Since the last decade, TiAl alloys have been regarded as a one of the strongest candidate materials for light and high temperature structural materials. These alloys have been studied by the automotive industries and aerospace communities for use in aircraft propulsion components and intake/exhaust valves of automobiles engines. One of the most important properties of these applications is creep resistance, in particular within primary creep regime.
It is well known that the steady state creep deformation mechanism of pure metals and certain solid solution alloys is the dislocation climb process with the help of self-diffusion. Therefore, the creep deformation activation energy in the steady state is measured and known to be equal to that of self-diffusion. In addition, within the primary stage, the dislocation density increases with creep strain, and the measured activation energy in this stage is lower than that of the steady state because of higher effective stress in primary regime than the steady state.
Lamellar TiAl alloys have much different microstructural features from pure metals and certain solid solution alloys. These microstructural features of the lamellar TiAl alloys affect the creep deformation mechanism within the primary creep regime resulting in different mechanism from that in pure metals and certain solid solution alloys.
In this study, the primary creep deformation mechanism of lamellar TiAl alloys, which are EPM alloy of Ti-46.6Al-1.4Mn-2Mo (in at.%) and K5CBS alloy of Ti-45.5-2Cr-2.6Nb-0.17W-0.1B-0.2C-0.15Si (in at.%), have been investigated. As in the case with metals, the normal primary creep stage was observed. In addition, as the creep strain increased within the primary regime, it was found that the creep activation energy increased from a low value and saturated to a high value at steady state. The normal primary behavior and the trend of activation energy variation with creep strain are the same manner with the usual metals. However, the activation energy at steady state (380kJ/mol for EPM and 390kJ/mol for K5CBS) is higher than that of diffusion (self-diffusion of Ti and/or interdiffusion in TiAl, 300kJ/mol). This result is different from that of metals. In addition, the observation result that the dislocation density decreased as the creep strain increases is a opposite phenomenon to that of usual metals. So that the effective stress reduction can not be the reason for the activation energy variation of the lamellar TiAl alloys. In addition, the climb is not the rate controlling process in the steady state in lamellar TiAl alloys.
New creep deformation mechanism for the lamellar TiAl alloys is suggested in this study under the basis of experimental results as follows.
(1) the activation energy at steady state is measured to be higher than that of diffusion about 100kJ/mol as mentioned before.
(2) as the creep strain increases within the primary regime, the activation energy for creep deformation increased from about 300kJ/mol and saturated to about 400kJ/mol at steady state.
(3) during the primary creep deformation, the dislocation density decreased.
(4) at the beginning stage of primary creep deformation is controlled by initial dislocation climb process.
(5) the dislocation generation process, which is needed for continuous creep deformation, is controlled by phase transformation from $α_2$- to γ-phase.
It is newly suggested that the phase transformation of $α_2$- to γ-phase is the creep deformation rate controlling process having higher activation energy than that of diffusion at the steady state. And the creep deformation rate controlling process within the primary stage shifts from the initial dislocation climb to the phase transformation as the creep strain increases.
It is expected that if the initial dislocation density can be reduced by some routes, the primary creep resistance of lamellar TiAl alloys will be enhanced because the newly suggested creep deformation mechanism says that the early stage of primary creep deformation proceeds by the initial dislocation glide assisted by climb. A small amount of prestrain at high temperature with slow tension rate, which is the same order with initial creep deformation rate, was applied to the lamellar TiAl alloy to reduce the initial dislocation density by simulating the early stage of primary creep deformation. As expected by newly suggested creep deformation mechanism, enhancement of primary creep regime was obtained, and the reduction of initial dislocation density by prestraining was found to be responsible for this beneficial result. In addition, the result that the activation energy of the prestrained specimen in early stage of primary creep is as high as the steady state clearly confirms the newly suggested creep deformation mechanism of the lamellar TiAl alloys.
The effects of N or C addition and heat treatment condition on the Ti-48Al-1.5Mo alloy have been studied also in this investigation. In addition, the effect of varying the manufacturing process of EPM alloy, and 0.3C or 2vol.$%TiB_2$ addition effect on creep resistance were investigated.
