys with increased content of iron. One of the recent trends is the processing by powder metallurgy, producing completely new grades of aluminium alloys. The aim of the present work was to test the properties of Al–Si–Fe alloy, which can be manufactured from the high-irony scrap, as a possible material for elevated temperature applications. Since chromium and nickel accompany the iron in the scrap when stainless steels are admixed, the effect of chromium and nickel on thermal stability and mechanical properties at room and elevated temperature was also studied. Al–Si–Fe–X alloys were rapidly solidified by using melt spinning technique. The rapidly solidified ribbons were pulverized by planetary ball mill and the obtained powders were used for compaction by means of Spark Plasma Sintering. It was found that nickel improves the thermal stability as well as the mechanical properties at the elevated temperature. On the other hand, the effect of chromium on the mechanical properties at elevated temperature was almost detrimental. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Al–Si alloys are desired for use in severe conditions, which require high wear-resistance and thermal stability at elevated temperature [1]. Rapid solidification allows the refinement of microstructure [2] and its significant modification and extended solid solubility of constituting elements. Furthermore, it is possible to produce metastable phases by cooling of the melts by rates about 104− 106 K·s−1 by rapid solidification [3]. So, the cooling rate plays an important role in controlling the morphology of primary silicon, eutectic silicon and other phases [4]. These effects lead to better mechanical properties, because new metastable phases are produced during this process. The rapid solidification powder metallurgy technology allows manufacture of powders, foils or ribbons [5,6,7]. Most transition metals exhibit a low solubility and diffusivity in solid Al. The low diffusivity is useful in stabilization of the microstructure at elevated temperature, slowing down the recrystallization processes [8]. Many of them also form thermally stable phases, which pin the grain boundaries and prevent grain growth. Combining rapid solidification with the addition of alloying elements could result in the structure refinement and excellent mechanical properties at elevated temperatures. These alloys can be used at temperatures up to about 250 °C and allow providing weight reduction and high efficiency in automotive and aerospace industry [9,10,11]. The addition of transition metals significantly improved the hardness ⁎ Corresponding author. E-mail addresses: [email protected] (A. Školáková), [email protected] (T.F. Kubatík).
http://dx.doi.org/10.1016/j.matdes.2016.06.069 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
and compressive strength of rapidly solidified Al–Si–Fe alloys [10]. Thus, new aluminium alloys with transition metals made by rapid solidification processing are promising materials for applications in automotive and aerospace industries. This present study is concerned with the effect of Cr and Ni additions on microstructure and thermal stability of the Al–Si–Fe base alloys processed using rapid solidification technique (melt spinning) and rapid consolidation by Spark Plasma Sintering (SPS) process. Iron, as one of the main constituents of the tested alloys, is usually taken as an impurity in aluminium alloys, because it forms hard and brittle Al–Fe–Si phases. A majority of aluminium waste is contaminated by certain level of iron. All wrought aluminium alloys always contain at least minor iron amount. Growing concentration of iron is caused by an increasing use of recycled aluminium, where iron comes for example from the iron-containing die casting alloys, from austenitic stainless steel parts which cannot be magnetically separated from aluminium. In rapidly solidified alloys, iron increases the thermal stability due to precipitation of intermetallic compounds with iron [5,10,12]. Formation of intermetallic phases during solidification depends on the levels of Fe and Si concentration and cooling rate [13,14]. Nickel in aluminium alloys is known to improve the thermal stability due to low diffusivity in aluminium. Therefore it is added to the Al–Si based alloys for the manufacture of the pistons of internal combustion engines. Chromium is a relatively cheap alloying element and has one of the lowest diffusion coefficients in aluminium. However, its solubility in aluminium is extremely low [15]. Cr modifies the morphology of the iron-containing intermetallic compound and refines the primary silicon particles [10].
