A collaborative research team from the Hefei Institutes of Physical Science (HFIPS) of the Chinese Academy of Sciences has recently investigated the relationship between microstructure evolution and property degradation of two representative second-phase dispersion-strengthened tungsten materials after electron beam thermal loading.

The results were published in Journal of Materials Science & Technology.

The optimal environment for human survival is 25 ℃. However, plasma-oriented tungsten (W) materials in magnetic confinement nuclear fusion devices are directly exposed to high temperature plasma, and usually suffers steady-state thermal load of 5-20 MW/m² and transient thermal shock of ~1 GW/m², which can raise the surface temperature of tungsten to more than 1800 ℃. High heat flow load on W will lead to some irreversible material damage, such as surface roughness, cracking and surface melting. Therefore, it is urgent to evaluate the thermal load resistance of W material.

In this study, the researchers performed repeated heat loads on a 30-kilowatt Electron-beam Material-research Platform (EBMP-30) facility. The platform is specifically designed to evaluate the thermal shock resistance of plasma-facing materials.

"It uses a 30-kilowatt welded electron beam with a maximum acceleration voltage of 100 kV," said XIE Zhuoming, who helped build the platform. "It can scan an area of 30 × 30 mm² with a maximum frame rate of 35 kHz, and the pulse duration can be changed 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 uniform distribution of ZrC particles and its CTE equivalent to the W matrix effectively avoid ZrC particle shedding and microcrack formation," said WU.

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

The work was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, and the Anhui Provincial Natural Science Foundation, etc.

A researcher is doing experiment on EBMP-30 equipment. (Image by ZHAO Weiwei) 

Tensile engineering stress-strain curves of (a) WZC, (b) WYO alloy samples and (c) ITER-W after exposed to cyclic heat loads with different APDs. (Image by WANG Hui)