Multimodal grain size distribution and high hardness in fine grained tungsten fabricated by spark plasma sintering

Graphical abstract: Cross-sectional micrograph of spark plasma sintered tungsten with multimodal grain size distribution: (a) using focused ion beam (b) Bright field TEM image. Highlights: → High applied external pressure during SPS led to high density of the samples. → The consolidated samples by SPS had a multimodal size distribution. → Ultrafine grains were present within the samples sintered at low temperatures. → High Vickers hardness was obtained compared to commercial tungsten. → The consolidated samples were proved to be pure by chemical analysis. - Abstract: Preparation of fine grained, hard and ductile pure tungsten for future fusion reactor applications was tested using the bottom-up approach via powder consolidation by spark plasma sintering (SPS) at different temperature (1300-1800 deg. C) and pressure (90-266 MPa) conditions. Pure tungsten powders with an average particle size of about 1 μm were sintered to high density (about 94%) with almost no grain growth at a temperature below 1400 deg. C and an applied pressure up to 266 MPa. These samples had a multi-modal grain size distribution (resembling the size distribution of the initial powder) and a very high Vickers hardness (up to 530 kg/mm2). Above 1500 deg. C fast grain growth occurred and resulted in a drop in hardness. XRD on the surface of bulk samples showed a small amount of tungsten oxides; however, XPS and EDS indicated that these oxides were only surface contaminants and suggested a high purity for the bulk samples. The results demonstrate that SPS can lead to ultrafine and nanocrystalline tungsten if used to consolidate pure nano tungsten powders.