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The development of recycling technologies has increased for the high-irony scrap of Al–Si alloys. One of the possible methods of processing this waste is powder metallurgy using rapid solidification methods and SPS. SPS process is a modern method of sintering, which allows consolidation of various materials by the combination of uni-axial pressing and passage of high electric current [16]. This method utilizes rapid heating and lower sintering temperatures than in conventional sintering techniques [17]. The compact specimen is obtained in a very short time. The advantages are low cost, minimal grain growth and maintaining fine-grained structure [18]. In this work, the production of Al–Si–Fe, Al–Si–Fe–Cr and Al–Si–Fe– Ni alloys by powder metallurgy of rapidly solidified particles was tested. Hardness of melt-spun ribbons and compact alloys, obtained by cryogenic milling and subsequent Spark Plasma Sintering (SPS) process, was tested. Vickers hardness was measured and differential thermal analysis was carried out in order to determine mechanical properties and thermal stability, respectively.
performed on a Setaram DSC 131 instrument under argon atmosphere in the temperature range of 25–600 °C. 3. Results and discussion 3.1. Microstructure of the cast alloys Cast alloys are composed of eutectic structure α-Al and Si in which the coarse intermetallic phases Al5FeSi (AlSi12Fe7 alloy and AlSi12Fe4Cr3
2. Experimental Cast AlSi12Fe7, AlSi12Fe4Ni3 and AlSi12Fe4Cr3 alloys were prepared by melting in an electric resistance furnace. Commercial master alloys (AlSi50, AlFe10, AlNi20, AlCr5 — in wt.%) and commercial purity elements (Al, Si, Fe, Cr) were used as components for the alloy preparation. Ribbons were produced by the melt spinning process by means of casting of molten alloy onto the rotating copper wheel (CuCr1Zr0.1) with a diameter of 300 mm rotating at 30 m/s. Melting was completed under argon protective atmosphere with temperature of the melt 950 °C. The rapidly solidified ribbons were pulverized by planetary ball mill Retch PM 100 CM in a stainless steel container (ball-to-powder ratio of 15:1) for 10 min at 400 ppm. At the beginning of the milling, liquid nitrogen was added to the container. The obtained powders, with the size of the particles at less than 50 μm, were consolidated to 20 mm diameter cylindrical samples by the means of SPS. SPS was carried out in the Institute of Plasma Physics AS CR using Thermal Technology SPS 10-4 device by the pressure of 80 MPa for 10 min at 500 °C with the heating rate of 100 K/min. The samples were ground using P80–P4000 sandpapers with SiC abrasive particles and polished using D2 diamond paste. Then the samples were etched by Kroll's reagent (5 ml HNO3, 10 ml HF, 85 ml H2O). Microstructure of all the samples was investigated by the optical microscope (Olympus PME3) with Axio Vision 4.8 software, as well as by the scanning electron microscope (SEM–TESCAN VEGA 3 LMU). Phase composition was determined by X-ray diffraction (PANanalyticalX'Pert Pro, CuKα radiation). The grain size of aluminium-based matrix was determined by Scherrer Calculator in X'Pert High Score Plus software. Vickers microhardness was measured by Carl Zeiss Neophot 2 metallographic light microscope equipped with Hanneman hardness testing unit with a load of 5 g (HV 0.005) on the melt spun ribbons in longitudinal and cross section, while the hardness of the cast and compacted samples was investigated by the Vickers method with the load of 5 kg (HV 5) due to the heterogeneity of the samples. The thermal stability of the melt spun ribbons and compact alloys was evaluated as the change in hardness (HV 0.005) during the shortterm or long-term annealing performed at 100–500 °C for 1 h, respectively at 300 and 400 °C for 400 h in air. Hardness was measured every 100 h. Mechanical properties were measured on compact samples by means of the universal testing machine LabTest 5.250 SP1-VM. The device is equipped with a laser extensometer for the accurate measurement of deformation of the sample. Tests were performed at room temperature and at 300 and 400 °C on samples prepared by SPS. Differential scanning calorimetry (DSC) was performed on the rapidly solidified ribbons. The temperature program was set to a linear increase of temperature at a heating rate of 10 K/min. DSC analyses were
Fig. 1. Light micrographs of a) AlSi12Fe7, b) AlSi12Fe4Ni3 and c) AlSi12Fe4Cr3.
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Fig. 2. X-ray diffraction patterns of cast alloys.
Fig. 3. Microstructure of a) AlSi12Fe7, b) AlSi12Fe4Ni3 and c) AlSi12Fe4Cr3.
