Papers by Keyword: NbAl₃

Abstract: The behavior of different process control agents (PCAs) during mechanical activation of Nb75Al powder mixtures was investigated. Mechanical activation by high-energy ball milling was carried out on a shaker mill (SPEX®8000) for 1 hour. Each PCA (Stearic acid, ethanol and methanol) was added to the powder charge in two proportions (1 and 2 wt%). Shape and microstructure of activated powders (aggregates) were analyzed by scanning electron microscopy. Milled powder mixtures were uniaxially pressed in cylindrical compacts that were further vacuum reacted at a constant heating rate (30°C/min) in order to produce NbAl3 intermetallic compound. The temperature of the samples was monitored by an S-type thermocouple. The results show that the shape and the microstructure of the milled powders were strongly affected by the type and quantity of PCAs, therefore changing the reaction behavior and the densification of the produced pellets. Although ethanol was more effective to control aggregate size, best densification results were attained with 2 wt% of stearic acid.

Abstract: In this work shake milling were used to mechanically activate Nb – Al powder mixtures at different relative proportions (Nb80Al, Nb65Al, Nb54Al e Nb42Al). All milling process parameters were unchanged, e.g., powders mass, ball/powder mass ratio, balls diameter, quantity and kind of process control agent. Uniaxially compacted cylindrical pellets of milled powders were vacuum reacted. After a two-step degassing treatment (290°C for 0.5 h and 400°C for 4 h), samples were heated at 30°C/min. Ignition and combustion temperatures were measured by a thermocouple inserted in a hole drilled into the pellets. The microstructure of milled powders and reacted pellets were characterized by X-ray diffraction and SEM analysis. Bulk density of the pellets was measured by water immersion (Archimedes). The results showed a decrease of both ignition and combustion temperature with mechanical activation as seen by comparison with reacted pellets of the same composition not mechanically activated (simple mixtures). By increasing the heating hate the completeness of the reactions were improved. The lower the aluminum contents the lower the ignition and combustion temperatures and also the densification. The decrease on ignition temperature was caused by a more effective dispersion (and so more activation) attained by samples with lower aluminum content.