Historical research guide · 0187 · 2009
DIRD Metallic Glasses: Status and Prospects for Aerospace Applications
DIA / AAWSAP contractor
In one minute
The Defense Intelligence Agency opened its AAWSAP reference series not with exotic physics but with a metal: metallic glass, an alloy cooled so fast that its atoms freeze in place before they can line up into a crystal. Losing the crystal changes everything, the author argues. With no lattice there are no dislocations — the mobile defects that let ordinary metals bend — so an aluminium-based metallic glass yields at 1,280 megapascals, about 80 percent of the theoretical strength of a perfect crystal and two to three times a conventional high-strength alloy. But with nothing to spread the strain, the material shears in a single band and snaps: essentially zero tensile ductility, and a fatigue limit near 5 percent of tensile strength where crystalline alloys reach 40. The fix the report backs is the composite — soft crystalline dendrites grown inside the glassy matrix — whose fracture toughness passes 170 megapascal root-metres, higher than virtually any other metal known. And because a glass softens rather than melting, parts can be injection-moulded, blow-moulded, foamed, or embossed down to 13 nanometres.
Connection
Chapter 1 reads the thirty-eight AAWSAP reference documents as a government survey of everything an advanced aerospace vehicle would need, and this is the one that starts with the airframe. It is also the plainest statement of chapter 13’s bottleneck: in a field with no exotic physics in it at all, a DIA report concludes that whether the technology arrives depends on finding alloys nobody has found yet — which is the thesis’s own claim that the limit on using the vacuum is material science, made here about ordinary matter.
From the source
Because an amorphous alloy has no crystal lattice it can have no dislocations, so its strength approaches the theoretical limit set by the atomic bonds themselves: an aluminium-based metallic glass yields at 1,280 megapascals — about 80 percent of the 1,600 megapascal theoretical maximum for defect-free aluminium, against 400 to 500 megapascals for a typical 7xxx-series high-strength alloy — and the best iron-based glasses reach roughly 4 gigapascals.Strength and Ductility: Plastic Deformation, p. 6; Table 2, p. 7
The same absence of dislocations is the material’s Achilles heel: metallic glasses do not strain harden but strain soften, so plastic deformation localises into a single shear band and fracture occurs there with essentially zero tensile ductility, and the fatigue limit is only about 5 percent of tensile strength where high-strength crystalline alloys reach about 40 percent — a critical limitation, since fatigue is estimated to cause up to 90 percent of in-service structural failures.Strength and Ductility, pp. 6-7; Fatigue, p. 9; Figure 5, p. 10
Growing ductile crystalline dendrites inside the glassy matrix restores what the glass lacks: the plane-strain fracture toughness of some zirconium-based in situ composites exceeds 170 megapascal root-metres — seven times that of the single-phase glass and greater than that of virtually any other metallic alloy — fatigue strength rises to 20 to 30 percent of tensile strength, and titanium-based in situ composites have been reported with 12 percent tensile elongation at about 30 percent greater strength than Ti-6Al-4V.Fracture and Fatigue, p. 16; Strength and Ductility: Plastic Deformation of composites, p. 16
Because they are glasses in the true sense, metallic alloys can for the first time be processed like thermoplastics — heated above the glass transition into a supercooled liquid whose viscosity is set by temperature, then injection-moulded, blow-moulded into thin-walled vessels, lithographically formed into microsprings, or foamed — and superplastic forming of metallic glass surfaces with features as small as 13 nanometres has been demonstrated.Casting and Molding, p. 4; Foams, p. 5; Other Applications, p. 19
The recommended aerospace use is not to replace aluminium but to replace high-strength steel: dendritic metallic glass matrix composites in load-limited structural components where space is tight, such as pylon structures and landing gear — though the report notes it has yet to be demonstrated that the composites can be fabricated in the sizes needed, and their corrosion and stress-corrosion cracking resistance has not been fully evaluated.Structural Applications, p. 17
The programme’s own bottleneck is alloy discovery: there are no good aluminium-rich glass-forming alloys despite the DARPA Structural Amorphous Metals programme and parallel work in Japan and China, the dendritic composite route has been demonstrated in only two closely related alloy systems, and the report expects 20 to 50 years of further work — with widespread aerospace use depending critically on the development of new, lightweight alloys.Alloy Design, p. 20; Composites and the Quest for Ductility, p. 21; Summary and Recommendations, p. 22
The way in
https://documents2.theblackvault.com/documents/dia/AAWSAP-DIRDs/DIRD_01_Metallic_Glasses_for_Aerospace_Applications.pdfHow to cite it
DIA / AAWSAP contractor (2009) DIRD Metallic Glasses: Status and Prospects for Aerospace Applications https://documents2.theblackvault.com/documents/dia/AAWSAP-DIRDs/DIRD_01_Metallic_Glasses_for_Aerospace_Applications.pdf