Diffusion bonding and interface structure of advanced carbide-dispersion-strengthened Cu and oxide-dispersion-strengthened W

Abstract

Metallurgical bonding was achieved between carbide-dispersion-strengthened Cu (CDS-Cu) and oxide-dispersion-strengthened W (ODS-W) after diffusion bonding via an interface tailoring method of anodization and deoxidation of ODS-W combined with the introduction of Cu wetting layer. The shear strength of the CDS-Cu/ODS-W joints fabricated by the interface-tailoring bonding is 124 MPa, which is higher than that of 99 MPa of the joint prepared by direct bonding. The increase in strength is due to the achievement of the metallurgical bonding between CDS-Cu/ODS-W, the increase of the debonding energy and the increase of propagation distance of the cracks. After metallurgical bonding, the second phase particles of WC in CDS-Cu and Y₂O₃ in ODS-W were dispersed at the phase boundary to reduce the lattice strain energy. This work paves the way for the preparation of advanced plasma-facing components comprised of high-performance CDS-Cu and ODS-W for China Fusion Engineering Test Reactor (CFETR).

Introduction

Divertor is an important part of the magnetic confinement fusion device. Bolt et al. (Bolt et al., 2002) reviewed that the plasma-facing components (PFCs) of the divertor include plasma-facing materials (PFMs) and heat sink materials. In the review of Linsmeier (Linsmeier et al., 2017), the development of advanced high heat flux and plasma-facing materials was reported. PFM is an armor material that must withstand high heat flux and radiation of hydrogen, helium, and neutron. Heat sink materials must have excellent thermal conductivity and can quickly transfer the heat load of the PFMs.

Investigated by Ueda et al. (Ueda et al., 2014), W and W alloys are promising materials used as PFMs, due to their high melting point, low tritium retention and high sputtering threshold. Hirai et al. (Hirai et al., 2009) reported that pure tungsten is brittle at low temperatures and has a high ductile-brittle transition temperature (DBTT). In addition, according to Hasegawa et al.’s report (Hasegawa et al., 2014), neutron irradiation causes the embrittlement of pure tungsten. The performances decrease and the DBTT increases. Oxide-dispersion-strengthened W (ODS-W) was prepared by adding some oxide nanoparticles in the W matrix. Liu et al. (Liu et al., 2016) prepared W-Y₂O₃ by sol–gel method. Chen et al. (Chen et al., 2021a) fabricated W-La₂O₃ via the solution combustion synthesis-based methods. Better performances such as high thermal stability and strong radiation resistance make ODS-W promising PFMs in future fusion devices. According to Butterworth and Forty’s (Butterworth and Forty, 1992) report, Cu and Cu alloys are widely used as heat sink materials owing to the excellent thermal conductivity. In Ke et al.’s study (Ke et al., 2021), the strength of pure Cu and Cu alloys decreases severely at high temperatures. Second phase disperse-strengthened Cu (DS-Cu) composites have been designed by adding oxide nanoparticles or carbide nanoparticles to form oxide-dispersion-strengthened Cu (ODS-Cu) or carbide-dispersion-strengthened Cu (CDS-Cu). DS-Cu is the promising candidate for heat sink materials in future PFCs. Huang et al. (Huang et al., 2019) prepared Cu-Y₂O₃ alloy for heat sink materials application. Li et al. (Li et al., 2005) prepared Al₂O₃ reinforced Cu composites by internal oxidation. At present, ODS-Cu is typically prepared by the internal oxidation method. However, in Lu et al.’s report (Lu et al., 2017), the process of internal oxidation is very complicated. Cu₂O is inevitably present during the preparation process, and the high-temperature performance is damaged. In Han et al.’s (Han et al., 2021) recent work, CDS-Cu was prepared by intermittent pulse electrodeposition combined with spark plasma sintering (SPS). The materials have high yield strength, high thermal stability and strong radiation resistance while maintaining high thermal conductivity.

In order to effectively transfer the thermal load of the PFMs, it is necessary to achieve the high-strength bonding of the PFMs (ODS-W) and the heat sink material (CDS-Cu). Merola et al. (Merola et al., 2002) reviewed the need to develop reliable and low-cost methods for joining PFMs and heat sink materials. However, W and Cu are immiscible, and metallurgical bonding is therefore difficult to be achieved between W and Cu, such as the report given by Pintsuk et al.’s (Pintsuk et al., 2007). Tang et al. (Tang et al., 2014) evidenced that the large thermal stress generated due to the very different thermal expansion coefficients of W and Cu under the high-heat loads will make the joint unstable. Barabash et al. (Barabash et al., 2000) also indicated the difficulty of achieving the high bonding strength of W and Cu. Many methods have been proposed for the joining of W and Cu. Liu et al. (Liu et al., 2022) fabricated W-Cu joint via hot explosive welding. Friction stir forming was used to prepare the interlocked Cu–W joint by Ahuja et al. (Ahuja et al., 2015). During the diffusion bonding process of Cu alloy and W, Batra et al. (Batra et al., 2004) introduced an interlayer of Ni, and Zhao et al. (Zhao et al., 2012) introduced an amorphous W–Fe coated copper foil as an interlayer. Yan et al. (Yan et al., 2019) introduced a Fe-Ni-Cu interlayer during the vacuum diffusion bonding of W and W-Cu alloy. Jiang et al. (Jiang et al., 2017a) fabricated micro/nano interface structures by femtosecond laser to enhance the bonding strength of W/Cu joint. It is however even more difficult to achieve a high-strength bonding of CDS-Cu (or ODS-Cu) and ODS-W. In Chen et al.’s (Chen et al., 2021b).study, the surface activity of W and Cu decreases due to the presence of the second phase particles. The particles at the interface may become the source of cracks and reduce the bonding strength. It is necessary to develop a reliable method for high-strength bonding of ODS-W and CDS-Cu and understand the joint interface structure in detail.

The key to achieve a high-strength bonding of CDS-Cu and ODS-W is the tailoring of the joint interface structure. In this paper, an interface tailoring method involving anodization and deoxidation of ODS-W combined with the introduction of a Cu wetting layer was employed to achieve the high-strength bonding of CDS-Cu and ODS-W. After interface tailoring, the activity of the interface increased. Intermixing of W and Cu occurred after diffusion bonding under hot pressing and a metallurgical bonding was achieved. The shear strength of the thus-fabricated CDS-Cu/ODS-W joint is higher than that of the joint fabricated by direct bonding due to the increase of the debonding energy and the propagation distance of the cracks. The second phase particles Y₂O₃ (in ODS-W matrix) and WC (in CDS-Cu matrix) migrated to the phase boundary of the metallurgical bonded ODS-W and CDS-Cu to reduce the lattice strain energy.