Due to its high specific strength at elevated temperature, the gamma TiAl alloy has been known to be one of the strongest candidate materials for light and high-temperature structural materials. It is also well known that the mechanical property of the gamma TiAl alloy is strongly dependent on its microstructure. Duplex structure has been developed to have sufficient room-temperature ductility, while this microstructure shows poor creep resistance and low fracture toughness compared with lamellar structure. Therefore, many researchers have concentrated on developing the lamellar TiAl alloy and evaluating its high-temperature mechanical property.
The lamellar TiAl alloy designed for light and high-temperature applications is generally exposed to creep-fatigue deformation. Therefore, for the purpose of safety and performance improvement, studies on creep- fatigue deformation are necessary. However, most investigators have focussed mainly on fatigue crack growth resistance and fatigue fracture behavior under high cycle fatigue condition.
In the present study, total strain range controlled creep-fatigue and continuous low cycle fatigue tests with lamellar Ti-46.6Al-1.4Mn-2Mo (at.%) alloy have been conducted to investigate high-temperature low cycle fatigue damage property. It is observed that fatigue life is drastically reduced in the creep-fatigue test compared with that of the continuous fatigue test. It is generally understood that this reduction of fatigue life is due to the introduction of creep deformation during hold time.
It is well known that the rate controlling process of creep deformation of most metallic materials is dislocation climb assisted by self-diffusion, and that the main creep damage is grain boundary cavitation. However, recent reports indicate that the lamellar TiAl alloy has a different creep deformation mechanism from that of most metallic materials. It has been reported that α$_2$ → γ phase transformation accompanying dislocation generation at the lamellar interface is the rate controlling process of creep deformation. Therefore, it can be assumed that the introduction of creep deformation in the creep-fatigue test causes α$_2$ → γ phase transformation at the lamellar interface, which implies that plastic deformation is easily concentrated on the phase-transformed γ phase in the matrix. Therefore, transgranular fracture mode is assumed in the creep-fatigue test.
However, the previously mentioned assumption is proved to be wrong by the observation of fatigue fracture surface. It is observed that intergranular fracture mode is predominant in the creep-fatigue test, while transgranular fracture mode is predominant in the continuous fatigue test. This suggests that the introduced creep deformation in the creep-fatigue test weakens a grain boundary rather than a matrix. Therefore, it is understood that creep-fatigue fracture behavior is controlled by a different mechanism from α$_2$ → γ phase transformation at the lamellar interface, i.e., pure creep deformation mechanism.
In order to investigate the reason for the grain boundary weakness under creep-fatigue deformation, microstructural and compositional analyses were conducted in the present study. The experimental results clearly indicate that the introduction of creep deformation during hold time causes α$_2$ → γ phase transformation at the grain boundary to induce the formation of grain boundary γ phase. Because the grain boundary γ phase is proved to be the region where plastic deformation is concentrated, it is confirmed that α$_2$ → γ phase transformation at the grain boundary reduces the fatigue life by inducing intergranular cracking and grain boundary weakening.
In addition, it is observed that increased strain assists α$_2$ → γ phase transformation at the grain boundary to result in the formation of grain boundary γ phase and intergranular cracking. Therefore, it can be concluded that high-temperature low cycle fatigue damage mechanism with hold time and strain is α$_2$ → γ phase transformation at the grain boundary. Furthermore, the method to extend low cycle fatigue life by retarding α$_2$ → γ phase transformation at the grain boundary is proposed, and the method is proved to be right from the results of low cycle fatigue tests with carbon-added alloy.
This investigation of grain boundary phase transformation with hold time and strain can be helpful to qualify lamellar TiAl alloy and to maximize the component reliability for the practical application. These results can also be the basis of developing new alloys with better high-temperature mechanical properties.