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DWA Aluminum Composites

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Yao B.,University of Central Florida | Patterson T.,University of Central Florida | Sohn Y.,University of Central Florida | Shaeffer M.,Johns Hopkins University | And 3 more authors.
TMS Annual Meeting | Year: 2011

Trimodal aluminum (Al) metal matrix composites (MMCs) consisting of a nanocrystalline Al (ncAl) phase, B4C reinforcement particles, and a coarse grain Al (cgAl) phase contain multiscale microstructure and interfacial features that contribute to exceptional mechanical properties. One of the hierarchal microstructural features within the composite, namely the ncAl+B4C agglomerates formed during cryomilling, is an important feature whose size can range from tens of micro-meters to millimeters. This paper reports a study on the influence of the ncAl+B4C agglomerate size on the dynamic compressive strength of the trimodal Al MMCs. A desired hierarchal microstructure for high-strength of Al MMCs is identified to contain uniformly distributed relatively small ncAl+B4C agglomerates. Commercial-scale fabrication process to achieve this microstructure includes size classification by sieving the cryomilled ncAl+B4C agglomerates.


Hofmeister C.,University of Central Florida | Yao B.,University of Central Florida | Sohn Y.H.,University of Central Florida | Delahanty T.,Pittsburgh Materials Technology Inc. | And 2 more authors.
Journal of Materials Science | Year: 2010

Trimodal aluminum (Al) metal-matrix-composites (MMCs), consisting of B 4C particulates, a nanocrystalline Al (NC-Al) phase, and a coarse-grain Al phase (CG-Al), has been fabricated. These MMCs exhibits extremely high compressive strength and tailorable ductility. Excellent thermal stability of NC-Al grains and high strength has been attributed partially to the nitrogen present within the trimodal Al MMCs, which is introduced during the cryomilling process in liquid nitrogen. This paper describes an investigation into the concentration and constituents of nitrogen within the trimodal Al MMCs. The structure of nitrogen-containing dispersoids was examined by analytical transmission electron microscopy (TEM), and secondary ion mass spectrometry (SIMS) was employed to determine the total concentration of nitrogen. The nitrogen concentration increased linearly with an increase in cryomilling time up to 24 h. Both crystalline and amorphous aluminum nitrides with very fine size, down to 5 nm, as dispersoids, have been observed by analytical TEM. Correlations between the cryomilling time, nitrogen concentration, NC-Al grain size, and composite hardness are presented and discussed. The presence of nitrogen as nitride-dispersoids can contribute to the outstanding mechanical properties of trimodal Al MMCs by inhibiting NC-Al grain growth during the high temperature consolidation and deformation process, and by dispersion- strengthening. © 2010 Springer Science+Business Media, LLC.


Yao B.,University of Central Florida | Heinrich H.,University of Central Florida | Smith C.,DWA Aluminum Composites | van den Bergh M.,DWA Aluminum Composites | And 2 more authors.
Micron | Year: 2011

This paper describes a methodology based on hollow-cone dark-field (HCDF) transmission electron microscopy (TEM) to study dislocation structures in both nano- and micro-crystalline grains. Although the conventional approach based on a two-beam condition has been commonly used to acquire weak-beam dark-field (WBDF) TEM images for dislocation structure characterization, it is very challenging to employ this technique to study nanocrystalline materials, especially when the grains are less than 100nm in diameter. Compared to the conventional two-beam approach, the method described in this paper is more conducive for obtaining high-quality WBDF-TEM images. Furthermore, the method is suitable for studying samples with both nanocrystalline and coarse-grains. A trimodal Al metal-matrix-composite (MMC) consisting of B4C particles, a nanocrystalline Al (NC-Al) phase, and a coarse-grained Al (CG-Al) phase has been reported to exhibit an extremely high strength and tailorable ductility. The dislocations in both NC-Al and CG-Al phases of the trimodal Al MMCs at different fabrication stages were examined using the HCDF method described. The influence of the dislocation density in both NC-Al and CG-Al phases on the strength and ductility of the composite is also discussed. © 2010 Elsevier Ltd.


Sohn Y.H.,University of Central Florida | Patterson T.,University of Central Florida | Hofmeister C.,University of Central Florida | Kammerer C.,University of Central Florida | And 5 more authors.
JOM | Year: 2012

