DOW-UAP-D117, AAWSAP DIRD, Metallic Glasses for Aerospace Applications, December 2009

PAGE 2

Metallic Glasses: Status and Prospects for Aerospace Prepared by: Acquisition Support Division (DWO-3) Defense Warning Office Directorate for Analysis Defense Intelligence Agency AAP Person 63 Administrative Note COPYRIGHT WARNlNG: Further dissemination of the photographs in this publication is not authorized. This product is one in a series of advanced technology reports produced in FY 2009 under the Defense Intelligence Agency, Defense Warning Office's Advanced Aerospace Weapon System Applications (AAWSA) Program. Comments or 1uestions pertaining to this document should be addressed to!AAP Person 1 , AAWSA Program Manager, Defense Intelligence Agency, ATTN: CLAR/DWO-3, Bldg 6000, Washington,

PAGE 3

Metallic Glasses...................................................................................................... 1 Glass-Forming Alloys...................................................................................... 2 Casting and Molding ....................................................................................... 4 Thin Films and Coatings ................................................................................. 5 Mechanical Behavior Near Room Temperature ............................................... 5 Stiffness: Elastic Deformation ........................................................................ 6 Strength and Ductility: Plastic Deformation ................................................... 6 Fracture Toughness........................................................................................ 8 Wear Resistance........................................................................................... 10 Corrosion and Stress-Corrosion Cracking ..................................................... 10 Mechanical Behavior at Elevated Temperature ............................................. 11 Other Properties: Magnetic, Electrical, Optical, Thermal, and Acoustic ........ 12 Metallic Glass Matrix Composites ......................................................................... 13 Processing and Structure of Composites .......................................................... 13 Ex Situ Composites........................................................................................... 14 In Situ Composites........................................................................................... 14 Mechanical Properties of Composites ............................................................... 15 Strength and Ductility: Plastic Deformation ..................................................... 16 Fracture and Fatigue ........................................................................................ 16 Aerospace Applications of Metallic Glasses .......................................................... 16 Other Applications............................................................................................ 19

PAGE 4

Current Challenges and Prospects for the Future ................................................. 20 Thermophysical Properties and Thermoplastic Processing ............................... 20 Composites and the Quest for Ductility ............................................................... 21 Summary and Recommendations ......................................................................... 22 3. Examples of Processing of Metallic Glasses........................................................ 4 5. Fatigue Limit of Metallic-Glass-Matrix Composites ........................................... 10 6. Deformation Map for a Metallic Glasses............................................................ 11 8. Microstructure of In Situ Metallic Glass Matrix Composite................................ 15 9. Materials Property Charts................................................................................. 18 1. Selected Bulk Glass-Forming Alloys.................................................................... 3 2. Comparison of Strengths of Amorphous and Crystalline Aluminum Alloys ......... 7

PAGE 5

Metallic Glasses: Status and Prospects for Aerospace Metallic glasses combine some of the advantageous mechanical properties of metals-strength, 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 processi'ng. 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 fide ity. 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. Therefore, in order for metallic glasses (and their composites) to be of broad utility in aerospace structural applications, progress in the following areas is • Development of new lightweight alloys and composite systems, preferably by computational and/or combinatorial approaches rather than by trial and • Understanding of mechanical behavior, especially:

PAGE 6

□The effect of alloy composition and structure on plastic deformation. - 0Microstructural 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. 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 7

Metallic Glasses The atomic-scale structure of most metals and alloys is crystalline; that is, the atoms are arranged in a highly ordered manner on a lattice that is periodic in three dimensions, as depicted in Figure l(a). In contrast to this crystalline structure, metallic glasses lack the long-range order of a lattice and are therefore said to be amorphous, as depicted in Figure l(b) . Although the word "amorphous" implies a complete lack of structural order, in fact the atomic structure of metallic glasses is not truly random. Constraints on atomic packing provide strong short-range order; for instance, on average the atoms have a particular number of nearest atomic neighbors at a well­ defined distance. But this short-range order persists only over distances of a few atoms; there is no long-range order as there is in a crystalline alloy. In many ways, the atomic-scale structure of metallic glasses more closely resembles the highly disordered structure of a liquid than the structure of a crystalline alloy. Cry ta ll in Amorph u (gla s) Figure 1, Amorphous Versus Crystalline Structure. Schematic atomic-scale structure of crystalline (a) and amorphous (b) metals. In a crystalline structure, order persists over long distances (many atomic dimensions}. In a glass, there is short range order but no long-range order. A corollary of this difference in structure is that the nature of structural defects is quite different between crystalline and amorphous alloys. Crystalline alloys, for example,