Abstract
Measurements of local strength are performed in-situ on individual silicon particles that constitute the second phase of aluminium alloy A356. Particles are shaped using Focused Ion Beam (FIB) milling such that, upon the application of a compressive force on the particle, a volume of material unaffected by FIB milling is subjected to bending. Silicon particles in this commercial aluminium casting alloy are shown to be capable of locally sustaining tensile stresses as high as 16 GPa, i.e., approaching theoretical strength. The reason why such strengths are not reached by most alloy Si particles is shown to be the presence of specific surface defects, the effect of which is assessed. The most deleterious defects are interfaces between merged silicon crystals; therefore, eliminating these might lead to significantly enhanced strength and ductility in this widely-used casting alloy family.
Highlights
In heterogeneous materials composed of hard brittle particles embedded in a soft ductile matrix, mechanical damage generally consists in particle fracture, particle-matrix interfacial decohesion, or matrix voiding
Another example is Specimen #2 (Fig. S3b1eb4 in the Supplementary Material), in which fracture originated in a spot subjected to significantly lower stress than the maximum stress in the specimen
In-situ measurements of local strength on individual, microscopic, silicon particles within cast aluminium Alloy A356 were performed using a micromechanical testing method that probes in tension a portion of the particle surface that is unaffected by artefacts arising from Focused Ion Beam (FIB) milling
Summary
In heterogeneous materials composed of hard brittle particles embedded in a soft ductile matrix, mechanical damage generally consists in particle fracture, particle-matrix interfacial decohesion, or matrix voiding. In Al-Si alloys, a prototypical example of such materials representing the vast majority of aluminium-based casting alloys, early stages of damage are typically dominated by the fracture of silicon particles. These constitute the main alloy second phase and can take a variety of forms, typically of intricate networked or isolated particles, depending on the alloy composition and processing history [1e5]. A classical approach consists in relating the estimated (average) stress in the silicon phase to the fraction of broken particles, itself measured along polished sections of the probed material or using X-rays or neutrons (for diffraction and/or tomography), after deformation to varying levels of macroscopic strain. Nishido et al [11] used an expression based on assimilation of the Si phase to an equivalent Eshelby inclusion, while Huber et al [12] used an extension of the Eshelby theory to estimate the stress in silicon particles, deriving from this the silicon particle strength by using it as the fitting parameter of their particle fracture-induced void nucleation model
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