14 December 2009 ICOD: 1 December 2009 UNCLASSIFIECff POii 8PPIQlsltL l!III 8HL'&' Intelligence Acquisition Threat Support Metallic Glasses: Status and Prospects for Aerospace Applications UNCLASSIFIEl:'//509 OFFIOiU L 'W&E IHH!Y UNCLASSIFIED,'/E'Olil OE'E'iEGI I k WliiE 8flllalf Metallic Glasses: Status and Prospects for Aerospace Applications Prepared by: l(bJ(SJ:10 use 424 Defense Intelligence Agency Administrative Note COPYRIGHT WARNING: 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, l(b)(3):10 usc 424 V\dvanced Aerospace Weapon System Applications (AAWSA) Program. Comments or questions pertaining to this document should be addressed to {b {3):10 use 424;(b)(6) , AAWSA Program Manager1 Defense Intelligence Agency, (b)(3):10 usc 424 g 6000, Washington, UNCLASSIFil:D,C;FliOR: 8FFIOll1k WGi 81'1klt UNCLASSIFIED//FOlil 8PfllliltL t!l!I! e,nx Mechanical Behavior Near Room Temperature ............................................... s Mechanical Behavior at Elevated Temperature ............................................. 11 Other Properties: Magnetic, Electrical, Optical, Thermal, and Acoustic 12 UNCLASSIFIEDl/POR 8PPIGl$tb llili la'-' UNCLASSIFIED/;'P8Fl 8PPllll1l W&li ,nlbl.f Current Challenges and Prospects for the Future ................................................. 20 Thermophysical Properties and Thermoplastic Processing ............................... 20 5. Fatigue Limit of Metallic-Glass-Matrix Composites ........................................... 10 8. Microstructure of In Situ Metallic Glass Matrix Composite ................................ 15 2. Comparison of Strengths of Amorphous and Crystalline Aluminum Alloys 7 UNCLASSIFIED//F8R 8FFl'il2k W&fii a,1b UNCLASSIFIEDJ,'&QA Q&&I&l'tk W&E tHUIJ Metallic Glasses: Status and Prospects for Aerospace Applications Metallic glasses combine some of the advantageous mechanical properties of metals-strength, stiffness, and in some cases toughness-with the processing flexibillty 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. 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: UNCLASSIFIED/ ,1f8ft 8FFIGIO! UNCLASSIFIED,C/PIHl 8PPIOll1k Ylil tUll!.l.f The effect of alloy composition and structure on plastlc 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. 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. UNCLASSIFIED//F8R: 8FFlliltlzk W6i 8fib'J UNCLASSIFIED}} I OR OPP!eIAL ltfil a,11a1ar 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. Crystalline Amorphous (glass) 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, have extended linear defects in the crystal structure, called dislocations, that are (in large part) responsible for determining mechanical behavior. The lack of crystalline order precludes the existence of dislocations in metallic glasses, but other sorts of defects can be present and may influence properties and behavior. From an applications point of view, the amorphous structure of metallic glasses has two principal implications. First, the mechanical properties of amorphous alloys are significantly different from those of their crystalline counterparts; some of these differences are advantageous, but others are not. Second, because metallic glasses are UNCLASSIFIED//FOA: 81iiFilll/,I! lt9I! 61\E I UNCLASSIFIED/ /SOR OSSJCI0! !PSS OD!P X glasses in the true sense of the word, rather than melting abruptly (as crystalline metals do), they soften and flow over a range of temperatures in a manner akin to common (oxide) glasses. This creates opportunities for tremendous flexibility in the processing of metallic glasses. PROCESSING Glass-Forming Alloys The key to making a metallic glass is to retain the disordered, liquid-like atomic scale structure during cooling from the melt. All materials have a tendency to crystallize upon cooling because the crystalline state is the most stable structure at any temperature below the melting point. But crystallization takes time, so if the cooling is fast enough, it is possible to bypass crystallization and form an amorphous structure at the glass transition temperature (Figure 2(a)). Glass formation and crystallization are therefore competitive processes; which one will occur depends on the material and the processing conditions. Temperature :\,felling 1cmr,cmtuIc Ciluss m111siliua tcmpcrntutc rurc nickd .... , "Conventional" metallic glasses I 0 ~ (max thickness< I mm) \ Bulk metallic glasses \ \ (max thickness> I mm) Reduced .gluss tran~ition temperature (T/T Figure 2. Critical Cooling Rate. (a) Effect of the cooling rate on glass formation -If the cooling rate is slow (path 1 ), then the melt crystallizes before going through the glass transition. If the cooling rate is fast enough (path 2), then the melt can form a glass. The critical cooling rate (path 3) is the slowest rate at which the melt can be cooled and still form a glass. (b) Critical cooling rates for various metallic alloys - The horizontal axis is the glass transition temperature normalized to the melting (liquidus) temperature. For some materials, such as silica (silicon dioxide) and most thermoplastic polymers, the crystallization process is slow because the crystal structures are complex and the bas[c structural units (for example, segments of polymer chains) are slow to rearrange into a crystalline form. These materials can therefore be