Introduction

Demands for reduced greenhouse gas emissions and for producing more fuel efficient vehicles have increased the use of light-weight materials in the automotive and aerospace industries.1 Weight reduction in the automotive industry is also important because of the increased weight of safety systems, luxury and comfort items that are required by the customers.2 Al–Si alloys are the most widely used aluminium alloys in the foundry industry because of their good fluidity, excellent castability, high strength-to-weight ratio, good corrosion resistance and good wear properties.3 However, the use of cast Al–Si alloys at temperatures exceeding 200 °C is challenging because of the significant reduction in the mechanical properties at elevated temperature.2 , 4 Therefore, researchers have focused significant efforts into improving the mechanical properties of Al–Si alloys at elevated temperatures through tailoring microstructure and alloy composition, and addition of transition elements has been shown to promote thermally stable phases.2 , 4,5,6, – 7 Both Co and Ni have high melting points and are well known as superalloys for high-temperature applications.8 , 9 Limited studies have been carried out on the combined effects of Co and Ni in Al–Si alloys. This paper reports the results of a study into the effects of Co and Ni on the microstructure and mechanical properties at room and elevated temperature of hypoeutectic Al–Si alloys produced by directional solidification in a Bridgman furnace.

Experimental Methods

Four hypoeutectic alloys were made in an electrical resistance furnace by melting together pure Al, pure Si, and master alloys (Al–20%Co and Al–20%Ni). The melt was held at a temperature of 730 °C for 1 h for homogenization. Next, about 250 ppm Ti was added as Al–5Ti–1B master alloy, 50–70 ppm P as Al–3P and about 300 ppm Sr as Al–10%Sr master alloy. All additions were wrapped in aluminium foil and pre-heated to 200 °C before addition to the melt at 730 °C. The chemical composition of each alloy was measured in an optical emission spectrometer (see Table 1). The melt was held for 30 min, stirred and poured into a pre-heated (200 °C) copper mould to obtain cylindrical rods (length 20 cm, diameter 1 cm). The rods were inserted into steel tubes coated with graphite which were inserted into the Bridgman furnace in argon atmosphere. The temperature was set to 730 °C, and the samples were held for 30 min. Directional solidification was carried out at a speed of 3 mm/s, generating a microstructure corresponding to that obtained in high-pressure die casting, i.e. secondary dendrite arm spacing (SDAS) ~ 10 µm.

Table 1 Chemical Composition (wt%) of all Alloys
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Tensile test specimens with a gauge length of 50 mm and a diameter of 6 mm were machined from the directionally solidified samples. The samples were tested in the as-cast condition and after treatment at 240 °C for 1000 h. Testing was carried out at room temperature and at 230 ± 2 °C with a constant cross-head speed of 0.5 mm/min, and three samples were tested for each condition. Strain was measured by a clip-on extensometer at room temperature and a laser extensometer at 230 °C. The samples tested at 230 °C were heated in an electrical resistance furnace and held for 30 min prior to testing. Microstructural investigations were carried out on representative regions within the gauge length of the as-cast specimens in an optical microscope. The SDAS was measured at ten locations in the longitude direction and ten locations from the transverse direction in all alloys. The grain size was measured by the intercept method according to ASTM E112-96 on an area of minimum 1.5 mm2 of inverse pole figure (IPF) maps from electron backscatter diffraction (EBSD) analysis in the scanning electron microscope. The EBSD measurements were done at an acceleration voltage of 15 kV and a step size of 2 µm. Hardness measurements were performed with a micro-Vickers (6 for each sample) and macro-Vickers (6 for each sample) following the ISO 6508 standard. Moreover, in order to understand the influence of Co and Ni on the hardness, nanoindentation tests (36 for each sample) were performed in a Micromaterials Nanotest Vantage instrument with a load of 200 mN.

Results and Discussion

Microstructural Analysis

Figure 1 illustrates the microstructures of the directionally solidified Al–7Si alloy that shows a typical plate-like, unmodified Si morphology (see Figure 1a).10 The addition of 0.4 wt% Co did not produce any significant changes in the microstructure (see Figure 1b), and the SDAS values are similar (see Table 2), but some Co-rich intermetallics could be observed. Kaya et al.11 reported a reduction of eutectic silicon interflake spacing with addition of 2 wt% Co, but there is no clear effect in the present samples, perhaps caused by the lower addition level and higher growth rate in the present study.

Figure 1
figure 1

(a) Microstructure of the Al–7Si alloy, (b) microstructure of the Al–7Si–Co alloy.

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Table 2 SDAS Results for all Alloys
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The combined addition of Co and Ni to the Al–7Si alloy caused a significant modification of the Al–Si eutectic microstructure (see Figure 2a) and a small reduction in SDAS compared to Figure 1. The main reason for the modification effect of Si needs further investigation. Co- and Ni-rich phases are observed in the vicinity of the α-dendrites. Stadler et al.1 reported that the addition of Ni creates a stronger interconnected 3D network of Al–Si eutectic and Al3Ni phases. However, it is difficult to make a direct observation of such an effect based on these micrographs. The microstructure of the Al–7Si–Co–Ni–Sr alloy exhibits a fully modified structure (see Figure 2b). Moreover, the Sr addition also caused a significant change in the size and morphology of the Co- and Ni-rich phases, reducing the size to about one-third (compare Figure 2a and b).

Figure 2
figure 2

(a) Microstructure of Al–7Si–Co–Ni alloy, (b) microstructure of Al–7Si–Co–Ni–Sr alloy.

