The local strength of microscopic alumina reinforcements

We measure, using an adaptation of a method designed for ceramic ball bearings, the local strength of a brittle second phase that serves to reinforce a metal. The method uses focused ion beam milling and a nanoindentation device, and is free of artifacts commonly present in micromachined specimens. It is demonstrated on Nextel 610 (TM) nanocrystalline alumina fibers embedded in an aluminum matrix composite. Results reveal a size effect that does not follow, across size scales, usual Weibull statistics: the fiber strength distribution differs between measurements at the microscale and macroscopic tensile testing. This implies that, in micromechanical analysis of multiphase materials, highly localized events such as the propagation of internal damage require input data that must be measured at the same, local, microscale as the event; the present work opens a path to this end. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd.

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 micromechanical strength of oxide ceramic composite reinforcements

Václav Pejchal

This work shows how one can probe the micromechanical strength of ceramic reinforcements used in metal matrix composites, which greatly influences the mechanical performance of the composite material yet has seldom been quantified with precision. More specifically, this study presents two methods by means of which one can measure the statistical strength distribution of microscopic, low-aspect-ratio, ceramic particles. Additionally, the study reveals the nature of specific defects that weaken such ceramic reinforcements and shows that, when those defects are absent, one can produce particles of near-theoretical strength, which have the potential to produce remarkably strong and tough metal matrix composites. In one developed method called here the Meridian Crack Test, individual spherical particles are compressed uniaxially between a pair of parallel elasto-plastic platens. It is shown that, by tailoring the platen hardness one can control the relative area of particle-to-platen contact during the test, thereby eliminating the initiation of contact microcracks that are often found to influence particle fracture when hard platens are used. It is shown how this method, coupled with the mathematics of statistical survival-analysis, can give unambiguous access to the particle statistical tensile strength as governed by surface flaws. The method is first demonstrated using microscopic fused quartz spheres 40±20µm in diameter and is then used to measure the strength controlled by surface and subsurface flaws in plasma-sprayed spherical amorphous and nanocrystalline near-eutectic "Eucor" alumina-zirconia-silica ceramic particles of diameter near 30 µm. Results show that nanocrystalline Eucor particles exhibit a characteristic Weibull strength of 1490 MPa, which is approximately 30% higher than in corresponding amorphous particles. The second developed method, called here the C-shaped sample test, combines focused ion beam milling, loading using a nanoindentation device, and bespoke finite element simulations to measure the local strength of ceramic reinforcements free of artifacts commonly present in micromachined specimens. The method is first demonstrated on Nextel 610TM nanocrystalline alumina fibres embedded in aluminium. Results reveal a size effect that does not follow, across size scales, the Weibull statistical strength distribution that is measured by tensile testing macroscopic samples of the fibres. This indicates that, in micromechanical analysis of multiphase materials, highly localized events such as the propagation of internal damage require input data that are measured at the same, local, micro- scale as the event. Finally, we implement the C-shaped sample test method with additional micro-cantilever beam testing to measure the local strength of vapour-grown ¿-alumina Sumicorundum® particles 15 to 30 µm in diameter, known to be attractive reinforcing particles for aluminium. Results show that, provided the particle surface is free of readily observable defects such as pores, twins or grain boundary grooves, the particles can achieve local strength values that approach those of high-perfection single-crystal alumina whiskers, on the order of 10 GPa. It is also shown that by far the most harmful defects are grain boundaries, leading to the general conclusion that alumina particles must be single-crystalline or alternatively nanocrystalline to fully develop their potential as a strong reinforcing phase in composite materials.

EPFL2017

Damage and fracture of particle-reinforced metal matrix composites

Aude Hauert, Andreas Mortensen, Andreas Rossoll

Particle Reinforced Metal Matrix Composites (PRMMCs) are produced by high-pressure infiltration of ceramic particle beds. Matrix materials are either pure Al or Al-4,5%Cu, and the reinforcement is made either of angular or of polygonal alumina particles. The resulting composites feature a high volume fraction (40-60%) of uniformly distributed reinforcement. These materials accumulate damage during deformation, in the form of particle cracking or matrix voiding; localization of damage induces final failure. Our work aims towards a better understanding of the link between this damage build-up and fracture. Mechanical tests are conducted on smooth or notched tensile, smooth compressive, as well as smooth or double-notched 4-pt bend bars. This large set of different specimen geometries allows a wide variation of the stress state and of the size of the stressed volume. Small bending specimens show the largest strain at failure and tensile specimens the lowest, which indicates a scale-dependence of fracture. Double-notched specimens are employed to observe damage just prior to failure: when one notch fails, the other is close to failure. Metallographic cuts of the unbroken ligament reveal a large stable process zone in pure aluminium composites, whereas failure behaviour is unstable in alloyed matrix composites. The final aim of this study will be the establishment of a micro-mechanically based model that describes how the accumulation of damage induces failure in these materials.