Plazmová a vakuová metalurgie speciálních slitin titanu

Abstract

Titanium aluminide-based intermetallic compounds show a very good combination of high strength, oxidation resistance at elevated temperatures and low weight. Thanks to these properties, these alloys have been the target of research for more than 20 years and are expected to be used as thermally and mechanically loaded components in the automotive, energy and aerospace industries. One of the biggest problems that prevents massive use is considered to be their problematic preparation. This is because melts of TiAl intermetallic compounds are highly reactive, especially with commercially available ceramic crucibles, and for their preparation, it is usually necessary to use the skull melting method with a water-cooled Cu crystallizer. However, this method of preparation is very energy intensive, which causes higher costs for the preparation of components and also does not allow significant overheating of the melt, which is necessary for accurate casting of thin-walled products. Another limitation of TiAl-based intermetallics is their low creep resistance at temperatures higher than 750 ° C. Carbon is an element for which it has been proven that its smaller additions significantly increase the high-temperature properties of TiAl alloys. Carbon alloying was usually applied by adding graphite or TiC powder to the charge. In this work, another approach was used, namely in-situ carbon alloying during vacuum induction melting using isostatically pressed graphite crucibles. The aim was to experimentally assess the possibilities of carbon alloying in the range from 0.3 to 2 % by the given method for two types of modern TiAl alloys (TNM and ABB). In this work, it was proved that the method of vacuum induction melting allows the alloying with carbon in a given range, but it is necessary to observe the individual parameters during melting. The maximum solubility of carbon in two types of TiAl alloys was determined, the microstructure was characterized and heat treatment was designed and performed in order to homogenize and achieve an almost lamellar type of microstructure. The mechanical properties and creep resistance of ABB carbon alloy in the range from 0.4 to 2 at. %, were also determined. It was found that from the point of view of creep resistance, carbon content of about 0.5 to 0.8 at. % appears to be the most suitable for ABB alloy for which a minimum creep rate of less than 10-8 s-1 at 800 °C and a load of 200 MPa was determined.