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Fig. 4. Detailed SEM micrograph of rapidly solidified AlSi12Fe7 alloys.
alloy) and Al2FeSi (AlSi12Fe4Ni3 alloy) are dispersed (Figs. 1a, b, c and 2). It shows that nickel stabilizes different types of Al–Fe–Si phase than occurs in ternary Al–Fe–Si alloys.
3.2. Microstructure of melt spun ribbons Fig. 3a–c shows the optical micrographs of the microstructure for melt spun ribbons. Observation reveals that microstructure is very fine and homogeneous. This is also proved by the SEM micrograph presented in Fig. 4 showing that the particle size is in sub-micron scale. For the rapidly solidified alloy AlSi12Fe7 it can be observed that the intensity of diffraction lines of intermetallic phase Al5FeSi is lower than in as-cast state (Figs. 2, 5). Therefore it can be assumed that the quantity of this intermetallic phase decreases in comparison with the cast alloy (Fig. 2). X-ray diffraction analyses of the Al–Si–Fe alloys revealed the presence of Si, Al and Al5FeSi. This phase also replaced Al2FeSi phase for Al–Si–Fe–Ni. In the case of Al–Si–Fe–Cr alloys, the weak
Fig. 6. Hardness of a) cast alloys and b) melt spun ribbons.
diffraction lines from Al–Fe–Si intermetallics indicate that the ribbon contains very low amount of this phase. The decrease of the amount of intermetallic phases is caused by the rapid cooling and leads to supersaturation of solid solution alloying elements. Small amount of alloying elements remains to create the intermetallic phases. It can also be seen that the microstructure of the air sides is coarser than that of the wheel sides. This is due to the fact that the cooling rate on the wheel side is higher than that of the air side.
Fig. 5. X-ray diffraction patterns of the rapidly solidified ribbons.
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Fig. 7. a) DSC heating curve and b) X-ray diffraction patterns of the annealed rapidly solidified alloy AlSi12Fe7.
3.3. Hardness of alloys The addition of nickel and chromium caused increasing hardness for cast alloys (Fig. 6a). Cast alloy containing nickel was the hardest and melt spun ribbons were harder than the cast alloys. Hardness also depends on the state of alloys. The alloys prepared by rapid cooling of the melt contained very fine structure, which led to significantly higher hardness. Higher hardness is probably caused by very fine solid solution and the presence of intermetallic phases. Rapidly solidified alloy containing chromium was the hardest (Fig. 6b). This shows that the increase in hardness can be attributed to ultrafine-grained supersaturated solid solution of α-Al, which results in the internal stress in the matrix and also to fine Al5FeSi particles. In addition, the alloy may contain ultrafine coherent particles of intermetallics, which are not detectable by XRD. Al–Si–Fe alloys were much less hard than alloys with chromium and nickel. It was further found that the hardness of the cross section
is greater than the hardness of the longitudinal section. This likely indicates the presence of the texture in the rapidly solidified ribbons due to unidirectional heat dissipation. So, by evaluating the results in Fig. 6b one can clearly see that the hardness values for cross section of ribbons are higher than values for longitudinal section and increase with the addition of alloying elements. Rapid solidification has a significant effect to increase the hardness. The positive influence of the transition metals on the hardness was also noticeable. 3.4. Thermal stability of rapidly solidified ribbons 3.4.1. Thermal analysis Differential thermal analysis allows the determination of the temperature which leads to phase transformation. The decomposition of the supersaturated solid solution in rapidly solidified ribbon was
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Fig. 8. a) DSC heating curve and b) X-ray diffraction patterns of the annealed rapidly solidified alloy AlSi12Fe4Ni3.
accompanied by an exothermic reaction. Exothermic peaks indicate that there is a precipitation of intermetallic phases and silicon from the supersaturated aluminium grains. Exothermic transformation takes place at 320 °C for AlSi12Fe7 (Fig. 7a). Al5FeSi phase precipitates from the supersaturated solid solution at these temperatures (Fig. 7b). The exothermic transformations occur at 200 °C and at 300 °C for AlSi12Fe4Ni3 (Fig. 8a). Silicon precipitates from the supersaturated solid solution in this case and the phases Al5FeSi and Al2FeSi appear there (Fig. 8b). Phase transformations take place at about 200 °C and 350 °C for AlSi12Fe4Cr3 (Fig. 9a). Only silicon precipitates from the supersaturated solid solution at 200 °C and a ternary intermetallic phase is formed at 300 °C (Fig. 9b). The endothermic effect occurs at about 580 °C in all studied alloys. This effect is associated with the eutectic transformation in Al–Si system.