The fabrication of hierarchical aluminum metal matrix composites (MMCs) begins with the cryomilling of inert gas-atomized AA5083 Al powders with B 4C particles, which yields agglomerates of nanocrystalline (NC) Al grains containing a uniform dispersion of solidly bonded, submicron B 4C particles. The cryomilled agglomerates are size classified, blended with coarse-grain Al (CG-Al) powders, vacuum degassed at an elevated temperature, and consolidated to form the bulk composite. This hierarchical Al MMCs have low weight and high strength/stiffness attributable to the (A) Hall-Petch strengthening from NC-Al (5083) grains, (B) Zener pinning effects from B 4C particulate reinforcement and dispersoids in both the NC-Al and CG-Al, (C) the interface characteristics between the three constituents, and (D) a high dislocation density. The hierarchical Al MMCs exhibit good thermal stability and microstructural characteristics that deflect or blunt crack propagation. A significant change in the microstructure of the composite was observed after friction stir processing (FSP) in the thermomechanically affected zone (TMAZ) due to the mechanical mixing, particularly in the advancing side of the stir zone (SZ). The NC-Al grains in the TMAZ grew during FSP. Evidence of CG-Al size reduction was also documented since CG-Al domain was absent by optical observation. Given the proper control of the microstructure, FSP has demonstrated its potential to increase both strength and ductility, and to create functionally tailored hierarchical MMCs through surface modification, graded structures, and other hybrid microstructural design. © 2012 TMS.


Yao B.,University of Central Florida | Hofmeister C.,University of Central Florida | Patterson T.,University of Central Florida | Sohn Y.-H.,University of Central Florida | And 3 more authors.
Composites Part A: Applied Science and Manufacturing | Year: 2010

Trimodal Al Metal-Matrix-Composites (MMCs), consisting of a nanocrystalline Al phase (NC-Al), B4C reinforcement particles, and a coarse-grain Al phase (CG-Al), were successfully fabricated on both lab and commercial scales. Multi-scale microstructural features contributing to the exceptional high strength of Trimodal Al MMCs were examined via comprehensive microstructural and spectroscopic analysis. Size and distribution of nanocrystalline Al grains, B4C particles, coarse-grain Al, and uniformity in distribution were examined and quantified. Other features such as dispersoids with and without nitrogen (e.g., Al2O3, Al4C3), dislocation density, and interfacial characteristics were also examined with due respect for their contributions to the strength of Trimodal Al MMCs. © 2010 Elsevier Ltd. All rights reserved.


Yao B.,University of Central Florida | Simkin B.,New Mexico Institute of Mining and Technology | Majumdar B.,New Mexico Institute of Mining and Technology | Smith C.,DWA Aluminum Composites | And 3 more authors.
Materials Science and Engineering A | Year: 2012

Grain growth of nanocrystalline aluminum ( ncAl) in trimodal Al metal-matrix-composites (MMCs) during hot forging was investigated. The ncAl phase formed through cryomilling of inert gas-atomized powders in liquid nitrogen has an average grain size down to 21nm, exhibits excellent thermal stability. However, substantial grain growth of ncAl up to 63nm was observed when the Al MMCs were thermo-mechanically processed even at relatively low temperatures. Grain growth of the cryomilled ncAl phase in trimodal Al MMCs after hot forging was documented with respect to temperature ranging from 175°C to 287°C, true strain ranging from 0.4 to 1.35 and strain rate ranging from 0.1 to 0.5s -1. Hollow cone dark field imaging technique was employed to provide statistically confident measurements of ncAl grain size that ranged from 21 to 63nm. An increase in forging temperature and an increase in true strain were correlated with an increase in grain size of ncAl. Results were correlated to devise a phenomenological grain growth model for forging that takes strain, strain rate and temperature into consideration. Activation energy for the grain growth during thermo-mechanical hot-forging was determined to be 35kJ/mol, approximately a quarter of activation energy for bulk diffusion of Al and a half of activation energy for static recrystallization. © 2011 Elsevier B.V.


Grant
Agency: Department of Defense | Branch: Navy | Program: SBIR | Phase: Phase I | Award Amount: 79.85K | Year: 2011

DWA Aluminum Composites proposes the use of powder-metallurgy based, silicon carbide particulate (SiCp) reinforced aluminum Metal-Matrix-Composites (MMCs) for the replacement of 4320 steel alloy bearing liners to achieve weight reduction in the gearboxes of the CH-53K helicopter. Ceramic particulate reinforced aluminum MMCs possess the properties of interest to the Navy, including 1) density on par with monolithic aluminum alloys, 2) increased tensile and compressive strength, 3) increased tensile and compressive modulus, 4) improved wear resistance, and 5) gearbox environmental compatibility. The Phase I program will concentrate on mechanical and wear screening tests for conventionally sized SiCp reinforcement and"ultra-fine"SiCp reinforcement in moderate and high strength alloys in simple extrusions. The primary questions the Phase I program will seek to answer are 1) what are the effects of SiCp size and matrix alloy composition on strength and wear properties and 2) are the strength and wear properties obtained sufficient for application to bearing liners. If the Phase I program is successful, it will help to establish lightweight MMC bearing liners with 1/3 the density of 4320 steel that can be mass produced using conventional thermo-mechanical processing of ring structures over a broad range of diameters, large and small.

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