Enhancement of hardness in niobium carbide composites

Traditional cemented carbides are based on tungsten carbide (WC). Among alternative hard phases niobium carbide (NbC) shows high potential. Mechanical properties and microstructures of binderless NbC composites with different amounts of secondary carbides and different sized starting powders were investigated. Different sintering techniques such as FAST/SPS and the conventional SinterHIP technique were used to produce completely dense samples at different temperatures. Especially with the addition of secondary carbides a hardness increase from 1600 HV10 to above 1800 HV10 was measured. This closes the hardness gap of binderless NbC to binderless WC composites with similar grain size. Furthermore, NbC composites with secondary carbides showed an increase of fracture toughness to values of 4.5 MPa*m1/2, which could lead to their use as cutting tools. Results also include a comparison of hot hardness and other functional properties of binderless NbC and binderless WC composites as a function of grain size

Stroemungsberechnung auf unstrukturierten Netzen mit der Methode der finiten Elemente

TIB: DR 8488 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Rate controlled sintering of binderless tungsten carbide

Prevention of the typical densification peaks of conventional constant-heating-rate sintering (CHR) by real-time densification rate control can provide better materials with less microstructural heterogeneities, residual stresses and abnormal grain growth inter alia. FAST/SPS spark plasma sintering systems are able to realize this control by a smooth adjustment of heating rate and applied force, making this technique ideal to investigate the effect of rate controlled sintering (RCS). Within this work nanoscaled binderless tungsten carbide samples were produced by conventional as well as densification rate controlled sintering realized by adjusting the applied force or heating rate during the respective sintering cycle. Especially by controlling the heating rate, the resulting samples showed an increase of density associated to around 250 HV10 higher hardness compared to the conventionally FAST/SPS sintered samples. Microstructural analysis revealed a slightly smaller grain size

Subglacial melt channels and fracture in the floating part of Pine Island Glacier, Antarctica

A dense grid of ice-penetrating radar sections acquired over Pine Island Glacier, West Antarctica has revealed a network of sinuous subglacial channels, typically 500 m to 3 km wide, and up to 200 m high, in the ice-shelf base. These subglacial channels develop while the ice is floating and result from melting at the base of the ice shelf. Above the apex of most channels, the radar shows isolated reflections from within the ice shelf. Comparison of the radar data with acoustic data obtained using an autonomous submersible, confirms that these echoes arise from open basal crevasses 50–100 m wide aligned with the subglacial channels and penetrating up to 1/3 of the ice thickness. Analogous sets of surface crevasses appear on the ridges between the basal channels. We suggest that both sets of crevasses were formed during the melting of the subglacial channels as a response to vertical flexing of the ice shelf toward the hydrostatic condition. Finite element modeling of stresses produced after the formation of idealized basal channels indicates that the stresses generated have the correct pattern and, if the channels were formed sufficiently rapidly, would have sufficient magnitude to explain the formation of the observed basal and surface crevasse sets. We conclude that ice-shelf basal melting plays a role in determining patterns of surface and basal crevassing. Increased delivery of warm ocean water into the sub-ice shelf cavity may therefore cause not only thinning but also structural weakening of the ice shelf, perhaps, as a prelude to eventual collapse

Getting around Antarctica: new high-resolution mappings of the grounded and freely-floating boundaries of the Antarctic ice sheet created for the International Polar Year.

The boundary of grounded ice and the location of ice transitioning to a freely floating state are mapped at 15-m resolution around the entire continent of Antarctica. These data products are produced by participants of the International Polar Year project ASAID using customized software combining Landsat-7 imagery and ICESat laser altimetry. The grounded ice boundary is 53 610 km long; 74% of it abuts to floating ice shelves or outlet glaciers, 19% is adjacent to open or sea-ice covered ocean, and 7% of the boundary are land terminations with bare rock. Elevations along each line are selected from 6 candidate digital elevation models: two created from the input ICESat laser altimetry and Landsat data, two from stereo satellite imagery, and two from compilations of primarily radar altimetry. Elevation selection and an assignment of confidence in the elevation value are based on agreement with ICESat elevation values and shape of the surface inferred from the Landsat imagery. Elevations along the freely-floating boundary (called the hydrostatic line) are converted to ice thicknesses by applying a firn-correction factor and a flotation criterion. The relationship between the seaward offset of the hydrostatic line from the grounding line only weakly matches a prediction based on beam theory. Airborne data are used to validate the technique of grounding line mapping, elevation selection and ice thickness derivation. The mapped products along with the customized software to generate them and a variety of intermediate products are available from the National Snow and Ice Data Center