Volume 52, Issue 5

In this contribution, the microstructure of two intermetallic γ-TiAl-based alloys with different Al contents were examined after high-temperature deformation. To investigate the dynamic recrystallization of these alloys, isothermal compression tests were performed using a Gleeble ® 3500 simulator. For the experiments, a temperature range of 1 150 °C to 1 300 °C and strain rates of 0.005 s −1 , 0.05 s −1 and 0.5 s −1 were applied, up to a true strain of 0.9. The deformed microstructural states, particularly the multiphase alloys' dynamically recrystallized grain sizes were characterized via Scanning Electronic Microscopy (SEM) and Electron Back Scatter Diffraction (EBSD). The recrystallized grain sizes obtained from the experiments could be linked with the calculated Zener-Hollomon parameter through a power law.

Owing to their excellent high temperature properties, intermetallic γ-TiAl based alloys are potentially suitable materials for applications in the aviation and automotive industry. In order to adjust a fine lamellar microstructure with optimum properties, the forging process is followed by a multi-stage heat treatment. During the final annealing process, a discontinuous precipitation reaction, a so-called cellular reaction, may occur which decomposes the fine lamellae in the α 2 /γ colonies. A stable microstructure is a crucial factor in the development of highly creep resistant alloys. In the scope of this work, various heat treatments were performed in order to adjust microstructures with a varying proportion of discontinuous precipitation. Furthermore, creep tests were performed at 800 °C and 150 MPa. Scanning electron microscopy in the back-scattered electron mode was used to characterize the microstructure's morphology. The respective phase fractions were determined by X-ray diffractometry. It could be shown that the creep resistance decreases with an increasing amount of discontinuous precipitation.

Solid-state phase transformations and grain growth in an intermetallic γ-TiAl based alloy were examined using a confocal laser-scanning microscope. This apparatus allows the in-situ visualization of the microstructural development at high temperatures. The sample is thus placed into a furnace and a predetermined temperature profile is followed, while the surface is scanned using a laser. The contrast is gained by the thermal etching.

A particular engine user experienced the repeated fracture of new fixing bolts in combustion chambers upon first tensioning. The heads of the bolts fractured from their shafts upon torque-controlled tensioning. Two affected screws were received from site by the original equipment manufacturer's (OEM) laboratory to perform a metallurgical failure investigation and to determine the metallurgical root cause of failure. From the findings of the failure investigation described in this paper it is concluded that the metallurgical cause of failure in both cases was quench cracking, i. e. the imperfections leading to failure were induced upon manufacture and pre-existed in the bolts, i. e. they did not develop in service. These pre-existing cracks eventually caused failure by torsional overload upon first tensioning in assembly.