Documents / Report
The Defense Intelligence Agency issued this Defense Intelligence Reference Document (DIA-08-0911-012), dated 14 December 2009, as one of its FY 2009 advanced technology reports under the Advanced Aerospace Weapon System Applications (AAWSA) Program. It reviews how metallic glasses are structured and processed, their mechanical properties, and metallic glass matrix composites. It concludes that dendritic composites could replace high-strength steels in some aerospace parts. It adds that wide aerospace use depends critically on developing new lightweight glass-forming alloys.
“metallic”29 pages
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Table 2. Comparison of strengths of Amorphous and Crystalline
Aluminum Alloys. Compared with tne theoretical maximum strength
(taken to be μ/30, whereμ is the,shear modulus of pure aluminum).
Theoretical
Strength (Defect-
Free Crystal)
Typical High-
~treligth
Aluminum Alloy
(7xxx Series)11
Best Crystalline
Aluminum Alloy12
Aluminum-Based '
•• Meta me Glass13
Yieid Stress (MPa)
400-500
770
1,280
0/o of Theoretical
strength
25-31%
48%
80%
Unfortunately, the lack of dislocations in amorphous alloys.is also theirAcl::lilles' heel. In
crystalline alloys, dislocations move a.nd multiply in response to applied stresses,
resulting in dislocation tangles that increase the resistc:1nce to further dislotatio11
motion. This process, called'strain hardening, is of crucial importance because'",t makes
plastic deformation stable. If one region of a crystalline materiaf yields and begins to
plastically deform, the deforming region strain hardens, and so another region will
deform instead. The result is that the plastic deformation is not concentrated but rather
spreads through a large volume of material. Metallic glasses, lacking dislocations, do
not strain harden and in fact strain soften in response to plastic deformation. This
means that as soon as any one region yields, any further deformation will occur in the
same region. This process, known as shear localization, leads to the formation of shear
bands {Figure 4). In any loading geometry where the metallic glass experiences
significant tensile loading, fracture occurs on a single dominant shear band with
essentially zero tensile ductility.2 Metallic glasses therefore fracture in an abrupt,
apparently brittle manner on the macroscopic scale (even though there can be
significant plasticity on a microscopic scale). This lack of ductility is of obvious concern
to designers interested in structural applications. Furthermore, it limits the ability to
fabricate metallic glasses into different shapes by deformation processing (by rolling or
forging, for instance) after casting.
2 This assumes there Is no geometrical constraint preventing fracture. Some geometries (such as simple bending)
can involve tensile loading, but there can still be significant plastic deformation because the geometrical constraints
inhibit propagating of shear bands across the specimen.
7
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Report, from the dia collection. The PDF is mirrored here; the original link is under it. 30 pages are in the text index: search them above, or from the library's search.