Massive Transformation in High Niobium Containing TiAl-Alloys
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Massive Transformation in High Niobium Containing TiAl-Alloys A. Bartelsa, S. Bystrzanowskia, H. Chladilb, H. Leitnerb, H. Clemensb, R. Gerlingc, and F.-P. Schimanskyc 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 ABSTRACT Massive transformation in high Nb bearing γ-TiAl-based alloys, Ti-45Al-7.5Nb and Ti-46Al9Nb (at.%), and the thermal stability of the resulting microstructure were investigated. Using a quenching dilatometer, a nearly complete massive transformation in Ti-45Al-7.5Nb was found at about 1050°C after annealing at 1305°C for 10min and subsequent cooling with a rate of 55K/s. Higher starting temperatures and higher cooling rates lead to incomplete massive transformation and small transformed areas situated at the grain-boundary triple points of the parent α-grains are observed. By means of EBSD only in one case the same orientation of the close-packed planes of parent α-grains and of massively transformed γM-areas was observed. The thermal stability of the microstructure of massively transformed Ti-46Al-9Nb sheet material was tested by annealing samples for 1 hour between 400 and 1200°C. Above 800°C a drop of hardness was measured and X-ray diffraction patterns show an increasing separation of (200)γ and (002)γ reflections as expected from a tetragonal γ-TiAl lattice. After annealing at 1100°C α2phase segregates at grain boundaries and after 1200°C α2-lamellae appear insides the γM-grains parallel to all four {111}γ-planes. INTRODUCTION Over the last years new concepts in alloy design of γ-TiAl based alloys were employed and a new alloy class exhibiting a high Nb content (5-10at.%) was developed [1,2]. These so-called TNB alloys combine high tensile strength at elevated temperatures with good ductility at room temperature. The high Nb content reduces the stacking fault energy and leads to widely dissociated superdislocations. As a consequence cross-slip and climb processes are effectively hampered [1]. Additionally, a high Nb-content diminishes diffusion processes in these alloys and thus decreasing the climb rate of dislocations [1,3]. Normally, in these new alloys a low Alcontent around 45at.% is adjusted. The resulting lower α-transus temperature in combination with decelerated diffusion due to niobium facilitates massive transformation during cooling from the α-phase field. The massive transformation in γ-TiAl based alloys is subject of intensive research and of special interest because technical alloys seldom show massive transformation behavior [4]. In TiAl alloys these microstructural features can be used to increase the strength or to establish special microstructures by subsequent heat treatments in the (α+γ)-phase field, e.g. fine γ-grains with crossed α2-lathes [5]. Therefore, the aim of this study was to investigate massive transformation in two high Nb-cont
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