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Metallic glasses have high strengths but are inherently brittle. To overcome this shortfall, metallic glass composites can be created by growing soft, crystalline particles in the glass to make it tougher. Processing these composites is difficult by any known method because they oxidize badly in open air and have high viscosity. This article describes a one-step casting process by which complex components can be made, opening the possibility for commercial and military hardware produced from highstrength toughened glassy composites.

INTRODUCTION

For more than two decades, bulk metallic glasses (BMGs) have been the subject of intense scientific study.¹ By retaining liquid-like structure in rapidly cooled solids, BMGs possess unique mechanical properties that have made them desirable candidates for structural applications.¹ Lacking a dislocation- based plasticity mechanism and having a low stiffness, BMGs exhibit high yield strengths and large elastic limits. They have high hardness, excellent corrosion resistance, and can be processed at low temperatures into non-sacrificial molds. These properties have been widely exploited in commercial applications such as electronic casings, sporting equipment, jewelry, materials for defense, and coatings.²

Despite a wide variety of commercial successes obtained from die-casting BMGs into copper molds, many structural applications have gone unrealized, owing to the low fracture toughness, low fatigue endurance limit, and shear localization observed in BMGs. For example, the most commercialized BMG (Vitreloy 1 or LM1) has been measured to have a plane-strain fracture toughness, K1C, equal to 18 MPa m^1/2,³ fatigue endurance limit detected at 4% of its yield strength,⁴ and 0% plastic deformation in quasi-static room-temperature tension testing.⁵ Compared with high-strength steels that exhibit the same ultimate tensile stress, BMGs have significantly inferior deformation characteristics.

To improve the brittle fracture endemic to monolithic (single-phase) BMGs, ductile-phase reinforced BMGs have been introduced.^6,7 With special attention paid to composition, these BMG “matrix composites” form two-phase alloys comprised of soft crystalline dendrites distributed within a glassy matrix. In 2001, these alloys were demonstrated to have up to 3% ductility in room-temperature tension testing, while still exhibiting over 1.0 GPa ultimate tensile stress.⁷ More recently, an improved understanding of the importance of microstructure, composition, and processing has lead to BMG matrix composites with strength, toughness, ductility, and fatigue endurance limit at the upper boundary of what is possible with crystalline metals.^5,8,9 These new alloys exhibit yield strengths as high as 1.6 GPa,⁸ K1C as high as ~170 MPa m^1/2,⁵ up to 15% tensile ductility, and fatigue endurance limits up to 25% of the yield strength.⁹

SEMI-SOLID FORGING DEVELOPMENT

The new BMG matrix composites were developed using a novel semisolid processing strategy, which involves holding the alloy isothermally between the solidus and liquidus temperatures to coarsen the microstructure before rapidly cooling.⁵ This strategy, developed at the California Institute of Technology (Caltech), was carried out using a water-cooled copper boat, diagrammed in Figure 1a. A BMG sample is heated while resting in an indentation formed in a water-filled copper tube. The sample is isolated in a titanium- gettered argon environment by a quartz tube and is heated by an induction coil. The high thermal conductivity of copper prevents destruction of the boat, while radio-frequency stirring and levitation homogenizes the sample. Several boats have been developed to process samples ranging in mass from 1–300 g. A 25 g sample of a BMG matrix composite within the copper boat is shown in Figure 1b.

While the mechanical properties of semi-solidly processed ingots from the copper boats have been shown to have excellent mechanical properties,^5,8,9 the geometry of the resulting ingots is highly restrictive. Samples for mechanical testing must be machined out of large semi-solidly processed ingots. The only net-shape part possible with this processing geometry is a cylinder, and these have been formed successfully. The major advantage of semi-solid processing is that tough BMG matrix composites can be formed in a one-step approach from a partial liquid. To commercialize these and similar alloys, a casting process needs to be developed to form net-shaped parts from semisolid BMG matrix composites. Typical die-casting into copper molds is challenging owing to the high viscosity of the semi-solid liquids. Thermoplastic forming has been demonstrated as an imprinting process for BMG matrix composites and monolithic glasses,^10,11 but complex geometries are not possible in this route. In the current work, we develop the concept of semi-solid induction forging, a hybrid casting process combining the metallurgical processes of squeeze casting with forging.

