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Abnormal decrease of Si particle size in 1100 °C MS Al–12.2at.%Si alloy

XRD analysis of MS Al–Si alloys

Figure 1a–d show the XRD patterns of the 800 °C and 1100 °C MS Al–3at.%Si, Al–7at.%Si, Al–12.2at.%Si and Al–20at.%Si alloys, respectively. The XRD patterns of all the investigated MS Al–Si alloys are present in the peaks of Al phase and Si phase, indicating that the microstructure of the MS Al–Si alloys is composed of the Al matrix phase and Si phase. The XRD intensity of the Al(111) main peak of both 800 °C and 1100 °C MS Al–3at.%Si, Al–7at.%Si, Al–12.2at.%Si and Al–20at.%Si alloys decreases monotonically with the increase of Si content, except the 1100 °C MS Al–12.2at.%Si alloy whose Al(111) peak intensity shows an abnormal backing increase, when compared with the Al(111) peak intensity of the 1100 °C MS Al–7at.%Si alloy. The volume fraction of Al matrix phase that takes the majority in the investigated MS Al–Si alloys decreases with increasing Si content, and there is a big decrease in the volume fraction of the Al matrix phase from 12.2at.%Si to 20at.%Si, which might lead to the general decrease of the XRD intensity of Al with increasing Si content and the small XRD intensity of Al at the MS Al–20at.%Si alloys.

Figure 1

X-ray diffraction patterns of the Al–Si melt-spinning alloys rapid solidified from melts at 800 °C and 1100 °C. (a) Al–3at.%Si melt-spinning alloy; (b) Al–7at.%Si melt-spinning alloy; (c) Al–12.2at.%Si melt-spinning alloy; (d) Al–20at.%Si melt-spinning alloy.

SEM analysis of MS Al–Si alloys

Figure 2a–d present the medium magnification (×15 k) SEM micrographs taken from the center of the cross-section of the 800 °C MS Al–3at.%Si, Al–7at.%Si, Al–12.2at.%Si and Al–20at.%Si alloy ribbons, sequentially, and Fig. 2e–h show the corresponding statistical size distribution of Si particles in Fig. 2a–d. Si particles in the 800 °C MS Al–Si alloys show a near lognormal size distribution, with particle size ranges of (0, 220 nm), (0, 260 nm), (0, 320 nm) and (0, 440 nm) for the Al–3at.%Si, Al–7at.%Si, Al–12.2at.%Si and Al–20at.%Si MS alloys, respectively. Figure 2i–l present the medium magnification (×15 k) SEM micrographs taken from the center of the cross-section of the 1100 °C MS Al–3at.%Si, Al–7at.%Si, Al–12.2at.%Si and Al–20at.%Si alloy ribbons, sequentially, and Fig. 2m–p show the corresponding statistical size distribution of Si particles in Fig. 2i–l. Si particles in 1100 °C MS Al–Si alloys also show a near lognormal size distribution, with particle size ranges of (0, 340 nm), (0, 400 nm), (0, 200 nm) and (0, 440 nm) for the Al–3at.%Si, Al–7at.%Si, Al–12.2at.%Si and Al–20at.%Si MS alloys, separately. The size of Si particles in the 800 °C MS Al–Si alloys increases monotonically with the increase of Si content. Different from the 800 °C MS condition, with increasing Si content, the size of Si particles in the 1100 °C MS Al–Si alloys increases in a non-monotonic way with an abnormal decrease occurs at Al–12.2at.%Si. Simultaneously, the abnormal increase of the number of Si particles was observed at Al–12.2at.%Si for the 1100 °C MS Al–Si alloys.

Figure 2
figure2

Medium magnification (×15 k) SEM micrographs taken from the cross-section center of the Al–Si melt-spinning alloy ribbons and corresponding statistical size distribution of Si particles in the Al–Si melt-spinning alloy ribbons rapid solidified from melts at (ah) 800 °C and (ip) 1100 °C: (a, e, i, m) Al–3at.%Si; (b, f, j, n) Al–7at.%Si; (c, g, k, o) Al–12.2at.%Si; (d, h, l, p) Al–20at.%Si.

