Ductile metallic glasses

Dr. Neil Canter, Contributing Editor | TLT Tech Beat June 2010

Researchers developed a material that combines excellent mechanical strength with ductility. 

KEY CONCEPTS
• Metallic glasses are a relatively new class of materials that are amorphous in structure and prepared by rapidly quenching multiple molten metal alloys.
• Metallic glasses are very strong, lightweight and exhibit better wear resistance than crystalline metals. But metallic glasses are extremely brittle and fail catastrophically.
• Smaller metallic glass samples with diameters less than 100 nanometers deform in a similar fashion to ductile metals while maintaining a high level of mechanical strength.

Many research efforts are engaged in developing materials that can provide better mechanical properties, which are more readily usable in structural applications. One trend that is readily seen is the drive to use structural materials with lower weight as a means to improve fuel economy in automobiles. This has led to the use of lighter-weight metals such as aluminum and magnesium that also has prompted the steel industry to develop ultra lightweight alloys.

One class of materials that has been examined quite closely is silicon. It is quite brittle and readily fractures when pressure is applied. A previous TLT article discusses the effect of crack propagation through silicon at low speeds (1). A combination of computer simulation and experimentation was used to show how the structure of the silicon atoms changes as the crack moves through them.

A relatively new class of materials is metallic glasses, which were first reported in 1960 (2). Julia R. Greer, assistant professor of materials science and mechanics at the California Institute of Technology in Pasadena, Calif., says, “Most metal alloys are crystalline and contain well-ordered, periodic arrangements of atoms. In contrast, metallic glasses are amorphous materials often prepared by the rapid quenching of multiple molten metals, thereby preventing crystalline structure formation.”

Metallic glasses are typically prepared from proper stoichiometric ratios of metals such as zirconium, titanium, copper and nickel. Greer says, “Metallic glasses are very strong and lightweight. In contrast to crystalline metals, they are mainly corrosion-resistant and do not tend to form oxide coatings on their surfaces. They also exhibit better resistance to wear than crystalline metals.”

Metallic glasses can be formed from several combinations of metals as dictated by their phase diagrams. But metallic glasses do have one huge Achilles heel, according to Greer. She says, “Under mechanical loads, metallic glasses are extremely brittle and they fail catastrophically. This occurs due to instantaneous, highly localized shear-band propagation under loads, causing the entire structure to fail.” Research has been conducted to determine how to improve the ductility of metallic glasses. Steps were taken to more uniformly distribute shear bands and to hinder their propagation in metallic glasses.

A secondary problem is the inability to produce metallic glasses in large quantities. Greer points out that a high level of stored energy is available in large samples to force the metal into a crystalline state. Metallic glasses are typically made into ribbons and wires.

But metallic glass mechanical properties are particularly sensitive to specific experimental conditions and to the sample geometry. Greer says, “Several research groups have created and tested tapered rather than perfectly cylindrical pillars, which significantly affects both the attained stresses in compression and pillar deformation, as the pillar top experiences a higher, inhomogeneous stress than its bottom.”

Metallic glasses are strong and brittle in a similar fashion to ceramics. In contrast, crystalline metals are ductile, which minimizes failure but do not have the strength of metallic glasses or ceramics.

There is great interest and need to prepare a material that combines excellent mechanical strength with ductility. Such a material has not been developed until now.

SIZE OF METALLIC GLASS PARTICLES
Taking special measures to remove the geometric and experimental artifacts, Greer along with Dongchan Jang, a senior CIT post-doctoral student, explored how a change in the size of the sample specimen down to 100 nanometers in diameter affects the mechanical properties of the metallic glass. In conducting this study, Jang fabricated and tested several samples with diameters between 1 micron and 100 nanometers and discovered that at 100 nanometers these metallic glasses maintained even higher-than bulk strength while acquiring ductility.

A zirconium-based bulk metallic glass sample was used as a precursor for sample fabrication for subsequent nana-tension testing. The results for specimen samples down to 200 nanometers showed an increase in strength but a failure by catastrophic shear-band formation as determined by in situ nanomechanical measurements.

The tensile yield strength increases from 1.7 to 2.25 gigapascals as the size of the specimen is reduced from 875 nanometers to 330 nanometers in diameter. But all of these samples display catastrophic failure at strain levels between 3% and 5%.

At the diameter of 100 nanometers, however, the results were drastically different. Greer says, “We found that these smaller samples were capable of carrying plastic strain of more than 20%, which means that they were deforming in a similar fashion to a ductile metal.” Stress values reached a maximum of 2.35 gigapascals during this work, which is higher than seen for larger diameter metallic glass samples.

A scanning electron micrograph of a typical 100 nanometer in diameter sample used in tensile testing is shown in Figure 2. Greer says, “The 100 nanometer metallic glass samples first necked, and only then failed by forming a shear band in the necked region at 53 degrees to the loading axis, which is typical of metallic glasses. This result means that 100 nanometer samples deform in a similar fashion to ductile metals but fail typically like metallic glasses.”


Figure 2. The 100 nanometer diameter metallic glass sample shown in this scanning electron micrograph deforms in a similar fashion to ductile metals. (Courtesy of the California Institute of Technology)

Greer believes that two competing energetic processes, localized shear-band propagation and homogeneous flow, affect how the metallic glass samples of different sizes react towards stress. At specimen diameters below a certain critical value, plastic deformation will dominate, leading to homogeneous flow until the metallic glass eventually fails when a higher, shear-band propagation stress is attained. At diameters above this critical value, shear-band propagation is more energetically favorable, and the sample behaves in a manner typical of metallic glasses.

This work shows that by utilizing size as a design parameter, materials combining ductility with a high level of mechanical strength can be prepared. Greer envisions that nanoscale components could be prepared with this unique combination of properties in structural applications. One potential use is combining metallic glass with particle diameters of 100 nanometers with ultrafine-grained ductile metal to prepare a new type of engineering composite.

Further information can be found in a recently published paper (3) or by contacting Greer at jrgreer@caltech.edu. 

REFERENCES
1. Canter, N. (2009), “Crack Formation in Brittle Materials,” TLT, 65 (2), pp. 26–27.
2. Klement, W., Willens, R. and Duwez, P. (1960), “Non-Crystalline Structure in Solidified Gold-Silicon Alloys,” Nature, 187, pp. 869-870.
3. Jang, D. and Greer, J. (2010), “Transition from a Strong-Yet-Brittle to a Stronger-and-Ductile State by Size Reduction of Metallic Glasses,” Nature Materials, 9, pp. 215–219.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.