AAWSAP DIRD, Metallic Glasses for Aerospace Applications, December 2009

Metallic glass foams (see above) also provide intriguing possibilities for structural applications. It has recently been shown that metallic glass foams with outstanding strength can be formed by controlling the size of the ligaments between pores. 40 This is a new development, and these foams have not been fully characterized, but it seems likely that optimized foams will have a specific stiffness (E/ρ) superior to that of polymer foams, along with high strength and acoustic damping. Such structural foams could be useful in applications requiring strength and stiffness under compressive loads, such as structural panels for extraterrestrial buildings. Conceivably, such structural foams might even be produced on site (from raw feedstock), reducing the volume of material that needs to be launched. A final possibility is that metallic glasses might be combined with polymer composites into metal-fiber laminate materials. Similar laminates (with crystalline aluminum alloys) are being employed in large quantities on the new Airbus 380 and are likely to find increased application in the future. 41 The use of metallic glasses in these laminates is appealing because of their high specific strength (although the specific stiffness is lower than that of aluminum). Furthermore, the individual layers in the laminate are sufficiently thin that a wide range of glass-forming alloys might be considered (in contrast to thicker structural sections, which will be limited by the glass-forming ability of the alloy). OTHER APPLICATIONS Monolithic metallic glasses are unique among metallic materials in having no microstructure at length scales of more than a few atomic spacings. In principle then, metallic glasses should be capable of replicating features down to this scale. This possibility is facilitated by the ability of metallic glasses to be formed in the supercooled liquid temperature range with controllable viscosity. Indeed, superplastic forming of metallic glass surfaces with features as small as 13 nanometers has been demonstrated. 42 This ability could be exploited for direct embossing of nanostructures in polymers or other materials. Structures on this length scale are also potentially useful as diffraction gratings for ultraviolet and soft x-ray radiation. In a related area, metallic glasses have a variety of useful properties for application in micro-electromechanical system (MEMS) actuators, including large elastic strains and high resilience (elastic strain energy storage), good corrosion and wear resistance, and an excellent surface finish. 43 The scale of these devices is smaller than the plastic zone size (Equation 1 above), making brittle fracture unlikely. Furthermore, a much wider variety of amorphous alloys can be made in thin film form (by vapor deposition) than is possible by casting. Finally, the magnetic properties of certain amorphous alloys have long been exploited. For instance, their low coercivity and high electrical resistivity make ferromagnetic amorphous alloys attractive as high-efficiency electrical transformers, particularly at high frequencies. Such applications are likely to continue well into the future. 19 UNCLASSIFIED//FOR OFFICIAL USE ONLY

UNCLASSIFIED//FOR OFFICIAL USE ONLY Current Challenges and Prospects for the Future ALLOY DESIGN A critical limitation of existing metallic glass technology (and related composites) is the relative dearth of alloys with good glass-forming ability. The best glass-forming alloys are either based on expensive elements (for example, palladium) or contain toxic elements (for example, beryllium in the best zirconium- and titanium-based alloys). 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. Attempts to make aluminum-based metallic glass components by consolidating amorphous powders have met with limited success. Similarly, all of the good iron-based metallic glasses contain considerable amounts of nonmetallic elements (notably carbon, boron, silicon, and/or phosphorus), which are thought to contribute to the very low fracture toughness of these alloys (Figure 9(b)). However, there is reason to expect that further progress is possible. Recent experimental results have shown that some of the empirical "rules" of glass-forming ability44 are actually quite flexible, and that glass-forming ability is much more sensitive to composition than had been previously appreciated.45 So it is highly probable that some excellent