Precipitation Phenomena and Strain Hardening of Intermetallic Titanium Aluminides
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Precipitation Phenomena and Strain Hardening of Intermetallic Titanium Aluminides J. Müllauer, F. Appel Institute for Materials Research, GKSS Research Center, Max-Planck-Strasse, D-21502 Geesthacht, Germany ABSTRACT In two-phase titanium aluminide alloys, the implementation of precipitation reactions is a widely utilized concept to control the microstructure and strengthen the material. A study has been made on the influence of carbide and boride precipitates on dislocation mobility and strengthening at 300 K. Compression tests were carried out for characterizing the mechanisms determining flow stress and dislocation glide resistance. The interaction mechanisms between the precipitates and dislocations were assessed by thermodynamic glide parameters and transmission electron microscopy. It has been shown that small titanium boride precipitates and carbide precipitates of perovskite type act as long-range dislocation glide obstacles. The interaction between the dislocations and the borides and carbides mainly leads to an athermal stress contribution. However, the dislocation-particle interactions are quite different. Small groups of borides are encircled by dislocations. This gives rise to the formation of loop structures the density of which increases with strain. On the contrary, the carbide precipitates are shearable and can be overcome without Orowan looping. This different behaviour is also reflected in the work hardening characteristics. Whereas the work hardening coefficient of the boron doped material increases with increasing B-concentration, it is independent of concentration in the case of the carbon-doped material. INTRODUCTION Intermetallic titanium aluminides based on the two phases γ-TiAl (L10) and α2-Ti3Al (D019), have a significant potential to be used in high-temperature structural applications in the automotive and aero-industry whenever specific strength and stiffness are of major concern [1, 2, 3]. In recent years, much emphasis has been placed on getting a good balance of mechanical properties from room temperature up to the moderately high service temperatures of about 700°C. In this respect, significant improvements have been achieved by microalloying with ternary elements, for example boron or carbon. It is well known that boron additions are beneficial for refining and stabilizing the microstructure [4]. An appreciable improvement in strengthening and creep resistance has been achieved by carbon additions and suitable thermal treatments [5-7]. Compared with the large amount of mechanical data, which has been established for these alloys, little information on work hardening is available. This essentially reflects the limited tensile ductility of the materials. However, in compression, it is possible to produce work hardening over large plastic strain ranges. For example, at room temperature values of σ/µ >3. 10-2 can be achieved which clearly presents a significant potential to strengthen the material. Analysis of work hardening in terms of dislocation mechanisms is hindered by the
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