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Figure 3 shows EBSD mapping results for all alloys. It is apparent that the addition of the different elements causes a change in the grain size of the alloy. Table 3 shows the results of the grain size measurements. The addition of Co causes a small increase in grain size in the Al–7Si alloy. Further addition of Ni increases the grain size much more, and the grain size remains the same upon addition of Sr. The addition of Co and Ni increases the solute content at the solidification front, thereby increasing the constitutional undercooling, which would be expected to cause a reduction in grain size. However, the grain size is observed to increase on addition of these elements and this effect is likely caused by the formation of Co-rich intermetallics prior to the nucleation of α-Al dendrites, as shown by ThermoCalc calculations (see Figure 4). These intermetallics may have nucleated on some of the TiB2 particles, thus rendering them inactive for nucleation of α-Al. However, this result is contradictory to the findings by Naeem et al.12 who found that grain refinement occurred on addition of Co and Ni to an Al–Zn–Mg–Cu alloy.

Figure 3
figure 3

EBSD mapping results showing the microstructure of (a) Al–7Si, (b) Al–7Si–Co, (c) Al–7Si–Co–Ni, (d) Al–7Si–Co–Ni–Sr.

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Table 3 Results from the Grain Size Measurements
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Figure 4
figure 4

ThermoCalc result of the Al–7Si–Co alloy.

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Mechanical Properties: Room Temperature

Figure 5 shows the mechanical properties at room temperature for all alloys in the as-cast and overaged condition. The figure shows that the mechanical properties of the overaged alloys are as expected, with lower ultimate tensile strength (UTS) and higher elongation to failure compared to the as-cast material. The addition of Co causes a limited improvement in UTS compared to the base alloy in both conditions (see Figure 5a). However, the combined addition of Co and Ni causes an 8% increase in UTS. This improvement could be a result of dispersion strengthening or due to higher degree of contiguity in the 3D network of Co- and Ni-rich phases and eutectic Al–Si. The elongation to failure decreased on addition of Co and Ni (see Figure 5b), from 17% in the base alloy to around 12%. Addition of Co and Ni causes a slight increase in yield strength (YS) compared to the base alloy (see Figure 5c). The results from tensile testing at room temperature for the Al–7Si–Co–Ni alloys show that the improvement in UTS remains after long-time exposure at elevated temperature, in accordance with results reported in the literature.1 , 13

Figure 5
figure 5

(a) UTS at room temperature, (b) elongation to failure at room temperature, (c) YS at room temperature of all alloys under various conditions. The error bars are showing standard deviation.

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Sr addition had a negligible effect on the UTS and elongation to failure, although the microstructure had become well modified. The addition of Sr expects to increase the ductility in the Al–Si alloy, accordance with results reported in the literature, which is not observed in this study.14 , 15

Mechanical Properties: Elevated Temperature

The results from tensile testing at 230 °C are shown in Figure 6. The UTS increases significantly with combined addition of Co and Ni in both the as-cast and the overaged alloys (see Figure 6a), while the elongation to failure decreases (see Figure 6b). The YS results (see Figure 6c) show a significant increase with the combined addition of Co and Ni in the as-cast condition. The combined addition of Co and Ni increases the UTS about 32% compared to the base alloy in the as-cast condition, and it remains ~ 23% higher after 1000 h to 240 °C (see Figure 6a). Choi et al.16 reported that an Al–1.1Si–Mg alloy with combined addition of Co and Ni shows a great potential for elevated temperature applications as the UTS at 450 °C only decreased by ~ 20% compared to room temperature.

Figure 6
figure 6

(a) UTS at 230 °C, (b) elongation to failure at 230 °C, (c) YS at 230 °C for all alloys under various conditions. The error bars are showing standard deviation.

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Mechanical Properties: Hardness

The hardness data are shown in Figure 7. It can be observed that the micro-Vickers hardness and the nanohardness follow the same trend (see Figure 7a). The addition of Co increases the hardness of the alloy, which further increases with the combined addition of Co and Ni. However, a decrease in hardness is associated with the addition of Sr. Since neither Co nor Ni has significant solid solubility in Al, the increase in hardness must be related to the formation of Co- and Ni-rich intermetallics. It is possible that an increase in contiguity in the 3D-network1 is responsible for the increase in hardness. These results are in accordance with the results reported by Kaya et al.11 The reduction in hardness observed with addition of Sr could be caused by the effect of Sr on the volume fraction of α-Al in the microstructure. Sr addition causes a shift of the eutectic point to higher Si concentrations and lower temperature17 and therefore increases the amount of α-Al.

Figure 7
figure 7

(a) Hardness results from the micro-Vickers and nanoindentation, (b) the hardness response of the alloys aged at 240 °C. The error bars are showing standard deviation.

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Figure 7b shows the variation in hardness as a function of time in the base alloy and the alloy containing combined Co and Ni addition exposed at 240 °C. The results show a rapid decrease in hardness during the first 10 h for both alloys. These results agree with the observations by Zamani et al.4 The alloy with combined Co and Ni addition has a hardness value about 12% higher than the base alloy. This could be because of the formation of thermally stable Co and Ni intermetallics in the Al–7Si–Co–Ni alloy.

Conclusions

This work has focused on the effect of Co, Ni and Sr in Al–7%Si. The results show:

  1. (1)

    The combined addition of Co and Ni modified the eutectic silicon morphology.

  2. (2)

    The size of the Co- and Ni-rich phases is significantly reduced by the addition of Sr.

  3. (3)

    The grain size increased with the addition of Co and Ni.

  4. (4)

    The combined addition of Co and Ni shows a good potential to improve the mechanical properties of Al–Si alloys at elevated temperature. The UTS increased by ~ 32% at 230 °C in the as-cast condition, and it remains at 23% higher after prolonged exposure for 1000 h at 240 °C.

  5. (5)

    The combined addition of Co and Ni gave an improvement in hardness by 34.5% in the Al–Si alloy.