Search

In this paper, cemented carbides with gradient surface enriched in Co phase and depleted of cubic phases were prepared by one-step vacuum sintering. The gradient cemented carbides of different WC grain size were used as the substrates of CVD coatings. The effects of WC particle size on the formation of gradient layer, microstructure and properties of the gradient cemented carbides were investigated. Besides, the influence of WC grain size and gradient layer on microstructure, growth and adhesion strength of the coatings were studied. The results showed that the thickness of surface gradient layer decreased with increasing WC particle size, which was attributed to the decreased diffusion paths and the increased diffusion distance. The interface between the surface gradient layer and the bulk was disordered due to abnormal grain growth of WC in ultrafine cemented carbide. The microhardness across the direction of the fcc-free (Face Center Cubic Free) surface layer to the bulk were similar in the three gradient cemented carbides, and could be expressed as: from the surface to the inner, the microhardness decreased firstly, then increased sharply around the interface, and subsequently dropped to the bulk level. The coating on the fcc-free surface layer was thicker than that on bulk, and the coating on the substrate with fine-sized WC grains is the thickest. The increase of the WC grain size in the substrate could improve the bonding strength of the coating. Furthermore, the presence of Co-rich layer could improve the bonding strength. However, bonding strength was poor for the grain size of ultrafine. [ABSTRACT FROM AUTHOR]

Direct melt infiltration does not produce sintered closed holes and considerably reduces the time required for preparation compared with pre-sintered WC skeleton followed by melt infiltration of Co. However, it may lead to alloy collapsing and deformation. In this study, WC and Co green parts were prepared via binder jet 3D printing. The two parts were stacked layer by layer then subjected to melt infiltration to prepare a WC–Co series of cemented carbides. The aim was to investigate the shape retention, infiltration conditions, and properties of the alloy after the equal distribution of Co and the direct infiltration of the Co melt into the WC green parts without pre-sintering. The effect of different Co amounts on the product was also studied. Results showed that the carbide prepared through direct melt infiltration of WC green parts without pre-sintered skeletons displayed excellent shrinkage and properties. The shrinkage of the prepared cemented carbide with a Co content of about 25 wt% was within 3% in all directions. The relative density, hardness, and fracture toughness of the alloy reached 96.65%, 11.5 GPa, and 23.5 Mpa m1/2, respectively. This work provides a simplified, and high-shape-retention method for other alloys that can be prepared through melt infiltration. • WC and Co green parts were prepared by binder jet 3D printing, and WC-Co was successfully prepared by melt infiltration method. • The melt infiltration after layering and stacking two green parts resulted in an alloy with good properties and shrinkage of less than 3%. [ABSTRACT FROM AUTHOR]

Trace titanium and yttrium (~0.1 wt.%) were doped to the novel W-K-Mo-Ti-Y alloys to investigate their performance under simulated transient heat loads in fusion reactor. The micro-structure, Vickers micro-hardness, cracks formation and surface roughness after transient heat loads were analyzed through X-ray diffraction analysis (XRD), Vickers Micro-hardness device, scanning electron microscopy (SEM) and three dimensional (3D) industrial optical microscope, respectively. It indicated that the transient heat loads would lead to surface cracking and roughening on the loaded surface, as well as a decrease on micro-hardness. Further, trace titanium and yttrium doping in the samples could largely mitigate surface roughening after transient heat loads. Both titanium and yttrium have significant influence on the surface cracking threshold and plastic deformation, indicating the improvement of transient heat loads resistance in this novel alloy. [ABSTRACT FROM AUTHOR]

Dense potassium-doped tungsten (W–K) ingots with homogeneously dispersed nanoscaled bubbles were fabricated by spark plasma sintering (SPS) using Al–K–Si (AKS) doped tungsten powder. Parameters such as sintering temperature, holding time and sintering pressure were considered to tune the grain size, density, microhardness, bubble formation and impurity contents. In addition, a two-step sintering scheme was designed to reduce Al/Si impurity contents and enhance the density. The sample prepared by the two-step sintering scheme displays both inter- and intra-granular nanocomposites, which is expected to have better creep resistance, high bending strength and fracture toughness. Potassium was detected inside the bubbles, determining the crucial role in the bubble formation process, and thus leading to compatible properties as plasma-facing material. [ABSTRACT FROM AUTHOR]