Confirmation of abnormal decrease of Si particle size

During the MS process, the cooling rate decreases gradually from the wheel side to the free surface of the MS alloy ribbons, which results in the gradual increase of the size of phases across the cross-section of the MS alloy ribbons from the while side to the free surface27,28. Therefore, the size of Si particles across the whole cross-section (near the wheel side, in the center and near the free surface) of the 1100 °C MS Al–3at.%Si, Al–7at.%Si, Al–12.2at.%Si and Al–20at.%Si alloy ribbons was studied, to confirm the abnormal decrease of Si particle size in the 1100 °C MS Al–12.2at.%Si alloy. Figure 3a–d,e–h,i–l,m–p show the detail cross-section SEM morphology of the 1100 °C MS Al–3at.%Si, Al–7at.%Si, Al–12.2at.%Si and Al–20at.%Si alloy ribbons, respectively. From the low magnification (×1 k) cross-section morphology shown in Fig. 3a,e,i,m, the thickness of the 1100 °C MS Al–3at.%Si, Al–7at.%Si, Al–12.2at.%Si and Al–20at.%Si alloy ribbons is ~ 55 μm. Figure 3b,f,j,n, Fig. 3c,g,k,o and Fig. 3d,h,l,p show the high magnification (×30 k) morphology of Si particles near the wheel side, in the center and near the free surface of the MS alloy ribbons, respectively. The size of Si particles in the 1100 °C MS Al–Si alloys increases gradually across the cross-section from the wheel side to the free surface due to the decreasing cooling rate. With the increase of Si content, the size of Si particles across the whole cross-section of the 1100 °C MS Al–3at.%Si, Al–7at.%Si, Al–12.2at.%Si and Al–20at.%Si alloy ribbons first increases, then decreases abnormally at Al–12.2at.%Si, after increases again. Thus the abnormal decrease of Si particle size at the 1100 °C MS Al–12.2at.%Si alloy is the fact across the whole cross-section of the 1100 °C MS Al–12.2at.%Si alloy, and it is not a fluke by loosely taking the size of finer Si particles near the wheel side of the 1100 °C MS Al–12.2at.%Si alloy while taking the size of larger Si particles far from the wheel side of the other 1100 °C MS alloys.

Figure 3
figure3

Cross-section SEM morphology of (ad) Al–3at.%Si, (eh) Al–7at.%Si, (il) Al–12.2at.%Si and (mp) Al–20at.%Si melt-spinning alloy ribbons rapid solidified from melts at 1100 °C: (a, e, i, m) low magnification (×1 k) cross-section morphology, and high magnification (×30 k) morphology of Si particles (b, f, j, n) near the wheel side, (c, g, k, o) in the center, and (d, h, l, p) near the free surface of the melt-spinning alloy ribbons.

For consistency, the medium magnification (×15 k) SEM micrographs taken from the center of the cross-section were used for the statistics of the size of Si particles in the investigated MS Al–Si alloy ribbons. Figure 4a presents the evolution of the statistical mean size of Si particles in the 800 °C and 1100 °C MS Al–Si alloys versus Si content. All of the investigated MS Al–Si alloys show the increase in Si particle size with the increase of both Si content and initial MS melt temperature, except for the 1100 °C MS Al–12.2at.%Si alloy that shows an abnormal decrease in Si particle size. Figure 4b shows the XRD intensity of the Al(111) main peak of all the investigated MS Al–Si alloys, basing on the XRD patterns shown in Fig. 1. For 800 °C MS Al–Si alloys, with increasing Si content, the Al(111) XRD intensity decreases monotonically, which is consistent with the monotonic increase of Si phase in the alloys observing by SEM. For 1100 °C MS Al–Si alloys, with increasing Si content, the Al(111) XRD intensity decreases in a non-monotonic way, and the abnormal increase of the Al(111) XRD intensity at Al–12.2at.%Si is self-consistent with the abnormal decrease of Si particle size under this condition observing by SEM. The self-consistent SEM and XRD analysis of the 800 °C and 1100 °C MS Al–3at.%Si, Al–7at.%Si, Al–12.2at.%Si and Al–20at.%Si alloys confirms the abnormal decrease of Si particle size and the abnormal increase of Si particle number in the 1100 °C MS Al–12.2at.%Si alloy, which demonstrates the disruption of Si-rich microstructure in the Al–12.2at.%Si alloy melt with the increase of melt temperature from 800 to 1100 °C.

Figure 4
figure4

Abnormal decrease of Si particle size in 1100 °C melt-spinning Al–12.2at.%Si alloy. (a) Evolution of statistical mean size of Si particles in the investigated 800 °C and 1100 °C melt-spinning Al–Si alloys versus Si content; (b) Evolution of the XRD intensity of the Al(111) main peak of the investigated 800 °C and 1100 °C melt-spinning Al–Si alloys versus Si content.

