DOW-UAP-D117: Metallic Glasses for Aerospace Applications - Record Strength, Poor Fatigue and a 20-50 Year Horizon

Source file: DOW-UAP-D117_AAWSAP-DIRD-Metallic-Glasses-for-Aerospace-Applications-December-14-2009.pdf Originating agency: Defense Intelligence Agency (DIA), Defense Warning Office, under the AAWSA Program Document type: Defense Intelligence Reference Document (DIRD), "Acquisition Threat Support" series; control number DIA-08-0911-012 Date: 14 December 2009 (information cutoff date, ICOD: 1 December 2009) Classification: UNCLASSIFIED//FOR OFFICIAL USE ONLY, with the FOUO caveat struck through on every page; released 2026 Page count: 30 VIRIN: 260918-D-D0360-1106 PURSUE Release: 6

Summary

"Metallic Glasses: Status and Prospects for Aerospace Applications" is a Defense Intelligence Reference Document dated 14 December 2009, prepared by the Acquisition Support Division (DWO-3) of the DIA's Defense Warning Office, Directorate for Analysis. The author's name is redacted and replaced by the label "AAP Person 63." An administrative note describes the paper as "one in a series of advanced technology reports produced in FY 2009" under the AAWSA Program, and directs questions to AAP Person 1, the program manager, at a DIA address in Washington. A second note warns that further dissemination of the photographs in the publication is not authorized, for copyright reasons.

In structure and tone the paper resembles a review article in a materials-science journal. After a two-page summary it covers the structure and processing of metallic glasses, their mechanical behavior at room and elevated temperature, their magnetic and other properties, metallic-glass-matrix composites, aerospace applications and open challenges, ending with a "Summary and Recommendations" section. It carries nine figures, two tables, one equation and 49 endnotes to the open scientific literature.

Nothing in the paper concerns UFOs, UAP or anomalous phenomena; it does not mention them at all. Its only gestures beyond ordinary aviation are coatings for "dry bearings for space applications" and foam panels for "extraterrestrial buildings." Among the technical areas in the program's Statement of Objectives (DOW-UAP-D110), it serves chiefly the materials area and, secondarily, configuration and structure.

Research Article

What a metallic glass is

Most metals are crystalline: their atoms sit on a lattice that repeats in three dimensions. A metallic glass is an alloy whose atoms lack that long-range order. The author stresses that "amorphous" does not mean random: there is strong short-range order over a few atoms, and in many ways the structure "more closely resembles the highly disordered structure of a liquid" (page 7). Two consequences follow. Mechanically, the absence of a lattice rules out dislocations, the line defects that govern how ordinary metals deform. For manufacturing, metallic glasses soften gradually over a range of temperatures, like window glass, instead of melting abruptly.

Making one is a race against crystallization. The key measure is the critical cooling rate, the slowest cooling that still yields a fully amorphous solid (Figure 2). The first metallic glasses, from the 1960s and 1970s, were binary alloys that needed about 10^4 to 10^7 K/s, which confined them to ribbons, foils, wires and powders less than 100 micrometers thick. Two decades of alloy design produced multi-component alloys that need 0.1 K/s or less and can be cast more than a centimeter thick; any alloy that can be cast as a glass at least 1 mm thick is called a "bulk" metallic glass. Table 1 lists seven outstanding glass formers: the record holder, Pd40Cu30Ni10P20, reaches 72 mm, and the zirconium alloy Zr41.2Ti13.8Cu12.5Ni10Be22.5 reaches 50 mm. The author notes that glass-forming ability still cannot be predicted, that identifying good alloys "is still mostly a matter of trial and error" (page 9), and that in some zirconium alloys it drops dramatically in the presence of oxygen.

The processing advantage is real. Heated above the glass transition, a metallic glass becomes a supercooled liquid whose viscosity can be tuned by temperature, so techniques from the polymer world, such as injection and blow molding, can be applied. Figure 3 shows a microspring made by lithography and a thin-walled bottle made by blow molding, credited to Professor Jan Schroers of Yale University. The window is short, "typically on the order of minutes" (page 10), before crystallization sets in. Welding is problematic for the same reason, and the paper singles out metallic-glass foams, where the high viscosity actually helps stabilize the pores, as "One particularly promising recent development" (page 11).

