New breeds of near-α titanium alloys such as Ti-1100 and IMI834 were developed to be used for the compressor parts of gas turbine engines. These alloys have an improvement of about 50℃ in operating temperature over the conventional titanium alloys. Compressor parts of gas turbine engine are subjected to cyclic and static loading simultaneously at high temperature and can be degraded by creep-fatigue interaction. In order to apply these materials in practical use, the resistance to the degradation caused by creep-fatigue interaction has to be considered. However, the effects of creep-fatigue interaction on the high temperature low cycle fatigue(HTLCF) behavior in these newly developed titanium alloys and their damage mechanisms under these complicated loading conditions are not clearly understood.
In this study the effects of microstructure and various superimposed hold times on HTLCF behavior of these materials were investigated and the damage process under these conditions was discussed. In continuous LCF tests, fatigue life of Ti-1100 with bi-modal structure is longer than that of lamellar structure. However, as the hold time is increased, bi-modal structure leads to a larger amount of stress relaxation during hold time to give a higher reduction rate of fatigue life. It can be explained that creep damage resulting from stress relaxation during hold time is more detrimental in the bi-modal structure. Creep deformation during hold time resulted in a change of dislocation structure from planar form to homogeneous distribution within the α lamellae. Apparent activation energy for creep deformation of about 500∼520 kJ/mol was obtained from the stress relaxation curve and was consistent with results of other near-α titanium alloys. In all cases failures were fatigue-dominated, although creep cavity and internal crack formation were observed in hold time specimens. In the lamellar structure, creep-fatigue life decreases with increasing α lamellae width. This can be ascribed to more significant creep damage accumulation related with higher stress relaxation. This can be explained by the gradual change of the damage mechanism from fatigue dominant crack propagation to creep dominant grain boundary failure. Since the lamellar structure in Ti-1100 alloy has a higher resistance to the additive creep damage during hold time, lamellar structure with finer α lamellae width is considered to have better performance for high temperature applications which require long-term creep-fatigue resistance.
The creep-fatigue characteristics of IMI834 were also investigated. It was found that in bi-modal structure of IMI834, low primary α volume fraction in the order of 10 % leads to better creep-fatigue resistance at 600℃.
This evaluation can be helpful to qualify these alloys for the use in gas turbine engine application and to maximize the operating time with good safety and performance of these alloys under operating condition. These results can be also bases of developing new titanium alloys which have good mechanical properties at high temperature. The knowhow of evaluating and developing new alloys used for aerospace industry is important because the interest is taken in aerospace industry.