glass-forming alloys compositions remain to be discovered, possibly including some low-density glasses based on aluminum. Identifying these good glass-forming alloys will be a challenge. Most alloy development to date has been done with a brute-force approach, but combinatorial techniques46 re likely to enable much more ra pid screening. One issue is identification of suitable metrics for glass-forming ability, since the combinatorial approaches use vapordeposited thin films, and it is not clear what characteristics of such a film correlate with glass-forming ability in the bulk. Similarly, continued development of ab initio molecular dynamics techniques should enable identification of candidate alloys from computer simulations, particularly as computers continue to increase in power. One area that has received insufficient attention is the influence of processing conditions on glass-forming ability. For instance, application of electromagnetic vibrations during cooling reportedly significantly enhances the glass-forming ability of magnesium-based metallic glasses.47 This approach could, in principle, be applied to other alloys, possibly greatly extending the range of alloys and compositions that can be produced as bulk metallic glasses. THERMOPHYSICAL PROPERTIES AND THERMOPLASTIC PROCESSING Most of the practical interest in single-phase (monolithic) metallic glasses centers on the potential for thermoplastic processing near to or above the glass transition temperature. However, the thermophysical properties and behavior of metallic glasses are not well understood. For instance, the viscosity of the metallic glass melt (or supercooled liquid) is of critical importance, but we do not know how and why alloy composition influences viscosity. From an engineering point of view, the practical aspects of molding of metallic glasses are just beginning to be explored. Certainly many 20 UNCLASSIFIED//FOR OFFICIAL USE ONLY

parallels can be drawn with thermoplastic forming of polymers, but there are certain to be many differences as well . Continued developments in this area are highly likely to result in the ability to produce complex net-shape parts in a single processing step. This ease of processing could offset the higher raw materials costs for metallic glasses, making them competitive in a much wider range of applications. Furthermore, as noted above, the ability to replicate extremely small features ( < 20 nanometers) in metallic glasses is likely to be exploited in the manufacture of nanostructured devices.48 49 Finally, development of metallic glass foams will continue and will be aided by improved understanding of thermophysical properties. It is highly likely that foams will be produced in a wide range of glass-forming alloys, and that techniques will be developed for precise control of the porosity, pore size, ligament size, and connectivity. This will allow the properties of these foams to be tailored to particular applications. COMPOSITES AND THE QUEST FOR DUCTILITY From the point of view of structural applications, localization of plastic deformation into shear bands is the single biggest challenge because this tendency limits the tensile ductility, fracture toughness, and fatigue crack resistance of metallic glasses. 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. Progress is likely to occur on two fronts. First, it is now well established that some alloys are inherently brittle, in the sense that they experience very little plastic deformation around a crack tip, while other alloys show extensive plastic deformation (albeit localized into shear bands). The precise reason for this difference is not understood at present, but it seems likely that it willl be resolved with continued work on fundamental aspects of plastic deformation and fracture . This is likely to lead to development of new alloys with reasonable fracture toughness, although not to tensile ductility. However, even this will be an important step if such alloys can be used as matrices for composites. Second, in order to achieve tensile ductility, it appears to be necessary to have some microstructural features to interact with shear bands. Furthermore, the length scale of the microstructure is clearly a critical parameter in arresting shear band propagation. Again, the precise reasons for this are not known, but continued research quite likely will lead to an improved understanding of the interactions between second-phase particles and shear bands. At present, the