L. Weber

On the electrical conductivity of metal matrix composites containing high volume fractions of non-conducting inclusions

Different predictive models-the Maxwell mean field approach, the differential effective medium scheme, the 2- and 3-phase self-consistent, and 3-point model-for the electrical conductivity of two-phase materials are assessed based on electrical conductivity measurements of metal matrix composites with non-conducting inclusions produced by gas pressure infiltration. The volume fraction of non-conducting phase, namely equiaxed or angular alumina particles of various sizes and size-distributions embedded in a matrix of pure aluminum, is varied between 40 and 70 vol.-%. For a given volume fraction, the equiaxed particles yield consistently higher conductivity than their angular counterparts, by as much as 40%. The Maxwell/Mori-Tanaka estimate and the 3-phase self-consistent model are consistently too high for the case of equiaxed particles (approximated by spheres), while for this particle shape the 3-point bounds for the limiting case of symmetric cell materials and the differential scheme give good agreement. For angular particles, approximated by randomly oriented oblate spheroids, only the differential scheme yields accurate predictions, whereas the Maxwell mean-field approach largely, and the 3-phase self-consistent approach for randomly oriented spheroids slightly, overestimate the effective conductivity. The 3-point bound for symmetric cell materials with spheroidal cells also overestimates the effective conductivity significantly. Overall, the differential scheme is found to exhibit very good predictive capacity over the ranges of geometry and volume fractions covered in this study, unlike all other models examined

Partitioning of chromium and molybdenum in super duplex stainless steels with respect to nitrogen and nickel content

Abstract Experimental data of elemental partitioning of Cr, Mo, and Ni in a 5% Mn containing SAF 2507 super duplex stainless steel (SDSS) with varying nitrogen and nickel mass fractions are presented. Experimental results on phase equilibria and alloying element partitioning are compared to values calculated using Thermo-Calc software revealing good agreement. A trilinear model is proposed to describe the partitioning ratio of Cr, Mo and Ni with nitrogen and nickel mass percent in the austenite and the annealing temperature as variables. While nitrogen is found to reduce the partitioning of chromium and molybdenum, nickel enhances the partitioning ratio of these to alloying elements. Based on this semi-empirical model, a guideline for the future development of improved SDSS is formulated. With respect to corrosion resistance, a higher nitrogen level is not beneficial by itself but needs to be paralleled by increasing molybdenum and decreasing chromium contents.

Influence of damage on the tensile behaviour of pure aluminium reinforced with ?40 vol. pct alumina particles

particle beds with high purityAl (99.99%). These materials feature 40–60 vol. pct reinforcement homogeneously distributed in a pore-freematrix. Their tensile behaviour is studied as a function of reinforcement size and shape. Internal damage, inthe form of particle fracture and matrix voiding, occurs from the onset of plastic straining. Its evolution withstrain is monitored through changes in (i) stiffness and (ii) peak stress after incremental plastic straining andannealing. The influence of damage on the flow curves of the composites can be accounted for using basicpostulates of continuum damage mechanics. Failure strains vary between 2 and 4%, and are a function ofthe rate of damage accumulation. An expression is derived to predict elongation to failure of damagingmaterials that fail by tensile instability, which gives good agreement with the experimental observations. 2001 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.Keywords: Composites; Dislocations; Aluminium; Stress–strain relationship measurements; Damage1. INTRODUCTION

Diamond-Based Metal Matrix Composites for Thermal Management Made by Liquid Metal Infiltration — Potential and Limits

Diamond-based metal matrix composites have been made based on pure Al and eutectic Ag-3Si alloy by gas pressure infiltration into diamond powder beds with the aim to maximize thermal conductivity and to explore the range of coefficient of thermal expansion (CTE) that can be covered. The resulting composites covered roughly the range between 60 and 75 vol-% of diamond content. For the Al-based composites a maximum thermal conductivity at room temperature of 7.6 W/cmK is found while for the Ag-3Si based composites an unprecedented value of 9.7 W/cmK was achieved. The CTE at room temperature varied as a function of the diamond volume fraction between 3.3 and 7.0 ppm/K and 3.1 and 5.7 ppm/K for the Al-based and the Ag-3Si-based composites, respectively. The CTE was further found to vary quite significantly with temperature for the Al-based composites while the variation with temperature was less pronounced for the Ag-3Si-based composites. The results are compared with prediction by analytical modeling using the differential effective medium scheme for thermal conductivity and the Schapery bounds for the CTE. For the thermal conductivity good agreement is found while for the CTE a transition of the experimental data from Schapery’s upper to Schapery’s lower bound is observed as volume fraction increases. While the thermophysical properties are quite satisfactory, there is a trade-off to be made in these materials between high thermal conductivity and low CTE on the one side and surface quality and machinability on the other.

