Perovskite Ti 3 AlC Carbide Splitting in High Nb Containing TiAl Alloys
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Perovskite Ti3AlC Carbide Splitting in High Nb Containing TiAl Alloys Li Wang, Heike Gabrisch, Uwe Lorenz, Frank-Peter Schimansky, Andreas Stark, Florian Pyczak Helmholtz-Zentrum Geesthacht, Institute of Materials Research, Max-Planck-Strasse 1, Geesthacht, D-21502, Germany
ABSTRACT Transmission electron microscopy has been used to investigate the morphological development of the perovskite (P-) Ti3AlC carbides in the γ matrix of a Ti-45Al-5Nb-0.75C alloy during annealing. P-Ti3AlC carbides in the γ matrix initially have a needle-like shape but during annealing at 800 °C they change to a plate-like shape. In the needle-like shape the carbides are orientated parallel to the [001] direction of the matrix. They extend along the [100]γ or [010]γ direction into plates later and subsequently split into sub particles after extended annealing. It is proposed that the elastic interaction energy between the split sub domains may be the reason that this decomposition into sub-particles is energetically favorable. INTRODUCTION The mechanical properties of γ-TiAl based alloys are improved by the addition of carbon via precipitation hardening by the Ti3AlC perovskite phase [1, 2]. Usually P-Ti3AlC carbides that precipitate in the γ matrix are coherent with the γ phase and have a needle-like shape. During extended annealing they coarsen and finally lose coherency [3]. In TiAl alloys with 0.5 mol% C Tian et al. [4] also reported a long-range ordering of carbon-vacancies in the P-Ti3AlC carbides and two ordered domains coexisting in a single needle after annealing at 800 °C. In order to raise the application temperature above 700 ºC, γ-TiAl alloys with a high amount of Nb have been developed, which possess high strength [5], excellent creep resistance [6] and good oxidation resistance [7]. For efficient precipitate-strengthening the stability of the precipitates, their shape and the character of the interface between the matrix and the precipitate are very important factors. However, there is little published work on these aspects for such high Nb containing TiAl alloys [3]. In order to obtain information on the shape development of perovskite carbides in this alloy system, a Ti-45Al-5Nb-0.75C alloy (in atomic percent) was produced by powder metallurgy (PM) and then heat-treated. The resulting microstructures were investigated using transmission electron microscopy. EXPERIMENT The alloy was made by a PM route and had the composition of Ti-45Al-5Nb-0.75C. The pre-alloyed powder (particle size < 180 μm) was consolidated by hot isostatic pressing at 1250 °C and 200 MPa for 2 h. Two heat treatment routes were used in the investigation. Some HIPed specimens were directly annealed at 800 °C for 24, 48, 96, 168, 336 and 1054 h (referred to as HIP+anneal. in the paper). Other specimens were annealed using the same conditions but after a solution treatment at 1250 °C for 5 h (oil quenched) (referred to as SOL+anneal.). The specimens were examined using a Philips CM200 transmission electron microscope (TEM) operating at 200 kV and a FEI Tita
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