Estudo do equilíbrio de fases no sistema Al-Fe-Nb pelo processo de mechanical alloying

Abstract

The knowledge regarding balance phases of Al-Fe-Nb system, and the understanding of its microstructures, are important for development of several families of alloys, such as niobium-based super-alloys for use in high temperatures, light metal alloys used mainly in vehicles bolide, and aluminum added steels, which request low density and high-resistance to corrosion materials, used in aerospace and aeronautic applications. Although, arc fusion and induction processes commonly used for manufacturing of these alloys present some restrictions, such as high temperatures, lengthy times of process, high steam pressure that cause severe mass loss by evaporation, significant fusion temperatures of elements and the impossibility to obtain materials in nanometric scale. Thus, the Mechanical Alloying (MA) process becomes a viable alternative, which allows to synthesize different phase types, as well as supersaturated solid solutions, and intermetallic phases, from elementary powders, in a solid state reaction, into high-energy ball milling that offers shorter process time when compared to conventional methods for obtaining of phases in alloys with refractory materials. Consequently, heat treatments subsequent to MA for phase formation and/or stabilization, in significant lower temperatures than those used for phase stabilization after fusion are reached. Thereby, it is possible to makes viable the study of phase balance in regions yet or few explored in diagram, such as Nb rich region, in temperatures under 1000 °C. Based on this premise, in this study there were produced samples in Fe-Nb and Nb-Al samples, with compositions that present isothermal sections with balance of three distinctive phases. In initial experiments, in order to define Mechanical Alloying parameters, powders of Al (min. 99.9%), Fe (min. 99.8%) and Nb (min. 99.8%) were used, with three different process control agents (methanol, hexane and stearic acid), under four milling times (20, 40, 60 and 80 hours). The MA was conducted in a high-energy planetary mill, under inert atmosphere of argon gas, in a rotation of 350 rpm. The analysis focused on powder morphology, distribution of particle size, phase formation, reductions of crystallite sizes and lattice parameter. After set the process parameters, these were applied to all studied compositions. Heat treatment after MA were developed for 600 °C, 800 °C and 1000 °C temperatures for 48 hours. The morphology analysis was made with Scanning Electron Microscopy, in Secondary Electron mode (SEM/SE). The phases were obtained by X-ray diffractometry, and phase volumetric fractions by Rietveld refining. Phase formation was reached in some systems just after MA, such as for Nb-60Al (%at.) alloy, which presented 33.0 %v. of NbAl3 phase for milled condition, and 40.3 %v. after heat treatment at 1000 °C. For ternary system, in Nb-55Al-5Fe (%at.) sample, it was possible to reach the expected phase balance for its isothermal section. Furthermore, it was possible to observe that for all studies, intermetallic phases were obtained, e.g. Laves from heat treatment at 600 °C for Fe-15Nb (%at.) composition, the phases Nb2Al and Nb3Al in heat treatment at 800 °C for Nb-23Al (%at.) composition, and phases Nb3Al, Nb2Al and NbAl3 after heat treatment at 600 °C for Nb-17Al-3Fe (%at.) composition.