Researchers reveal mechanism of fatigue damage by high thermal loads in tungsten composites

The relationship between microstructure evolution and property degradation of two representative second-phase dispersion strengthened tungsten materials after electron beam thermal loading was investigated recently by a collaborated research team from Hefei Institutes of Physical Science (HFIPS), Chinese Academy of Sciences (CAS).

The related research was published in Journal of Materials Science & Technology.

The best environment for human survival is 25 ℃. The plasma-facing tungsten (W) materials in the magnetic confinement nuclear fusion devices, however, are directly exposed to high-temperature plasma and are typically subjected to steady-state thermal loads of 5–20 MW/m² and transient thermal shocks of ~1 GW/m², which can raise the surface temperature of tungsten to above 1800 ℃. The high heat flux load to W leads to some irreversible material damages, such as surface roughening, cracking and surface melting. Therefore, it is urgent to evaluate the thermal load resistance of W materials.

In this study, researchers carried out repetitive heat loads on an electron beam device 30 kW Electron-beam Material-research Platform (EBMP-30). This platform was specially built to evaluate the thermal shock resistance of plasma-facing materials (PFMs).

“It adopts a 30 kW welding electron beam with a maximum acceleration voltage of 100 kV,” explained XIE Zhuoming, who helped to build the platform, “it can scan 30 × 30 mm² area with maximal frame rate of 35 kHz, and its pulse duration can change from 100 ms to a continuous state.”

Based on the EBMP-30 device, two representative W-0.5wt% ZrC (WZC) and W-1.0wt% Y₂O₃ (WYO) composites were selected to study the damage behavior induced by repeated steady-state heat loads with absorbed power density (APD) in the range of 10–30 MW/m².

The results show that the microstructures and tensile properties of WZC and WYO specimens do not change significantly when APD ≤ 20 MW/m². However, when APD ≥ 22 MW/m², full recrystallization and grain growth in WYO specimens and Y₂O₃ particles shedding from the W matrix were detected.

Moreover, the ultimate tensile strength and total elongation of WYO decreased from 861 MPa to 510 MPa and from 15% to near zero, respectively.

“Due to the different coefficients of thermal expansion (CTEs) of the Y₂O₃ phase and W, irreversible plastic deformation of the W matrix occurs, especially around the coarse Y₂O₃ particles,” said Wu Xuebang, who led the team, “which leads to the interface debonding between Y₂O₃ particles and the W matrix.”

After thermal loads at 22 MW/m², WZC specimens maintained the high ultimate tensile strength of 816 MPa due to its high recrystallization temperature (~1300 ℃).

“The fine and uniform distribution of ZrC particles and its comparable CTE to the W matrix,” Wu added, “which effectively avoids ZrC particle shedding and the formation of microcracks.”

“This study reveals the correlations between the microstructure evolution and performance degradation in two representative second-phase dispersion strengthened tungsten materials, as well as the mechanism of fatigue damage by high thermal loads,” said WU, “which provides an important reference for the further development of high-performance tungsten materials.”