Double Boat Design
Semi-solid induction forging, a technique developed in parallel by Caltech Liquidmetal Technologies and the University of California (U.C.) at San Diego, is a containerless processing strategy used to semi-solidly process a BMG matrix composite in an inert environment and then forge the slurry into a mold, creating a tough net shape. In one embodiment, employed at U.C. San Diego, the induction forging process is carried out by a “double boat” design. In this strategy, copper molds are welded into stacked water-cooled copper tubes. Both copper tubes and the molds are encased in a quartz tube and surrounded by the heating coil. Once the sample is processed sufficiently on the lower mold, flexure is used to snap the mold shut on the ingot, forging a plate. Figure 1c-d shows the copper molds in the open and closed position, while Figure 1e shows a Ti-based BMG matrix composite undergoing semisolid processing. The resulting plate is shown in the inset. Although the geometry is still restrictive, the double boat design allows for concentrated heating and a rapid quench rate (~104 K/s), which permits the vitrification of weak glass forming alloys (such as non-beryllium containing composites). The plates, which are typically 5 cm long by 2 cm wide and 2 mm thick, are sufficiently sized to perform compression, tension, bending, fracture toughness, and fatigue tests, making them a highly desirable geometry. To illustrate the benefits of the design, a non-beryllium bearing BMG/nanocrystalline matrix composite was formed in the Ti-Ni-Cu-Mo-Sn-Si system with the optimal microstructure for toughening (Figure 1g). Room-temperature compression testing, shown in Figure 1f, demonstrates that the composite structure exhibits ~20% total strain whereas the parent glass exhibits none. The high cooling rate of the double boat design will be critical in the development of high-melting-point, weak-glass-forming composites.

Casting Chamber Method
In the second embodiment, developed at Caltech, the quartz tube is eliminated and semi-solid forging is done in an argon filled casting chamber. In this design, two large water-cooled copper plates serve as both the platform for semi-solid processing and as the mold. By performing the casting in a vacuum chamber, sample geometries are limited only by the diameter of the induction coil, which range from 3–6 inches. After processing a large ingot on the bottom plate, the upper plate is lowered with a force, either through hydraulics or a handle, as shown in Figure 2a. However, owing to the larger mass of the molds and slower forging time, cooling rates are lower than the previous design (~103 K/s). This typically either limits samples to thin dimensions or forces them to have large glass-forming ability. The chamber contains five ports used for water lines, heating coils, viewing, plunging, and vacuum, as shown in Figure 2b and d. Samples ranging in mass from 6–200 g have been produced, with the only limitations being the diameter of the induction coil and the size of the chamber. Figure 2c shows a 100 g ingot of a highly processable berylliumbearing BMG matrix composite undergoing semi-solid processing. Shims are used to achieve a certain thickness, in this case, 1 cm.

To investigate the success of the semi-solid induction forging process in comparison to samples made on the water-cooled copper boat of Figure 1b, tension tests were cut from a 5 mm thick, 100 g plate of the alloy DV1 (Ti_44.3Zr_35.2V_11.8Cu_6.1Be_2.6 in wt.%), from Reference 8. The typical geometry of such a plate is that of an ellipse, with an 8.5 cm major axis and 5.5 cm minor axis, as shown in Figure 2b. Five-millimeter thick square strips were cut from a similar plate of DV1 and 3 mm diameter gauge sections were machined for tension testing.

After numerous tension tests, the plates all exhibit 10–14% total strain at ultimate tensile stresses of at least 1.4 GPa. A nominal tension test from the semi-solidly forged plate in comparison with an ingot from the copper boat is shown in Figure 2a. As shown in the inset, every sample exhibits significant necking. Within statistical error, the tension tests from the semi-solidly forged plates match those from the more idealized copper boat, indicating the process was successful. To demonstrate, the semi-solid processing technique is also successful at toughening commercial-grade material, a 3.5 mm thick plate was produced from commercially available LM2 (Zr_71.9Ti_9.3Nb₆Cu_6.15Ni_4.6Be_1.6 in wt.%) with ~5,000 ppm oxygen content. After processing, a 3.5 mm square beam was bent to nearly 90° without fracturing, a demonstration which is not possible with any monolithic BMG at that dimension. It should be noted that the plates are initially in contact with the cold copper molds, and there exists a small “cold spot” on the plates. This crystalline region has been shown to be only several micrometers thick and has no effect on the overall mechanical properties of the plates.

Typical microstructures of the semisolidly forged plates were investigated through scanning electron microscopy (SEM). As demonstrated previously, the length scale of the dendrites in relation to the critical flaw size of the matrix is the fundamental mechanism for toughening.⁵ It has been shown that BMG matrix composites cooled on an arc melter have a variation in dendrite size that changes by an order of magnitude or more, from sub-micrometer to hundreds of micrometers. Semi-solid ingots from the water-cooled boat always display a coarsened microstructure, with variation in size of less than 10% across the ingot (see Supplementary Information, Reference 5). In the semi-solidly forged plates in this work, two distinct microstructures are observed, shown in Figure 3d–g for DV1.

