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

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.

Strength and fracture of silicon second-phase particles in aluminium casting alloys

Martin Guillermo Mueller

The mechanical behaviour of metallic materials that have a microstructure composed of a brittle phase embedded in a ductile matrix is dictated by a complex interplay of factors such as local phase properties, cohesive properties, geometrical characteristics, and specific damage mechanisms. An important example of such two-phase materials are Al-Si casting alloys, which are widely used in automotive applications. In the micromechanics of these alloys, fracture of the particulate brittle phase plays a dominant role. Namely, when the alloy is deformed, the brittle silicon particles within the aluminium matrix start fracturing, leading ultimately to fracture of the alloy. In this thesis, micromechanical methods to measure local fracture toughness or strength of individual brittle microscopic particles within alloys and metal matrix composites (MMCs) are developed. The methods are based on coupling experimental techniques such as Scanning Electron Microscopy (SEM), Focused Ion Beam (FIB) milling and nanoindentation with finite element modelling (FEM). Special attention is put in probing portions of the particles that are left unaffected by the FIB micromachining process, and are thus representative of the particlesâ intrinsic properties. These novel methods are also used to study the fracture of silicon particles within Al-Si casting alloys. To measure fracture toughness at a small scale, a microscopic chevron notch test is developed and demonstrated on benchmark materials. The main advantage of this method with respect to most existing small-scale fracture toughness test methods is that in chevron-notched samples the crack growth resistance is measured on a real crack instead of a pre-notch. The main difficulty, on the other hand, is achieving crack initiation at applied loads low enough to allow for subsequent stable crack growth. This was found to be particularly challenging in silicon. Local strength measurements on individual microscopic silicon particles within Al-Si alloys were achieved through two different, novel, methods. The first is a microscopic 3-point bending test by which the large facets of plate-like particles extracted from an Al-Si alloy can be probed. In the second approach, particles that are only partially exposed by deep-etching from an Al-Si sample are carved by FIB milling into such a shape that by compressing its top, bending is produced on a well-defined portion of the particle. The main finding of the local strength measurements is that silicon particles within Al-Si alloys can achieve extremely high strength values, yet fracture early when an Al-Si alloy is deformed because most of them feature stress-concentrating defects on their surfaces. The most important stress-limiting defects are found to be grooved interfaces between contacting silicon crystals, followed by surface pinholes, this being a defect identified here. Using FIB cross-sectioning and EDX examination it was revealed that these pinholes originate from alloy impurities. The insights gained on the intrinsic strength of silicon particles unveil the great potential of silicon as a reinforcing phase in Al-Si alloys. This, together with the acknowledgement of particle strength-limiting defects, may be used to devise strategies that, through the avoidance of those defects, should lead to improved alloy mechanical properties.

EPFL2017

Processing and Mechanical Properties of Alumina Reinforced Copper-Aluminum Composites

Carmen Krüger

Copper-based composites reinforced with alumina particles are produced by two different pro- cesses, namely gas pressure driven infiltration of a porous alumina preform and a two-step in-situ process developed in the course of this work. The resulting composites are characterized for their microstructures and mechanical properties. It is shown that during gas-pressure infiltration of angular alumina particles with liquid copper- aluminum alloys at 1300◦C, rounding of the alumina particles occurs. Rounding is quantified by measurements of local curvature, and shown not to be found after infiltration of the same particles with pure copper at 1150◦C. Transport pathways necessary for rounding are examined by analysis of the thermodynamics and kinetics of the copper-aluminum-alumina system, drawing from the literature on wetting of alumina by copper and its alloys. Analysis leads to two different possible explanations for the phenomenon: (i) the presence of aluminum in the melt increases the equilibrium oxygen concentration and thus accelerates volume diffusion or (ii) segregation of aluminum in the liquid alloy to the alloy-alumina interface causes accelerated interfacial diffusion. The pressure infiltration of four different copper-aluminum alloys with varying titanium content (Cu8wt%Alxwt%Ti with x=0, 0.2, 1, 2) into preforms of two types of alumina particle leads to sound, pore-free composites reinforced with 56-59 vol% alumina. Depending on the titanium content of the matrix alloy, a new phase appears at the interface between the alumina particles and the metallic matrix; this phase was identified as the mixed oxide Ti3(Cu,Al)3O. A very thin (5-10 nm) titanium-rich layer between alumina and the ternary oxide is responsible for a mechanically strong interface between the copper-alloy matrix and alumina. Peak mechanical properties in these composites were obtained by infiltration of equiaxed Al2O3 particles with a Cu8wt%0.2wt%Ti alloy. Their measured properties displayed a Young’s modulus near 235 GPa, a density of 5.5 g/cm3, a tensile strength and elongation of 600 MPa and 1% respectively, together with a compressive strength near 1100 MPa reached at 9% strain. Copper matrix alumina composites are also produced via a process combining in-situ formation of alumina and deformation processing. Elemental powders are uniaxially compacted and subjected to a two-step heat treatment that leads to the formation of alumina films in a copper-aluminum solid solution matrix. During deformation processing, the composite is consolidated and the alumina films are broken up into particles of average diameter 1-2 μm. Further hot working in the form of upsetting and extrusion further refines the alumina particles and improves their distribution, which improves the composite mechanical properties. Cu7wt%Al matrix composites reinforced with 26 vol% alumina particles were obtained by this process. In optimum processing conditions of this work, these composites have a tensile strength of 846±44 MPa and a tensile ductility of 2.2±0.8%, with likely room for significant further improvement of these properties after more extensive deformation processing.

EPFL2012

Microscopic strength of silicon particles in an aluminium–silicon alloy

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

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.