Miglioramento della resistenza ad ossidazione di una lega y-TIAL mediante nanocristallizzazione superficiale ed effetto gettering.

?-titanium aluminide alloys possess many attractive properties such as low density and good high temperature strength which recommend their use in moving structural parts at elevated temperatures. At present , applications as blades in gas turbines and as valves, turbocharger rotors, etc. in car engines are the aim of development in industry. The most limiting factor for their use as high temperature operating components is the weak oxidation resistance at temperatures in excess of 700 °C Two are the aims of this work : understanding the oxidation mechanism of TiAl alloys in relation with the influence of various parameters ( composition , microstructure ) on it and the development of new methods in order to increase the oxidation resistance. In order to perform the first task the most promising methodologies are here presented : the chlorine effect , aluminizing , siliconizing , pack diffusion coatings. We will finally propose two methods for oxidation resistance improvement : nanocrystallization via laser ablation and superficial chrome enrichment via vacuum interdiffusion of a chrome layer ( deposited via laser ablation ).The final result is the development of a new promising and versatile methodology which will extend the use of these materials at temperatures in excess of 900 °C . Much attention is focused on the "numbers" of those phenomena : a finite - difference oxidative model will be developed in order to predict the oxide scale composition , the effect of the grain size on the oxidative behaviour and to calculate the thermal treating duration for the chrome interdiffusion step. The result is a very flexible and versatile approach.