The first microstructure, comprised of two different length scales, is observed when plates are heated for the minimum time to allow successful forging. In these alloys, dendrite coarsening is interrupted by forging, which prevents the semi-solid slurry from reaching equilibrium. The matrix material is supersaturated with solute during quenching, causing a finer scale of dendrites to precipitate. However, because the larger-scale dendrites are still homogeneously distributed throughout the matrix, no change in mechanical properties is observed with the presence of the smaller dendrites. This supports previous observations that smallscale dendrites have little or no effect on toughening BMGs.⁵

To eliminate the second length scale, the isothermal processing time is increased by ~1 min. These plates exhibit the typical microstructure found in BMG matrix composites directly from the water-cooled boat. Large, coarsened dendrites are distributed evenly throughout the matrix (Figure 3f, g). The full size of a single dendrite can be found by sectioning the plate near a dendrite’s primary axis, shown in Figure 3f. The length scale of each dendrite tree, which is essentially a single crystal of body-centered-cubic (b.c.c.) material, is on the order of hundreds of microns. Dendrite arms have a diameter of ~10 μm, as shown in Figure 3g.

To cast net-shaped parts using the semi-solid induction forging technique, copper molds can be attached to the water-cooled copper blocks shown in Figure 2a. Owing to the high thermal conductivity of copper, the molds do not need to be in direct contact with cooling water but need to be in large surface area contact with the cooled copper blocks. This allows many lowcost molds to be attached to the apparatus, without changing the basic casting platform. A diagram of net-shape casting is shown in Figure 4a. An ingot, initially resting on the mold, is heated to a desired point in the semi-solid region before the mold is closed, casting a part. The first net-shaped part demonstrated is the disk shown in Figure 4b. In this simple case, an ingot of the BMG matrix composite DH1 (Zr_55.3Ti_24.9Nb_10.8Cu_6.2Be_2.8 in wt.%) was forged into a copper o-ring, making a 2 mm thick plate, 5 cm in diameter. After polishing, the part takes on a mirror finish, shown in Figure 4c.

To create a more complex shape, concentric rings were lathed into an aluminum block, 3 mm thick. An ingot of DH1 was placed over the block, which was then put on the lower platform in the forging machine. The ingot was processed and forged over the block, creating the rings shown in Figure 3d. Partial wetting of the aluminum to the BMG matrix composite occurred. To create a three-dimensional part, where an upper and lower mold close onto the sample, corrugations were carved into two copper molds. An ingot of DH1 was placed across these corrugations in the machine as demonstrated in Figure 4f. After forging, the 8 mm diameter rods from the arc-melter are forged into 5 cm diameter corrugated plates, shown in Figure 4e. The strut thickness of the plates can be varied by changing the semi-solid processing temperature or the forging pressure and are between 0.5–2 mm (Figure 4g). In another design, an ingot of DH1 was forged into a brass mold with a “waffle” pattern. The tips of the wedge-shaped corrugations are < 100 μm and smaller dimensions are certainly possible, based on prior work.¹¹ Micro-replication of molds is therefore possible with the semi-solid forging technique. In addition to forging net-shapes, more complex structures can be assembled from parts made using the process. In Figure 4i, a honeycomb structure has been assembled by cutting and soldering together strips of the plates from Figure 4g. These structures have high strengths and toughnesses with low relative densities

Another useful part that can be made using semi-solid induction forging is a thin plate. We have observed that there is high demand for BMG composite plates in dimensions less than or equal to 1 mm and yet these parts are difficult to die-cast. Thermoplastic forming has been used for many years to flatten ingots of BMGs into submillimeter thicknesses, but this has not been accomplished with toughened composites. Semi-solid induction forging was used successfully to produce lowoxygen content plates of BMG matrix composites in thickness from 0.25–10 mm. To assure that the thin plates are two-phase composites, x-ray scans were performed on two 1 mm plates of DH1 and DV1, shown in Figure 5a,b. In both scans, b.c.c. peaks are superimposed on a glassy background, indicating the thin plates have been processed without heterogeneous nucleation of another phase or without total vitrification.

A 0.5 mm plate with diameter ~5 cm is shown in Figure 5c. To illustrate the high elastic limit of the thin plate, Figure 5d,e shows the 0.5 mm thick plate of DH1 in a three-point bending fixture undergoing elastic deformation. The plate can be bent substantially, but when the force is removed, it elastically returns to a flat plate. To demonstrate that the thin plates still possess the coarsened microstructure necessary for toughening, the same plate was clamped in a vise and bent plastically with repeated hammer strikes, Figure 5f. Thin plates of beryllium-bearing BMG matrix composites can be bent to more than 90? without fracturing. Non-beryllium BMG/nanocrystalline matrix composites can also be formed during the forging process. Figure 5g shows a Ti-Ni-V-Si composite forged into a 2 mm thick plate and Figure 5h shows a Zr-Nb-Cu-Ni-Al alloy from Reference 12 forged into a 1 mm thick plate. In non-beryllium composite systems, it is challenging to quench the matrix material as a glass without heterogeneously nucleating crystals. As such, we have observed that most of these composites have a partially crystalline matrix and are subsequently more brittle than the beryllium-bearing versions.