3.4.2. Short term annealing Thermal stability was determined so that the rapidly solidified alloy was annealed for 1 h in the temperature range of 100–500 °C. Good thermal stability was observed for alloys which were alloyed with chromium and nickel (Fig. 10). Mechanical properties of the Al–Si–Fe alloys decrease after exceeding 300 °C. The hardness of quaternary alloy was much higher in both cases above at this temperature. Furthermore, alloy containing chromium was the hardest. XRD patterns show (Figs. 7b, 8b, 9b) that the phases Al5FeSi or Al2FeSi precipitate during annealing of all alloys. Differences in thermal stability are likely caused by the different amounts of the thermally stabilizing (slowly diffusing) transition metals, which are contained in the supersaturated solid solution of α-Al. Higher supersaturation allows precipitation of more hardening phases during thermal exposure. The
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Fig. 9. a) DSC heating curve and b) X-ray diffraction patterns of the annealed rapidly solidified alloy AlSi12Fe4Cr3.
hardness decreased after the temperature exceeds 300 °C for all alloys. It is caused by gradual coarsening particles, which cause the hardness decrease.
3.4.3. Long term annealing Rapidly solidified alloys were annealed for 400 h at temperatures of 300 and 400 °C. Hardness was measured every 100 h. Alloys, that were annealed at 300 °C, were harder than alloys annealed at 400 °C. This fact was proved in all cases (Fig. 11a–c). The hardness of alloy with nickel had been always the highest after 100 h and the hardness was constant throughout the annealing (Fig. 11b). Therefore, this alloy can be considered as thermally stable. The hardness of the alloys Al–Si–Fe increased after 400 h (Fig. 11a). Probably intermetallic phase Al5FeSi precipitated from the supersaturated solid solution (Fig. 12). However, its hardness
Fig. 10. Vickers microhardness vs. annealing temperature.
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on the hardness was noticeable. Microstructure of melt-spun ribbons was very fine and homogeneous. In the case of chromium-containing alloy, rapid solidification suppressed formation of intermetallics. The hardness decreased after the temperature exceeds 300 °C for all alloys during short-term annealing but chromium and nickel improved thermal stability. Alloys annealed at 300 °C for 400 h ware harder then alloys annealed at 400 °C. The hardness of Al–Si–Fe–Ni alloys was almost constant throughout the annealing and the hardness of alloy with chromium only decreased. Therefore, the addition of Ni to Al–Si–Fe improved the thermal stability as expected. 3.4.4. Microstructure of compact alloys The compaction of the powder obtained from rapidly solidified ribbons was done by Spark Plasma Sintering. The grains did not grow noticeably during the process (Table 1). The structure is very fine and homogeneous (Fig. 14a–c). Uniform and fine intermetallic phase is dispersed in the structure, even though a very slight coarsening of the primary intermetallics can be observed (Fig. 15). The XRD patterns show the presence of Al5FeSi phase in the structure of AlSi12Fe7 and AlSi12Fe4Cr3 alloys (Fig. 16). Very small quantities of this phase appeared in the structure of the rapidly solidified alloy with chromium (Fig. 5). This indicates that this phase formed during SPS. Alloy with nickel contained Al2FeSi phase (Fig. 16), which was identified in rapidly solidified alloy annealed for 1 h at 300 °C but not in the structure of rapidly solidified alloy before annealing. According to the XRD pattern, the needle-shaped phase distributed in the ribbons was analysed as a Al5FeSi compound (Fig. 5), and it was transformed partially to a stable Al2SiFe compound after annealing (Fig. 8b). This phase (Al2FeSi) is very fine and it causes the increase of hardness of compact alloy. Its average dimension is about 0.069 μm. There was no notable change in XRD pattern test between cast and consolidated powder. The structure is composed by supersaturated solid solution of α-Al and intermetallic phases, which are stable and very fine.
