Zhangjian Zhou

Development of functionally graded plasma-facing materials

Abstract Three different processing technologies are described for the fabrication of SiC/C functionally graded material (FGM), B 4 C/Cu coating FGM and W/Cu FGM. The microstructure and physical properties of the FGMs are evaluated. Some plasma-relevant performances of these three FGMs show their prospect as plasma-facing materials in fusion reactors.

Performance of yttrium doped tungsten under 'edge localized mode'-like loading conditions

Spark plasma sintered tungsten grades, with an yttrium content varying between 0.25 and 1 wt%, were characterized and exposed to transient thermal loads. The samples were cyclic tested at room temperature applying 1 ms long heat pulses using a Nd:YAG laser beam and the electron beam facility JUDITH 1. The absorbed power density of these pulses varied between 0.37 and 1.14 GW m−2. The material modifications were analysed with scanning electron microscopy, optical microscopy and laser profilometry. Comparison showed an improvement of the thermal shock resistance with increasing yttrium content. Additionally, three samples were tested at an elevated base temperature at 400 °C. The two materials with highest yttrium content cracked, indicating still brittle behaviour at the elevated base temperature when adding yttrium.

Overview of R&D on Plasma-Facing Materials and Components in China

Abstract A project to realize, in several years, a W/Cu divertor on Experimental Advanced Superconducting Tokamak (EAST) with ITER-like plasma-facing component (PFC) configuration was launched at Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP) in 2010. The ITER-like configuration should withstand the rapid increase in particle and power impact onto the divertor and demonstrate the feasibility of the ITER design under practical long-pulse tokamak plasmas. The project could help not only EAST experiments, but also realize ITER PFC technology validation and bring answers in a timely manner for the ITER full-W divertor for the nuclear phase. Southwest Institute of Physics (SWIP) will have 10% of the first wall (FW) procurement package of the enhanced heat flux (EHF) type. The materials have been developed and characterized according to the ITER-grade material specifications, including vacuum hot pressing (VHP)-Be, CuCrZr alloy, and 316L(N)-IG forged blocks, and qualification testing of the VHP-Be tiles joining to the CuCrZr heat sink by hot isostatic pressing (HIP) has been carried out. Some Chinese universities have started to explore new grades of W materials, e.g., carbide or oxide dispersion strengthened fine grain W materials, and investigated their behavior under high heat loads.

Evaluation of ultra-fine grained tungsten under transient high heat flux by high-intensity pulsed ion beam

Abstract Pure tungsten, oxide dispersion strengthened tungsten and carbide dispersion strengthened tungsten were fabricated by high-energy ball milling and spark plasma sintering process. In order to evaluate the properties of the tungsten alloys under transient high heat flues, four tungsten samples with different grain sizes were tested by high-intensity pulsed ion beam with a heat flux as high as 160 MW/(m 2 ·s −1/2 ). Compared with the commercial tungsten, the surface modification of the oxide dispersion strengthened tungsten by high-intensity pulsed ion beam is completely different. The oxide dispersion strengthened tungsten shows inferior thermal shock response due to the low melting point second phase of Ti and Y 2 O 3 , which results in the surface melting, boiling bubbles and cracking. While the carbide dispersion strengthened tungsten shows better thermal shock response than the commercial tungsten.

Annealing behaviour and transient high-heat loading performance of different grade fine-grained tungsten

The annealing-induced grain growth behaviour and the transient high-heat loading performance of several W grades, including laboratory fine-grained W, oxide dispersion strengthened (ODS) W and commercial W, were measured and compared. The results show that the starting temperature causing grain growth for pure W (both laboratory fine-grained W and commercial W) is not very high, about 1300° C. The finer the initial grain size of pure W, the stronger the grain growth after annealing at high temperature. But for the pure W with a very fine initial grain size, the grain size after high-temperature annealing is still not very coarse; it is less than that of commercial W. For the ODS W, only slight grain growth occurred even after annealing at a temperature of 1900° C. Transient high-heat loading tests show that the surfaces of fine-grained W and ODS W tend to surface melting. For fine-grained pure tungsten, this is attributed to the low material density, which leads to a relatively bad heat transfer to the bulk and, for the ODS W, to a lower melting point of the oxide phase.

Performance of the different swaged tungsten grades under transient high heat loads

Transient high heat loads simulations by using the electron beam facility have been performed on two swaged tungsten grades at several power loads with a pulse duration of 5 ms. The cracking patterns of the two swaged tungsten grades are quite similar. All cracks occurred along the grain boundary and located across the loaded area. The cracks can be distinguished with two levels, major cracks with larger crack width but lower crack density and microcracks with smaller crack width but higher crack density. The higher the deformation level of swaged tungsten and heat loading power density, the smaller the major crack density will be, but there is no obvious difference in the microcracks pattern. No melting occurred for both swaged tungsten grades after transient heat loading at power density of 0.88 GW m−2.

