Fracture toughness determination of high volume fraction particle reinforced aluminium matrix composites

The influence of particle size and particle type on the fracture toughness of aluminium matrix composites is investigated using model composites of 99.99% purity aluminium reinforced with particles of alumina and boron carbide. These composites are produced by gas pressure infiltration, and feature a volume fraction of reinforcement near 50 vol pet. The average particle size is varied between 5 and 35 mum. Fracture properties are determined using the single specimen J-integral test method. Observations of crack propagation mechanisms are conducted by optical microscopy using the arrested crack technique. Equivalent initiation fracture toughness values (K-JC) up to 24 MPa .m(1/2) for the toughest composites are measured, a comparatively high value for a material consisting of 50 vol pet ceramic. In both composites systems, it is found that the average particle size exerts a significant influence on fracture initiation and crack propagation resistance. Microscopical investigations of the mode of fracture show that cleavage of angular particles is present, and becomes increasingly dominant in alumina reinforced composites as the average particle size increases. For composites reinforced by rounded alumina or angular boron carbide particles, matrix voiding is dominant. Particle cracking results in a lower fracture initiation roughness, while increasing particle diameter results in increased crack initiation toughness and tearing modulus.

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

Probing the local strength and toughness of microscopic hard second phases in metals and composites

Marco Cantoni, Raphaël Charvet, Cyril Dénéréaz, Willy Dufour, Marta Fornabaio, Lionel Michelet, Andreas Mortensen, Martin Guillermo Mueller, Václav Pejchal, Goran Zagar

Many engineering alloys and composites combine a ductile matrix with discrete microscopic reinforcing phases, e.g. silicon particles in aluminium alloys, carbides in steels or ceramic particles in metal matrix composites. It is well understood that the mechanical properties of such alloys and composites are strongly influenced by those of their reinforcing particles; yet not much is known of the nature and size of strength-limiting flaws within such microscopic second phases. To investigate the strength and fracture toughness of these microscopic particles directly, we have developed test methods that combine focused ion beam (FIB) milling with microtesting techniques and bespoke finite element simulation. Local strength is probed by micromachining individual reinforcing particles in such a way that tensile stresses are produced in a volume of material free of FIB and polishing artifacts upon the application of a load via nanoindenter. Fracture toughness is measured by the chevron-notch fracture test technique adapted for microscopic samples. Testing methods are established using nanocrystalline alumina fibers as the testbench material, so as to compare data thus obtained with known fiber properties from tensile tests on long sections of the fibers. These test methods are currently being transposed towards the microscopic testing of more irregularly shaped and anisotropic brittle second phases, with the objective to measure their local mechanical properties and identify microstructural flaws that govern the strength of such particles, using in-depth electron microscopy characterization combined with microtesting.

2015

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

2014