Fig. 11. Vickers microhardness (HV 0.005) of rapidly solidified alloys a) AlSi12Fe7, b) AlSi12Fe4Ni3 and c) AlSi12Fe4Cr3 vs. annealing time.
does not exceed the hardness of alloy with nickel. In contrast, the hardness of alloy containing chromium only decreased (Fig. 11c). The structure is composed of a supersaturated solution before annealing and although the precipitation of intermetallics was detected during heating, its effect is not sufficient in this case. XRD pattern shows (Fig. 12) very remarkable process. Phase Al5FeSi had precipitated in the case of AlSi12Fe7 alloys. The hardness of this alloy increased after 400 h of annealing. Alloy AlSi12Fe4Ni3, that was annealed at 300 °C, was composed of new phases Al5FeNi and Al8Fe2Si. These phases caused the increase of hardness after long-term annealing. Phase Al8Fe2Si also appeared during annealing at 400 °C, but phase Al5FeNi was replaced by phase Al5FeSi, which was identified previously (Figs. 8b, 12). The size of the primary particles of intermetallics slightly increased (Fig. 13), as compared with Fig. 4. Microstructure of the cast alloys was composed of large intermetallic phases. The coarse microstructure of the cast alloys resulted in a low hardness. The addition of nickel and chromium caused the increase of hardness of cast alloys. The positive influence of the transition metals
3.4.5. Hardness of compact alloys Hardness of compact alloys is shown in Fig. 17a. Hardness increased up to three times. It is caused by the very fine structure of these alloys. The high hardness for all compact alloys compared with their cast counterparts can be attributed to finer structure. The highest hardness belongs to the Al–Si–Fe–Ni alloys and the lowest hardness was measured for Al–Si–Fe–Cr. Results show, that the compact alloy with chromium has lower hardness in comparison with the rapidly solidified alloy with chromium (Figs. 6b, 17a). The same trend was observed for Al–Si–Fe–Ni alloys but not for Al–Si–Fe alloys. 3.5. Thermal stability of compact alloys 3.5.1. Short term annealing Compact alloys were annealed for 1 h at 100–500 °C. Hardness of compact alloys is lower in comparison with the rapidly solidified alloy in the whole temperature range. Compact alloy AlSi12Fe4Ni3 is the hardest and the hardness is constant at 100–500 °C (Fig. 17b). Alloying by nickel thus positively affects the thermal stability of the alloy. On the other hand, chromium-alloyed compact material has the lowest hardness during annealing. There can be a strong increase of hardness observed at 300 °C, attributed probably to the precipitation of Al5FeSi intermetallics, as proved by DSC and XRD in the case of rapidly solidified ribbons (Fig. 9a, b). At the temperatures higher than 300 °C, the hardness decreases. It can be concluded that the effect of chromium on thermal stability of Al–Si–Fe alloys is rather negative. Basic compact Al–Si–Fe alloys are harder than Al–Si–Fe–Cr throughout the testing interval and show almost the same trends in change of the hardness with annealing temperature.
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Fig. 12. X-ray diffraction patterns of the annealed rapidly solidified alloy for 400 h.
3.5.2. Long term annealing The alloys were also annealed for 400 h at 300 and 400 °C and the hardness was measured every 100 h. The difference between the hardness of the alloys, which were annealed at 300 and 400 °C, is small (Fig. 18a–c) in comparison with
the hardness of the melt spun ribbons, when the hardness was significantly higher for the alloys annealed at 300 °C (Fig. 11a–c). Hardness of alloy AlSi12Fe7 gradually increased for temperature 400 °C and decreased for 300 °C (Fig. 18a). This alloy is also harder than alloy with chromium (Fig. 18b). Hardness of alloy with chromium decreased only slightly and can be considered almost constant (Fig. 18b), unlike melt spun ribbons with chromium, when hardness changed very sharply (Fig. 11c). Alloy with nickel was the hardest throughout thermal exposure again (Fig. 18c). The addition of nickel increases thermal stability while the addition of chromium decreases hardness but its value is also almost constant. Compact alloys retained very fine structure with submicron size intermetallics. Phase composition was same, as in the case of cast alloys.