Quantification of microdamage phenomena during tensile straining of high volume fraction particle reinforced aluminium

Particle reinforced composites are produced by infiltrating ceramic particle beds with 99.99% Al. Resulting materials feature a relatively high volume fraction (40-55 vol. pet) of homogeneously distributed reinforcement. The evolution of damage during tensile straining of these composites is monitored using two indirect methods; namely by tracking changes in density and in Youngs modulus. Identification and quantification of the active damage mechanisms is conducted on polished sections of failed tensile specimens: particle fracture and void formation in the matrix are the predominant damage micromechanisms in these materials. The damage parameter derived from the change in density at a given strain is found to be one to two orders of magnitude smaller than the parameter based on changes in Youngs modulus. A simple micromechanical analysis inspired by the observed damage micromechanisms is used to correlate the two indirect measurements of damage. The predictions of this analysis are in good agreement with experiment

On the influence of the shape of randomly oriented, non-conducting inclusions in a conducting matrix on the effective electrical conductivity

The influence exerted by the shape of randomly oriented non-conducting inclusions on the electrical conductivity of a metal is investigated using the aluminum-sit icon eutectic alloy. By varying the time and temperature of heat treatment after solidification, the shape of the silicon inclusions is varied between plate-like and close-to-spherical. The influence of the geometry of the inclusions on the resistivity of the composite is separated from the influence of varying amounts of silicon in solid solution by centering the analysis on the temperature coefficient of the electrical resistivity rather than on the resistivity itself. The experimentally determined influence of inclusion geometry at fixed volume fraction on resistivity is compared to analytical predictions by extending the dilute solution for random-oriented spheroids using either the Maxwell/Mori-Tanaka approach or the differential effective medium approach. As a third predictive scheme, the three-phase self-consistent scheme for randomly oriented ellipsoids developed by Miloh and Benveniste is used. It is shown that the differential scheme and the three-phase self-consistent model capture the experimental results quite well, while the Maxwell/Mori-Tanaka approach underestimates both the resistivity and the influence exerted by the inclusion aspect ratio

Thermal conductivity and interfacial conductance of AlN particle reinforced metal matrix composites

Aluminum nitride (AlN) particle reinforced metal-matrix-composites produced by pressure infiltration are characterized in terms of their thermal conductivity. The composites are designed to cover a wide range of phase contrast between the dispersed particles and the matrix; this is achieved by changing the matrix conductivity using Cu, Al, Sn, and Pb as the matrix. The interface thermal conductance (hc) between AlN and the matrix metals is determined by varying the size of the AlN particles using the Hasselman–Johnson approach and the differential effective medium (DEM) model to calculate hc from measured composite conductivity values. In addition, hc is measured directly at the AlN/Al interface using the transient thermoreflectance (TTR) method on thin aluminum layers deposited on flat AlN substrates to find good agreement with the value derived directly from Al/AlN composites of variable particle size and thus confirm the approach used here to measure hc. Data from the study show that hc at AlN-metal in...

The electrical conductivity of microcellular metals

The electrical conductivity of metallic foams varies strongly with porosity. Data are presented for replicated open-pore microcellular metal over a wide range of relative density. These data, together with data from the literature for different foam structures, are compared with previously suggested models for the conductivity of porous materials. We show that clear differences exist between the behavior of foams according to their different structural types and that simple models exist to capture these differences.

Qualitative link between work of adhesion and thermal conductance of metal/diamond interfaces

We report Time-Domain ThermoReflectance experiments measuring the Thermal Boundary Conductance (TBC) of interfaces between diamond and metal surfaces, based on samples consisting of [111]-oriented diamond substrates with hydrogen or with sp2 carbon surface terminations created using plasma treatments. In a concurrent theoretical study, we calculate the work of adhesion between Ni, Cu, and diamond interfaces with (111) surface orientation, with or without hydrogen termination of the diamond surface, using first-principles electronic structure calculations based on density functional theory (DFT). We find a positive correlation between the calculated work of adhesion and the measured conductance of these interfaces, suggesting that DFT could be used as a screening tool to identify metal/dielectric systems with high TBC. We also explain the negative effect of hydrogen on the thermal conductance of metal/diamond interfaces.

Nextel? 610 alumina fibre reinforced aluminium: Influence of matrix and process on flow stress

Abstract Continuous alumina fibre reinforced aluminium matrix composites are produced using two different liquid metal infiltration methods, namely direct squeeze casting and gas pressure infiltration. Net-shape fibre performs for longitudinal parallel tensile bars are prepared by winding the Nextel™ 610 alumina fibre (3M, St Paul, MN) into graphite moulds. High purity aluminium, two binary (Al–6% Zn and Al–1% Mg) and one ternary (Al–6% Zn–0.5% Mg) aluminium alloys are used as matrix materials. The composite is tested in uniaxial tension–compression, using unload–reload loops to monitor the evolution of Youngs modulus. A linear dependence between Youngs modulus and strain is observed; this is attributed, by deduction, to intrinsic elastic non-linearity of the alumina fibre. This conclusion is then used to compare on the basis of the in situ matrix flow curve the influence of matrix composition and infiltration process on the composite stress–strain behaviour.