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TECHNICAL ARTICLES
MICROSTRUCTURALLY DESIGNED HIERARCHAL ALUMINUM METAL MATRIX COMPOSITES
[Yongho Sohn]
Professor, Materials Science and Engineering
University of Central Florida, Orlando
INTRODUCTION
Nanostructured Al-alloy based MMCs have gained considerable interest due to their excellent properties (i.e., high strength, low
density, good corrosion resistance), and their technical and economical ease of manufacturing. Recently, a hierarchal MMC consisting
of a nanocrystalline Al phase (NC-Al), boron carbide (B4C) reinforcement particles, and a coarse-grain Al phase (CG-Al) has
been successfully fabricated and reported to exhibit extremely high compressive yield strength ( > 1 GPa) and tailorable ductility
(~ 15%) [1,2]. In particular, high strain-rate dependent materials have found their way into aerospace, automotive, biomedical, and
defense application. These properties are achieved through control of the microstructure that is dependent on parameters of materials
processing and manufacturing. The hierarchical Al-MMCs exhibit good thermal stability and microstructural characteristics
that deflect or blunt or bridge crack propagation. The microstructure and properties can be further customized through the use of
friction stir processing [3] which offers a variety of possibilities for joining components of dissimilar geometry, modified and graded
microstructure and composition.
MATERIALS PROCESSING AND MANUFACTURING
The commercial gas-atomized AA5083 Al powder has an average particle size of less than 45 μm with a grain size ranging from 0.2
to 2 μm. The B4C powder has a starting particle size between 1 to 7 μm. The 5083 Al powder is combined with B4C powder and cryomilled
for up to 24 hours. The powders are ball-milled in the presence of liquid N2 to increase friability, prevent oxidation and mitigate
dynamic recrystallization. Commercially available stearic acid is utilized as a process-control agent during milling to prevent excessive
agglomeration of the powders and improve yield. Cryomilling is a key step that produces the Al grain size down to 20-30 nm
[4,5], disperses the B4C into the Al matrix and creates the NC-Al/B4C bond. Other strengthening mechanisms are generated during
this process including dislocation entanglement, dispersoids strengthening and complex interface development. After cryomilling,
additional AA5083 powder (CG-Al) is blended with the cryomilled mixture, which in turn creates the hierarchal MMC powders.
The powder samples are degassed
through a rotary dynamic vacuum
degassing system. The degassed
powders are then consolidated into
billets with established powder metallurgy
techniques such as vacuum
hot pressing, cold isostatic pressing,
or hot isostatic pressing. Secondary
processing is achieved through
conventional wrought processing
techniques such as extrusion, forging,
and/or rolling. Figure 1 illustrates
the complete manufacturing
processing steps of the hierarchal Al
Figure 1. Manufacturing sequence of hierarchal Al metal matrix composites
MMCs examined in this study.
NOVEL MICROSTRUCTURAL FEATURES OF HIERARCHAL COMPOSITES
Extensive microstructural and spectroscopic analyses were carried out as a function of processing parameters. The properties of
hierarchal Al MMCs were influenced by multiple microstructural features including: (1) volume (or weight) fraction of each constituent,
NC-Al, CG-Al and B4C; (2) grain size and distribution of the NC- and CG-Al; (3) size and distribution of the B4C particle
reinforcement; (4) size and distribution of hierarchal microstructural domains, e.g., CG-Al and B4C/ NC-Al agglomerates; (5)
dislocation densities the in NC- and CG-Al; (6) composition, distribution and structure of nitrogen incorporated during cryomilling
either in solution or as a dispersoid reinforcement; (7) characteristics of grain boundaries and interfaces (e.g., NC-Al and B4C,
NC-Al and CG-Al, CG-Al and B4C); (8) other impurity-associated strengthening by solution or dispersion reinforcements.
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KSEA LETTERS Vol. 40 No. 3 April 2012