Fracture behaviour of cast TiAl based intermetallic alloys

The γ -TiAl based intermetallic alloys have received considerable attention as candidate materials for high-temperature aerospace applications, power generation and automotive industry, with the efforts being directed toward the replacement of Ni-based superalloys in many interesting applications. In fact TiAl-based alloys’ density is about 4 g/cm3 which is about half of that of commonly used Ni-based superalloys, and therefore these alloys have attracted broad attention as potential candidate for high-temperature structural applications in the fields for example of turbocharger manufacture. Specific composition/microstructure combinations are believed to be capable of improving mechanical properties while maintaining satisfactory oxidation resistance, creep resistance and high temperature strength for targeted applications. Different casting methods such as conventional sand casting, investment casting, low pressure casting, centrifugal casting, shell mould casting have been used for producing TiAl based alloys. In our experimental work specimens were produced by means of centrifugal casting, the main advantage of this method being the production of castings completely free from any porosity. Tests carried out on several samples characterized by different alloy compositions highlighted that solidification shrinkage and solid metal contraction during cooling produce the development of relevant residual stresses that are sufficient to fracture the castings during cooling or to produce a delayed fracture. In this work cracks initiation and growth have been analysed in order to identify the factors causing these very high residual stresses that often produce explosive crack propagation throughout the casting.