The Effect of Impact Damage on the Mechanical Behaviour of Gamma TiAl

Abstract

Due to its outstanding high-temperature properties, TiAl is a promising candidate for applications up to 700 degrees Celsius in the high pressure compressor and the low pressure turbine. One of the main problems which represent a barrier for application in gas turbines is its low ductility, especially in relation with impact behaviour. To clarify the influence of high-speed impact damage on the mechanical behaviour of a cast gamma-TiAl based alloy impact tests were performed. Flat shaped specimens of different thicknesses, namely 1.5 mm, 2.5 mm and 5 mm, were used for these tests. Impact tests were carried out making use of a ballistic gun shooting steel spheres with a diameter of 1.5 mm to 5 mm at the specimens. The impact speed was varied from 100 m/s to 200 m/s. Two different angles of impact were chosen; 90 degrees and 45 degrees to the specimen and further the location of impact was varied (at the specimen edge or centre). For comparison, the same tests were performed with the near-alpha- alloy Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6242). The higher ductility and toughness caused the extend of damage to be lower in Ti-6242. However, for the selected test parameters no material blow-off could be observed in the TiAl-alloy specimens. The extend of damage in the two materials is described as a function of the impact energy, impact angle and the specimen thickness. Preliminary microstructural studies showed that at small impact energies and higher thicknesses, similar to the Ti-alloy, only the impact indentation was visible in the TiAl-alloy. Conversely, at higher impact energies a network of cracks could be observed at the back of the TiAl-specimens. The influence of impact damage on the mechanical behaviour of the TiAl-specimens is determined with the aid of tensile tests and fatigue tests. The results are still under evaluation.