Overview of processing technologies for tungsten-steel composites and FGMs for fusion applications

Tungsten is a prime candidate material for the plasma-facing components in future fusion devices, e.g. ITER and DEMO. Because of the harsh and complex loading conditions and the differences in material properties, joining of the tungsten armor to the underlying construction and/or cooling parts is a complicated issue. To alleviate the thermal stresses at the joint, a sharp interface may be replaced by a gradual one with a smoothly varying composition. In this paper, several techniques for the formation of tungsten-steel composites and graded layers are reviewed. These include plasma spraying, laser cladding, hot pressing and spark plasma sintering. Structure, composition and selected thermal and mechanical properties of representative layers produced by each of these techniques are presented. A summary of advantages and disadvantages of the techniques and an assessment of their suitability for the production of plasma-facing components is provided

Microstructure and Properties of Spark Plasma Sintered Al-Zn-Mg-Cu Alloy

The microstructure of an aluminum alloy containing 53 wt% Zn, 2.1 wt% Mg and 1.3 wt% Cu as main alloying elements has been studied with the focus on the precipitation behavior during the spark plasma sintering process. The starting material was an atomized Al-Zn-Mg-Cu powder with the particle size below 50 μm. The particles showed a solidification microstructure from cellular to columnar or equiaxed dendritic morphology with a large fraction of the alloying elements segregated in form of intermetallic phases, mainly (Zn,Al,Cu)₄₉Mg₃₂ and Mg₂(Zn,Al,Cu)₁₁, at the cell and dendrite boundaries. The microstructure of the sintered specimens followed the microstructure of the initial powder. However, Mg(Zn,Al,Cu)₂ precipitates evolve at the expense of the initial precipitate phases. The precipitates which were initially continuously distributed along the intercellular and interdendritic boundaries form discrete chain-like structures in the sintered samples. Additionally, fine precipitates created during the sintering process evolve at the new low-angle boundaries. The large fraction of precipitates at the grain boundaries and especially at the former particle boundaries could not be solved into the matrix applying a usual solid solution heat treatment. A bending test reveals low ductility and strength. The mechanical properties suffer from the precipitates at former particle boundaries leading to fracture after an outer fiber tensile strain of 3.8%

Microstructure and Properties of Spark Plasma Sintered Al-Zn-Mg-Cu Alloy

The microstructure of an aluminum alloy containing 53 wt % Zn, 2.1 wt % Mg and 1.3 wt % Cu as main alloying elements has been studied with the focus on the precipitation behavior during the spark plasma sintering process. The starting material was an atomized AlZnMgCu powder with the particle size below 50 µm. The parti-cles showed a solidification microstructure from cellular to columnar or equiaxed dendritic morphology with a large fraction of the alloying elements segregated in form of intermetallic phases, mainly (Zn,Al,Cu)49Mg32 and Mg2(Zn,Al,Cu)11, at the cell and dendrite boundaries. The microstructure of the sintered specimens followed the microstructure of the initial powder. However, Mg(Zn,Al,Cu)2 precipitates evolve at the expense of the initial precipitate phases. The precipitates which were initially continuously distributed along the intercellular and in-terdendritic boundaries form discrete chain-like structures in the sintered samples. Additionally, fine precipitates created during the sintering process evolve at the new low-angle boundaries. The large fraction of precipitates at the grain boundaries and especially at the former particle boundaries could not be solved into the matrix applying a usual solid solution heat treatment. A bending test reveals low ductility and strength. The mechanical proper-ties suffer from the precipitates at former particle boundaries leading to fracture after an outer fiber tensile strai

Heat load and deuterium plasma effects on SPS and WSP tungsten

Tungsten is a prime choice for armor material in future nuclear fusion devices. For the realization of fusion, it is necessary to address issues related to the plasma–armor interactions. In this work, several types of tungsten material were studied, i.e. tungsten prepared by spark plasma sintering (SPS) and by water stabilized plasma spraying (WSP) technique. An intended surface porosity was created in the samples to model hydrogen/ helium bubbles. The samples were subjected to a laser heat loading and a radiation loading of deuterium plasma to simulate edge plasma conditions of a nuclear fusion device (power density of 108 W/cm2 and 107 W/cm2, respectively, in the pulse intervals up to 200 ns). Thermally induced changes in the morphology and the damage to the studied surfaces are described. Possible consequences for the fusion device operation are pointed out

Mechanical and Thermal Properties of Individual Phases Formed in Sintered Tungsten-Steel Composites

Tungsten is a prime candidate material for plasma facing components in fusion devices, thanks to its advantageous properties with respect to interaction with hot plasma. For its bonding to the supporting structure, composites and graded layers can be used for the reduction of stress concentration at the interface. When tungsten and steel are processed at elevated temperatures, e.g. hot pressing or spark plasma sintering, intermetallic phases may form and their presence and properties will affect the properties of the composite. In this work, mechanical and thermal properties of the individual phases, i.e. steel, tungsten and Fe-W intermetallics are investigated. Mechanical properties were determined by instrumented indentation. Thermal conductivity was determined by the xenon flash method on a range of samples with varying composition, from which the conductivities of each constituent were estimated. The results can be used for the optimization of compositional profiles and processing conditions for manufacturing of plasma facing components

Behavior and microstructural changes in different tungsten-based materials under pulsed plasma loading

In this study, morphological, microstructural and phase changes of four types of tungsten materials after exposure to dense deuterium plasma were examined. The microstructures of the prepared materials mutually differ by the porosity, grain size and phase content. It was found that inherent porosity of sintered materials leads to a specific mechanism of erosion and might be a significant source of dust in the case of materials with higher porosity. Further, a preferential erosion of the dispersed particles by melting and evaporation and subsequent formation of thin film on the surface of W-Y2O3 was described as well

Nano-hardness, EBSD analysis and mechanical behavior of ultra-fine grain tungsten for fusion applications as plasma facing material

Tungsten and its alloys have been extensively studied in order to be used in plasma facing components for future fusion nuclear reactors such as ITER and DEMO. In this work, an evaluation of nano-hardness, microstructure/texture and mechanical behavior using nano-indentation, electron backscatter diffraction (EBSD) and tensile test was performed. The investigated materials were ultra-fine grain lab-scale tungsten and ITER-specification commercial tungsten products, taken as reference in the as-received and annealed (at 1300 °C for 1 h) conditions. Three ultra-fine grain (UFG) tungsten grades were produced under different spark plasma sintering conditions, namely at 2000 °C and 70 MPa, at 1700 °C and 80 MPa and, finally, at 1800 °C and 80 MPa. EBSD analysis provides very relevant data as it is known that the crystallographic orientation affects some features of the surface damage caused by fusion-relevant plasma exposure. The present results will serve as a reference for future studies that will be carried out using plasma-exposed samples in order to correlate the observed damage with the microstructural characteristics and mechanical behavior