Numerous government and commercial entities are currently engaged in research toward net-energy-positive nuclear fusion electricity generation technology, which if successfully developed would revolutionize the energy economy and have a significant beneficial impact on carbon emissions and anthropogenic climate change. One fusion subsystem with significant remaining engineering challenges comprises the multitude of divertor modules installed on the plasma-facing surface of modern nuclear fusion reactors. These divertors must absorb/conduct steady-state heat fluxes on the order of 10–20 MW/m² in an extremely harsh environment, including high-energy neutron bombardment. One common material choice for the plasma-facing portion of these components is tungsten, which provides a high melting point, high sputtering resistance, low tritium retention, and other benefits. As the thermal conductivity of tungsten is somewhat low, however, these tungsten components are often paired with a heat sink made from a more thermally-conductive material, such as copper-chromium-zirconium alloy, CuCrZr. While these materials can be joined by a number of methods, such as diffusion bonding and brazing, the dramatic mismatch in coefficient of thermal expansion (CTE; ~4 μm/m/K and ~17 μm/m/K for tungsten and CuCrZr, respectively) has a significant detrimental effect on the joint strength under these high-temperature, high-heat flux conditions. Development of a means for mitigating this CTE mismatch is imperative for the use of these materials to be feasible in a practical fusion device.

This talk will present results to date on development of an electrodeposited functionally-graded iron-tungsten (Fe-W) interlayer system for mitigation of the CTE mismatch between tungsten and CuCrZr in brazed joints. In theory, an interlayer with a CTE functionally graded between “tungsten-like” and “copper-like” values should significantly mitigate the thermally-induced stresses at the tungsten/CuCrZr joint. A key objective of the work is to demonstrate sufficient control over the local interlayer composition, residual stress, and other properties by adjusting primarily the pulse/pulse-reverse electrical waveform parameters, so as to allow interlayer preparation from a single electrodeposition bath. Results from electrodeposition trials and mechanical testing of brazed joints under ambient conditions as an initial screen for interlayer performance will be discussed, in anticipation of high-heat flux testing of joint performance under fusion-relevant conditions, where the ultimate goal is demonstration of increased tungsten/CuCrZr joint lifetime under thermal load via inclusion of graded interlayers, as compared to similar joints without interlayers.

Figure Caption

(Left) Tungsten substrate with electrodeposited Fe/W interlayer. (Right) Brazed tungsten/CuCrZr sample with Fe/W interlayer.