Table 1 Grain size vs. state of the alloys. State of alloys
Alloys
Grain size of Al matrix [μm]
Cast alloys
AlSi12Fe7 AlSi12Fe4Ni3 AlSi12Fe4Cr3 AlSi12Fe7 AlSi12Fe4Ni3 AlSi12Fe4Cr3 AlSi12Fe7 AlSi12Fe4Ni3 AlSi12Fe4Cr3
0.261 0.234 0.236 0.159 0.169 0.131 0.171 0.171 0.225
Rapidly solidified alloys
Compact alloys Fig. 13. Detailed SEM micrograph of annealed rapidly solidified AlSi12Fe7 alloys.
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Fig. 15. SEM micrograph of AlSi12Fe7 alloys.
3.5.3. Mechanical properties of compact alloys Mechanical properties were measured on compact samples and it was found that alloy with nickel has yield strength at room temperature only slightly higher (474 MPa) than alloy Al–Si–Fe (467 MPa). Yield strength dramatically decreased with increasing temperature (Fig. 19). The yield strength of alloy with chromium was the worst at laboratory temperature and at 300 °C. Moreover, yield strength of all alloys measured at 400 °C was about the same. Yield strength (measured at 400 ° C) was only slightly higher than that of pure aluminium after annealing (20 MPa) [19].
4. Conclusion
Fig. 14. Microstructure of a) AlSi12Fe7, b) AlSi12Fe4Ni3 and c) AlSi12Fe4Cr3.
Nickel alloying resulted in significantly higher hardness of the compact powder metallurgy prepared sample. Hardness of compact alloys was lower at comparison with rapidly solidified alloys during short term annealing. Nickel stabilized mechanical properties at elevated temperatures. Chromium affected the thermal stability positively, keeping the hardness constant throughout the test duration, but its effect on the mechanical properties at 300 and 400 °C was rather negative.
This study was focused on the microstructure, phase composition and thermal stability of Al–Si–Fe–X alloys prepared by melt spinning process and Spark Plasma Sintering. Microstructure of the cast alloys was composed of large intermetallic phases (Al5FeSi and Al2FeSi). Microstructure of melt-spun ribbons was very fine and homogeneous. It implies that the hardness of the ribbons was remarkably superior to that of the cast alloy. X-ray diffraction analyses of the Al–Si–Fe alloys revealed the presence of Si, Al and Al5FeSi. This phase also replaced Al2FeSi phase for Al–Si–Fe–Ni. In the case of Al–Si–Fe–Cr alloys, the weak diffraction lines from Al–Fe–Si intermetallics indicated that the ribbon contains very low amount of this phase. Compact alloys retained very fine structure. Uniform and fine intermetallic phase is dispersed in the structure. Phase composition was the same as in the case of cast alloys. The XRD patterns showed the presence of Al5FeSi phase in the structure of AlSi12Fe7 and AlSi12Fe4Cr3 alloys. Alloy with nickel contained Al2SiFe phase. The needle-shaped phase distributed in the ribbons was analysed as a Al5FeSi compound, and it was transformed partially to a stable Al2SiFe compound after annealing. This phase was very fine and it caused the increase of hardness of melt-spun ribbons. Nickel stabilized mechanical properties at elevated temperatures. Chromium affected the thermal stability positively, keeping the hardness constant throughout the test duration, but its effect on the mechanical properties at 300 and 400 °C was rather negative. Results showed that the alloys, especially the Al–Si–Fe–Ni alloys, could find possible application at elevated temperatures, e.g. in car engines. When assumed that this alloy could be manufactured from iron-rich scrap of Al–Si alloy, it can be highly beneficial for this industrial sector.
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Fig. 16. X-ray diffraction patterns of compact alloys.
Fig. 17. a) Hardness and b) microhardness of compact alloys vs. temperature.
Fig. 18. Vickers microhardness (HV 0.005) of compact alloys a) AlSi12Fe7, b) AlSi12Fe4Cr3 and c) AlSi12Fe4Ni3 vs. annealing time.
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Fig. 19. Yield stress of compact alloys.
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