Effects of consolidation conditions on microstructures and properties of tungsten-vanadium alloy

To face the harsh environment of intense irradiations, tungsten vanadium alloys (W-5wt.% V) were fabricated by spark plasma sintering (SPS) of mechanical alloying powder under different consolidation conditions. X-ray diffraction (XRD) was used to study the phase structures of the alloy powder and the sintered material. To investigate the improvements in mechanical properties and microstructures, the sintering temperatures and dwell times were taken into account for similar composition of tungsten vanadium alloy. The mechanical properties were estimated using micro-hardness and bending strain tests. The detail morphology analysis of the polished cross sectional surface and fracture surface of the samples were performed by scanning electron microscopy (SEM) and quadrant back scattering detector (QBSD). A highest relative density was achieved by alloy material sintered at 1600 °C for 5 min. The two alloy specimens, one sintered at 1100 °C for 2 min and then at 1600 °C for 3 min and the other sintered at 1600 °C for 3 min, have relatively low density but show better mechanical properties. The grain growth of tungsten and vanadium was influenced by consolidation temperature and not much by the difference in dwell time at same peak temperature.

Thermal stability evaluation of microstructures and mechanical properties of tungsten vanadium alloys

The thermal stability is important for tungsten based alloys as plasma facing materials to survive against high heat flux in fusion reactors. In this work, the thermal stability of W-5%V alloy fabricated following a powder metallurgy route by spark plasma sintering technique has been studied. To investigate the impact of temperature on the mechanical properties and microstructures, the alloy was subjected to heat treatment for 2 h over the temperature range 900–1500°C in a pure argon furnace. The micro-hardness values of the heat treated alloys were highly stable as compared to pure tungsten. A slight decrease flexural strength was observed with increasing annealing temperature. The maximum change flexural strength at the highest treated temperature was noted about 14% lower. The morphology analyses of the crack surfaces by scanning electron microscopy did not identify a drastic change in tungsten grain size, after heat treatment. The results indicate that the addition of vanadium in tungsten improves the overall thermal stability of microstructures and mechanical properties.

Surface modification and deuterium retention in reduced-activation steels under low-energy deuterium plasma exposure. Part I: undamaged steels

In this paper, reduced-activation ferritic/martensitic (RAFM) steels including Eurofer (9Cr) and oxide dispersion strengthening (ODS) steels by the addition of Y2O3 particles with different amounts of Cr, namely, (9-16)Cr were exposed to low energy deuterium (D) plasma (~20–200 eV per D) up to a fluence of 2.9 × 1025 D m−2 in the temperature range from 290 K to 700 K. The depth profile of D in steels was measured up to 8 µm depth by nuclear reaction analysis (NRA) and the total retained amount of D in those materials was determined by thermal desorption spectroscopy (TDS). It was found that the D retention in ODS steels is higher compared to Eurofer due to the much higher density of fine dispersoids and finer grain size. This work shows that in addition to the sintering temperature and time, the type, size and concentration of the doping particles have an enormous effect on the increase in the D retention. The D retention in undamaged ODS steels strongly depends on the Cr content: ODS with 12Cr has a minimum and the D retention in the case of ODS with (14-16)Cr is higher compared to (9-12)Cr. The replacing of Ti by Al in ODS-14Cr steels reduces the D retention. The formation of nano-structure surface roughness enriched in W or Ta due to combination of preferential sputtering of light elements and radiation-induced segregation was observed at incident D ion energy of 200 eV for both Eurofer and ODS steels. Both the surface roughness and the eroded layer enhance with increasing the temperature. The surface modifications result in a reduction of the D retention near the surface due to increasing the desorption flux and can reduce the overall D retention.

Effects of vanadium alloying on the microstructures and mechanical properties of hot-pressed tungsten material

Tungsten and vanadium (W–V) alloys (with 1, 5 and 10 wt.% V) are fabricated by hot pressing (HP) at 1800∘C under 20 MPa for 2 h. The effects of V content on the microstructures and mechanical properties of W–V alloy are investigated. The results indicate that with increasing V content, (i) the formation of W–V alloying phase is enhanced and the grain size of W-matrix is significantly refined; (ii) the relative density gradually increases from 92.16% to 97.72% in the case of pure W to W-10 wt.% V; (iii) the hardness rises linearly while the bending strength decreases, which is related to the enhanced alloy phase formation.