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This work concerns the sintering of tungsten-based (i.e WMoTaNb) high entropy alloy (HEA) powders using the spark plasma sintering (SPS) technique and their mechanical properties. The synthesis was performed by a self-propagating high-temperature synthesis (SHS) type reaction in which the mixture of metallic oxides (WO3, MoO3 ...) is reduced by magnesium. For this, a specific reactor has been developed. Different conditions including the addition of a moderator were tested. These powders are then densified by SPS technology which allows for keeping the initial microstructure of the powder. The optimization of sintering conditions was performed with the objective to control simultaneously the chemical composition, the grain growth and the densification stages. [ABSTRACT FROM AUTHOR]

Abstract: Spark plasma sintering (SPS) was used to sinter gas-atomized Zr57Cu20Al10Ni8Ti5 amorphous powder. Systematic analyses were performed to study particle size and annealing time effects on the parts structure and properties. Partial devitrification and particles welding were observed and correlated to particle size and thermal conditions. Mechanical testing, through compression and micro-hardness, reveals that the sintered parts show strength similar to a quenched bulk metallic glass and damaging before failure. However, the pulsed current input does not seem the most relevant way to sinter amorphous powders: during the sintering initial stages (when necks are small), excessive over-heating is generated in the vicinity of particles necks, and is responsible for partial devitrification; further current input at large necks leads to complete densification. Effects of the stress, the thermo- and electro-transports on the sintering are evaluated to provide a better understanding of the SPS mechanisms of densification of metallic glasses. [ABSTRACT FROM AUTHOR]

Copper nanopowders were sintered using Spark Plasma Sintering (SPS). Special attention was paid to the presence of natural surface oxide and reductive thermal treatments under H 2 atmosphere were carried out to remove it. The sintering mechanisms were investigated by the means of disrupted experiments at various temperatures to reach different shrinkage rates. Optimized experiments were also carried out to reach a maximum densification. The samples were characterized and compared using density measurement, X-ray diffraction, mechanical tests (tension and compression) and analysis of the fracture surfaces with scanning electron microscope. On the one hand, it results that during the SPS treatment, the raw powder undergoes a series of oxide transformations from CuO to Cu 2 O. On the other hand, reductive treatment drastically decreases the sintering temperature, from 650 °C to 260 °C, to reach 90% density without grain growth. Higher mechanical strength, up to 750 MPa, also shows better consolidation. [ABSTRACT FROM AUTHOR]

The chemically architectured alloys were proposed as a new concept of microstructure, with a multi-scale architecturation. They are composed of a solid-solution at the atomic scale, grain boundaries and composition gradients at the micronic scale and finally a 3D network of gradients at the mesoscale. Composition gradients are between two phases with the same crystalline structure but different compositions. As a first application of this concept, powders of pure Ni and CoCrFeMnNi high entropy alloy were successfully densified by Spark Plasma Sintering. Then the microstructure and mechanical properties were characterized by scanning electron microscope coupled with energy dispersive spectroscopy and electron backscattered diffraction, X-ray diffraction, nanoindentation and compression tests. The obtained chemically architectured alloys present a homogeneous distribution of the two phases with a 10 μm wide chemical gradient in between. By comparison with reference non architectured materials, the chemical architecturation induces a 35% increase of the yield strength. Thus the concept of chemically architectured metallic alloys and composition gradients was proven as very promising: processing of this complex microstructure is possible and it induces a significant mechanical strengthening. Image 1 • A mixture of CoCrFeMnNi and Ni powders was fully densified by spark plasma sintering. • A chemical gradient of around 10 μm was formed between HEA and Ni phases. • This material is called a chemically architectured alloy. • Mechanical strength is improved compared to an ideal mixture of Ni and CoCrFeMnNi. [ABSTRACT FROM AUTHOR]