Another part that can be made using the semi-solid induction forging technique is a multi-layered laminar composite, shown in Figure 6. In this strategy, an ingot of BMG matrix composite can be forged onto a layer of another material, such as a soft b.c.c. material or hard carbide. As shown in Figure 6a, the forging process can be repeated to either encase the other material in a BMG composite or to build up multi-leveled structures. These “ex-situ” type composites have been demonstrated for more than a decade with monolithic BMGs. Wires or particles have been infiltrated with BMGs to make composites. This is the first example of an ex-situ composite made with in-situ BMG matrix composites as one of the layers. Ex-situ BMG composites are typically made to toughen the brittle monolithic glass. By using a toughened BMG composite, new types of tough armored materials are possible. A three layer part is diagrammed in Figure 6b showing this concept. To demonstrate the excellent wetting obtained using the forging technique, DV1 was forged onto a layer of soft, crystalline Zr-Ti-Nb, which is essentially the dendrite material of DH1 and LM2.

The two-layer structure was bent in a three-point bending configuration to 12.5° before delamination was observed (see Figure 6c). Scanning electron microscopy was used to image the interface, shown in Figure 6d, which is intimate but has little reaction. The process can also be modified to other layers, such as low-melting point aluminum, or SiC, for example. Figure 6e shows an ingot of LM2 forged over a 1 mm thick layer of aluminum. Figure 6f demonstrates the “encasing” process by which a material is completely confined by the BMG composite. A three-layered composite, 3 mm thick, was formed by forging 1 mm thick layers of DV1 over a 1 mm layer of Zr-Ti-Nb. The cross section, shown in the inset, demonstrates that the soft layer has been completely encased in the composite.

To conclude, the semi-solid induction forging process has been demonstrated to be one possible route for commercially manufacturing toughened BMG components. This work started with the development of BMG matrix composites that exhibited tensile ductility. Next, semi-solid processing was used to coarsen the microstructure to obtain benchmark mechanical properties. Now, semi-solid induction forging has been used to make a wide variety of net-shapes that have the same excellent properties. This technique should provide the fundamentals for future commercial processing and for fabrication of new alloys.

1. W.L. Johnson, MRS Bull., 24 (1999), pp. 42–56.
2. See Liquidmetal Technologies, 30452 Esperanza, Rancho Santa Margarita, CA 92688, www.liquidmetal.com.
3. P. Lowhaphandu et al., Scripta Mater., 38 (1998), pp. 1811–1817.
4. C.J. Gilbert et al., Scripta Mater., 38 (1998), pp. 537–542.
5. D.C. Hofmann et al., Nature, 451 (2008), pp. 1085–1089.
6. C.C. Hays et al., Phys. Rev. Lett., 84 (2000), pp. 2901–2904.
7. F. Szuecs et al., Acta Mater., 49 (2001), pp. 1507–1513.
8. D.C. Hofmann et al., Proceedings of the National Academy of Sciences, United States of America, 105 (2008), pp. 20136–20140.
9. M.E. Launey et al., Proceedings of the National Academy of Sciences, United States of America, 16 (2009), pp. 4986–4991.
10. D.C. Hofmann et al., Scripta Mater., 59 (2008), pp. 684–687.
11. G. Duan et al., Adv. Mater., 19 (2007), pp. 4272–4275.
12. U. Kühn et al., Appl. Phys. Lett., 80 (2002), pp. 2478–2480.

Douglas C. Hofmann, visiting scientist, Henry Kozachkov, graduate student, Joseph P. Schramm, graduate research assistant, Marios D. Demetriou, senior research fellow, and William L. Johnson, Professor of Engineering and Applied Science, are with Keck Laboratory of Engineering Materials, California Institute of Technology, 1200 E. California Blvd., Pasadena CA, 91125; Hofmann is also a research scientist with Liquidmetal Technologies, 30452 Esperanza, Rancho Santa Margarita CA, 92688. Hesham E. Khalifa, Ph.D. candidate, and Kenneth S. Vecchio, Professor and Chair of the department, are with the Department of NanoEngineering, University of California at San Diego, La Jolla, CA 92093. Dr. Hofmann can be reached at D.C.H. dch@caltech.edu.