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Microstructure and Mechanical Characterization of ZrC-Mo Cerments Produced by Hot Isostatic Pressing

OVERVIEW: Microstructure analysis and mechanical characterization were performed on ZrC–Mo composites with 20, 30, and 40 vol% Mo produced by hot isostatic pressing. The composites reached >98% relative density after processing at 1800 ◦C and 200MPa for 1 h. The ZrC grain size was ~1–2m after densification. The Mo appeared to form clusters that increased in size from 15 to 54 micrometers with increasing Mo content. Analysis of mechanical property data indicated that the Mo clusters acted as the critical flaws during  fracture. Hardness decreased from ~17 to ~8GPa with increasing Mo content, and was related to the effective hardness of each of the constituent materials. The elastic moduli also decreased with Mo additions from 392 GPa (corrected for porosity) to ~380 GPa. Flexure strength and fracture toughness increased with increasing Mo content from 320 to 480MPa and 1.0 to 6.6MPa √m, respectively. The elastic moduli, flexure strength, and fracture toughness were all found to follow a volumetric rule of mixtures. Download Full Article.

Solid Solution Carbides are the Key Fuels for Future Nuclear Thermal Propulsion

OVERVIEW: Nuclear thermal propulsion uses nuclear energy to directly heat a propellant (such as liquid hydrogen) to generate thrust for space transportation. In the 1960’s, the early Rover/Nuclear Engine for Rocket Propulsion Application (NERVA) program showed very encouraging test results for space nuclear propulsion but, in recent years, fuel research has been dismal. With NASA’s renewed interest in long-term space exploration, fuel researchers are now revisiting the RoverMERVA findings, which indicated several problems with such fuels (such as erosion, chemical reaction of the fuel with propellant, fuel cracking, and cladding issues) that must be addressed. It is also well known that the higher the temperature reached by a propellant, the larger the thrust generated from the same weight of propellant. Better use of fuel and propellant requires development of fuels capable of reaching very high temperatures. Carbides have the highest melting points of any known material. Efforts are underway to develop carbide mixtures and solid solutions that contain uranium carbide, in order to achieve very high fuel temperatures. Binary solid solution carbides (U, Zr)C have proven to be very effective in this regard. Ternary carbides such as (U, Zr, X) carbides (where X represents Nb, Ta, W, and Hf) also hold great promise as fuel material, since the carbide mixtures in solid solution generate a very hard and tough compact material. Download Full Article.