Creep Behavior and Microstructural Stability of Ti-46Al-9Nb with Different Microstructures
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Creep Behavior and Microstructural Stability of Ti-46Al-9Nb with Different Microstructures S. Bystrzanowskia, A. Bartelsa, H. Clemensb, R. Gerlingc, F.-P. Schimanskyc, G. Dehmd a Materials Science and Technology, TU Hamburg-Harburg, D-21073 Hamburg, Germany b Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, A-8700 Leoben, Austria c Institute for Materials Research, GKSS-Research Centre, D-21502 Geesthacht, Germany d Max-Planck-Institut für Metallforschung, D-70569 Stuttgart, Germany ABSTRACT In this paper the creep behavior and the microstructural stability of Ti-46Al-9Nb (in at.%) sheet material were investigated in the temperature range of 700°C to 815°C. The study involves three different types of microstructure, namely fully lamellar with narrow lamellar spacing, duplex and massively transformed. Short-term creep experiments conducted at 700°C and 225 MPa confirmed that the lamellar microstructure with narrow lamellar spacing exhibits a much higher creep resistance when compared to the massively transformed and duplex ones. During longterm creep tests up to 1500 hours stress exponents (in the range of 4.4 to 5.8) and apparent activation energies (of about 4 eV) have been estimated by means of load and temperature changes, respectively. Both, stress exponents and activation energies suggest that under the applied conditions diffusion-assisted climb of dislocations is the dominant creep mechanism. The thermal stability of the different microstructures under various creep conditions has been analyzed by electron microscopy and X-ray diffraction. Our investigations revealed considerable stress and temperature induced microstructural changes which are reflected in the dissolution of the α2 phase accompanied by precipitation of a Ti/Nb - rich phase situated at grain boundaries. This phase was identified as a ω-related phase with B82-type structure. It was shown, that in particular the duplex microstructure is prone to such microstructural instabilities. INTRODUCTION Since γ-TiAl based alloys are considered as structural materials for engineering applications in the temperature range of 650 to 850°C large efforts have been made to improve their oxidation and creep properties. In the frame of a new alloying strategy - based on addition of a high amount of Nb - a new class of γ-TiAl alloys has been established. These so-called TNB alloys exhibit significantly improved tensile strength, creep properties and oxidation resistance when compared to previous generations of TiAl-alloys [1-4]. At a fixed chemical composition the mechanical properties can be further improved by adjustment of appropriate microstructures, which is usually achieved by high-temperature heat treatments followed by fast cooling. However, such heat treatments lead often to constituting phases being far from thermodynamic equilibrium, which consequently makes the microstructure prone to thermal instability. The objective of the present paper was to investigate the influence of three different microstructures (du
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