Ti-based bulk metallic glasses (BMGs) and matrix composites (BMGMCs) are a subset of the class of materials known synonymously as amorphous metals, liquid metals, and glassy metals, described by their majority element (in atomic percent) being that of titanium. BMGs are non-crystalline metal alloys based in a wide variety of elemental systems, including zirconium, iron, nickel, hafnium, gold, platinum, palladium, and silver, among others. The vast majority of commercially utilized BMGs are based in Zr-Ti-Cu-Ni-Be or Zr-Cu-Ni-Al due to their relatively low-cost elements and large glass-forming ability (GFA), typically greater than 1 cm. BMGs have long been considered to be a material without a clear application, as the density of BMGs fits squarely between two common, highperformance crystalline alloys that BMGs are usually thought to be replacements for: steel (density = 7.8 g/cm3) and titanium (density = 4.5 g/cm3). For example, Zr-based BMGs generally fit into the range of 6 to 6.5 g/cm3, which makes them difficult to use as direct replacements for conventional materials.

(Left) A 30-g ingot of the BMG Ti40Zr20Cu5Al5Be30 with a measured density of 4.76 g/cm3 after alloying in a plasma arc melter from pure elements. The ingot is fully glassy after melting. (Right) The ingot from the left cast into a 16-mm diameter cup-and-cone insert for latching operations on a proposed spacecraft.

The motivation for creating Ti-based BMGs is to reduce the density of BMGs to more closely match crystalline Ti alloys, such as Ti-6Al-4V, which has a density of 4.4 g/cm3. Prior research demonstrated a class of Ti-based BMGMCs with ductility, toughness, and fatigue limits similar to Ti-6Al-4V, but with a minimum density to allow net-shaped casting of 5.2 g/cm3 based on limitations in the balance among density, mechanical properties, and viscosity/formability. The focus of developing Ti-based BMGs and BMGMCs has always been to achieve properties as similar to Ti-6Al-4V as possible but with the lowest density. However, for many applications, mechanical properties such as ductility, toughness, and fatigue limit are not the driving design factors. For example, gears, bearings, mechanical fasteners, and inserts for spacecraft may require properties such as high hardness, low thermal expansion, high wear resistance, and low density considerably more than they require large fracture toughness. As such, there has been a lack of alloy development for Ti-BMGs and BMGMCs with low density and high GFA as the primary design requirements.

Research efforts towards this end were aimed at pushing the limits of Ti-based BMG glass forming alloys to densities below 5 g/cm3 while retaining the ability to be formed into net-shaped parts at least 15 mm in thickness. One optimized alloy, Ti40Zr20Cu5Al5Be30 with a measured density of 4.76 g/cm3, has been developed with an excellent combination of low density, high hardness, high strength, and very high GFA. Ti-based BMGs with still lower density can also be developed, but with a tradeoff between density and GFA. The lower limit for this alloy system, which approaches 4.5 g/cm3, cannot be made into BMGs greater than 5-6 mm thick.

This work was done by Douglas C. Hofmann of Caltech for NASA’s Jet Propulsion Laboratory. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Technology Transfer at JPL
Mail Stop 321-123
4800 Oak Grove Drive
Pasadena, CA 91109-8099
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Refer to NPO-49905.