New frontiers in superalloy development

June 21, 2007
Researchers at Sandia National Labs specializing in inorganic synthesis and characterization, modeling, and radiation science have designed a radical system of experiments to study the science of creating metal and alloy nanoparticles.

The lightweight, corrosion-resistant materials the team creates are needed for weapons casings, gas turbine engines, satellites, aircraft, and power plants, says Tina Nenoff, project lead.

The team devised a process to make superalloy materials using radiation to break down the molecular structure of substances and form nanoparticles. The method being studied is known as radiolysis. It is a flexible and versatile tool for making large quantities of superalloy nanoparticle compositions that can't be easily created otherwise, says Nenoff.

Metal alloys are combinations of two or more elements — at least one of which is metal. The resulting material has metallic properties different from the properties of its components. For example, steel is stronger than iron, its primary component. Superalloys, as the name implies, stand out from the general population of alloys because of they're remarkable mechanical strength and resistance to corrosion, oxidation, and deformation at high temperatures that would destroy conventional metals like steel and aluminum. In the past, superalloy development depended on chemical and technological innovations mainly driven by aerospace and power plant applications where superalloys are in high demand, says Jason Jones, Sandia researcher.

The team is focusing its research on the science that happens in the "novel metastable phase spaces" that are not accessible with traditional alloy production methods, such as melting, Nenoff says. These "phase spaces" are possible points in a given path, or orbit, that represent the motion of a particle over a period of time. Each potential state of that particle's system corresponds to one unique point in a phase space. Understanding these spaces is important for determining what alloys are created and how they form.

The team combines solvent molecules with molecules or ions and dissolves them in water. The solution undergoes radiolysis. Varying the reaction conditions and using alcohols as agents limits particle size. High-resolution transmission electron microscopy shows that the nanoparticles are nearly identical and defect-free.

The team performs the highly specialized experiments using their in-house Gamma Irradiation Facility (GIF) in combination with the Ion Beam Materials Research Laboratory (IBMRL). "Target solutions are placed in testing cells at GIF and exposed to a variety of gamma irradiation tests and controlled radiation dose rates," says Don Berry, GIF supervisor.

In their study of the particle growth that takes place, the researchers expose test solutions to even higher radiation doses at the IBMRL. "The ion beam irradiation experiments take place in a custom-built cell at the external beam end-station of the Tandem Van de Graff accelerator and result in intense dose rates in the solution," says ion beam researcher Jim Knapp. "A beam of protons exits the vacuum and passes through a thin Kapton film before entering the target solution. The system can exposes targets up to several hours, but the exposures needed in these experiments are usually only fractions of a second."

After irradiation at the GIF or IBMRL, samples are studied using ultraviolet-visible spectroscopy and high-resolution transmission electron microscopy to determine what effects time and experimental variables have on particle formation, size, shape, and composition. Depending on the combination of reactants, dose, and dose rate of radiation, researchers have created nanometer-sized particles of gold in a variety of shapes including spheres, rods, and pyramids.

Researchers are also translating the results of these experiments into computer simulations. Kevin Leung, Sandia researcher, is leading the effort to use abinitio molecular dynamics, along with other methods, to interpret and understand the controlling factors in the researchers’ experiments.

"Using the results from the experiments and computer analysis, the team can simulate the structure of the nanocrystal initiation," Leung says. "By examining the free energy present in the interface between the different materials, we will be able to understand what factors govern the size of these metal alloy nanocrystals."

Single gold nanoparticle crystals formed using radiolysis at Sandia's Gamma Irradiation Facility (approximately 30 nm in size).

A gold nanoparticle cluster formed using radiolysis at Sandia's Gamma Irradiation Facility (approximately 300 nm in size).

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