Strength and its Achilles' heel

The central numbers are in Table 2 (page 13). Taking the theoretical strength of defect-free aluminum as 1,600 MPa (the shear modulus divided by 30), a typical high-strength 7xxx-series aluminum alloy yields at 400-500 MPa (25-31 percent of theoretical), the best crystalline aluminum alloy at 770 MPa (48 percent), and an aluminum-based metallic glass at 1,280 MPa (80 percent). The best iron-based glasses reach about 4 GPa, two or three times conventional high-strength steels. Stiffness, by contrast, is 20-30 percent lower than in comparable crystalline alloys: a drawback for structures, but an advantage in springs and fastener-free snap-fit assembly.

Then comes the problem the author calls the "Achilles' heel." Without dislocations, a metallic glass cannot strain-harden; it strain-softens, so deformation concentrates in thin shear bands (Figure 4), and a specimen under tension fails on a single band with "essentially zero tensile ductility" (page 13). Fracture toughness varies widely: zirconium-based glasses reach about 20 MPa·m^1/2, below the roughly 55 of crystalline zirconium alloys but far above the 1-5 of ceramics, while iron- and magnesium-based glasses are "intrinsically brittle" (page 14). Heating can embrittle even a tough glass, and "there is no known way to reverse embrittlement once it occurs" (page 15).

Fatigue is the most serious limit. By the estimate the paper cites, up to 90 percent of structural failures in service are caused by fatigue. While high-strength crystalline alloys have a fatigue limit of about 40 percent of tensile strength, metallic glasses manage about 5 percent (Figure 5), because their featureless structure has nothing to stop a crack. Designers would have to overdesign, losing "much of the advantage of having a high-strength material in the first place" (page 15). The paper also corrects two common assumptions: corrosion resistance is not always excellent (zirconium glasses are very susceptible to stress-corrosion cracking in environments containing chloride ions), and metallic glasses are not transparent but shiny like any metal (Figure 7). A further page covers other properties: very low magnetic coercivity, hence use in high-frequency transformers; poor but nearly temperature-insensitive electrical conduction; superconductivity near absolute zero; and very low acoustic damping, potentially useful in vibrating-structure gyroscopes.

The proposed fix: dendritic composites

The paper's answer is to embed a ductile crystalline phase in the glass. Ex situ composites are made by adding particles to the melt or casting around a fiber preform (up to about 80 percent crystalline by volume), but viscosity and interfacial reactions limit them. In in situ composites, ductile crystalline dendrites grow directly from the melt (Figure 8). The dendrites must be ductile, have a lower shear modulus than the matrix, and preferably be branched; so far only zirconium- and titanium-based alloys meet the criteria.

Equation 1 explains why the approach works: the plastic zone ahead of a crack tip is approximately R_p ≈ (1/2π)(K_Ic/σ_y)^2, from about 1 micrometer in brittle glasses to about 1 mm in tougher ones. A microstructure on that scale arrests shear bands before they become cracks. The reported results are striking: titanium-based composites with tensile elongation up to 12 percent, comparable to the common alloy Ti-6Al-4V but about 30 percent stronger; zirconium-based composites with fracture toughness above 170 MPa·m^1/2, seven times that of single-phase glass and "greater than that of virtually any other metallic alloy" (page 22); and fatigue strength of 20-30 percent of tensile strength, comparable to conventional structural alloys.

Aerospace applications

The applications chapter lists seven criteria (strength, stiffness, density, fracture toughness, fatigue resistance, corrosion resistance and cost) and places metallic glasses against other materials in two property charts (Figure 9, page 24). The charts show the glasses and composites stronger for their weight than almost any crystalline metal, but also show iron- and magnesium-based glasses "as brittle as any ceramic" (page 23). The most realistic role is replacing steel in components where strength is critical but space is limited, such as pylon structures and landing gear, with two explicit caveats: it has not yet been shown that the composites can be made in the necessary sizes, and their corrosion and stress-corrosion cracking resistance "has not been fully evaluated" (page 23).

