Static and Dynamic Strain Ageing in Two-Phase Gamma Titanium Aluminides
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Static and Dynamic Strain Ageing in Two-Phase Gamma Titanium Aluminides U. Christoph, F. Appel GKSS Research Centre, Institute for Materials Research, D-21502 Geesthacht, Germany ABSTRACT The deformation behaviour of two-phase titanium aluminides was investigated in the intermediate temperature interval 450-750 K where the Portevin-LeChatelier effect occurs. The effect has been studied by static strain ageing experiments. A wide range of alloy compositions was investigated to identify the relevant defect species. Accordingly, dislocation pinning occurs with fast kinetics and is characterized by a relatively small activation energy of 0.7 eV, which is not consistent with a conventional diffusion process. Furthermore, the strain ageing phenomena are most pronounced in Ti-rich alloys. This gives rise to the speculation that antisite defects are involved in the pinning process. The implications of the ageing processes on the deformation behaviour of two-phase titanium aluminide alloys will be discussed. INTRODUCTION Gamma based titanium aluminide alloys are promising candidates for high temperature structural applications. Current investigations focus on Ti-rich alloys close to the stoichiometric composition of γ(TiAl) with modest amounts of several ternary elements [1,2] which are mainly composed of the intermetallic phases α2(Ti3Al) and γ(TiAl). The deformation behaviour of the two-phase alloys is characterized in the temperature interval 450-750 K by discontinuous yielding and a negative strain-rate sensitivity. These attributes have been associated in several studies with the Portevin-LeChatelier effect [3-8]. The effect has been closer investigated by static strain ageing experiments. Accordingly, the diffusion mechanism which underlies ageing is characterized by an activation energy of about Qa = 0.7 eV [5-8]. On the other hand, the self diffusion energy for Ti in γ(TiAl) has been estimated as Qsd = 3.01 eV [9] while recent measurements have led to Qsd = 2.6 eV [10]. This energy is significantly higher than Qa and indicates that the defect transfer onto the mobile dislocations cannot be explained by a conventional vacancy exchange mechanism. Unfortunately no values for the migration energies of fast diffusing elements in γ(TiAl) have been definitely established yet. Thus it is difficult to speculate about the transfer of impurity atoms onto the dislocations. Therefore further investigations have been necessary to identify the defect species responsible for ageing. Information on this matter can be obtained through the variation of the defect densities by varying the alloy compositions. Therefore a wide range of alloy compositions has been investigated. EXPERIMENTAL DETAILS The compositions (in at. %) and thermomechanical treatments of the alloys are described in detail elsewhere [11]. The alloys are inserted in the TiAl phase diagram (Fig. 1) in accordance with the volume fractions of the phases α2(Ti3Al) and γ(TiAl) and the temperatures of thermomechanical treatment thereby indicating that single-phase α2, two-ph
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