Scientists at Stony Brook University have designed nanocrystalline tungsten alloy films with enhanced thermal stability, which can meet performance requirements in extreme environments, like fusion reactors. The use of titanium and chromium as alloying elements eliminated abnormal grain growth and adverse metastable phases and led to the production of thin alloy films that were able to withstand temperatures up to 1000°C.

“The development of high temperature plasma-facing components for typical diverter and first wall applications [in fusion nuclear cores] is a critically important task for the realization of commercially viable fusion power,” says Brian Wirth, nuclear energy expert and professor at the University of Tennessee, Knoxville.

Tungsten, the element with the highest melting point at 3422°C, is often used as a plasma-facing material for such components. Its high temperature strength and good thermal conductivity, its resistance to sputtering, and its chemical compatibility with radioactive tritium are only some of the properties which make it endure exposure to high heat flux and the bombardment from plasma particles that occur during fusion.

While alloying can increase tungsten’s strength, the size of the grains in the alloy plays a significant role in material properties at elevated temperatures. Especially, grains in the ultrafine and nanocrystalline regimes have been shown to enhance tungsten’s mechanical behavior, but grain boundaries are also preferred sites for the precipitation of second phases.

Jason Trelewicz and his research team at the Department of Materials Science and Engineering at Stony Brook University decided to address these issues by testing unalloyed nanocrystalline tungsten as well as tungsten-titanium (W-Ti) and tungsten-chromium (W-Cr) alloy thin films with compositions of W-20 at.% Ti and W-15 at.% Cr. After preparing the films with magnetron sputtering deposition, the researchers used in situ transmission electron microscopy (TEM) to track the evolution of selected grains as a function of the annealing temperature. They also analyzed phase transformations by means of precession electron diffraction, a technique based in a TEM for the collection of diffraction patterns.

Unalloyed films were heated at a rate of 5°C/min to 300°C, 450°C, and 650°C, where they remained for 5–10 min before being cooled to room temperature. Alloy films were heated up to 1000°C and held there for 20 min prior to cooling.

In the unalloyed films, two phenomena were observed: grain growth occurring through a discontinuous process at temperatures up to 550°C, and the phase transformation of metastable β-tungsten with A-15 cubic structure, to stable bcc α-tungsten. At 650°C, a complete transformation to the bcc α-phase occurred, accompanied by the development of a homogenous grain structure.

Alloy films exhibited only the stable bcc α-phase throughout the process, a result that was “both intriguing and exciting,” according to Trelewitzc. Another important finding was the contrasting thermal stability between W-Ti and W-Cr alloys. “In the W-Cr system, less solute atoms of Cr were available to accumulate to the grain boundaries, due to the presence of a solute-rich second phase upon annealing. Thus, it became more difficult to stabilize the nanocrystalline grains and it was manifested in the W-Cr films as enhanced grain growth,” Trelewitzc says. “Establishing the correlation between the observed phase transformation and the onset of abnormal grain growth was particularly challenging,” he adds. 

For Chris Schuh, department head and professor of materials science and engineering at the Massachusetts Institute of Technology, “This work is an exciting contribution because it extends the concept of nanostructure stabilization in a new direction: they show that alloying elements in tungsten [Ti and Cr] improve stability against competing metastable phases that form during film growth, which in turn helps the system avoid abnormal coarsening processes that compromise stability upon heating.”

Wirth believes that this research demonstrates a promising alloy design path. “Of course, additional work will be required to thoroughly test these ideas, especially in extreme environments and to be certain that this alloying approach does not have detrimental effects on other, lower-temperature properties or performance,” he adds.

Trelewicz is now exploring, with his colleagues, the chemistry of nanocrystalline ternary W alloys with the simultaneous addition of Ti and Cr. “We aim to address the current limitations of tungsten as a plasma-facing material by precisely tailoring the volume fraction, chemistry, and structural state of grain boundaries,” he says, adding that his research team is also studying the effect that the stabilized nanocrystalline grains have on the recrystallization temperature of tungsten, with an eye toward designing materials for future fusion devices. 

Read the abstract in the Journal of Materials Research.