More speculative ideas follow. Metallic-glass foams could serve as compression-bearing structural panels for "extraterrestrial buildings," and might "even be produced on site (from raw feedstock), reducing the volume of material that needs to be launched" (page 25). Metallic glasses could also join the metal-fiber laminates already used "in large quantities on the new Airbus 380." And because they have no microstructure beyond a few atomic spacings, they can replicate features as small as 13 nanometers, pointing to nanoscale embossing tools, diffraction gratings for ultraviolet and soft x-rays, and actuators in micro-electromechanical systems (MEMS).

Challenges and the 20-50 year forecast

The closing chapter is candid. The best glass formers depend on expensive palladium or toxic beryllium, and "the most glaring lack" for aerospace is the absence of any good aluminum-based glass former, despite efforts in the United States (DARPA's Structural Amorphous Metals program), Japan and China (page 26). The good iron-based glasses contain carbon, boron, silicon or phosphorus, which are thought to explain their brittleness. The author is cautiously optimistic: recent work shows glass-forming ability is more sensitive to composition than was believed, so better alloys probably remain undiscovered, "possibly including some low-density glasses based on aluminum." Combinatorial thin-film screening, ab initio molecular dynamics simulations and processing techniques such as applying electromagnetic vibrations during cooling (reported to improve magnesium alloys) are named as ways forward.

On ductility the paper is blunt: for single-phase glasses "There may well be no solution." Dendritic composites have been demonstrated "in only two, closely related alloys" (page 27). The recommendations call for developing lightweight alloys by computational and combinatorial methods rather than trial and error, for better understanding of plastic deformation and composite microstructure, and for improved processing, including foams. The closing forecast: "It is highly likely that continued work over the next 20-50 years will result in significant advances," but widespread aerospace use "depends critically on the development of new, lightweight alloys" (page 29).

Significance

This is one of the most conventional papers in the AAWSAP series: a careful, well-referenced survey of a real and actively researched class of materials, with no speculative physics and no reference to UAP. Its value to the archive lies in what it shows about the program's scope. Although it is filed in the "Acquisition Threat Support" series, it contains almost no threat analysis: foreign research appears in a single sentence noting alloy-design efforts in Japan and China, and there is no assessment of any adversary's capability. What it offers is a baseline in the sense the official description uses: a snapshot of the field in December 2009 and of what would have to change for metallic glasses to find a place in flight hardware.

Its judgments are measured. The author separates what has been demonstrated (bulk glasses up to 72 mm thick, 12 percent elongation in titanium-based composites, fracture toughness above 170 MPa·m^1/2) from what has not (large composite parts, stress-corrosion behavior, aluminum glasses), and the 20-50 year horizon is explicitly conditional on lightweight alloys. The paper gives 1 December 2009 as its information cutoff date, and this article does not assess how the field has developed since. One small administrative inconsistency: the note calls the paper an FY 2009 product, although a 14 December 2009 date falls in U.S. federal fiscal year 2010, which began on 1 October 2009.

Key People

Locations

Key Concepts

Notable Quotes

"Unfortunately, the lack of dislocations in amorphous alloys is also their Achilles' heel." -- page 13

"The fatigue limit for high-strength crystalline alloys is typically about 40 percent of the tensile strength, but for metallic glasses, it is only about 5 percent of the tensile strength (Figure 5)." -- page 15

"A common misperception among those hearing about metallic glasses for the first time is to think they are transparent. This is not the case; amorphous alloys are highly reflective, with a shiny luster similar to that of other metals (Figure 7)." -- page 18

"The most obvious applications would be to replace steel in certain components where strength is critical but space is limited. These might include pylon structures and landing gear, although it has yet to be demonstrated that the composites can be fabricated in the sizes necessary." -- page 23

"Such structural foams could be useful in applications requiring strength and stiffness under compressive loads, such as structural panels for extraterrestrial buildings." -- page 25

"For aerospace applications, the most glaring lack is that, despite significant alloy design efforts in the United States (through the DARPA Structural Amorphous Metals program), Japan, China, and elsewhere, there are no good glass-forming alloys based on aluminum." -- page 26

"There may well be no solution to this problem for monolithic metallic glasses, for the simple reason that they lack any microstructure to interact with shear bands." -- page 27

"It is highly likely that continued work over the next 20-50 years will result in significant advances in all these areas, and that metallic glasses and metallic glass matrix composites will see increasing acceptance as structural materials. Whether or not they achieve widespread use in aerospace applications, however, depends critically on the development of new, lightweight alloys." -- page 29