Metales de transición de estructura hexagonal. Autodifusión y estabilidad de fases compuestas

Abstract

Tesis para optar al título de Magister en Ciencia y Tecnología de Materiales

Phases of hexagonal structure are present in the binary systems which are components of the ternary hafnium-zirconium-tin, i.e. the compact alpha phases with tin, rich in hafnium and / or zirconium which correspond to the stable terminal solid solutions at low temperatures, and the Mn5Si3 type intermetallics. The same hexagonal structure of the terminal olid solutions of hafnium and zirconium with tin, alpha(Hf,Sn) and alpha(Zr,Sn), is in the binary hafnium-zirconium, where there is total solubility between both transition metals alpha(Hf,Zr). In the consequent ternary system this phase is identifies as alpha(Hf,Zr,Sn). The ternary titanium-zirconium-tin system has a stable hexagonal intermetallic compound (Ti,Zr)5Sn3 where titanium and zirconium are mutually interchanged. An initial hypothesis of this work is that a similar compound (Hf,Zr)5Sn3 is formed in the ternary hafnium-zirconium-tin system. From the experimental point of view, the equilibrium phases of three alloys of nominal compositions of about 20 at. per cent of tin and variable in the relation hafnium / zirconium have been characterized. For this purpose the elements were melted in an arc furnace and thermically treated al 900 and 1100 C degree. Optical microscopy techniques, microanalysis and X-ray analysis were applied for the phase characterizations and the lattice parameter determinations. On the other hand, the alpha phases of pure hafnium and zirconium were studied by computer simulation techniques, like molecular static (EM) and molecular dynamic (DM). Many-body interatomic potentials of the embedded atom type were used to represent the atomic interactions. In particular, self diffusion via vacancy mechanism has been studied in a wide temperature range. Finally, ab-initio calculations were carried out to determine theoretically the lattice parameters of different stable hexagonal phases. Results are compared with experimental and other calculated values.