Probing the micromechanical strength of oxide ceramic composite reinforcements

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.

High volume fraction particulate and interpenetrating phase composites (IPC)

Ghodratollah Roudini

The role of reinforcing phase contiguity in high volume fraction particulate composites is investigated using model composites of pure aluminium reinforced with α-Al2O3 particles. To produce the composites, alumina particle (5µm) preforms were either loosely packed or, alternatively, packed and CIPed; these were then sintered, varying the sintering time and temperature, and infiltrated with pure liquid aluminium using gas pressure infiltration. The microstructure of the resulting composites was characterized using image analysis in terms of reinforcement phase contiguity (β), defined as the ratio of particle surface that is in contact with other particles to that in contact with the matrix. The influence of this parameter on several characteristics of the composites and the preforms they were made from was then studied, using theoretical models in the literature to isolate the influence of this parameter from that of other variables, such as the volume fraction ceramic. To this end, the thermal conductivity of the sintered preforms, basic composite mechanical properties (hardness, tensile strength, and damage evolution during straining) and the thermal expansion of the composites were characterized and compared with theory; furthermore, quenching and neutron diffraction were used to probe in more depth the thermal expansion behaviour of the composites. The results show that the thermal conductivity of the preform increases, as expected, upon increasing the relative density (volume fraction) and the contiguity of alumina particle after sintering. Comparison of data with theory shows that, while experimental values for the CIPed samples agree with the prediction by Argento and Bouvard for the thermal conductivity of powder preforms as a function of densification, neither this model, nor that of Montes et al. , fits all experimental values. Rather, when pooled the data show that the thermal conductivity of powder preforms, normalized for relative density using the differential effective medium model, obeys relatively well a simple linear relation between contiguity and conductivity. The hardness of the composite increases with increasing relative density and increasing contiguity. Here again, after normalization for the dependence of hardness on ceramic volume fraction using current theory, a nearly linear relation is found between hardness and contiguity. Tensile testing results show that the strength of the composite increases with increasing fraction ceramic and increasing contiguity while the elongation decreases, composites with an interconnected ceramic phase rapidly becoming quite brittle, with elongations falling below half a percent. No clear relationship is found between contiguity and Young's modulus after normalization for the volume fraction ceramic. Measurements of internal damage evolution show that damage increases very rapidly with deformation in the present interconnected composites, in agreement with their brittle character. The thermal expansion results show that the CTE of the present composites often varies with temperature at a rate higher than that predicted to result from physical parameter variations by existing theoretical models. With regard to their thermal expansion, the composites can be separated into three classes that are characterized by the level of contiguity of the low expansion phase: low contiguity (i.e. β ≤ 0.045), medium contiguity (i.e. 0.05 ≤ β < 0.074) and high contiguity (i.e. β ≤ 0.074 ). The CTE values during heating for the low contiguity composites lie between Schapery's upper bound and the rule of mixtures, starting upon heat-up from the Schapery upper bound at low temperature and transiting above roughly 150°C to the rule of mixture prediction; upon cool-down the same behaviour is observed in reverse. The CTEs of the composites with medium contiguity at low temperatures are somewhat below the Schapery upper bound, increasing somewhat faster than predicted for this bound with increasing temperature to reach Shapery's upper bound at around 200°C. For the composites with high contiguity, the CTEs lie between elastic and plastic bounds predicted using the self-consistent model, the latter model being obtained by assigning a zero shear stiffness to the matrix. Physical reasoning and comparison of data with theory indicate that these transitions in expansivity of the composites are mostly a result of matrix yield following fully elastic deformation past a certain thermal excursion, this transition being prevented by increased ceramic phase contiguity. This interpretation is confirmed by neutron diffraction results. Increased reinforcement contiguity in the composites also affects the amplitude of strain hysteresis during thermal cycling, the magnitude of hysteresis decreasing in nearly linear fashion with increasing contiguity, reaching zero near β = 15%. Samples that were quenched into liquid nitrogen after a relaxation heat treatment and reheated to room temperature yielded data globally similar to those of as-processed samples. Nitrogen-quenched samples with higher contiguity (β ≥ 0.05) exhibit furthermore signs of ratcheting upon cycling, which was not observed in the composites with low contiguity.

EPFL2008

Tensile flow stress of ceramic particle-reinforced metal in the presence of particle cracking

Andreas Mortensen, Andreas Rossoll, Ludger Weber

A simplified model is proposed to quantity the effect of damage in the form of particle cracking on the elastic and plastic behaviour of particle-reinforced metal matrix composites under uniaxial tensile loading: cracked particles are simply replaced, in a mean-field model, with as much matrix. Pure aluminium reinforced with 44 vol.%., alumina particles, tested in tension and unloaded at periodic plastic deformations, is analysed by neutron diffraction during each reloading elastic step, at 30%, 50%, 701% and 90% of the tensile flow stress. The data give the evolution of the elastic matrix strains in the composite and also measure the progress of internal damage by particle cracking. The test gives (i) the evolution of the in situ matrix flow stress, and (ii) the evolution of load partitioning during elastic deformation with increasing composite damage. Predictions of the present model compare favourably with relevant results in the literature, and with results from the present neutron diffraction experiments. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

2008

SAMARIS D26 - Modelling of UHPFRC in composite structures

Emmanuel Denarié, Hamid Sadouki

The extremely low permeability of Ultra-High Performance Fibre Reinforced Concretes (UHPFRC) associated with their outstanding mechanical properties make them especially suitable to locally "harden" reinforced concrete structures in critical zones subjected to an aggressive environment and to significant mechanical stresses. UHPFRC materials can be applied on new structures, or on existing ones for rehabilitation, as thin watertight overlays in replacement of waterproofing membranes, as reinforcement layers combined with reinforcement bars, or as prefabricated elements such as kerbs. The successful rehabilitation of existing structures is a major challenge for civil engineers. When existing concrete needs to be replaced, a new composite structure formed of the new material cast on the existing substrate will result from the intervention. Both the protective function and the mechanical performance of the composite system have to be guaranteed over the planned service life. These requirements can be fulfilled by a sound understanding of the origin of deteriorations, and by proper design and application. Further, to take the full benefit of the potential of UHPFRC to rehabilitate or reinforce structures, it is often needed to get better insights of the performance of composite UHPFRC-Concrete members by means of numerical models such as finite element simulations. The numerical modelling of multiple layer systems involves comprehensive models of the time dependent behaviour of the materials at early-age and long term. UHPFRC require special consideration of their tensile hardening behaviour. This document gives the theoretical background, and provides validations of a constitutive model for UHPFRC under tensile loading. This model is then applied to the inverse analysis of test results in bending, and to the simulation of various configurations of application of UHPFRC in composite structural members.