ffects of Carbon Content, Annealing Condition and Internal Defects on the nucleation, growth and coarsening of P-type Ca
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Effects of Carbon Content, Annealing Condition and Internal Defects on the nucleation, growth and coarsening of P-type Carbides in High Niobium Containing TiAl Alloys Li Wang1, Florian Pyczak1,2, Heike Gabrisch1, Uwe Lorenz1, Mathias Münch2, Frank-Peter Schimansky1, Andreas Schreyer1, Andreas Stark1 1 Helmholtz-Zentrum Geesthacht, Institute of Materials Research, Max-Planck-Strasse 1, Geesthacht, D-21502, Germany 2 Brandenburgisch Technische Universität Cottbus-Senftenberg Konrad-Wachsmann-Allee 17, Cottbus, D-03046, Germany ABSTRACT The nucleation, growth and coarsening of carbides is investigated in high niobium containing TiAl alloys by diffraction and transmission electron microscopy. Higher carbon content increases the dissolution temperature of carbides. The solubility of carbon is much higher in a /2-phase alloy than in the phase alone. Hereby no significant influence of Nb on carbon solubility is found. Crystallographic defects as grain boundaries and dislocations promote carbide nucleation which results in a carbide precipitation sequence starting first at grain boundaries, then at dislocations and only later in the matrix away from crystallographic defects. The consumption of carbon by grain boundary carbides or neighboring 2 grains also generates a precipitate free zone in grains near the grain boundary. INTRODUCTION TiAl alloys are increasingly used especially in aero engine applications due to their superior specific strength and creep resistance up to about 700 °C compared to Ni-base superalloys [1]. Nevertheless, to widen the application range of TiAl alloys, a further increase of the creep and high temperature strength is desirable [2,3]. The addition of carbon is a means to achieve this either by carbon solid solution hardening or by precipitation hardening via carbides [4,5]. Unfortunately there are few investigations of carbide precipitation in actual TiAl alloys which exhibit aluminum contents of 45 at.% or below and higher amounts of phase stabilizing elements like niobium or molybdenum. These alloys differ in a number of ways from established variants. They typically exhibit a finer grain structure and higher fraction of internal interfaces as well as slower diffusion due to the higher amount of heavy alloying elements [6]. Due to this also diffusion controlled processes take more time. In addition an increased solubility of carbon in the phase of these alloys caused by niobium addition is reported [7]. This paper presents how carbon content influences the temperature stability of carbides, how much carbon can be dissolved in the phase and how grain boundaries influence the nucleation of carbides in high niobium containing fine grained TiAl alloys. EXPERIMENT Two TiAl alloys with the compositions Ti-45Al-5Nb-0.5C and Ti-45Al-5Nb-0.75C (all atomic percent) were produced by powder metallurgy. These alloys are in the following called 0.5C and 0.75C. Alloy powder was made by the plasma melting induction guiding gas
atomization (PIGA) method [8]. The particle size fraction < 180 m
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