Andreas Mortensen

Size dependent strengthening in particle reinforced aluminium

The tensile behaviour of composites produced by infiltrating ceramic particle beds with high purity (99.99%) At is studied as a function of reinforcement size and chemistry (Al2O3 and B4C). The yield stress is higher in composites containing B4C particles, increasing with decreasing interparticle distance in both composite systems. The flow stress of the composites, when corrected for damage, displays the same dependence on interparticle distance as the yield stress. The overall strain hardening exponent, however, is independent of the microstructural scale. These observations are rationalized based on the theory of geometrically necessary dislocations

Deformation of open-cell aluminum foam

The mechanical properties of high-purity aluminum foams produced by replication from salt precursors are measured in compression. These foams have homogeneous open-porosity, cell sizes equivalent to the particle size of the precursor salt (∼500 μm in this case) and relative densities near 25%. Deformation is uniform and strain hardening similar to the bulk material is observed without a plateau stress. A simple analytical model based on beam theory is employed to describe the flow stress and the change in stiffness of the foams as a consequence of compression. This model leads to a modified scaling law for the flow stress of metallic foams.

Infiltration of fibrous preforms by a pure metal. Part I. Theory

General expressions are derived to describe fluid flow and heat transfer during infiltration of fibrous preforms by a pure metal. Analytical solutions to the problem are given for the case of unidirectional infiltration into a uniform preform of aligned fibers under constant applied pressure. Calculations are carried out for infiltration kinetics (including total infiltrated length) and temperature distribution, using as an example alumina fiber/aluminum composites. Limiting cases leads to very simple expressions. Initial fiber temperatures both above and below the metal melting point are considered. In the case of fibers at a temperature significantly below the metal melting point, it is concluded that the factor most strongly influencing infiltration is the solidification of metal in the interfiber region. In the calculations, it is assumed that this solidification is in the form of a uniform solid metal sheath around the fibers. Metal superheat, when present, serves to progressively remelt the solidified sheath from the upstream end of the preform. Fiber volume fraction and initial temperature are predicted to have a major effect on infiltration kinetics, while metal superheat exerts a relatively minor influence. When no external heat extraction is present and a constant pressure is applied to the metal, flow through the preform continues indefinitely. For the case of external heat extraction, flow ceases when sufficient solidification occurs to block flow.

ON THE INFILTRATION OF METAL MATRIX COMPOSITES

Keywords: FIBER REINFORCEMENT ; INFILTRATION ; LIQUID METAL FRONT ; PRESSURE DIFFERENTIAL ; METALS AND ALLOYS Note: MIT, Cambridge, MA, USA, MIT, Cambridge, MA, USA03602133 (ISSN) Reference LMM-ARTICLE-1987-001View record in Scopus Record created on 2006-10-09, modified on 2017-05-10

Infiltration processing of fibre reinforced composites: governing phenomena

All three classes of fibre reinforced composite materials (polymer, metal and ceramic matrix) may be produced by flow of liquid matrix into the open spaces left within pores of a fibre preform. Even though several specific issues arise from the nature of each composite matrix class, governing phenomena apply to all infiltration processes, and include in particular: (i) capillary phenomena, (ii) transport phenomena, and (iii) the mechanics of potential fibre preform deformation. These phenomena and their governing laws are reviewed for the case of isothermal infiltration with no phase transformations. Four basic functional quantities, which need to be known to model the processes, are identified, and addressed in turn. The paper concludes with some examples of modelling methodologies and comparison with experimental data.

On plastic relaxation of thermal stresses in reinforced metals

Abstract Silver chloride containing alumina fibers or glass microspheres is used as a model material to study matrix plasticity induced by thermal mismatch in metal matrix composites. Resulting matrix dislocations are decorated at room temperature in the bulk material and observed by optical microscopy. Plastic deformation of the matrix around the inclusions is found to take the form of (i) rows of prismatic dislocation loops puched into the matrix and/or (ii) a plastic zone containing tangled dislocations surrounding the inclusions. From the number of loops punched by spheres, the temperature interval over which slip of prismatic loops is operative is calculated to be 100 ± 30 K wide. The stress in the plastic zone around fibers is determined from the radius of curvature of pinned dislocations, leading to the conclusion that the matrix is locally strain-hardened. A simple model taking this fact into account is proposed to predict the radius of the plastic zone around embedded cylinders and spheres and is compared to the experimental data.

Infiltration of fibrous preforms by a pure metal. Part II. Experiment

In a previous paper, a theory was developed to describe the flow of a pure metal into a fibrous preform. This paper presents experimental data to test the results of the theory for pure aluminum flowing into fibrous alumina preforms. An apparatus was designed and built for unidirectional infiltration under constant pressure and carefully controlled temperature parameters. A sensor was also developed to measure the position of the liquid metal in the fibrous preform during the experiment. This technique enabled quantitative comparison of theory and experiment. Experimental data are reported for the infiltration by 99.999 and 99.9 wt pct pure aluminum of SAFFIL alumina fibers fabricated into two-dimensionally random preforms. Fiber volume fraction was varied from 0.22 to 0.26, fiber preheat temperature was varied from approximately 483 to 743 K, and metal superheat was varied from 20 to 185 K. Infiltration pressure was varied from 1 to 4.5. MPa (145 to 650 psi). Agreement between theory and experiment was very good under all the experimental conditions studied for the 99.999 wt pct pure matrix. The impurity level of the metal was found to influence infiltration significantly. The measured perform permeability for 99.9 wt pct aluminum was much lower than that for 99.999 wt pct aluminum.

Interfacial phenomena in the solidification processing of metal matrix composites

Capillary phenomena pertinent to solidification processing of metal matrix composite materials are reviewed. Recent research on the design of wettability measurement methods that are pertinent to metal matrix composite fabrication and overcome limitations of the sessile drop technique is presented. The discussion also addresses non-chemical aspects of wetting of a reinforcement by liquid metal, with emphasis on the infiltration process, and reviews engineering approaches for improved wettability and control of interface microstructure.

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

Infiltration of fibrous preforms by a pure metal: part III. Capillary phenomena

Infiltration experiments on aluminum cast into SAFFIL alumina fiber preforms containing a silica binder and of fiber volume fraction varying from 10 to 25 pct are reported. Data are compared with the theory presented in Part I and used to characterize wettability of the preforms by plotting the infiltrated length of composite divided by the square root of time as a function of applied pressure. The intercept of the resulting curves with the abscissa axis is shown to be a measurement of the capillary pressure needed to fully infiltrate the fiber preforms. Resulting experimental values of this capillary pressure are then used with Brunauer, Emmett, and Teller (BET) adsorption isotherm measurements of the preform’s specific surface to derive an apparent wetting angle of the fibers by aluminum during infiltration. In this manner, the effective wetting angle of pure aluminum on the alumina/silica fibers is found to be 106 ±5 deg, independent of fiber preform temperature. We also propose a mechanism for preventing preform compression during infiltration.