Papers by Keyword: Refractory Metals

Abstract: Vanadium alloys are highly promising as structural materials of fusion reactor blanket, owing to their excellent high-temperature strength, and good compatibility with liquid metal lithium, which functions as both a coolant and a fuel tritium breeder material. Chemical composition of V-4Cr-4Ti has been selected as the primary candidate after systematic investigations into its neutron irradiation properties. Since V and Cr do not produce long-lived radioactive isotopes emitting high-energy gamma rays even under intense neutron irradiation conditions, low-activation characteristics are primarily governed by Ti and detrimental high-activation impurities, such as Co, Cu, Fe, Mo, Nb, and Ni. Very early material recycling, such as remote recycling within ten years, and re-use even in the same fusion reactor is achievable through effective impurity removal and minimization of Ti concentration. This paper discusses the progress in and mechanisms of vanadium metal refining. Additionally, the present paper reviews recent results and current status of redesign efforts for the Cr and Ti concentration balance to identify a new high-Cr and low-Ti composition, maintaining various attractive properties of the V-4Cr-4Ti alloy.

Abstract: The work investigates refractory metals (bulk W, W produced via plasma spraying, W-1% La₂O₃ and Mo) of interest as plasma facing materials in future nuclear fusion reactors. They have been irradiated by a single Nd:YAG laser pulse to simulate the effects of transient thermal loads of high energy occurring in a tokamak under operative conditions and then examined by SEM observations. In all the materials the laser pulse induces a crater in the central area of laser spot surrounded by a ridge due to movement of molten metal while in a more external area a network of cracks is observed. Diameter and depth of the crater, ablated volume and morphological features of the surrounding area exhibit differences depending on the specific metal, its physical and microstructural characteristics which affect vaporization, melting and heat propagation from the irradiated spot.

Abstract: A procedure has been developed for calculating the geometric, kinematic and energy-power parameters of helical rolling, which allows a comparative analysis of the processes carried out in stands of various designs. Based on the results of this analysis, a helical rolling mill was designed and manufactured which allows to roll materials with high deformation resistance at high temperatures. The results of the study of the mechanical characteristics of molybdenum bars and wires made by new technology are presented.

Abstract: While studying activation sintering of tungsten, Evans [5] and Ito and Furusawa [6] revealed that W-Cr-Pd alloys exhibit unexpected oxidation resistance at elevated temperatures. The role of palladium in stimulating oxidation resistance in W-Cr alloys is the main aim of the present contribution. As previously observed, at 800 °C these alloys form a relatively dense protective scale that consists of an inner layer of Cr₂O₃, an intermediate layer of Cr₂WO₆ and an external layer of WO₃. At 1200 °C only Cr₂WO₆ layer is found, since the Cr₂O₃ and WO₃ evaporate. To determine the role of paladium, W and W-Pd alloys were coated with Cr layers and undergone diffusion experiments. An extraordinary affinity between the Cr and Pd was revealed, manifested by extremely fast inward diffusion of Cr along grain boundaries. In a second experiment the dissolution of Cr into W grains at 1300°C was followed and found to take place preferentially along grain boundaries. These observations assess that the Pd segregated at grain boundaries provides fast diffusion channels for Cr to the free surface and it imparts the significant improvement of the oxidation resistance of W alloys, as suggested by Lee and Simkovitz [10-12].

Abstract: In the scope of this work, 2 mm thick TZM sheet metal is butt welded by electron beam welding (EBW) without filler material and a systematic investigation of the most relevant welding parameters to improve the weld quality is conducted. With the aid of design of experiment (DoE), it is shown that with careful selection of the welding parameters it is possible to considerably reduce the size of the fusion zone and the heat affected zone and the grain size of both. Furthermore, the influence of the parameters on the quality of the weld and the characterizing values ultimate tensile strength and hardness of fusion zone is presented. It is concluded, which parameters influence the quality of the weld and suppress pores and cracks.

Abstract: Brazing Mo/Mo and Mo/porous W using 60Mo-40Ru in weight percent braze alloy are evaluated in the experiment. Sound Mo/60Mo-40Ru/Mo joint is obtained from brazing at 1970 and 2000 °C for 600 s, respectively. The brazed joints are comprised of σ and Mo-rich phases. For Mo/60Mo-40Ru/porous W joint, sound bonding is achieved only from brazing at 2000 °C for 600 s, and similar microstructure of the joint is obtained. The 60Mo-40Ru braze alloy shows potential in bonding refractory metal(s) for high-temperature applications.

Abstract: Engineering materials with better high temperature oxidation properties are needed to increase the thermodynamic efficiencies of the energy production and transportation systems. Because of their high melting temperatures, refractory metals like Nb or Mo are brought together with intermetallic compounds as two components of a new class of composite materials. To acquire a balanced high temperature mechanical and oxidation properties, these materials generally have multiphase and multicomponent structures. Borides of some transition elements are also being considered as high temperature structural materials for new aerospace vehicles. These materials are also required to have sufficient high temperature oxidation resistance in order to provide reliable and long service lives.

Abstract: Planning for a U.S. test blanket module to operate in the internationally-sponsored ITER reactor has focused attention on the many coating and compatibility issues that will need to be solved before fusion energy moves from concept to commercial reality. Examples are given for (1) a dual-layer, electrically-resistant coating as a potential solution to reduce the magnetohydrodynamic pressure drop with liquid Li and (2) materials compatibility issues with eutectic Pb-Li for conventional alloys and SiC/SiC composites. Because of the reduced activity of Li in Pb-Li, a wider range of functional materials can be considered in this system. Nevertheless, an Al2O3 scale on FeCrAl was transformed to LiAlO2 after exposure to Pb-Li at 800°C.