most promising approach to producing composite materials with the proper microstructural length scale is the formation of dendritic composites, as discussed above. A critica'I limitation is that this process has been demonstrated in only two, closely related alloys and does not appear to be a general phenomenon . Unfortunately, our understanding of thermodynamics and phase formation in complex multicomponent alloys is not such that we can predict a priori which alloys are capable of producing ductile dendrites in a glass-forming matrix. Until that understanding is developed, discovery of new dendritic composite materials will remain a matter of trial 21 UNCLASSIFIED/ /FOR OFFICIO! 11SF AN! X

and error. The potential benefits are significant, however, because there exists the possibility of making materials with exceptionally high strength, fracture toughness, and fatigue resistance. Summary and Recommendations Metallic glasses combine some of the advantageous mechanical properties of metalsstrength, stiffness, and in some cases toughness- with the processing flexibility usually associated with thermoplastic polymers. The absence of crystalline defects allows metallic glasses to be much stronger than conventional alloys but also means they have near-zero tensile ductility and poor fatigue resistance. In structural applications, therefore, metallic glasses are most likely to be useful in the form of composites consisting of ductile crystalline dendrites in a metallic glass matrix. These dendritic composites sacrifice some strength but can have exceptionally high fracture toughness, as well as good fatigue resistance, and could replace high-strength steels in certain load-limited structural components in aerospace vehicles where space is limited. Because they are true glasses, thermoplastic forming near the glass transition temperature affords metallic glasses tremendous flexibility in processing. For instance, metallic glass components can be formed in a single step (for example, by injection molding) in complex geometries that would be difficult or impossible to produce with conventional alloys. In addition, metallic glass foams can be made with relative ease, raising the possibility of making structural foams with high strength and stiffness. Finally, because they lack a crystalline grain structure, metallic glasses can be used to form nanoscale features with high fidelity. This may make metallic glasses useful in a variety of micro-electromechanical systems (MEMS) applications. Metallic glasses also have significant limitations for aerospace applications, however. Foremost among these is a lack of good glass-forming alloys; in particular, there are no good aluminum-rich glass-forming alloys, the known titanium-based alloys are either relatively dense ( owing to high concentrations of alloying elements) or contain beryllium, and the known magnesium- and iron-based alloys are all quite brittle, with low fracture toughness. Although metallic glass matrix composites can have outstanding properties (particularly strength and fracture toughness), the number of good composite systems known at present is also quite limited. For metallic glasses (and their composites) to be of broad utility in aerospace structural applications, progress in the following areas is required: • Development of new lightweight alloys and composite systems, preferably lby computational and/or combinatorial approaches rather than by trial and error. • Understanding of mechanical behavior, especially: - The effect of alloy composition and structure on plastic deformation . - Microstructural design of composites for optimal toughness . • Development of processing techniques, including thermophysical processing of complex and/or nanoscale features as well as production of metallic glass foams. 22

UNCLASSIFIED/ /FOR OFFICIAL USE ONLY 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. 1 Z. P. Lu, Y. Liu, and C. T. Liu, Chapter 4 in Bulk Metallic Glasses, M. Miller and P. K. Liaw, eds. (Springer, 2009). 2 H. Men, W. T. Kim, and D. H. Kim, Mater. Trans. 44, 2142 (2003). 3 A. Peker and W. L. Johnson, Appl. Phys. Lett. 63, 2342 (1993). 4 N. Nishiyama and A. Inoue, Mater. Trans. JIM 37, 1531 (1996). 5 W. Zhang, Q. S. Zhang, and A. Inoue, Mater. Trans. 50, 679 (2009). 6 J. Schroers and W. L. Johnson, Appl. Phys. Lett. 84, 3666 (2004). 7 F. Guo, H. Wang, S. J. Poon, and G. J. Shiflet, Appl. Phys. Lett. 86, 091907 (2005). 8 V. Ponnambalam, S. J. Poon, and G. J. Shiflet, J. Mater. Res. 19, 1320 (2004). 