Small angle neutron scattering of Al–12.2at.%Si alloy melt

Small angle X-ray scattering (SAXS)29,30 and small angle neutron scattering (SANS)5,31,32 have been applied to study the micro-heterogeneous structure in solid materials and solutions especially the colloidal suspension system, as the measured intensity of small angle scattering (SAS) is proportional to the contrast that is given by the difference in the scattering length density between the micro-heterogeneous structure and the matrix, and there is SAS signal for the micro-heterogeneous system, while there is no SAS signal for the homogeneous system without micro-heterogeneous structure embedded in the matrix33,34. Considering the abnormal decrease of Si particle size in the 1100 °C MS Al–12.2at.%Si alloy, SANS was applied to further explore the microstructure evolution in the Al–12.2at.%Si alloy melt.

Figure 5 shows the size distribution of the micro-heterogeneous structure in the Al–12.2at.%Si alloy melt at 800 °C and 1100 °C measuring by SANS, and the nanoscale micro-heterogeneous structure in the Al–12.2at.%Si alloy melt comes from the aggregation of Si atoms, as it has the contrast difference with the Al-rich melt matrix under SANS. The Si-rich micro-heterogeneous structure in the Al–12.2at.%Si alloy melt exists in two size families, i.e., large quantities of small Si-rich micro-heterogeneous structure ranging between 0 and 10 nm and small quantities of large Si-rich micro-heterogeneous structure ranging between 10 and 240 nm. Large quantities of small (0–6 nm) micro-heterogeneous structure was measured in the Sn–26.1at.%Pb alloy melt by SANS5, therefore the size of the micro-heterogeneous structure in alloy melts depends on the alloy system. With the increase of the Al–12.2at.%Si melt temperature from 800 to 1100 °C, the proportion of the small (0–10 nm) Si-rich micro-heterogeneous structure in the alloy melt increases, while the proportion of the large (10–240 nm) Si-rich micro-heterogeneous structure in the alloy melt decreases significantly, which further confirms the disruption of large Si-rich micro-heterogeneous structure in the Al–12.2at.%Si alloy melt into small Si-rich micro-heterogeneous structure under the high temperature of 1100 °C.

Figure 5
figure5

Small angle neutron scattering results showing the distribution of the Si-rich microstructure in the Al–12.2at.%Si alloy melt at 800 °C (blue curve) and 1100 °C (red curve). (a) Distribution of small Si-rich microstructure bellow 10 nm, (b) Distribution of large Si-rich microstructure above 10 nm.

TEM analysis of MS Al–Si alloys

The above mentioned MS and SANS study demonstrates the Si-rich micro-heterogeneous structure in Al–12.2at.%Si alloy melt at 800 °C, so the 800 °C MS Al–12.2at.%Si alloy was further studied by TEM. Among the eight MS conditions, Si particles in the 800 °C MS Al–3at.%Si alloy are the smallest and closest to the Si-rich micro-heterogeneous structure in the molten state, therefore the 800 °C MS Al–3at.%Si alloy was also analyzed by TEM. Si-rich particle aggregates were observed in the Al grains of the 800 °C MS Al–3at.%Si and Al–12.2at.%Si alloy under TEM, as shown in Fig. 6. Figure 6a presents the scanning TEM (STEM) image of a particle aggregate (160 nm) in the Al grain of the 800 °C MS Al–3at.%Si alloy, and the magnification (Fig. 6b) shows that the particle aggregate in Fig. 6a comprises multiple small particles with the sizes smaller than 30 nm. Figure 6c presents the STEM composition mapping of the particle aggregate shown in Fig. 6a,b and the Al matrix surrounding the particle aggregate, the enrichment of Si and the infertility of Al can be found in the area of the particle aggregate when compared to the surrounding Al matrix, while the concentration of oxygen (O) in the particle aggregate is uniformly the same as that in the surrounding Al matrix, and the concentration of O is much lower than that of Al and Si, which demonstrates that the particle aggregate is not the Al-based or Si-based oxide and the particle aggregate is rich in Si. The uniform appearance of trace O in the STEM mapping was due to the inevitable oxidation of the surface of the TEM sample. Figure 6d shows the bright-field TEM image of a Si-rich particle aggregate in the Al grain of the 800 °C MS Al–12.2at.%Si alloy, and the particle aggregate also comprises multiple small nanoscale Si-rich particles below 50 nm, while the size of small Si-rich particles in the particle aggregate of the 800 °C MS Al–12.2at.%Si alloy is slightly larger than that of the 800 °C MS Al–3at.%Si alloy.

Figure 6
figure6

TEM images showing the Si-rich particle aggregates in the Al grains of the melt-spinning Al–Si alloys. (a) Scanning TEM image of the Si-rich particle aggregate and (b) its magnification in the 800 °C melt-spinning Al–3at.%Si alloy; (c) Scanning TEM composition mapping of the Si-rich particle aggregate in (a) and (b); (d) Bright-field TEM image of the Si-rich particle aggregate and (e) its magnification in the 800 °C melt-spinning Al–12.2at.%Si alloy.



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