The effect of potassium doping on the thermal shock behavior of tungsten is investigated in this paper. Traditional W–K samples with different potassium contents were tested by an electron beam at an absorbed power density of 0.37 GW/m 2 . It shows that the damage after thermal shock test is smaller with potassium content increasing, but the cracking threshold is not improved. It suggests that for traditional W–K sample the potassium bubbles do have some pinning effects on tungsten but not enough to prevent cracking in transient high heat load. The thermal shock behavior of spark plasma sintered W–K (SPS–WK) was comparably studied. In SPS–WK, potassium bubbles not only distribute along the grain boundaries but also retain inside the grains. Size of the potassium bubbles inside the grains is in the range of 20–100 nm. This microstructure leads to higher thermal shock resistance and no obvious damage occurred after transient high heat load at 0.37 GW/m 2 . This may provide some important information to improve thermal shock resistance for W–K PFM. [ABSTRACT FROM AUTHOR]

Nano‑potassium bubble strengthened tungsten (W-K) alloy with excellent overall performance has been proven to be one of the most desirable plasma facing materials (PFMs). However, some key properties, e.g., thermal heat resistance, ductile-brittle transition temperature (DBTT), strength and toughness need to be further improved to meet the requirements of fusion reactor. Herein, pure yttrium (Y) was doped in W-K alloy to achieve multiple strengthening effects. The grains of W-K alloy have been significantly refined, and the tensile strength was improved along with the introducing of Y. Thermal shock cracking threshold has increased to exceed 0.62 GW/m2. Interestingly, the existing form and size of the precipitates differed with the change of doping amount, leading to the nonlinear evolution of DBTT. The W-K alloy with 0.1 wt%-Y doping content (W-K-0.1Y) showed the best match of strength and toughness. Furthermore, coupled with relatively good thermal conductivity, W-K-0.1Y alloy displayed prominent thermal shock resistance. This work may pave a way for designing PFMs with higher performance. • Pure yttrium (Y) was doped in W-K alloy to realize multiple strengthening effects. • The grains of W-K alloy were significantly refined by Y doping. • The existing form of the precipitates differed with the change of doping content. • The DBTT of W-K-0.1Y alloy has been reduced to ~200 °C. • The thermal shock cracking threshold of W-K-0.1Y alloy exceeded 0.62 GW/m2. [ABSTRACT FROM AUTHOR]

The shrinkages presented in sintering process of tungsten alloys are extremely detrimental to their practical applications in fusion reactors. Positively in this work, we effectively reduced the shrinkages via synergistically applying Ti-doping and annealing process on tungsten‑potassium (WK) alloys. This shrinkage-fill effect only works in the one with trace amount of titanium doping, such as 0.05 wt%Ti or 0.5 wt%Ti. Due to such effect, the compressive strength was improved by ~18%. Based on morphological and elementary analyses, we speculated that the Ti-rich particles along the grain boundaries of WK alloys were activated and migrated into shrinkages during annealing. Further Ti doping (e.g. 1wt.%Ti) led to nanoscale grain refinement in WK alloys, which almost doubled the compressive strength. In this case, an unconventional grain-refinement model was proposed. Furthermore, anomalous grain growth was observed in WK-3wt.%Ti, which was attributed to the formation of Ti-rich regions and the corresponding raise of energy in grain boundaries. The fabricating strategies in this work may benefit the design of refractory tungsten alloys with less shrinkages and ultra-fine grains. • The annealing induced shrinkage-fill effect was put forward for the first time. • Shrinkages in WK alloys were significantly reduced due to this effect. • The compressive strength of WK-0.5Ti was notably improved after annealing. • The schematic mechanism of shrinkage-fill effect was proposed. • The explanation of anomalous grain growth in WK-3Ti was presented. [ABSTRACT FROM AUTHOR]