9 A. H. Brothers and D. C. Dunand, Scripta Mater. 54, 513 (2006). 10 X. J. Gu, S. J. Poon, and G. J. Shiflet, J. Mater. Res. 22, 344 (2007). 11 Data from MatWeb, <www.matweb.com>. 12 Data from MatWeb, <www.matweb.com>. 13 Y. He, G. M. Dougherty, G. J. Shiflet, and S. J. Poon, Acta Metall. Mater. 41, 337 (1993). 14 Graphic reproduced from C. A. Schuh, T. C. Hufnagel, and U. Ramamurty, Acta Mater. 55, 4067 (2007) and used with the permission of Elsevier, Ltd. Original micrograph is from R. D. Conner, W. L. Johnson, N. E. Paton, and W. D. Nix, J. Appl. Phys. 94, 904 (2003). 15 P. Lowhaphandu and J. J. Lewandowski, Scripta Mater. 38, 1811 (1998). 16 Data from MatWeb, <www.matweb.com>. 17 C. J. Gilbert, V. Schroeder, and R. O. Ritchie, Metall. Mater. Trans. A 30, 1739 (1999). 18 B. Menzel and R. H. Dauskardt, Acta Mater. 54, 935 (2006). 19 M. E. Launey, D. C. Hofmann, W. L. Johnson, and R. O. Ritchie, Proc. Nat. Acad. Sci. 106, 4986 (2009). 20 A. L. Greer, K. L. Rutherford, and I. M. Hutchings, Int. Mater. Rev. 47, 87 (2002). 21 B. A. Green, P. K. Liaw, and R. A. Buchanan, Chapter 8 in Bulk Metallic Glasses, M. Miller and P. K. Liaw, eds. (Springer, 2009). 22 V. Schroeder and R. O. Ritchie, Acta Metall. 54, 1785 (2006). 23 Graphic reproduced from C. A. Schuh, T. C. Hufnagel, and U. Ramamurty, Acta Mater. 55, 4067 (2007) and used with the permission of Elsevier, Ltd. 24 A. Hernando and M. Vazquez, Ch. 17 in Rapidly Solidified Alloys, H. H. Liebermann, ed. (Marcel Dekker, 1993). 25 T. Richmond and H. J. Guntherodt, Ch. 14 in Rapidly Solidified Alloys, H. H. Liebermann, ed. (Marcel Dekker, 1993). 26 C. Haan, D. Camel, B. Drevet, and J. M. Pelletier, Metall. Mater. Trans. A 39, 1791 (2008). 27 Photograph by Todd Hufnagel. 28 C. C. Hays, C. P. Kim, and W. L. Johnson, Phys. Rev. Lett. 84, 2901 (2000). 29 C. Fan, R. T. Ott, and T. C. Hufnagel, Appl. Phys. Lett. 81, 1020 (2002). 30 D. Hofmann, J.-Y. Suh, A. Wiest, G. Duan, M.L. Lind, M. D. Demetriou, and W. L. Johnson, Nature 451, 1085 (2008). 31 D. Hofmann, J.-Y. Suh, A. Wiest, G. Duan, M.L. Lind, M. D. Demetriou, and W. L. Johnson, Nature 451, 1085 (2008). 32 D. Hofmann, J.-Y. Suh, A. Wiest, G. Duan, M.L. Lind, M. D. Demetriou, and W. L. Johnson, Nature 451, 1085 (2008). 33 Graphics reproduced from C. A. Schuh, T. C. Hufnagel, and U. Ramamurty, Acta Mater. 55, 4067 (2007) and used with the permission of Elsevier, Ltd. Original artwork provided by Charlie Hays, Caltech. 34 M. F. Ashby and A. L. Greer, Scripta Mater. 54, 321 (2006). 35 D. C. Hofmann, J.Y. Suh, A. Wiest, M.L. Lind, M. D. Demetriou, and W. L. Johnson, Proc. Nat. Acad. Sci. 105, 20136 (2008). 36 D. Hofmann, J.-Y. Suh, A. Wiest, G. Duan, M.L. Lind, M. D. Demetriou, and W. L. Johnson, Nature 451, 1085 (2008). 37 M. E. Launey, D. C. Hofmann, W. L. Johnson, and R. O. Ritchie, Proc. Nat. Acad. Sci. 106, 4986 (2009). 38 N. Barrington and M. Black, Ch. 1 in Aerospace Materials, B. Cantor, H. Assender, and P. Grant, eds. (Institute of Physics, 1998). 39 Data for metallic glasses are from X. J. Gu, S. J. Poon, and G. J. Shiflet, J. Mater. Res. 22, 344 (2007); D. Hofmann, J.-Y. Suh, A. Wiest, G. Duan, M.L. Lind, M. D. Demetriou, and W. L. Johnson, Nature 451, 1085 (2008); M. F. Ashby and A. L. Greer, Scripta Mater. 54, 321 (2006); D. C. Hofmann, J.Y. Suh, A. Wiest, M.L. Lind, M. D. Demetriou, and W. L. Johnson, Proc. Nat. Acad. Sci. 105, 20136 (2008); J. J. Lewandowski, W. H. Wang, and A. L. Greer, Phil. Mag. Lett. 85, 77 (2005). Data for other materials are from Cambridge Materials Selector, <http://www.grantadesign.com/>. 23 UNCLASSIFIED/ /FOR OFFICIAL USE ONLY

UNCLASSIFIED/FOR OFFICIAL USE ONLY 40 M. D. Demetriou, C. Veazey, J. S. Harmon, J. P. Schramm, and W. L. Johnson, Phys. Rev. Lett. 101, 145702 (2008). 41 C. Vermeeren, ed. Around Glare: A New Aircraft Material in Context (Kluwer, 2002). 42 D. V. Louzguine-Luzgin, D. B. Miracle, and A. Inoue, Adv. Eng. Mater. 10, 1008 (2008). 43 A. L. Greer, Materials Today 12(1-2), 14 (2009). 44 D. V. Louzguine-Luzgin, D. B. Miracle, and A. Inoue, Adv. Eng. Mater. 10, 1008 (2008). 45 Y. Li, Q. Guo, J. A. Kalb, and C. V. Thompson, Science 322, 1816 (2008). 46 Y. Li, Q. Guo, J. A. Kalb, and C. V. Thompson, Science 322, 1816 (2008). 47 T. Tamura, K. Amiya, R. S. Rachmat, Y. Mizutani, and K. Miwa, Nature Mater. 4, 289 (2005). 48 G. Kumar, H. X. Tang, and J. Schroers, Nature 457, 868 (2009). 49 A. L. Greer, Materials Today 12(1-2), 14 (2009). 24 UNCLASSIFIED//FOR OFFICIAL USE ONLY