Silicon particle pinhole defects in aluminium\u2013silicon alloys

Martin Guillermo Mueller,A Mortensen,M Fornabaio

doi:10.1007/s10853-016-0381-y

Martin Guillermo Mueller, A Mortensen + Show 1 more

Open Access

https://doi.org/10.1007/s10853-016-0381-y

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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.

Highlights

Silicon particles play a key role in the solidification, microstructural development and fracture processes of Al–Si-based alloys [1,2,3,4,5,6,7,8,9]
In the heat-treated standard-purity Al-12.6 %Si alloy, a total of 14 silicon particles embedded within the spectra a3 and b3 correspond to the intermetallic particles indicated with an arrow on images a2 and b2, respectively
It is likely that many other particles of this kind were missed due to the relative thick sectioning that was used. On their Energy-Dispersive X-ray Spectroscopy (EDXS) spectra, a small peak corresponding to Si was present; it is not possible to tell whether that Si signal originated from the small intermetallic particle, from the silicon particle around it or whether it is an artefact

Summary

Introduction

Silicon particles play a key role in the solidification, microstructural development and fracture processes of Al–Si-based alloys [1,2,3,4,5,6,7,8,9] When these alloys are mechanically strained, it is typically observed that silicon particles within the a-aluminium matrix start fracturing gradually, essentially as soon as the alloy starts to deform plastically. As the number of fractured particles increases, nucleated microcracks start to grow, and link by tearing the aluminium matrix that connects fractured silicon particles. This lowers the rate of work hardening of the alloy, in turn hastening the onset of the tensile instability; the coalescence of such microcracks produces macrocracks that can drive final fracture of the material. The average stress in the particles as a function of macroscopic alloy strain is calculated through micromechanical

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