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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. Source


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. Source


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. Source


Grant
Agency: Department of Defense | Branch: Air Force | Program: SBIR | Phase: Phase I | Award Amount: 99.94K | Year: 1999

N/A


Grant
Agency: Department of Defense | Branch: Air Force | Program: SBIR | Phase: Phase I | Award Amount: 99.95K | Year: 1998

The lack of fully developed extrusion design tools for Discontinuously Reinforced Aluminum (DRA) such as finite element simulation codes will continue to impede market penetration of this promising aerospace material. There is limited motivation on the part of the aluminum extrusion industry to develop finite element codes given the low cost of aluminum and the use of common tool steels for extrusion dies. The design tools this proposal seeks to establish will reduce the lead-time and cost of developing DRA extrusions, expand the manufacturing infrastructure, and grow the market for DRA. The objectives of Phase I are, 1)documentation of critical extrusion process parameters such as ram velocity, extrusion pressure, billet and container pre-heat temperatures, and extrusion die temperatures through the use of thermocoupled tooling and extrusion equipment, 2)a verified finite element simulation model for DRA shear face die extrusions based on DEFORM software, 3) an initial DRA property database specific to deformation processing composed of compression flow stress data that will aid not only the modeling of extrusion but any bulk fabrication process such as rolling, forging, etc., and 4)Preliminary n, as well as preliminary guidelines for die bearing land lengeh.

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