Silicon particle pinhole defects in aluminium–silicon alloys

Abstract

It was recently shown that silicon particles in heat-treated Al-Si casting alloys can contain flaws such as surface pinholes and grooves, which cause varying degrees of reduction in the in situ particle fracture strength and hence influence the mechanical properties of this class of alloys. In this work, we show that the formation of one class of such strength-limiting flaws in solidified and coarsened Si particles, namely surface pinholes, is caused by alloy impurities such as Fe and Ti in both binary eutectic Al-Si alloys and also in casting alloy A356. This is evidenced by using Focused Ion Beam serial sectioning tomography coupled with Energy-Dispersive X-Ray Spectroscopy, and confirmed by the observation that a high-purity Al-Si alloy presents a significantly lower proportion of pinholes along the surface of the silicon phase than does an alloy of commercial purity. A similar correlation between alloy purity and the formation of another, more severe strength-limiting particle defect, namely grooved interfaces, was on the other hand not found.

Electrochemical corrosion performance of Si-doped Al-based automotive alloy in 0.1 M NaCl solution

“…Selective dissolution of primary aluminium phases over a depth of a few micrometers creates the pinholes [39]. The images prove that there are localized corrosion and discrete pits on the surfaces of the samples.…”

“…Increased surface pinholes can be discerned on the surfaces of the hypereutectic alloy, which may be attributed to intermetallic particles created by high impurity. These surface pinholes indicate an excessive pitting corrosion mechanism [39].…”

The Effect of Solution Treatment on the Si Particles’ Morphology Evolution and the Thermal Conductivity and Tensile Properties of Sb-Modified Al-8Si-0.6Mg Alloys

“…As shown in Figure 3g, some Si particles have not begun to fuse after squeezing in the alloy, and there are dents on the surface caused by squeezing with other particles (marked by green arrows). In Figure 3d,h, holes on the Si particles (marked by red arrows) can be seen, these are caused by impurity elements, such as Ti and Fe, and have nothing to do with the evolution of Si particles [23]. The high magnification secondary electron images of the Si particles during the solution treatment are shown in Figure 3, which more intuitively shows the other two evolution mechanisms.…”

“…Therefore, it is necessary to determine the action time of each mechanism and formulate a reasonable heat treatment system for Al-Si alloys. The spheroidization of Si particles has been widely demonstrated: the decrease of surface energy is its main driving force, and it usually occurs in a relatively short time (<1 h) [23]. In the following sections, splitting, fusion, and coarsening will be discussed.…”

Intrinsic and Extrinsic Effects of Microstructure on Properties in Cast Al Alloys

“…Mueller et al also determined that Si particles were weakened by pinhole defects and interfaces visible on their surfaces. They also [110] found Ti-and Fe-rich intermetallics at the bottom of the pinhole defects, embedded in the Si particles, which significantly weakened the particles. In Figure 7, the limits determined by Caceres et al are also indicated, as well as the cumulative probabilities associated with them.…”