|Some aspects of the high-temperature behaviour of an in situ Ti-Ti5Si3 eutectic composite|
VOJTECH, D., CIZOVA, H., MAIXNER, J.
vol. 43 (2005), no. 5, pp. 317 - 337
The work describes some aspects of the high-temperature behaviour of an in situ Ti-Ti5Si3 eutectic composite at 750–1050°C. The alloy was annealed under vacuum and its changes induced by high-temperature expositions were monitored using a microstructural examination, XRD and hardness measurement. The cyclic oxidation was conducted in CO2 for 108 hours, and its kinetics was investigated by the way of weight gains measurements and by a detailed examination of the scales. The in situ Ti-Ti5Si3 eutectic composite was found to correspond to the as-cast TiSi8 (in wt.%) alloy. The Ti3Si phase was not found in the as-cast state due to the kinetic factor and the presence of carbon. The high-temperature exposition at 750–850°C induced slow coarsening of the eutectic Ti5Si3 silicide and its precipitation from a supersaturated solid solution. No phase transformation, e.g. into Ti3Si, was detected, which was consistent with the stabilizing effect of carbon. The structural changes were accompanied with a very slow hardness reduction. The measurements of the oxidation kinetics and its comparison with pure Ti proved a strong protecting effect of the Ti5Si3 silicide at 850–1050°C. The slow oxidation of Ti-Ti5Si3 at 850–950°C was governed by diffusion through the scales, and was described by the parabolic law. On the other hand, pure Ti and the Ti-Ti5Si3 alloy oxidized at 1050°C did not obey the parabolic law due to the scales porosity, cracking, and poor adherence to the substrate. The WDS and XRD analyses proved that the main component of the scales on both pure Ti and the Ti-Ti5Si3 alloy was rutile. The scales on the Ti-Ti5Si3 alloy also contained amorphous silica with a small amount of carbon. Silica, which was responsible for a strong oxidation rate reduction, originated from the oxidation of the Ti5Si3 silicide.
titanium, titanium silicide, high-temperature material, metal matrix composite, high-temperature oxidation, eutectic
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