Abstract
The spheroidization behavior of the nano-primary silicon phase induced by Nd under high-current pulsed electron beam (HCPEB) irradiation was investigated in this study. The study results revealed that, compared to the Al–17.5Si alloy, spheroidized nano-primary silicon phase emerged in the alloy’s HCPEB-irradiated surface layer due to the presence of Nd. Because Nd was abundantly enriched on the fast-growing silicon crystal plane, its surface tension was reduced under the extreme undercooling caused by HCPEB irradiation, causing the growth velocity of each crystal plane to be the same and spherical nanometers of silicon to appear. The spheroidization of nano-primary silicon phases occurred in the remelted layer. The microhardness test revealed that Nd could depress the microhardness of the Al matrix at the same number of pulses, but conversely increase the microhardness of the primary silicon phase, compared to the Al–17.5Si alloy. The tribological test showed that the presence of spherical nano-primary silicon could significantly improve the alloy’s tribological property.
Highlights
In previous studies [16,17], our group found that the unmodified Al–17.5Si alloy was refined into the nano-primary silicon phase because of the rapid heating and cooling effects of high-current pulsed electron beam (HCPEB)
A nanometer primary silicon phase appears on the surface of an HCPEB-irradiated Al–17.5Si alloy, with an angular plate-like morphology (Figure 1a)
The results demonstrated that the composite modification using Nd and HCPEB could refine the primary silicon phase while spheroidizing it, which was attributed to the abundantly enriched Nd on the fast-growing silicon crystal plane, reducing its surface tension under the extreme undercooling by the HCPEB treatment, causing the same growth velocity of each crystal plane and leading to the formation of spherical nanometer silicon
Summary
Hypereutectic aluminum–silicon alloy has been widely used in components requiring weight reduction, such as aircraft and automobile fittings, pistons, because of its advantages of low density, high specific strength, low thermal expansivity and good tribological property [1,2,3]. Among these alloys, A390 alloy (silicon content: 16–18 wt.%), with tensile strengths of 275 MPa and 102.1 MPa at room temperature and 300 ◦ C, respectively, has been used in cylinders and pistons in key engine components [4,5]. As a result, controlling the size and morphology of the primary silicon has always been one of the primary research interests in hypereutectic Al–Si alloys
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