Microscopic strength of silicon particles in an aluminium–silicon alloy

A microscopic three-point bending test that measures the strength of faceted particles of high aspect ratio is developed and used to probe individual coarsened plate-like silicon particles extracted from the eutectic Al-12.6%Si alloy. Focused ion beam milling is used in sample preparation; however, the tapered beam cross-section and multistep preparation procedure used here ensure that the particle surface area subject to tension in mechanical testing is free of ion beam damage. Results show that coarsened silicon particles in aluminium can reach strength values on the order of 9 GPa when they are free of visible surface defects; such high strength values are comparable to what has been reported for electronic-grade silicon specimens of the same size. By contrast, tests on eutectic silicon particles that feature visible surface defects, such as pinholes or boundary grooves, result in much lower particle strength values. Reducing the incidence of surface defects on the silicon particles would thus represent a potent pathway to improved strength and ductility in 3xx series aluminium casting alloys.

In-situ strength of individual silicon particles within an aluminium casting alloy

Andreas Mortensen, Martin Guillermo Mueller, Goran Zagar

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. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd.

Elsevier2018

Flexural strength of micron-scale plate-like silicon in aluminum

Marta Fornabaio, Andreas Mortensen, Martin Guillermo Mueller, Goran Zagar

Mechanical properties of composite materials and alloys are strongly influenced by the intrinsic mechanical properties of the reinforcing phases they contain; however, due to the irregular shape and small size of particulate reinforcements, measuring their intrinsic properties is a substantial methodological challenge. Here we present a method with which we probe the local flexural strength of individual microscopic plate-like silicon particles, which constitute, together with aluminium, the eutectic microconstituent in AlSi alloys. Silicon particles are extracted from the cast and heat-treated alloy by deep-etching the aluminium matrix. The plates are then are dispersed on a steel substrate, where irregular plate- like particles rest lying on one of their large flat facets. A beam of well-defined dimensions is micro-machined out of individual particles using focused ion beam (FIB) milling perpendicular to the substrate. In this way, the particle surface which is in contact with the substrate and that will later on be subjected to tension upon beam bending, i.e. where strength will be measured, is not affected by the FIB nor by redeposition. The FIB is also used to produce a hole in the steel substrate nearby the micro-machined beam so that the silicon beam can be transported and placed on top of the hole using a micromanipulator. The beam is tested in 3- or 4-point bending until fracture by applying a force with a nanoindenter equipped with a diamond tip featuring one or two small rounded ridges that are longer than the width of the beam. Fractography is carried out after testing to locate and characterize the fracture initiation point. Test results are coupled with Finite Element simulations for interpretation. Error in the measurement, including the influence of possible misalignments, of the measured strength is evaluated.

2015

Measuring the local strength of alumina reinforcements

Marco Cantoni, Andreas Mortensen, Martin Guillermo Mueller, Václav Pejchal, Andreas Rossoll, Goran Zagar

Ceramic particles in metal matrix composites, or hard second phases in alloys, often serve as reinforcements, raising its stiffness, hardness and strength. Their efficiency depends on their strength, a quantity that is statistically distributed over microscopic volumes and seldom quantified because it is, with the exception of long fibres, difficult to measure. We combine focused ion beam (FIB) milling methods with micro-testing techniques and finite element simulation to probe the strength of alumina fibres or particles that are used to reinforce aluminium. We present results from microscopic in situ tests conducted on Nextel ™ 610 nanocrystalline alumina fibres produced by 3M and used to reinforce continuous aluminium matrix composite wires. After exposing alumina fibres from the composite wire, we first use FIB milling to machine a wide rectangular notch in the fibre together with a line of load application along its top surface. Then, the notched fibre is loaded until failure by applying compressive force via a nanoindenter equipped with a flat tip: tensile stresses are thus created in the material, by bending under compressive loading. The deformation of each notched fibre sample is modelled using second-order elastic finite element analysis in order to obtain the three-dimensional stress state in the bent ligament prior to failure. The resulting strength distribution data are then used, knowing the local fracture toughness of the material, to estimate the nature and size distribution of local defects that govern the microscopic in-situ strength of the ceramic reinforcement. It is found that this defect population differs from that which governs the macroscopic tensile strength distribution of the fibres. We conclude with a discussion of the transposition of this methodology towards quantification of the microscopic strength of irregularly shaped ceramic particles or second phases that have potential for the reinforcement of aluminium.

2014

Measuring the local strength of alumina reinforcements

Marco Cantoni, Andreas Mortensen, Martin Guillermo Mueller, Václav Pejchal, Andreas Rossoll, Goran Zagar

2014