produced in glassy form even at very low cooling rates; in fact, it can be difficult to crystallize them at all. Metals and alloys are another matter because the crystal structures are relatively simple and the basic structural units are individual atoms, which are highly mobile. Metallic crystals nucleate and grow quickly, making production of a metallic glass more challenging. UNCLASSIFIED/,' P8fl 8PPl@IAL ltOI! 8Hl!'t! UNCLASSIFIEDl,SflHl 8Pfl81t ltDI! 8Hl1f One way to quantify the ability of a metallic alloy to be produced in glassy form is through the critical cooling rate-the slowest rate at which a metallic liquid may be cooled and still produce a fully amorphous structure, as shown in Figure 2(a). The critical cooling rate for a variety of metallic glass-forming alloys is shown in Figure 2(b). Early metallic glasses (discovered in the 1960s and 1970s) were binary alloys with critical cooling rates typically on the order of 104 to 107 K/s. Achieving such high cooling rates requires specialized techniques (such as melt spinning) and limits the maximum thickness of the metallic glass to < 100 m because of the need to rapidly extract heat from the melt. As a result, these early metallic glasses could be produced in only a limited range of forms, including ribbons, foils, wires, and ,powders. Extensive research efforts in alloy design over the past two decades have resulted in the development of multi-component alloys with much lower critical cooling rates (0.1 K/s or even lower). This has enabled the production of metallic glass specimens in larger sizes-in some cases exceeding 1-cm section thickness. Common practice in the field is to refer to any alloy capable of being cast into a section at least 1-mm thick as a metallic glass. These alloys may be cast or molded into forms suitable for structural applications. At present, it is not possible to predict a priori the glass-forming ability of an alloy of arbitrary composition. A variety of empirical rules for selecting alloying elements and compositions have been proposed, and techniques have been demonstrated for efficient searching of composition space. But identification of alloys with good glass-forming ability is still mostly a matter of trial and error. As a result, the number of truly outstanding glass-forming alloys (loosely defined as being able to be cast as a glass to a thickness of at least 1 cm) is quite limited (see Table 1). Table 1. Selected Bulk Glass-Forming Alloys. Selected alloys reported to have excellent glass forming ability, quantified here as the maximum thickness of a fully amorphous casting. Composition Mg6sCU1sAgsPdsGd10 Zr41.2Ti13,aCu12.sNi10Be22.s Pd40Cu30Ni10Pio Cu47Zr4sAg4Al4 pts1.sCU14.7Nis.3P22.s Ti4oZn.sNi3 Cu 12Be20 fe4sCr1sM014Er2C1sB6 Maximum Thickness Moving from the laboratory to industrial practice, it is important to note that factors besides alloy composition can affect glass-forming ability. In particular, some alloys are sensitive to the presence of impurities; for example, the glass-forming ability of some zirconium-containing alloys is dramatically reduced by the presence of oxygen. Processing conditions also influence the ability to make a glass; these may include the material and surface finish of the mold and the temperature of the liquid prior to casting. finally, glass-forming ability can be quite sensitive to small variations in composition which may be difficult to control in industrial practice. UNCLASSIFIED//EOR Qliiliilil/il: WOI 8HL'I' UNCLASSIFIED//FOR: OFFl&lllil!: WIH! et'' Casting and Molding Like other alloys, metallic glasses can be cast into net-shape or near-net-shape geometries. Die casting into a permanent (metal) mold-because it provides the rapid heat transfer needed to meet the requirement for relatively rapid cooling-is the most common casting technique. In mast cases, casting is done in either a vacuum or an inert atmosphere to prevent formation of oxide particles that promote crystallization. Conventional casting, however, does not take advantage of the flexibility afforded by the glassy nature of these alloys. If a metallic glass is heated to a temperature above its glass transition temperature, it becomes a supercooled liquid. In this state, the viscosity drops with increasing temperature over a wide range, making it possible to control the viscosity by controlling the temperature. This ability to control the viscosity enables many of the processing techniques commonly used in molding thermoplastic polymers to be applied ta metallic glasses (Figure 3). figure 3. Examples of Processing of Metallic Glasses. (a) Microspring produced by lithography and (b) thin- walled bottle produced by blow molding. Images are courtesy of Professor Jan Schroers (Yale University). There are two important limitations on processing of metallic glasses in the supercooled liquid region. First, supercooled liquids are metastable and have a tendency to crystallize, so there is a limited window of time (typically on the order of minutes) in which the processing must be completed if the glassy structure is to be maintained. Second, the viscosity of many glass-forming alloys near the glass transition temperature is too high for convenient processing. The viscosity can be reduced by increasing the processing temperature, but higher temperatures promote c